Optronic Laboratories, Inc. LED Measurement ALI 2003 LED Measurement Dr. Richard Young Optronic...

Optronic Laboratories, Inc. Optronic Laboratories, Inc. LED MeasurementLED Measurement ALI 2003 ALI 2003Optronic Laboratories, Inc. Optronic Laboratories, Inc. LED MeasurementLED Measurement ALI 2003 ALI 2003

LED Measurement

Dr. Richard Young

Optronic Laboratories, Inc.


Techniques & Types of Measurement

Several types of light measurement are possible. These define WHAT you measure.

For each type of measurement, there are several possible techniques. These define

HOW you measure.



Techniques• Photometry &

colorimetry

• Radiometry

• Spectroradiometry

Types• Total Flux

• Angular Intensity

• At a surface

• At the source




colorimetry

Types• Total Flux


• At a surface

• At the sourceHow does it look to humans?

Quantities start with “photopic” or “luminous”



Techniques• Radiometry

Types• Total Flux


• At a surface

• At the sourceHow much energy is produced?

Quantities start with “radiometric” or “radiant”



Techniques• Spectroradiometry

Types• Total Flux


• At a surface

• At the sourceHow is the energy distributed?

Quantities start with “spectral” or “spectroradiometric”




colorimetry

• Radiometry


Types• Total Flux

Light emitted in ALL directions

Quantities end with “flux”




colorimetry

• Radiometry


Types• Angular Intensity

Light emitted in SPECIFIED directions and angles

Quantities end with “intensity”




colorimetry

• Radiometry


Types• At a surface

Light falling onto areas of an object

Quantities end with “irradiance” or “illuminance”




colorimetry

• Radiometry


Types• At the source

Light emitted from areas within the source

Quantities end with “radiance” or “luminance”



Photometry + Total Flux = Total Luminous Flux

unit: lumensRadiometry + Total Flux = Total Radiant Flux

unit: WattsSpectroradiometry + Total Flux = Total Spectral Flux

unit: Watts/nm



Photometry + Angular Intensity = Luminous Intensity

unit: candelas = lumen/srRadiometry + Angular Intensity = Radiant Intensity

unit: Watts/srSpectroradiometry + Angular Intensity = Spectroradiometric

Intensity

unit: Watts/(sr nm)



Photometry + at a surface = Illuminance

unit: lux = lumen/m²Radiometry + at a surface = Irradiance

unit: Watts/m²

Spectroradiometry + at a surface = Spectral Irradiance

unit: Watts/(m² nm)



Photometry + at a source = Luminance

unit: candelas/m² = lumen/(sr m²)Radiometry + at a source = Radiance

unit: Watts/(sr m²)

Spectroradiometry + at a source = Spectral Radiance

unit: Watts/(sr m² nm)


LEDs

• Emission from LEDs generally depends critically on temperature.– Ambient temperature affects results.

– Heat-sinking, which includes how and where electrical connections are made, affects results.

• Emission from LEDs also depends on supplied current.– Use current regulated rather than voltage regulated

supplies where possible.

General Considerations for all measurementsGeneral Considerations for all measurements


LEDs

• LED chips are virtually ideal light sources.– Very small, almost point sources– Reasonably uniform– Lambertian, except at high angles– Almost monochromatic in most cases

• All types and techniques of measurement are easily employed.


LEDs

• LED packages are very useful, but...– They do not behave like small sources.

– They are generally non-uniform.

– They have highly angular emission.

– They are almost monochromatic in most cases.

• Unique difficulties are found with most types and techniques of measurement.– Standard conditions are required for agreement

between laboratories.


LEDs

• Total luminous flux, Total radiant flux, Total spectral flux

• Luminous intensity, Radiant intensity, Spectroradiometric intensity

• Illuminance, Irradiance, Spectral irradiance

• Luminance, Radiance, Spectral radiance

Here is a list of measurements that might be required:


LEDs





The only measurements with standard conditions are in bold.



Averaged LED Intensity

Mechanical Mechanical axisaxis


1 cm1 cm22 circular circular apertureaperture


31.6 cm31.6 cm31.6 cm31.6 cm

dd = 0.001 sr = 0.001 srdd = 0.001 sr = 0.001 sr

Condition ACondition ACondition ACondition A

Conditions specified in CIE Publication 127


Averaged LED Intensity





10.0 cm10.0 cm10.0 cm10.0 cm

dd = 0.01 sr = 0.01 srdd = 0.01 sr = 0.01 sr

Condition BCondition BCondition BCondition B

Conditions specified in CIE Publication 127

CIE committee TC2-46 is currently working on acceptable tolerances in recommended conditions, with the aim of creating an ISO/CIE standard

for this type of measurement.

CIE committee TC2-46 is currently working on acceptable tolerances in recommended conditions, with the aim of creating an ISO/CIE standard

for this type of measurement.


LEDs





How should total flux be measured?



Total Flux

dddd

We can map the We can map the angular properties angular properties

of a source by of a source by measuring at all measuring at all

values of values of and and ..

We can map the We can map the angular properties angular properties

of a source by of a source by measuring at all measuring at all

values of values of and and ..

Adding up the Adding up the values for all values for all

directions gives the directions gives the total flux.total flux.

Adding up the Adding up the values for all values for all

directions gives the directions gives the total flux.total flux.

A more common A more common method is to use an method is to use an integrating sphere, integrating sphere,

which gives the which gives the total in all total in all

directions with one directions with one measurement.measurement.

A more common A more common method is to use an method is to use an integrating sphere, integrating sphere,

which gives the which gives the total in all total in all

directions with one directions with one measurement.measurement.


Total Flux

• The LED is placed in the sphere center.

• A baffle prevents direct light hitting the detector.

• The sphere walls and baffle are highly reflective.

LEDLED

BaffleBaffle Cosine DetectorCosine

Detector


Total Flux

• A sphere has areas of uniform response (green).

• And non-uniform areas (red).

• If the source is highly directional, it should be pointed at a green area for the best results.


Total Flux

• The LED flux is calculated from signals with the LED and with a standard (known) flux source.

• But, anything placed in the sphere affects its throughput.

• The lamp or LED used in calibration and the LED to be measured are rarely the same.

• Changes in throughput between these lamps will mean results will be wrong unless the changes are also measured.


Total Flux

• An auxiliary lamp, which is housed permanently in the sphere, is used to measure changes in throughput.

• For photometers or radiometers, best results are with an auxiliary lamp the same as the LED to be measured.

• For spectroradiometers, a white light source is best.

Auxiliary Auxiliary LampLamp



Total Flux

• The auxiliary lamp is powered up while the standard or test lamp is in the sphere.– But not switched on.

• The ratio of signals is the change in throughput.– This is part of the

calibration procedure.




Total Flux

• Good total flux measurements require:– A large high reflectivity sphere– Small, well designed, baffles– A cosine collection detector at the sphere wall– An auxiliary lamp

• LEDs present no problem to this type of flux measurement.


Total Flux

• So why do we need standard conditions for measurement?

• Another, more common measurement is forward-looking or 2 flux.– Flux is measured with the LED at the sphere

wall.– It is NOT the same as total flux.– It is generally confused with total flux.


Forward-looking or 2 FluxSo what So what shouldshould be be

measured for 2measured for 2 luminous flux?luminous flux?

So what So what shouldshould be be measured for 2measