WORKSHOP: DISPLAY METROLOGYcormusa.org/.../2018/...Ed_Kelly_Display-Metrology.pdf · WORKSHOP:...

WORKSHOP: DISPLAY METROLOGYCORM — Council for Optical Radiation MeasurementsMay 9-11, 2010Las Vegas NevadaLas Vegas, Nevada

Edward F. Kelley, Ph.D.KELTEK, LLC.ed@keltekresearch com

Portions of this presentation are copyrighted.© E F Kelley of KELTEK 2010

[email protected] www.keltekresearch.com(or … 4keltek.com)

© E. F. Kelley of KELTEK, 2010.

KELTEK

OUTLINEWORKSHOP TITLE: Display MetrologyWORKSHOP TITLE: Display MetrologyABSTRACT: Display characterization presents some challenges to

the application of radiometry, photometry, and colorimetry that metrologists would normally work to avoid. We discuss the g ydifficult measurement problems associated with displays and how such measurement results are presently obtained. Difficult contrast, reflection, and detail measurements are discussed.

INTRODUCTION TO DISPLAY INDUSTRYDETECTOR PROBLEMSSTRAY LIGHT MANAGEMENT

SMALL-AREA MEASUREMENTSSETUP OF DISPLAYSETUP OF DISPLAYTYPES OF DISPLAY METRICS (A FEW)

KELTEK 2

INTRODUCTION TO DISPLAY INDUSTRY

Management sometimes wonders similarly withwonders similarly with the addition: “Why does this cost so much?!”

I was telling a non-technicalf i d th t I i l d ifriend that I was involved inmaking display measurements.He laughed and exclaimed, "Wh ' h d b h !""What's so hard about that!"Um... it was very hard to explain. How embarrassing!

KELTEK 3

p g

Complications:Not as Simple as It Looks!!!

Complications:Photometry-colorimetry uncertainties START at 1 % level.

Calibration from NRC or NIST may provide you with 0.5% relative uncertainty with a coverage factor of 1 (one sigma).What happens if it is dropped in shipping or you bump it?pp pp pp g y p

Our eye response quasi-logarithmic yet our measurement instruments are linear.

S ’t f th bl iSo, we won’t see some of the problems we are measuring.Sound and acoustics got it right using decibels, too bad we don’t have a decibulb to use.

Display output can drift in time both in color and luminance.

Again our eyes won’t notice but our instruments will

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Again, our eyes won t notice, but our instruments will.They build displays for our eyes.

Confusion in Display Industry

Luminance & illuminance (cd/m2 vs. lx)

Flux vs illuminance (lm vs lx)Flux vs illuminance (lm vs. lx)

Calling candela a unit of luminance

“ANSI l ” [ !] “ANSI fl ”“ANSI lumen” [wrong!] vs. “ANSI flux”

Specular confusion

Lambertian confusion

Diffusion confusion

Vision confusion

KELTEK 5

We've never needed to measure a contrast over 100:1 — after all it's all the 100:1 after all, it s all the eye can see anyway.

REDNECK METROLOGY

KELTEK 6

DETECTOR PROBLEMS

FILTERED DEVICES

SPECTRORADIOMETERS

ARRAY DETECTORS (CAMERAS)

DETERMINATION OF SCREEN NORMAL

SUBTENSE OF DETECTOR (ANGULAR APERTURE)

KELTEK 7

Filtered Devices1.41.61.8

(λ)

Detectors, Cont.

Luminance meters, illuminance meters, tristimulus colorimeters, photopic array cameras 0 2

0.40.60.8

11.2

z(λ)

x(λ)

y(λ)

photopic array cameras...0

0.2

350

400

450

500

550

600

650

700

750

800

850

λ (nm)

Small errors in the filters that duplicate th l t hi

0.8

1.0

T tLCD

nce

the color matching functions can affect narrow-band source measurements more 0.4

0.6Tungsten

CRTrmal

ized

Rad

ian

(Ugly!)

than broad-band source measurements.

0 0

0.2

Nor

KELTEK 8

400 450 500 550 600 650 700 7500.0

Wavelength (nm)

DETECTORS

Colorimeter (<$6k)

Color Meter (~$11k)

Colorimeter (<$6k)

Color Meter (~$11k)

Spectroradiometer (~$25k)Spectroradiometer (~$25k)

Tube and glossy frusta to eliminate Tube and glossy frusta to eliminate

Integrating-Sphere Source with tungsten-halogen

Integrating-Sphere Source with tungsten-halogen

stray lightstray light

Narrowband and broadband filters to test effectiveness of Narrowband and broadband filters to test effectiveness of

We compare how well different types of color-measurement devices

g gbulb and R, B, G, and W LEDs

g gbulb and R, B, G, and W LEDs

color measurementscolor measurements

KELTEK 9

We compare how well different types of color-measurement devices perform color measurements on different sources.

Detectors, Cont. Detectors X02

SOURCETungsten-Halogen

BROADBAND FILTERSCIE 1931 (x,y)

0.7

0.8

0.9

BROADBAND FILTERS Colorimeter

Tungsten-Halogen

0.5

0.6

0.7 Color Meter Spectroradiometer

0.3

0.4y

0.1

0.2

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

x

Notice how the all hit white (~Illuminant A, 2856 K)

KELTEK 10

correctly. The color meter and the spectroradiometer agree very well.


R, G, B, AND W LEDCIE 1931 (x,y)

0.7

0.8

0.9

0.5

0.6

0.7

0.3

0.4y

0.1

0.2

Not bad showing a little disagreement between

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

x

KELTEK 11

Not bad, showing a little disagreement between the color meter and the spectroradiometer.


NARROWBAND FILTERS CIE 1931 (x,y)0 9 CIE 1931 (x,y)

0.7

0.8

0.9

NARROWBAND FILTERS Colorimeter

C l M t

0.5

0.6

Color Meter Spectroradiometer Peak NB Wavelengths

0.3

0.4y

400

0.1

0.2

700

These are interference filters with 10 nm

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

x

700

KELTEK 12

These are interference filters with 10 nm bandwidths. Depressing! The spectroradiometer does the best job, but not as well as we’d wish.


LCD FPD CIE 1931 (x y)0 9

CONCLUSION:CIE 1931 (x,y)

0.7

0.8

0.9

LCD FPD R, G, B, W Colorimeter Color Meter

Spectroradiometer

Filter-based colorimeters tend to have trouble with narrowband sources. Spectroradiometers work the best.

0.5

0.6

SpectroradiometerLCD

0.008

0.009

the best.

0.3

0.4

y

0.005

0.006

0.007

ctra

l Rad

ianc

e

0.1

0.2

0.002

0.003

0.004

Rel

ativ

e Sp

ec

Also depressing! Whom (or what) do you trust? B d th b d lt ’d h t

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

x 0

0.001

350 400 450 500 550 600 650 700 750 800Wavelength (nm)

W B G R

KELTEK 13

Based upon the narrowband results, we’d have to trust the spectroradiometer.

W B G R

Color Diagnostics Using IFsInterference Filters Test Color Measurement: Assuming white point

Detectors, Cont.

Interference Filters Test Color Measurement: Assuming white point calibration is accurate, the nearness of the (x,y) of narrow-band interference filters to the spectrum locus provides an indication of instrument’s accuracy within the spectrum locus.

0.9

0.8

0.7

INTERFERENCEFILTER NDF

0.6

0.5

0.4y

DETECTOR APERTURE0.3

0.2

0.1 Ideal distance from spectrum locus is

DETECTOR DIFFUSESOURCE

0.80.70.60.50.40.30.20.10

0x

pdetermined by bandwidth of filter and curvature of locus.

Watch out for pinholes in the interference filters!!!

If your white point is correct and if the data are close to the spectrum l th t t th

KELTEK 14

the interference filters!!! Cover them up or get new filters.

locus, then you can trust the instrument over the gamut.

NOTE

Array Detector ProblemsDetectors, Cont.

Photopic ResponseSensitivity to IR can seriously corrupt what was intended to be a l i t Th filt b i (f

Flat-Field Correction (FFC)

luminance measurement. These filters can be expensive (from $500 to $1000 [US] or more).

Flat-Field Correction (FFC)Nonuniformity partially corrected by FFC. FFC may change with lens and object configurations.

We are assuming a background subtraction is performed before the FFC. The FFC can change for the type of lens used, the f-stop, the focus, the size of the light-area measured and its distance, etc. Very diffi lt t t l t b t l if fdifficult to accurately create because a truly uniform source of sufficient size is hard to obtain and because the correction needed can change so much with conditions. Be careful. What will serve as a FFC for one configuration may not for another!!

KELTEK 15

a FFC for one configuration may not for another!!

Raw pixel output: Rij

Array Detectorp p ij

Background (no light): Bij

Flat-field correction: Fij

Fi l i l S F (R B )Final signal: Sij = Fij(Rij – Bij)

The flat-field correction (FCC) attempts to fcorrect for:

Pixel-to-pixel sensitivity nonuniformty 1/cos4θ falloff for lens or aperturepIrregularities in the lens

The FFC may only be partially successful Any pixel to pixel nonlinearityThe FFC may only be partially successful. Any pixel-to-pixel nonlinearity, differing linearity, and spectral sensitivity nonuniformity will affect the success of using the FFC in addition to the problems stated previously. Obtaining a 1 % uncertainty using a FFC may be a challenge.

KELTEK 16

Array Detector Problems, Cont.1.0

Detectors, Cont.

cos4θ Falloff0.6

0.8

0.2

0.4

cosθcos4θ

±10°

-80 -60 -40 -20 0 20 40 60 800.0

Angle (°)

1° = width of little finger0.99

1.00

cosθ 1 width of little finger at arm’s length or width of thumbnail at arms length ( lib ti i d

0.97

0.98cosθcos4θ

(calibration required before use!).

½° = sun, moon angular diameter.0.95

0.96

KELTEK 17-10 -8 -6 -4 -2 0 2 4 6 8 10

0.94

Angle (°)

Array Detector Problems, Cont.

Spatial Aliasing (Moiré Patterns)

Detectors, Cont.

KELTEK 18

Determination of Screen Normal ±1° i l 1° th l idth f th b il littl

Detectors, Cont.

±1° is sloppy — 1° = the angular width of your thumbnail or little finger with an out-stretched arm, twice the angular width of the sun or moon (0.5°). Try for ±0.3° or ±0.1°.

Methods to find normal [FPDM A115] :

1. aligning virtual image of detector lens (if visible) with center of eyepiece, orcenter of eyepiece, or

2. centering reflection of small bulb in horizontal and vertical (if virtual image is not visible), etc.

(1) Visible virtual image of detector

(2) No virtual image, with flashlight in two positions

KELTEK 19

Note: Not all screens should be measured at normal. It depends upon design and task.

Be aware of rays of light contributing to the signal. Some displays have aSubtense of Detector & Region Measured

Detectors, Cont.

Be aware of rays of light contributing to the signal. Some displays have a viewing-angle dependence, and we can inadvertently measure what our eyes don’t see. [FPDM A102-1]

θL is roughly the angular aperture (most are not cognizant of its affects) L g y g p ( g )θF is the measurement field angle

The measurement field angle quoted for a luminance meter refers

KELTEK 20

The measurement field angle quoted for a luminance meter refers to the angle θF measured at infinity focus. For nearer subjects the angle θF may be different (often smaller).

Subtense of Lens a FactorSubtense of Detector & Region Measured, Cont.Detectors, Cont.

Top photo at f/2.8 gathers light from many directions through the diffusing front glass of the FPD. Bottom photo at f/32 is more the way the eye sees things with a much narrower

f/2.8

the eye sees things with a much narrower angular aperture. (Lens f# = f/D=focal-length/diameter: At f/2.8 f=60 mm lens has D=21 mm whereas at f/32 D=1.9 mm.) Diagram i i t l t l W t bis approximately to scale. We must be concerned about just what the detector is seeing and measuring.

f/32

LensCCD60 mm at

KELTEK 21FPD

60 mm at f/2.8 or f/32

Large Solid Angle of DetectorNote how much

Subtense of Detector & Region Measured, Cont.Detectors, Cont.

Note how much lighter the black pixels are at the top compared totop compared to the bottom or central regions.

90 mm Lens Close to FPD90 e s C ose to

What does this tell you about using a microscope to view the surface and then generalizing what the

KELTEK 22

Magnifier Configurationg g

display looks like under normal viewing conditions? Be careful!

STRAY LIGHT MANAGEMENT

St Li ht Withi Di l D iStray Light Within Display Device at the Region of the Pixel Surface

White px Black px White px White px

FPD f t f i l it tFPDs — front surface near pixels permits strong diffusing surface with some resulting internal scattering and reflections.

CRTs — front surface significantly separated from pixels provides more reflection plus internal scattering and beam halation behind pixel surface.

KELTEK 23

Stray Light Within Detector – Veiling GlareStray Light Management, Cont.

Complex LensShutter CCD with

Cover Glass

PhotopicFilter

ObjectIris Image

ObjectOriginal Veiling Glare Lens Flare

Reflection off of internal lens structure

Reflection between lens surfaces

KELTEK 24

Veiling Glare Can Affect Full-Screen Measurements

Stray Light Management, Cont.

1.5° Subtense with MaskMeasurement of Full-Screen WhiteWhiteComparison of two identical luminances h i diff t 15° Subtense without Maskhaving different angular sizes. Same screen with & without

Increase in measured luminance

mask (1.5° or 15°vertical angular diameter of white area from lens of with mask removed:

Instrument #1 0.4 %Instrument #2 1.3 %Instrument #3 4 8 %

area from lens of detector)

KELTEK 25

Instrument #3 4.8 %

Veiling Glare Can Affect Box Measurements


Measurement of Black Rectangle on White

1.5° Subtense with Mask1.5° Subtense with Maskon WhiteThis shows how important it is to anticipate veiling

l i thglare in the detection system. Same screen with & without mask

15° Subtense without Mask15° Subtense without Mask

(1.5° mask hole, 15° vertical angular diameter of white area from

Increase in measured luminance of white area from lens of detector)

with mask removed:Instrument #1 50 %Instrument #2 325 %Instrument #3 1200 %

KELTEK 26

Instrument #3 1200 %

Use of Masks — Flat and Frustum


DETECTOR VIEWUSING FRUSTUMUSING FRUSTUM

DetectorFrustum Aperture

Measurement Field

Side view with flat mask Side view with frustum maskSide view with flat mask Side view with frustum mask

KELTEK 27

With some very high contrast displays, the back reflections off flat masks can cause substantial errors.

Frustum Mask Compared to Flat Mask


Only if the flat mask is placed very near or on the surface of the FPD screen can it compare with the frustum mask. (CRTs have thick glass faceplates.)

Flat mask near or on screen may cause heatingFlat mask near or on screen may cause heating.

1.2

305%

RelativeError in Lb

z

Frustum Mask

1.0

ack

(cd/

m )2 305%

Felt Flat Mask0.8

ance

of B

la

0 4

0.6

Lum

ina

17%

z

KELTEK 28

0.4

Distance from Screen (mm)200 40 60 80 100

, z (mm)

Avoid Vignette (vin-yet´) from MaskStray Light Management, Cont.

DISPLAYSURFACE DETECTOR< 45°θ

Keep in mind that if the mask is too close to the lens it can interfere with the measurement (especially when the hole is smaller than the lens).

w

FRUSTUM APERTURE

u

s

p

UR

ED A

REA

LENS

z maxz =

= d (s - u)w - u

zmax

NORMAL USE: <z z max

MEA

SU

OK Marginal Vignette

d

KELTEK 29

You will generally want the hole of the frustum to be as close to the display as possible without touching the display surface.

Halation — With and Without Masks (LCD Display)


1800TS

Halation With and Without Masks (LCD Display)

120014001600

CD

CO

UN

T

600800

1000

CE

IN C

C

no mask

200400600

LUM

INA

N

with masks

00% 20% 40% 60% 80% 100%

PERCENTAGE OF DIAGONAL

L

KELTEK 30

Large Frustum RequiredStray Light Management, Cont.

FPDDetector

Frustum

The frustum (gloss black) must be huge if the display is moderately large (even a computerdisplay is moderately large (even a computer monitor) in order to keep all stray light away from the detector.

Detector Measurement Field

Region needed to be k d f

Frustum Apex Hole

KELTEK 31

masked from detector’s viewpoint.

Simple SLET Instead of a Large Frustum


FPD This simple SLET is only for detectors that use a lens and have a measurement field

The front frustum is gloss black However the interior of this simple

have a measurement field within the frustum apex hole.

The front frustum is gloss black. However, the interior of this simple stray-light-elimination tube (SLET) is matte black for luminance meters, spectroradiometers, etc., detectors with clean lenses. For illuminance measurements a more complicated SLET is needed that has a gloss-black i t i di it i th j ti t tiinterior; we discuss it in the projection-measurement section.

Detector Measurement

Required minimum end of tube from

Frustum Apex Hole

Fielddetector’s lens

Outer Diameter

KELTEK 32

Frustum Apex HoleOuter Diameter of Front Frustum

MASKSStray Light Management, Cont.

KELTEK 33

Replica MasksSMALL-AREA MEASUREMENTS

BLACKPLASTIC

BLACKRECTANGULARREPLICA

Replica Masks

PLASTICSTRIP MASK

Look I've been measuring Look, I ve been measuring contrast for years, and I've never had to worry about veiling glare!

KELTEK 34RUSTIC METROLOGYREDNECK

Small Area Measurements, Cont.

Replicas, Same Size As Black RegionReplica masks must be close to (+0%, -10 %) the linear size of the black

t b d tharea to be measured—the closer the better.

Um The contrastUm... The contrastof the display wentup a factor of tenwhen we used areplica mask. Um...

KELTEK 35

phave you sent thoseresults out yet???


Filt K d k W tt

Replica Mask with Diagnostic Filter Mask**NOTE: A neutral

Lfc Filter calibration

LhWhite pixels

Filter, e.g., Kodak Wratten Neutral Density 1.0 or 2.0**

NOTE: A neutral density filter of 2.0 might be better. Camera-system nonlinearities can

l

Lm L Filter white

LdBlack pixels L

cause large errors. Also, any reflected light might escape detection using a #1 NDF.

Mask Lfwc Filter-white calibration

Corrected white: LW = Lh - Lm

LfFilter Transmission: τ = Lfc/Lfwc (filter

material has temperature dependence). Use frustum maskdependence). Use frustum mask to measure luminances here in a uniform part of screen.

Corrected black: LK = Ld - Lm

Contrast: C = LW / LK

Check: Does (Lf - Lm)/LW = τ ???

If so, measurement is probably good. (At least a lot better than if we didn’t do anything!)

KELTEK 36

NOTE: If using a CCD camera, it is always a good idea to have from 10 to 20 CCD pixels (or more) covering any line or area you wish to measure. Don’t be stingy! Note

BUT…

What was wrong with the previous slide???Suppose we wanted to measure a black line on a white screen. pp

Which configuration below is better and why?

Detector in position A will reflect light back onto the screen and contaminate the black level of the

D t t

Detector position

A

line because of reflections!!!

Detector position

B

A

Detector in position B, because it is considerably farther from the screen, will reflect less light back onto the display.reflect less light back onto the display.

The previous slide showed the detector too close to the screen.NOTE that this measurement result is also sensitive to reflections from other items in the room in addition to the detector!

KELTEK 37

Line Replica Mask for Line & Grille Contrast MeasurementsSmall Area Measurements, Cont.

Line contrast is:

C Sh - SgC = Sh Sg

Sd - Sg

Line mask material canLine mask material can be obtained from graphic-arts supply stores (black charting tape) or c t from blacktape), or cut from black plastic sheets. We can also cut a very narrow slightly tapered wedge

S is the veiling glare corruption

g y p gof black opaque plastic and measure Sg across the part with the appropriate width )

KELTEK 38

Sg is the veiling glare corruption.

S is the signal (proportional to luminance) as from a CCD camera.appropriate width.)

SLET St Li ht Eli i ti T b

Dark regions can be seriously corrupted by glare (n X 100 %). Correction for glare or the

Narrow Frustum SLET (NFS)Small Area Measurements, Cont.

SLET = Stray Light Elimination Tube elimination of it is required to make accurate measurements.Display

CCD

Lensd = effective size of lens

(d = f#/f, used f/16) FullS

dzd

Open

( )

d ≅ 3.8 mm, Screen

CCD Camera View

NFS #1

dd

zd ≅ 328 mm

Magnified

NFS #2

Magnified

KELTEK 39Credit: A. Badano and M. J. Flynn, “Method for measuring veiling glare in high

performance display devices,” Applied Optics, Vol. 39, No. 13, pp. 2059-2066, May 2000.

Magnified

Open Configuration


NFS with additional frustumNFS with additional frustum to prevent reflections from apparatus.

Reflection of light back onto the

KELTEK 40Close-up showing 120° additional frustum

gscreen is minimal despite its proximity to the screen.

Effects of background change on NFS#1 results


White Black

Gray BackgroundNFS#1

63952234210630000

35000

Gray BackgroundBlack Background

101277421231

180255015985

639522342106

20000

25000

Black BackgroundWhite BackgroundAverage

unts

14853170116201

23319124421138

10000

15000

CC

D C

ou

5000

10000

Using NFS#1, the background has virtually

no effect on the result.

KELTEK 41

0 50 100 150 200 250 3000

Gray Level

no effect on the result.

G B k d

Effects of background change on NFS#1 resultsSmall Area Measurements, Cont.

D i i f A

Gray Background Shown

CONCLUSION: The FPD is working well with no profound cross-coupling or shadowing. Everything stays within ±4% of average.

0.03

0.04

Gray BackgroundBlack Backgroundna

l)

Deviation from Average

180255015985

639522342106

0 01

0.02

gWhite Background

ge (f

ract

io

23319124421138

101277421231

180255015985

0 01

0.00

0.01

from

Ave

ra 14853170116201

-0.02

-0.01

Dev

iatio

n f

KELTEK 42

0 50 100 150 200 250 300-0.03

D

Gray Level

Effects of background change on NFS#1 results vs. OpenSmall Area Measurements, Cont.

Open NFS#1

35000

40000

Backgrounds

25000

30000CCD Open: White (Starting) Gray Black White (Ending)NFS#1 Ant

s

10000

15000

20000 NFS#1 Average:

CC

D C

oun

Inversions!

0

5000

10000

Measured Area

KELTEK 43

0 50 100 150 200 250 300

Gray Level

Measured Area

D t N li d t

Effects of background change on NFS#1 results vs. OpenSmall Area Measurements, Cont.

Data Normalized to NFS#1 Average

16

4

5

BackgroundsCCD Open: White (Starting) Gray

BlackAve

rage

1300% Error

12

14

16

BackgroundsCCD Open: White (Starting)ra

ge 2

3 Black

White (Ending)NFS#1 on Gray:

zed

to N

FS

#1 A

8

10

12 Gray Black White (Ending)NFS#1 on Gray:

FS

#1 A

ve

0 50 100 150 200 250 3000

1

Nor

mal

iz

Gray Level

4

6

8

ized

to N

F Gray Level

0

2

4

Nor

mal

i

KELTEK 44

0 50 100 150 200 250 3000

Gray Level

EXPECTATIONS TO HIGH??? !!!!


If a ≅1000% error (without using a SLET or frustum) in measuring dark areas amidst a white screen shocks us then our expectations are too

Open

areas amidst a white screen shocks us… then our expectations are too high! We don’t understand the limitation of our equipment, and our use of it with impunity can yield disastrous results.

How much better it is to be alerted to the complications and takeHow much better it is to be alerted to the complications and take measures to correct the problem… or don’t attempt the measurement.

KELTEK 45

SETUP OF DISPLAYProper Setup Depends Upon Display Task.p p p p p y

How will the display be used?What environment (ambient, surround)?Are there manufacturing setup specifications?

Mi ht l t f ll if i t t t k

Are there manufacturing setup specifications?Gray scales near black and near white are often useful, but may not be sufficient. [FPDM 301-3A]

A face may show up problems that are difficult to see with other patterns simply because our vision processing is very sensitive to faces

Might also try a face as well as a scene, if appropriate to task.

faces.

KELTEK 46Available: http://www.fpdl.nist.gov/patterns.html as NISTDP02*.zip

Types of Measurements, Cont. Full-Screen & Box: Scales & Gammas

Gray Scale

INPUT OUTPUT

Gray Level

INPUT

Gray ShadeOUTPUT

(Command Level)Vi = 0, 1, 2, … 255 (8-bit)

V1 ≡ VK = 0 V256 ≡ VW = 255

L = LK, L2, L3, …, LWL1 ≡ LK, L256 ≡ LW

V1 ≡ VK 0, V256 ≡ VW 255

Typical form: Li = aVi + LKγ

, a = LW-LK

255γ

…

S th G S l & C l S l M t i S ti f th i ti f

KELTEK 47

See the Gray-Scale & Color-Scale Metrics Section for the prescription of selecting a subset of levels from the full number of available levels.

KELTEK 48

SEB01

KELTEK 49

SEK01

Look for color changes and unevenness in the grayscale.

KELTEK 50

KELTEK 51

SSKE

KELTEK 52

KELTEK 53SSWE

KELTEK 54

Setup, Cont. The eye is especially critical of faces.

KELTEK 55Patterns & faces available: www.fpd.nist.gov → Patterns

FacesCS

Setup, Cont.Face Matrix: Do the freckles or highlights change across the screen?

FaceMX13

KELTEK 56Patterns & faces available: www.fpd.nist.gov → Patterns

TYPES OF DISPLAY METRICS

FundamentalGray & Color ScalesS ti lSpatialUniformityViewing Angleg gTemporalReflection.)M iMotionPhysical, Mechanical, & ElectricalFront ProjectorFront ProjectorFront Projection Screen3D & Stereo

KELTEK 57

FUNDAMENTAL METRICS

Full-screen white, black, primary colors, peak whiteProblems with white primaries

Full-screen white

Imagery white

Peak white

Problems with black measurementsFull-screen black

Imagery blackImagery black

Box measurements, checkerboards, loading, halation

Contrast measurements & calculations&

Color Gamuts

KELTEK 58

Full-Screen Center MeasurementsFundamental Metrics, Cont. Full Screen & Box

Measurands: L, x, y; or L, u’, v’

What could possibly go wrong with s ch a straight for ard meas rement?such a straight-forward measurement?

Luminance — A few percent!

KELTEK 59

Because of Veiling Glare[FPDM 302-1 through -8, -10]

Box MeasurementsFull Screen & BoxFundamental Metrics, Cont.

With colors on black normally get few veiling-glare problems.

Veiling-glare (VG) problems especially arise with...CHECKERBOARDS [FPDM 304-9]

LOADING [FPDM 304-8]

HALATION [FPDM 304-7]

KELTEK 60

... etc. Can get color contamination from VG.

[FPDM 304-1 through 304-11]

FULL SCREEN WHITE:Often reported for displays that can sustain a bright

Full Screen & BoxFundamental Metrics, Cont.

Often reported for displays that can sustain a bright full screen without a loss of luminance. May include a white primary.PEAK WHITE (HIGHLIGHT WHITE):For screens that cannot maintain their brightest luminance at full screen because of power loading. May include a white primary. Often this is not distinguished from full-screen white, unfortunately.distinguished from full screen white, unfortunately.

LOADING:Method to determine power loading. Measure increasing white box.

…

L = 100%( Lmax – LW)/LW

L = 63%

KELTEK 61

LW = Full-Screen White

WHITE PRIMARIES (RGBW DISPLAYS):Great for making the display bright but including the white


RGB

RGBWGreat for making the display bright, but including the white primary (white subpixel) for imagery will make the saturated colors look dark and make other colors look washed out. Not good for imagery, unless ONLY used for highlights (requires RGBimage analysis in real time — future, maybe).

RGBPhotograph of RGB Display

RGBWPhotographs of RGBW Display

ORIGINALSignal Sent to Display

KELTEK 62Photographs made to look as close as possible to what is seen.Macbeth ColorChecker®

IMAGERY WHITE (WITH NO WHITE PRIMARY):


If the display employs a white primary, and if the white primary can be turned off, then you can measure a full-screen white. If the white primary cannot be turned off, then use the nonatile ,trisequence patterns measuring at the center screen. This is an attempt to eliminate or reduce the use of a white primary as much as possible.

3x3 matrices of maximum RGB

LIW = LR + LG + LB

The name “imagery white” is not yet fixed as a standard term It

KELTEK 63

The name “imagery white” is not yet fixed as a standard term. It is proposed in the ICDM DMS, but has not been voted in yet.

Gamuts, Cont.

Apparent Gamut Reduction from White Primary Addition

Fundamental Metrics, Cont.

CIELUV 1976 (L*, u*, v*)

100

150

G

This is a real color diagram with real color data from one projector showing the difference in color rendering between a

50

100

R

in color rendering between a Movie Mode (the blue dots) and a Bright Mode (the dark blue dots) This shows the real effect of introducing the white primary in

-50

0-100 -50 0 50 100 150 200v*

introducing the white primary in reducing the color gamut of the displayed image. Because of the white primary, the saturation of the colors appear to be reduced.

-100

the colors appear to be reduced. In a 3D diagram, they would also be darkened relative to the white level.

However, if the gamut is

-200

-150

u*

B

However, if the gamut is shown in u’,v’ or x,y diagram, we wouldn’t see a gamut reduction because those spaces don’t account for the white point.

KELTEK

u

Reduction of RGB gamut with the addition of a substantial white primary.ΦRGB = 292 lm (movie mode), ΦRGBW = 1458 lm

account for the white point.

WHITE PRIMARY DISTORTION OF GAMUT

RGB

RGBW

RGB

CREDIT: “Analysis of the proposed IEC ‘Color Illuminance’ digital

KELTEK 65

projector specification metric, using CIELAB gamut volume,” Karl Lang, Lumita, Madison, WI, Proceedings of the 15th Color Imaging Conference, Albuquerque, NM November 2007.

Color GamutsGamuts

0 60.8

0.9

Fundamental Metrics, Cont.

0 4

0.5

0.6

0.5

0.6

0.7

y

0 2

0.3

0.4u'

0.2

0.3

0.4

0.1

0.2

0

0.1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8x

People are concerned with

sRGB is often used with displays at the present time.

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7

v'Gamut Color x y u' v'

Red 0.64 0.33 0.4507 0.5229Green 0 3 0 6 0 1250 0 5625

sRGB, HDTV (ITU R BT 709 5)

People are concerned with gamut areas and overlaps, but unfortunately they often compare with the NTSC gamut th t h ’t b d f Green 0.3 0.6 0.1250 0.5625

Blue 0.15 0.06 0.1754 0.1579Red 0.67 0.33 0.4769 0.5285Green 0.21 0.71 0.0757 0.5757Blue 0.14 0.08 0.1522 0.1957Red 0.63 0.34 0.4330 0.5258

NTSC (FCC 1953, ITU-R BT.470-2 NTSC (1979),

(ITU-R BT.709-5)that hasn’t been used for many years. And worse! they do it in the 1931 color diagram rather than the u’v’ diagram—

KELTEK 66

Red 0.63 0.34 0.4330 0.5258Green 0.31 0.595 0.1303 0.5625Blue 0.155 0.07 0.1756 0.1785Red 0.64 0.33 0.4507 0.5229Green 0.29 0.6 0.1206 0.5613Blue 0.15 0.06 0.1754 0.1579

NTSC (1979), SMPTE 170M/240M, PAL/SECAM (UK, Russia)

gnaughty! Should use u’v’. [FPDM 302-4A, DMS 5.15?]

DISPLAY GAMUT 0.6

Gamuts, Cont. Fundamental Metrics, Cont.

COVERAGE

How well does 0 4

0.5

the display reproduce the required signal

0.3

0.4u'

Really BAD Display for Accurate Color

gamut?

0.1

0.2 Rendering!!!

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7

v'

The idea is to capture the color gamut that is associated with the input signal provided to the display. Having a larger color gamut than the input signal is not valuable unless the display gamut is properly adjusted to the input gamut Otherwise the colors may be incorrect

KELTEK 67

adjusted to the input gamut. Otherwise the colors may be incorrect.

GRAY-SCALE & COLOR-SCALE METRICS

GRAY SCA EINPUT OUTPUT

GRAY SCALE

Gray Level(C d L l)

Gray Shade(Command Level)

Vi = 0, 1, 2, … 255 (8 bit)L = LK, L1, L2, …, LW

γ LW-LKTypical form: Li = aVi + LKγ

, a = LW LK

255γ

COLOR SCALER, G, or B Levels R, G, or B Color

COLOR SCALE

KELTEK 68Typically gamma is 2.2 (γ = 2.2)

New Gray Scale for Adjusting & Measuring Displays

Patterns, Cont.Gray & Color Scales, Cont.

(Now you are seeing parts of the new ICDM DMS.)In the past, we usually used 8, 16, or 32 stepped gray scales. The new arrangement uses 9, 17, 33, etc. stepped gray scales. Advantages: (1) better uniformity in step size & (2) they are subsets of one another (not so with 8, 16, and 32). [Credit: Dr. Don-Gyou Lee of LG]

Here is a 33 step grayHere is a 33 step gray scale. Every other one is the 17 step gray scale. Every third one is the 9 step gray scale.

Pattern name: SCPL17 for snaking constant-picture-level; one pattern, the 17th of 33 patterns

KELTEK 69

17th, of 33 patterns.

SCPL17

New Gray Scale for Adjusting Displays, Cont.

Patterns, Cont.Gray & Color Scales, Cont.

KELTEK 70

The ΔVi for M=6, 16, and 18 are uniform, but not for M=8, M=32, etc. M=8 and M=16 have different Vi, there is no replication in M=8, 16, 32, etc.

SIMPLE “GAMMA” MODELThis functional relationship between the luminance and gray level has a variety

f d di h ’ i h l i l

Gray & Color Scales, Cont. Gammas

L = aV + Lγ a = (L - L )/(V – V )γ

of names depending upon the group you’re with: tone curve, electro-optical transfer function (EOTF), gamma (just the word), gamma curve, gray-scale curve, tone scale, etc.

Li = aVi + LKγ a = (LW - LK)/(VW – VK)

log(Li – LK) = γ log(Vi – VK) + log(a)Usually VK = 0Usually VK 0

2.5

3.0

350

400

/m2 )

Linear Regression (y = mx + b):γ = 2.25 ±0.012

1 5

2.0

2.5

g(L

- LK )

200

250

300

Lum

inan

ce, L

(cd/ b = log(a) = -2..84 ±0.026

r = 0.9999

0 5

1.0

1.5lo

50

100

150

Gra

y Sh

ade

L

b = log(a) is where it crosses the vertical axis at log(V) = 0.

KELTEK 71

0.51.4 1.6 1.8 2.0 2.2 2.4 2.6

log(V)

00 50 100 150 200 250 300

Gray Level, V

350

400

/m2 )

COLOR SCALES & GAMMAS:Each primary has a color scale To


150

200

250

300

de L

umin

ance

, L (c

d/Each primary has a color scale. To compare them, we need to normalize the luminance data to the maximum. For these data:

0

50

100

0 50 100 150 200 250 300

Col

or S

had

γR = 2.30, b = -3.52γG = 2.25, b = -2.82γB = 2.53, b = -4.32

Gray Level, V

0 5

0.0

0 8

0.9

1

L/L m

ax

γB 2.53, b 4.32

-1.0

-0.5

- L K

) / L

max

]

0.5

0.6

0.7

0.8

ed L

umin

ance

, L

-2.0

-1.5lo

g[( L

0.1

0.2

0.3

0.4

Col

or N

orm

aliz

e

KELTEK 72

-2.51.4 1.6 1.8 2.0 2.2 2.4 2.6

log(V - V0 )

00 50 100 150 200 250 300

Gray Level, V

DIFFERENT PATTERNS FOR DIFFERENT TECHNOLOGIES:GLOBAL DIMMING DISPLAYS: Displays that can change the luminance of the entire


GLOBAL DIMMING DISPLAYS: Displays that can change the luminance of the entire backlight (LCD) or the luminance of the full screen to suit the imagery. May be able to achieve absolute black as with full-screen black. To avoid this, use a constant average-picture-level (APL) pattern that cycles the different gray shades through the center region (c, d & f) or adjusts the center and background (e) for the same APL.center region (c, d & f) or adjusts the center and background (e) for the same APL.

LOCAL DIMMING DISPLAYS: Even more problematic—displays (LCD) that have a low-resolution LED backlight to modify contrast in different regions. Large APL patterns (c) may not work, still might get too black. Pattern (d) may work better.

LPreview of ICDM DMS…

BEST IDEA for both local and global dimming may be to turn the feature off, if possible, for gray-scale and color-scale measurements.

LS

LX

(a) (b) (c) (d) (e)

KELTEK 73

INTRO SCPL01 SCPL02 SCPL03 SCPL04 SCPL05 SCPL06 (f)

GammasDifferent Gammas:

Gray & Color Scales, Cont.

Here are three patterns that have a normal gamma. We are going to change the gammas a little See whichgammas a little. See which pattern tells you what has happened.

KELTEK 74Macbeth ColorChecker®

Different Gammas:Gray & Color Scales, Cont. Gammas

What changed, and which patternand which pattern best displayed the change? Why?


Clearly, to our eyes, ramps are not very useful


not very useful, colored patterns may be useful, but we quicklysee problemssee problems with faces. Why? Because we have been looking at faces since wefaces since we were born, and that imagery is deeply ingrained in our brainsin our brains.

Let’s try images, another pattern, and the facesand the faces.

KELTEK 77

FULL SCREEN BLACK:Often reported for displays that exhibit a visible black


p p yluminance LK > 0 when the entire screen is at the black level VK = V1 = 0.IMAGERY BLACK:

F di l th t ith t ff th i l ti l ( OLED di l )For displays that can either turn off the pixel entirely (e.g. OLED displays) or turn off the backlight as in global dimming or local dimming displays. We use a pattern that may allow light from a surrounding bright area to leak or bleed into the black area. That pattern has not been agreed upon. Candidates:

Star Patterns

SCPL01

Halation Patterns:

…

H = 100%(Lmax - LK)/LW.

KELTEK 78

( max K) W

The choice of size of the black box is critical!

Full Screen & Box

ZERO-LUMINANCE BLACK — OR CLOSE TO ITFundamental Metrics, Cont.

Some displays can exhibit a zero-luminance black for a full screen.Some displays may be able to exhibit a zero-luminanceSome displays may be able to exhibit a zero-luminance black for even a small black area amid a bright white screen.I ll h th hibit i fi it t t th t iIn all such cases, they exhibit an infinite contrast that is independent of how dim or bright the white is! (Discussed next.)Must be careful to properly identify if we really have such a black.

We must be able to not see any luminance with dark-adapted eyes in a GOOD d k (D k d t d i i l t t l d k f 20darkroom. (Dark adapted eyes mean you remain in nearly total darkness for 20 min to 45 min while avoiding all sources of light, LED instrument lights, computer screens, etc.)If we can see it, then we also need to be able to measure such luminance when it is NOT a zero luminance if we are going to claim an extraordinary contrast

KELTEK 79

it is NOT a zero luminance if we are going to claim an extraordinary contrast. That will require good instrumentation. The instrumentation MUST be able to measure the low luminance correctly.

Contrast Measurement ProblemsL

Contrasts, Cont.Fundamental Metrics, Cont.

How do we deal with enormous contrasts, even infinite contrasts? Whenever LK = 0 then C = ∞ no matterK

W

LLC =

Whenever LK 0 then C ∞ no matter how small LW may be!

CRT (cathode ray tube) displays can be configured to do this.CRT (cathode ray tube) displays can be configured to do this.We are now seeing LCD displays that can turn their backlights off for a full black screen giving an infinite contrast in a darkroomdarkroom.OLED displays are appearing where the pixel can be turned completely off also giving an infinite contrast in a darkroom.Some LCD displays have a locally adaptive backlights that canSome LCD displays have a locally adaptive backlights that can reduce local blacks to almost zero luminance giving enormous or infinite contrasts.

KELTEK 80

How to deal with this has not been decided.

Contrast Measurement ProblemsL

Contrasts, Cont.Fundamental Metrics, Cont.

What kind of problems can occur? Are there solutions?

K

W

LLC = → ∞

Even when a very small amount of light is available in the black, the luminance meters employed may not be able to measure low-light levels accurately, and those trusting such instruments will be reporting false values. Not good.What about using a level just above black? Many displays render such levels as black, L1 = L2 = … = L5 = etc. = LK. Not good.Wh t b t 50% l l t t??? S iti t !What about a 50% level contrast??? Sensitive to gamma!Try a dark-gray contrast: Cg = LW/Lg where the level “g” means the dark gray level associated with 1/16 of the total number of levels. For 0-255 levels it’s level 15; for N total levels its g = (N/16)-1 Maybe but sensitivelevels it s level 15; for N total levels its g = (N/16)-1. Maybe, but sensitive to gamma?Use a star field of single white pixels separated by 100 black pixels. This might work, might not.

KELTEK 81How to deal with this has not been decided.

g , gOther methods???

Contrasts, Cont.

Types of Contrasts, ExamplesFundamental Metrics, Cont.

Sequential Contrast: C = LW/LK for full screens.Knowing this can be useful for scene transitions.

I C t t C L /L i i hit &Imagery Contrast: CI = LIW/LIK using imagery white & black.

For contrast of images (still and video) with “normal” screen g ( )loading.

Peak Contrast: CP = LP/max(LK – LIK) using peak white.May be useful for star fields outer space imagery dark scenesMay be useful for star fields, outer-space imagery, dark scenes with bright objects, night scenes.Displays that exhibit a much higher peak contrast than other contrasts may be useful in rendering highlightscontrasts may be useful in rendering highlights.

Line & Grille Contrasts: CG = LW/LGUseful for the rendering of text and similar detail on white

KELTEK 82

screens. VERY HARD TO MEASURE CORRECTLY.

MOTION METRICS There are numerous types of motion artifacts Here isThere are numerous types of motion artifacts. Here is a sampling:

Judder

Moving Edge BlurringThis is often normalized to a response time called moving-This is often normalized to a response time called movingedge response time (name is standardized in FPDMUPDT [an update document]). In the literature you can also find it called motion picture response time (MPRT). Moving Line Spreading & Contrast Degradation

Wireframe Flicker or Moving Line Flicker

p p ( )

Wireframe Flicker or Moving Line Flicker

Some of these are artifacts of the display Some can also be artifacts

Color Breakup, Color Smearing, etc. …

KELTEK 83

Some of these are artifacts of the display. Some can also be artifacts from the combination of the display and how the eye sees things.

JudderJ dd J ki i ti i i f ti th t d

Motion Artifacts, Cont.

Judder: Jerkiness in motion arising from motion that does not cause blur (can arise from low frame rate). Probably the frame rate of the projection display is high enough to cause only bl H th f h ill b i t t d b f th ti tblur. However the refresh will be interrupted because of the operating system computations; so a jerkiness may be observed that simulates judder.

KELTEK 84(Animated)

Boxes Moving Across Screen at Different SpeedsMotion Artifacts, Cont. Try to follow and see blur; stare in one place appears sharp?

CAN

YOU

READ

THIS

WELL?

KELTEK 85

WELL?

(Animated) 85KELTEK

CRT Di l


CRT Display

Smooth-Pursuit Eye Tracking

KELTEK 86(Animated)

FPD H ld T Di l


FPD Hold-Type Display

Smooth-Pursuit Eye Tracking

KELTEK 87(Animated)

FPD H ld T


Retinal ImageTrace

Hold-Type Display with perfect Trace

of Edgetransitions between levels

ME

levelsTI

M

KELTEK 88

POSITION

(Animated)

Lines Moving Across ScreenMotion Artifacts, Cont. Try to follow and see blur; stare in one place appears sharp?

KELTEK 89KELTEK (Animated) 89

Wireframe FlickerSome displays exhibit a pronounced flicker when a


Some displays exhibit a pronounced flicker when a wireframe is slowly moved across the screen.Probably won’t work here because the motion is too fast.

KELTEK 90(Animated)

Color BreakupC l b k i f l ti l di l if


Color breakup can arise from color sequential displays if their sub-frame refresh frequency is too slow.

This may not be how it looks, it just gives you the idea

KELTEK 91(That’s supposed to be a thrown snowball.)

it just gives you the idea.

Color Smearing


Color Smearing

Motion

KELTEK 92(Animated)

Motion Artifacts, Cont. Pursuit Cameras

There are ways to record motion other than pursuit cameras.

For these measurements we need accurate lightneed accurate light, spatial, and temporal measurements… NOT EASY!

Linear Pursuit Camera Rotational Pursuit Camera

Exposure must be longer than 1/10 s. Must capture integral number of frames.

KELTEK 93

Performing diagnostics on these cameras to be sure that they are measuring the motion artifact correctly is NOT trivial.

3D STEREO METRICSThere are so many different technologies we can’t

Stereo Extinction Ratio & CrosstalkH ll i i t d f th th ?

There are so many different technologies… we can t cover them! Here are some common metrics.

How well is one eye image separated from the other?Stereo Contrast

Full-screen contrast for each eyeyLuminance & Luminance DifferenceLuminance UniformityColor UniformityViewing-Angle BehaviorH d TiltHead Tilt

How are all these results affected by head tilt?Color Differences

KELTEK 94

Color Differences… These are all extensions to two eye positions.

3D & Stereo Displays, Cont.

Binocular Vision Considerations (Human Factors)Binocular Vision Considerations (Human Factors)Interocular Spatial Frequency Difference Interocular Temporal Asynchrony Interocular Luminance DifferenceInterocular Luminance DifferenceInterocular Focus DifferenceVertical MisalignmentHorizontal DisparityHorizontal DisparityImage Size DifferenceConvergence MismatchConvergence DistanceConvergence DistanceFocal DistanceInterocular DistanceField CurvatureField Curvature

ICDM DMS will feature stereo displays and discuss in detail the binocular vision problems encountered.

KELTEK 95

p

3D & Stereo Displays, Cont.

ILLUSTRATION OF SOME OF THE PROBLEMS

Some can cross their eyes and combine these images into one image. The eyes will lock onto the images.

If you can then you will see a stereo imageIf you can, then you will see a stereo image.

It may feel a little strange (huge horizontal disparity!).

It will get a lot stranger when we move one of the imagesIt will get a lot stranger when we move one of the images.

KELTEK 96

The feeling you get from these disparities is the kind of thing to be avoided — and needs to be measured.

We apologize for any headaches, dizziness, or nausea.

CROSSTALKNearby objects show bleeding y j gfrom one eye image to the other.

KELTEK 97

3D CROSSTALK

3D & Stereo Displays, Cont. Stereo Problems, Cont.

3D CROSSTALK

3D CONTRAST

3D TEST PATTERNS

L R

DISPARITY

L RImage for one eye bleeds into other.Contrast not same for both eyes.

3D GRAYSCALEGrayscale not same for both eyes.

3D GRAYSCALE DISPARITY

KELTEK 98

HORIZONTAL DISPARITY


KELTEK 99The eyes will have to cross or go wall-eyed to align the images.

VERTICAL DISPARITY


KELTEK 100One eye will have to go up the other down to align the images!

INTEROCULAR MAGNIFICATION DIFFERENCE


OCU G C O C

KELTEK 101Only aligned in the center region.

EXAMPLES OF DIFFICULT PROBLEMS HORIZONTAL DISPARITY


HORIZONTAL DISPARITY

VERTICAL DISPARITY

L R INTEROCULAR MAGNI-FICATION DIFFERENCE

Red cross hairs mark the location of the eye gaze point if the 3D display were perfect.

FICATION DIFFERENCE

KELTEK 102

eye gaze point if the 3D display were perfect.

Camera may need to be synchronized with frame rate or if iti lt l ill b


nonuniformities may result — or long exposures will be required (1 s or longer).

View ofView of One

Camera

Grayscale Dithering Integration y gUpon Camera Array with Sync Error

KELTEK 103

Sync. Error

REFLECTIONOversimplified Models — Possible AmbiguityOversimplified Models Possible Ambiguity

E

Lambertian (“Diffuse”) component assumption:Display surface measured as if it were matte paint.

β

Lβ = luminance factor, q = luminance coefficient,

E = illuminance, L = observed luminance.

Strictly speaking, this equation is usually for a Lambertian material (if β is constant for all angles)—at least many people use it that way; “diffuse”

EqELπβ

==

Specular component assumption:

constant for all angles) at least many people use it that way; diffuse means “scattered out of the geometrical specular direction” and is notlimited to Lambertian materials, but many THINK diffuse = Lambertian! Note

sθ

sθ

LsSpecular component assumption:

Display surface treated as if it were a mirror.

ζ = specular reflectance, Ls = source luminance

KELTEK 104sLL ζ= L

ζ p s

Specular & Lambertian — Easy to Measure if That Is All!Reflection Measurements, Cont. Oversimplified, Cont.

Unfortunately, many FPDs are not well characterized by just these two components — an oversimplified model.

FPDs Can Permit Diffusing Surface Near PixelsFPDs Can Permit Diffusing Surface Near PixelsLike wax paper over printing...

Some FPDs allow diffusing surface close to pixels.

Backlight

Legibility depends upon distance of strong diffusion layer from surface containing information

Problem: Simple Models Inadequate for All Surfaces

KELTEK 105

Problem: Simple Models Inadequate for All SurfacesNeither Lambertian nor specular models may work!

Three-Component Model of ReflectionReflection Measurements, Cont.

1. SPECULAR (producing a distinct virtual image of the source)2. & 3. DIFFUSE (has TWO components): 2. LAMBERTIAN (like matte paint) & 3. HAZE (fuzzy ball in specular direction)

Reflectance can be thought of as having three components:ρ = ρL + ρS + ρH, where the diffuse reflectance is ρd = ρL + ρH.

KELTEK 106

4 MATRIX SCATTER…and Add a Fourth Component

Reflection Measurements, Cont.

4. MATRIX SCATTERWhen the front surface is smooth producing a specular reflection (a distinct virtual image of source isdistinct virtual image of source is visible), then...um… the interference reflection is under study. It is not exactly like haze and not exactly like specular. (Haze reflection is proportional to the illuminance only. Specular is proportional to the luminance of the source only.)

When the front surface has a microstructure that diffuses light so there is not distinct virtual image of the source: Then matrix scatter is like haze, and we know how to measure it (BRDF measurements make sense—BRDF discussed

KELTEK 107

make sense—BRDF discussed below).

ObservedL i = Lambertian

C tSpecularC t+ Haze

C t+

Reflection Measurements, Cont. Three Component Model, Cont.

.d)cos(),(L),,,(H),(LqE),(L iiiirrii

2/2

rrsrrr Ωθφθφθφθ+π±φθζ+=φθ ∫∫ππ

Luminance = Component Component+ Component+

00∫∫

Background gray

Distinct image

Fuzzy ball

zθ θ

dL dEi irr

Remember

y

i ir Remember that not all components must be

x

φ iφr present.

KELTEK 108

x

Simple BRDF (In Plane) θ = 5° Shown

Reflection Measurements, Cont. Three Component Model, Cont.

p ( )Extremes:

Lambertian (flat)Specular (spike)

Detectorθ = 5 Shown

θ

θ

θi < 0

p ( p )Haze is in between.

Haze characteristics:Proportional to

Source FPD Sample

θ

θi > 0

pilluminance

Directed in specular direction

100Specular

100 Specular

Peak

SPECIALLY PREPARED SAMPLE (DL + S + DH)

e!

NOTE: 3 to 5 orders of ( 1

10

Component

Haze(1/s

r )

-10 -5 0 5 100.1

1

10 Haze

Peak

Peak

te lo

g sc

ale

magnitude possible (or more!—your eye has no trouble seeing this range!) 0.1

1 Haze

Component

BR

DF

(

RESOLUTION< 0.1°

Not

KELTEK 109

-80 -60 -40 -20 0 20 40 60 800.01

Lambertian Component

Light Source Angle (degrees)

See next slide…

Need Simple Measurement MethodsReflection Measurements, Cont.

BRDF measurements are hard; simpler and faster methods are desired (required).

Acceptable Methods Must Be...

Robust: Results not subject to small apparatus

L( )θ

qEsubject to small apparatus imperfections or irregularities or choice of equipment

Reproducible: Same

L

LrSSρ

θ

θ

Reproducible: Same results obtained with same displays around the world

U bi

Lr

S

Unambiguous:Apparatus configuration and requirements clearly presented and all important

OBJECTIVE: To find the minimum set of measurements

to adequately quantify reflection

KELTEK 110

presented and all important concerns made obvious

q y q yperformance for a variety of

applications.

Which Measurement Methods are Most Robust?Reflection Measurements, Cont.

θsθr

θs

θd

θs

A. Ring E. Small Specular

s

θdB. Small Side

F. Large Specular

θs

θ

θs

θd

θr

C. Large SideG. Proximal Specular

KELTEK 111θd

D. View Port H. Diffuse

Many say, “not practical,” “not realistic,”…. Uniform Diffuse Source (H):

Reflection Measurements, Cont. Summary, Cont.

y y, p , ,NOT TRUE! Beach on a cloudy day. Snow field on a cloudy day. Light living room with light furniture and even illumination. Not so uncommon after all. Even a dark room with low ambient illumination is like diffusedark room with low ambient illumination is like diffuse illumination with small illuminance!!! People don’t like it because it can be tough on their displays giving low contrast values with large illuminance levels.

Ring Light Source (A):More work is needed to determine if there are better settings (e.g., larger source subtense) that will secure more robustness — perhaps two

Ring Light Source (A): ?

More work is needed to determine if there are better

p psettings will be useful (e.g., 20° and 45°).

?Large Specular Source (F):

Note that these three methods integrate about the normal

settings (e.g., larger source subtense) that will secure more robustness—perhaps two settings.

KELTEK 112

g(φ axis) or specular direction thus integrating the effects of

complicated haze or specular reflection profiles.Note

Method under research; no

Large-Hole Diffuse:Reflection Measurements, Cont. Summary, Cont.

Late News!Method under research; no results yet. This is hypothetical, based upon experience. Likely it will do rather well because the large specular source does welllarge specular source does well as does the uniform diffuse source. (This is really like the subtraction of the large specular source result from the uniformsource result from the uniform diffuse result.)

N t th t thi th d l i t t b t th

We anticipate that the hole will subtend 30° (±15° about the normal).

KELTEK 113

Note that this method also integrates about the normal (φ axis) thus integrating the effects of

complicated haze or matrix scatter.Note

Large Aperture DiffuseMethod under research

Alternatives & Implementation, Cont.Reflection Measurements, Cont.

Method under research.Spherical cap permits hemispherical diffuse reflectance measurements to be performed in addition to this. ±15° Hole

Add Frustum

be performed in addition to this. ±15 Hole(30° Apex Angle

Rdi RLADLAD

R

?RLSS

KELTEK 114Rdi = RLAD + RLSS?

Diffuse Ambient

Alternatives & ImplementationReflection Measurements, Cont.

ALTERNATIVES & IMPLEMENTATION

It is “easy” to characterize emissive displays, but reflective displays depend upon the surround

Diffuse Ambient

p pconditions. Comparing the two types of displays can be a problem. Certainly,

i t b d dRing Illumination

comparisons must be made under carefully controlled ambient conditions to which both displays are subjected, especially if a non-trivial

θr

j , p yhaze component is present.

Diffuse with Sunlight Addition

θs = 45°

Other configurations are being considered.

KELTEK 115θd

Directed Hemispherical Reflectance (β = ρ ) Using Uniform Diffuse IlluminationHemispherical Reflectance

Reflection Measurements, Cont.

Directed Hemispherical Reflectance (βd/θ = ρθ/d) Using Uniform Diffuse Illumination

A Worst-Case Situation: Uniform light surround with normal of display tilted approximately 8° to 10° from axis of measurement p y pp yhole.Reproducible: A variety of apparatus can be used to reproduce sufficiently

fthe uniform hemispherical surround conditions.Robust: Results tend to be insensitive t t fi ti d lto apparatus configuration and angular alignment.

Many say, “not practical,” “not realistic,”…. NOT TRUE! Beach on a cloudy day, snow field on a cloudy day, light living room with light furniture and even illumination. Not so uncommon after all. Most all environments have a baseline or background uniform diffuse illumination level—even in a theater. People don’t like it because it is tough on their displays, can give low contrast values.

KELTEK 116

p g p y g

IF THERE IS GOING TO BE ONLY ONE MEASUREMENT TO MAKE, THIS IS IT!!! (… my opinion…)

Effects of Distance from and Size of Measurement Port

Hemispherical ( βd/θ = ρθ/d ), Cont.Reflection Measurements, Cont.

55

60

m 2 )

50

le, L

(cd/

40

45

of S

ampl

63 cm from MP (Sample #1)

30

35

min

ance



25

30

Lum

KELTEK 117

20 25 30 35 40 45 50 55 60Measurement Port (MP) Diameter, DMP (mm)

Two General Types of InterestReflection Measurements, Cont. Hemispherical ( βd/θ = ρθ/d ), Cont.

Specular Included Specular Excluded

E E

NOTE: CAUTION!!!

θ θ

If there is a nontrivial haze component, then the specular-excluded result will

LRL

LRLR

excluded result will depend upon the size and distance of the hole!

KELTEK 118

LR

Eπρdi/θ=

LR

Eπρde/θ=

Reflection Measurements, Cont. Hemispherical ( βd/θ = ρθ/d ), Cont.

Specular included is Specular excluded is like Specular included is like viewing a display off normal on an overcast day.

viewing a display along the normal on an overcast day with a white shirt on. y

Both are useful measurements.BUT! The specular excluded measurement is robust ONLY for specular-Lambertian reflection properties It is NOT robust when there is nontrivial

KELTEK 119

Lambertian reflection properties. It is NOT robust when there is nontrivial haze or nontrivial matrix scatter.


Diffuse reflectance measurement depends upon the net reflected luminances that does not include the emitted luminance from an emissive display. p y

Darkroom luminances are needed:(Display taken out of the sphere or the sphere is removed!!!)

GW = darkroom luminance of white

NOTE:

WGK = darkroom luminance of black

(GW and GK are zero for a reflective display! — Duh!)

NOTE:1. GW and GK must be measured from the same θ = 10° from the normal to

account for any viewing angle properties of the screen.2. GW and GK must be measured at the same place (normally the screen

KELTEK 120

W Kcenter) as L'W and L'K are measured when in the sphere — because of possible nonuniformities of the screen.


Net reflected luminance is difference between luminance measured in the sphere and the darkroom luminance (measured outside the sphere). p ( p )

LR

L'W - GW

EWπρW = Diffuse reflectance

of the white screenR

Eπρ = W

L'K - GK

EπρK =

Diffuse reflectance of the black screen

NOTES: 1. In general, the reflectances ρK and ρW will not be the same.

EK

KELTEK 121

g , ρK ρW2. Difference in numerator must not be small or measurement will be very

uncertain — this requires that there be a lot of light — a large E.

Scale to Desired Illuminance Levels& Calculate Ambient Contrast


& Calculate Ambient ContrastIt may be difficult to achieve exactly the desired illuminance directly by adjusting the bulb brightness (especially if the sphere is large and the desired illuminance is large). Therefore, we measure the display under laboratory

E0 = Design or desired illuminance

illuminance is large). Therefore, we measure the display under laboratory illuminance conditions and then scale the results to the design illuminance or desired illuminance.

E0 Design or desired illuminance

Predicted (scaled) luminances for desired illuminance are calculated from the measured reflectances and the desired illuminance:

KW = LW + ρWE0πKK = LK + ρK E0

The luminances LW and LK are darkroom luminances taken in the design viewing direction — often

l th l ( t G d G )KK LK E0π

Resulting ambient contrast for the desired (design) illuminance:

K

along the normal (not GW and GK).

KELTEK 122

CA = KWKK

Problems with Integrating Spheres1 Need very large integrating sphere for a


1. Need very large integrating sphere for a large display. Expense is great for such a large sphere and it takes up a great deal of space within a room. (Rule of thumb: Diameter of sphere must be seven times the $Diameter of sphere must be seven times the size of the object being measured.)

2. Then the displays get larger and you have to get a larger sphere!

3 A large display filling a substantial portion

$3. A large display filling a substantial portion

of the interior may affect the uniformity of the interior luminance distribution and affect the measurement results.

Most will want a different solution…[drum roll…]

KELTEK 123

Hence, the sampling sphere…

Sampling SphereOnly a portion of the display is subjected to uniform diffuse illumination

Reflection Measurements, Cont. Sampling Sphere

Only a portion of the display is subjected to uniform diffuse illumination.

1 Size is much smaller than a large1. Size is much smaller than a large integrating sphere

2. Cost is much less than a large integrating sphere.

3. Much brighter interiors are more readily attained with smaller sphere.

4. Caution: Be careful about heating the display—avoid interior bulbs

KELTEK 124

display—avoid interior bulbs.

SAMPLING SPHERE

Sampling SphereDetector to

Sample Normal

Sample Port be placed at 10° from Normal

Sample Port

Illuminance

Measurement PortSample Holder

Illuminance Meter

White Standard

KELTEK 125

Reflection Measurements, Cont. Sampling Sphere

Differing Photopic Responses with Strong ColorsUsing haze sample, H: ρH = 0.0440

J

Photopic Photodiode Calibration:EJ = α J

IF th h t i f th l iJ

LEDs

IF the photopic response of the luminance meter is the same as the photopic response of the photopic photodiode, then the illuminances should be the same. LEDs

RGB & W For strong colors they are not!

LED ρ = πL/EJ Deviation from ρ

ρ = ρwallL/Lwall Deviation from ρ

Lwall

from ρH from ρH

RGB .0375 -14.8 % .0433 1.6 %

R .0500 13.7 % .0430 2.3 %G 0427 2 9 % 0427 2 9 %wall

LG .0427 -2.9 % .0427 2.9 %B .0263 -40.2 % .0434 1.4 %W .0407 -7.5 % .0431 2.1 %

KELTEK 126

Wall Calibration:Ewall = Cwall Lwall

Thus, wall measurements are safest.

Size of Measurement P t i I t t

Sampling Sphere, Cont.Reflection Measurements, Cont.

Port is Important

The measurement port cannot beThe measurement port cannot be too small so it intersects rays that should contribute to the measurement result.

Because the measurement port will be out of focus and fuzzy inwill be out of focus and fuzzy in the viewfinder, you may not notice when the measurement port intersects the rays by

i i h h h i fi d

Measurement Fields

viewing through the viewfinder.

SampleOK, but very close!

SampleBAD! Rays

KELTEK 127

Good alignment here is critical!

intersected by measurement port.

Alignment Is CriticalSampling Sphere, Cont.Reflection Measurements, Cont.

When measuring any object at the sample port or when making a measurement of the wall target

Wrong! Correct!Contributing rays obscured

a measurement of the wall target, be sure that the alignment is correct.

rays obscured by edge of measurement port.

The luminance meter is focused on the sample surface. Thus, the measurement port will be out of focus and fuzzy in thefocus and fuzzy in the viewfinder.

Be sure that the measurement

Focus on display surface.

field circle (on the sample) is symmetrically oriented in the fuzzy measurement port.

KELTEK 128

Measurement of Sample Shown

Avoiding Glare


Luminance meter is too close.Bright interior

Back further away to avoid bright walls in

When measuring anything other than the wall luminance, be sure the luminance meter is far may corrupt

measurement of samples darker than walls

making sample measurements.

luminance meter is far enough away so that none of the rays from the bright surround at the sample port than walls.

Focus on sample (display surface).

can be seen in the viewfinder. Otherwise those rays may contaminate the measurement resultsmeasurement results because of veiling glare.

KELTEK 129

Sample

sesc

MeasurementP t

Wall target is tilted a a from sample

Luminance measurements


DM

Port away from sample port so that any light from the sample (e.g., an emissive display) does not directly

measurements are made through the measurement

port.

θcθe

does not directly shine on the wall target. If the interior wall luminance is very large compared

Sample

RWall Target

very large compared to the screen luminance this may not be important, and a simple wall

DS Sample

SamplePort

Wall Target

Illuminance E upon sample can be determined by measuring the wall

a simple wall measurement will suffice.

Illuminance E upon sample can be determined by measuring the wall luminance Lwall or based upon a calibrated photopic photodiode. Advantage of wall measurement is that the wall and the sample are measured with the same luminance meter and same photopic response.

KELTEK 130

Need to determine radius R of sphere based upon design parameters.

Calibration of Wall Target (or Photodiode Monitor)


Wall Target LuminanceLcal Luminance Lcal

L td

Meter

Lstdρstd

Calibration Constant Photopic

k = ρstdLcal

πLstd

Photopic Photodiode MonitorJ

Illuminance at Sample Port

E = k Lwall

k = ρstd Jcal

πLstd

KELTEK 131

wall

E = k J

Size Requirements sesc

MeasurementD t b l (30%+) th th


θc

DM

PortDM must be larger (30%+) than the size of the diameter of the lens of the detector, e.g., for a 30 mm lens…

D = 40 mm θcθe

Rθc = 10° (center location)

θ = 6° ( d l ti )

DM = 40 mm

DS Sample

SamplePort

Wall Target

θe = 6° (edge location)

Approximately

(Want minimal perturbation of measurement port hole [black] on the measurement result.)

Approximately…

Sc – Se ≅DM

2≅ 2R(θc – θe) (angles in radians, not degrees)

4(θc – θe)DMR ≅ = 3.6 DM

For DM = 40 mm, rounding up…

R = 150 mm

KELTEK 132

( c e)Sphere diameter 300 mm

(about 12 inches)

DS

Displays with Recessed Pixel SurfacesSampling Sphere, Cont.Reflection Measurements, Cont.

F t

DS

θFrontGlass hθg

Pixel Surface

θg = tan-1(2h/DS)Some displays have a front glass (or plastic) to protect the pixel surface so that the pixel surface is recessed from the front surface of the display.

How big must the diameter D of the sampling

g ( S)

How big must the diameter DS of the sampling port be to not seriously affect the measurement result for the worst case display?

KELTEK 133

Need to consider the bidirectional reflectance distribution function (BRDF) for such displays.

Conclusion: DS /h ≥ 15 then error <0.2%for worst case displays (not Lambertian)


Error in luminance measurement for sampling port displaced from the pixel surface

for worst-case displays (not Lambertian)

sampling port displaced from the pixel surface

θ (°) θg (°) DS /h = 2/tanθg

Error for Lambertian

Error for Quasi-Lambertian

θg = π/2 - θ

Model Model85 5 22.8 0.59 % 0.02 %83 7 16.3 1.2 % 0.08 %83 6 3 % 0 08 %80 10 11.3 2.4 % 0.32 %75 15 7.46 5.2 % 1.3 %70 20 5.49 9.2 % 3.5 %65 25 4.29 14.0 % 6.8 %60 30 3.46 19.6 % 11.4 %

KELTEK 134

60 30 3.46 19.6 % 11.4 %

Example D 40 i


DM = 40 mm gives R = 150 mm and diameter D = 300 mm.

Suppose DS = 100 mm and h = 10 mm,and h 10 mm, then DS/h = 10.

Sample port 1/3 diameter asan example only. Not a requirement!

Lambertian model gives about 3% error.(But no displays are really Lambertian.)

Quasi-Lambertian model gives about 0.5% error.(And that is more realistic for such displays )

KELTEK 135

(And that is more realistic for such displays.)

Sphere DiagnosticReflection Measurements, Cont.SPHERE DIAGNOSTIC — THE “SPOON”

CameraUse with either integrating sphere or sampling sphere when you want to see the interior uniformity of the sphere.

Camera

Processed ImageShows a little increase on the left side.

Split Line

* *

Back of Sphere*

Worry only about interior seen by sample.

SPOON = polished stainless steel or chrome hemisphere (or sphere [large polished bearing])

KELTEK 136

sphere [large polished bearing]) such as a ladle, potato scoop, ice-cream scoop, etc.

SCALING TO DAYLIGHT LEVELS & SPECTRAScaling & SpectraReflection Measurements, Cont.

Desire to be able to make reflection measurements in the lab and then scale the results to daylight levels or any desired level.

IT IS OK TO SCALE REFLECTION MEASUREMENTS!

It is also a good idea: Here are the reasons:

Outdoor daylight conditions change quickly (sun moves 15°/h, clouds).

Lab offers controlled ambient conditions.

Diffuse-source and directed-source (simulating sun) measurements can be separated to give separate reflection parameters.

Combine darkroom measurements (emissive displays) with reflection measurements scaled to daylight levels.

KELTEK 137

Spectra & Daylight Scaling Cont.

1 Tungsten-

Reflection Alternatives, Cont.

0 70.80.9

a

TungstenHalogen (TH)

0.50.60.7

Spe

ctra

V(λ)Overcast

Sunlight

0.20.30.4

rmal

ized

16500 K6500 K5500 K

Skylight

00.1

300 350 400 450 500 550 600 650 700 750 800 850

Nor 5500 K

2856 K3380 K IR-Blocked

TH

Daylight spectra, tungsten-halogen (2856 K) spectrum, and tungsten-

300 350 400 450 500 550 600 650 700 750 800 850Wavelength, λ (nm)

KELTEK 138

y g p , g g ( ) p , ghalogen (2856 K) with infrared-blocking filter (KG-3) giving 3380 K.


0 91

ed ∫ λλλ d)(S)(V


0.70.80.9

Nor

mal

ize

16500 K

∫ λλλ d)(S)(V D

CCT (D) Integral16500 K 102.266500 K 102 08

0.40.50.6

V(λ

) — N 16500 K

6500 K5500 K2856 K3380 K

6500 K 102.085500 K 102.592856 K 104.753380 K 97.31

0 10.20.3

pect

ra X

3380 KAverage = 101.80Std. Deviation = 2.7%Max. Deviaton = 7.3%

00.1

300 350 400 450 500 550 600 650 700 750 800 850Wavelength λ (nm)

Sp

Normalized spectra times V(λ) then normalized again. Given a gray screen, this shows that using a wide range of spectra will provide the same reflection result within 5 % to 10 %. If we want 1 % results, then we have to

Wavelength, λ (nm)

KELTEK 139

reflection result within 5 % to 10 %. If we want 1 % results, then we have to pay attention to the spectra. For reflected colors, the correct spectra will probably be important unless the measurements are spectrally resolved.


0.08 0.009How important might the spectra be? — Less than you might think.


0.040.050.060.07

Ref

lect

ance

ρK(λ)

ρW(λ)

0 0040.0050.0060.0070.008

ce F

acto

rs

βW(λ)

00.010.020.03

Diff

use

R

00.0010.0020.0030.004

Rad

ianc

βK(λ)

350 400 450 500 550 600 650 700 750 800Wavelength, λ (nm)

0350 400 450 500 550 600 650 700 750 800

Wavelength, λ (nm)

Luminance Factor of White vs. Illumination Spectra

Spectrum:(D):CCT:

Tungsten Halogen(tung)2856 K

Tunsten Halogen with IR Blocking

(THIRB)3380 K

Sunlight(sun)

5500 K

Overcast Skylight(over)6500 K

Skylight(sky)

16 500 K

Reflection spectra associated with a LCD for black and white. Whereas the spectral

βW = 0.00534 0.00538 0.00555 0.00552 0.00566

KELTEK 140

diffuse reflectances are “grayish” the spectral radiance factor for white is not flat. Even so, the white luminance factor changes with a relative standard deviation of 2.4 % and maximum deviation of 5.8 %


0 91


0.70.80.9

pect

ra

0.40.50.6

aliz

ed S

p

6502 K

0 10.20.30.4

Nor

ma 6502 K

5487 K15577 K16500 K6500 K

00.1

300 350 400 450 500 550 600 650 700 750 800 850

6500 K5500 K

Wavelength, λ (nm)Dashed lines are the correct daylight spectra. Solid lines are an attempt to filter a 2856 K tungsten-halogen source to get close to the CCTs. The tail

f th filt d li ht i th i f 700 t 850 f t f

KELTEK 141

of the filtered light in the region of 700 nm to 850 nm goes up a factor of six or seven times the height of this graph.

Radiance Factor of Emissive Display — General Form (Source and detector geometry completely specified)

Spectra & Daylight Scaling Cont.Reflection Alternatives, Cont.

)](G)('L[π)(R HH λ−λ=λ

* GH(λ) Must be measured at same

Darkroom Radiance *Radiance with Reflections Included

=⎪⎪⎬

⎫

⎪⎪⎨

⎧λρλρ

=λ)()(

)(R K

W

)(E)(R

HH λ

=λ

H = Color (W,K)

measured at same angle from normal and at same place on screen where L' (λ) i d

Source Irradiance

=

⎪⎪⎭

⎬

⎪⎪⎩

⎨

λβλβ

=λ

)()(

)(R

K

WH

( , ) L'H(λ) is measured. (GH is zero for purely reflective displays).

NOTE: L'H(λ) must be significantly greater than GH(λ) or we will be subtracting similar radiances in the numerator giving large errors. This can mean that the light source needs to be very b i ht t bt i li bl t lt

)(R)(Lπ)(E std

λλ

=λ

p y )

If we don’t have an irradiance meter, we can use a white standard where its radiance factor is calibrated for the employed source-detector

bright to obtain reliable measurement results.

)(Rstd λca b ated o t e e p oyed sou ce detectogeometry used in the above measurement.

Note: To avoid complicated subscripts, radiometric quantities appear with explicit wavelength dependence Normally this would be done with a subscript “v” or “e” and

KELTEK 142

wavelength dependence. Normally this would be done with a subscript v or e and non-radiometric quantities with a wavelength dependence would be written as R(λ) —reflection parameters are not radiometric quantities but distributions, see below.


Diffuse Reflectance (sky): Directed-Source Radiance Factor: (sun)Determination of Ambient Contrast for Daylight (Sunlight + Skylight)


)](G)('L[π)(E

)](G)('L[π)(h

WuWuW

λλλ

λ−λ=λρ

Diffuse Reflectance (sky):

)](G)('L[π)(E

)](G)('L[π)( WWW

λλλ

λ−λ=λβ

Directed Source Radiance Factor: (sun)

)(E)](G)(L[π)(

d

KuKuK λ

λ−λ=λρ

“u” denotes uniform illumination.

)(E)](G)(L[π)( KK

K λλ−λ

=λβ

E(λ) will not be affected by light from display.

)()( λβλρ

Radiance from normal (or design viewing direction)Gi are darkroom radiances taken under the same angle as L’Wu and L’Ku, etc.

ssunH

skyH

HH cos)(Eπ

)()(Eπ

)()(L)(K θλλβ

+λλρ

+λ=λ

d)(E)()(Vcoskd)(E)()(VkLK s ∫∫ λλλβλθ

+λλλρλ+= .d)(E)()(Vπ

d)(E)()(Vπ

LK sunHskyHHH ∫∫ λλλβλ+λλλρλ+=

ssunH

skyH

HH cosEπ

Eπ

LK θβ

+ρ

+=

KELTEK 143K

W

KKD =Daylight Ambient Contrast Ratio:

CONTRAST ISN’T EVERYTHING C t t l i l ti t th di diti d

Spectra & Daylight Scaling Cont.Reflection Alternatives, Cont.

Contrast, luminance relative to the surrounding conditions, and character size all contribute to how readable a display is. We have only shown how to derive the contrast ratio under ambient conditions—especially under daylight conditionsconditions—especially under daylight conditions.

DAYLIGHT CONDITIONS:Just pointing a sunlight level source at a display from a largeJust pointing a sunlight level source at a display from a large angle from the normal does not constitute sunlight readability. The diffuse contribution from daylight conditions is often the largest contribution to a reduction in contrast and readability.

FLUORESCENCEIf the display exhibits a non trivial fluorescence then we

g y

If the display exhibits a non-trivial fluorescence, then we must use the correct illumination spectra or account for the fluorescence in another spectrally-resolved manner.

KELTEK 144

CHARACTER CONTRASTS UNDER AMBIENT CONDITIONS

Character ContrastReflection Alternatives, Cont.

AMBIENT LIGHT — RARELY WITHOUTNeed to measure display characteristics under reproducible p y pambient illumination environments.Text use — need accurate character-contrast measurements. Ergonomic and vision researchers need to make accurateErgonomic and vision researchers need to make accurate measurements to perfect models.

COMPARE EMISSIVE & REFLECTIVED i t b bl t fl ti di l ith i iDesire to be able to compare reflective displays with emissive displays on an equal playing field particularly for high ambient conditions.N d t b bl t l l b t t lt tNeed to be able to scale laboratory measurement results to any ambient illumination level encountered.

KELTEK 145

Provide Uniform Ambient Illumination Integrating

Sphere

Character Contrast, Cont.Reflection Alternatives, Cont.

(θ = 8° to 10°)

Vertical Character Stroke Upper-Case "i"

ELETTERS

Stroke, Upper Case i

θ

ELETTERS

I H H H

Black Material Same

Measurement Port[Top View]

H H H

White Reflectance

as Replica Mask

Array Detector

(Sphere shown smaller than

required.)Replica Mask Same

Standard

KELTEK 146

pSize as "I"

Avoid Measurement-Port Vignette


Focusing on items near sphere center makes the measurement port out of focus.

160

180

100

120

140

alue

UNIFORM REGION

40

60

80

100

Pixe

l Va

VIGNETTE REGION

Always be sure you are measuring within the uniform region

0

20

40 REGION region.

KELTEK 147

0 100 200 300 400Pixel Position, y

Whit St d dLuminances for Character Contrast

R li M t i l


White Standard Replica MaterialE = πLstd/ρstd

ρM = πLM/E

LstdLM stdGives E

R liCharacterLR = LM + LG

ReplicaLd

Character

KELTEK 148

White Screen LG = LR – LMLh

Reflective Displays? — Need No More Measurements


Correct character black by subtracting glare:

LK = Ld – LG & LW = Lh – LG

Ambient contrast:

C = L /LCCA = LW/LK

Ambient contrast is independent of illuminance level for purely reflective displays.

Emissive Displays? — Need More Measurements

p y p y

Unless we just happen to have used the exact illuminance level required for testing, we are not done. The results don’t apply to other E levels Must account for self-luminance of display

KELTEK 149

other E levels. Must account for self luminance of display.

Emissive-Display Darkroom MeasurementsCharacter Contrast, Cont.Reflection Alternatives, Cont.

Replica for small characters: Long tapered thin opaque black plastic positioned near character “I” to be measured.

LETTERSI

M L

L′Darkroom luminances:

L′d

L hWhite: L′W = L′h – L′R

Bl k L′ L′ L′

KELTEK 1500L′R

L dBlack: L′K = L′d – L′R


Emissive Displaysp y

Determination of Hemispherical Reflectances:

ρW = π(LW – L′W)/E

ρK = π(LK – L′K)/E

Luminances measured under illuminance E

Darkroom luminances

N t fl t d l iNet reflected luminance

KELTEK 151


Scale Results to Any Environment Illuminance E0

L′W + ρW E0π

Emissive Displays:

CCA =π

L′K + ρK E0ππ

Darkroom l i

Luminance t ib ti fluminance

contributionscontributions from reflected light

CCA = ρW / ρK

Reflective Displays:

KELTEK 152

Contrast for reflective displays does not depend upon environment illuminance E0.


Results for a Reflective Display

30%

20%

25%

hite

(LG/L

W)

10%

15%

Gla

re v

s. W

h

Amount of Glare in Percent of White Measured by Array Detector

0%

5%

Perc

ent G

KELTEK 153

0%0 10 20 30

Line Width (px)

4.0Corrected: CA = LW/LK Your eye


3.5

ytells you that the top curve is more

2.5

3.0

Con

tras

t Uncorrected: CA = Lh/Ld

is more correct!

1.5

2.0

C

Contrasts

1.00 5 10 15 20 25 30

Line Width (px)

ρ = π L /E = 0 450 ± 2 %ρM = ρstd LM/Lstd = 0.0418

Line Width (px)

KELTEK 154

ρW = π LW/E = 0.450 ± 2 %ρK = π LK/E = 0.124 ± 2 %

FRONT PROJECTION: Accounting for Stray Light

FRONT-PROJECTOR METRICSFRONT PROJECTION: Accounting for Stray Light

Projector should not be blamed for the less than perfect viewing conditions of the screen and room. GOAL: Obtain i t i i f f j tintrinsic performance of projector

IMAGE PROJECTOR

AMBIENTLIGHT

VIEWING SCREENBACK-REFLECTEDLIGHT

VIEWER

KELTEK 155

Even in a black-walled darkroom using a black screen with a checkerboard displayed, significant errors of several tens of percent can be made if we are not careful.

Accounting for Stray Light in Room – Projection Mask

Front-Projection Metrics, Cont. Projection Mask

g y g jUse a projection mask (wider than the lens diameter) placed from 35 cm to 60 cm from the screen. Objects in room and room walls reflect light from the white screen back into black area. This can be a serious corruption of pthe black even in a darkroom and even using a black screen!

IMAGEPROJECTOR

PROJECTIONMASK

Ill i d b hi d th

ILLUMINANCE

Illuminance measured behind the mask must be subtracted from the measurement without the mask to obtain an accurate measurement of bl k hitILLUMINANCE

METERblack or white.

Compares well with SLET (± 1 %) in a darkroom and

KELTEK 156

VIEWING SCREENcan possibly be used in a darkened room.

Stray-Light Elimination Tube (SLET)C it t t i hi h bi t li hti

SLETFront-Projection Metrics, Cont.

Can permit accurate measurements even in high-ambient lighting. Must make cosine correction if tilted away from screen normal:

E0 = ESLETcosθ

SLETPROJECTOR

Can permit accurate measurements even in high ambient

IMAGE

PROJECTOR

in high-ambient lighting.

VIEWING SCREEN

GLOSSY BLACK CYLINDER

LIGHT FROMPROJECTOR

ILLUMINANCEMETER

KELTEK 157

This is one kind of design for a SLET.GLOSSY BLACK CONES

Making Luminance Measurements

Luminance MeasurementsFront-Projection Metrics, Cont.

Screen gain is very directional. Can avoid screen effects by using calibrated diffuse white standard and converting to illuminance.

PROJECTIONMASK

IMAGEPROJECTORMASK

BE CAREFUL in using the white standard. It must be properly calibrated for this configuration to

WHITE DIFFUSESTANDARDS

accurately obtain illuminance from luminance.

VIEWING SCREENLUMINANCE METER

)(R φθφθ

KELTEK 158

),(Lπ

),,,(R),(E rrrrii

ii φθφθφθ

=φθ

Grille Measurements to Establish Resolution

Front-Projection Metrics, Cont.

PROJECTOR

LINE MASK

SHADOW

KELTEK 159

[See FPDM 303-7: Resolution from Contrast Modulation]

Direct Measurement Using Slit Illuminance Meter

Front-Projection Metrics, Cont. Grille, Cont.

PROJECTOR

Small Stray Light Elimination Tube

(SLET)

Glossy Black Cylinder

Light from Projector

Slit Illuminance

Meter

KELTEK 160

Glossy Black Frustums InsideMeter

Don’t forget cosine correction if tilted away from the normal.

THANKS FOR LISTENING!THANKS FOR LISTENING!

Edward F. Kelley, Ph.D.; KELTEK, LLC.; [email protected] F. Kelley, Ph.D.; KELTEK, LLC.; [email protected]© E. F. Kelley of KELTEK, 2010, portions of this presentation are copyrighted.

KELTEK

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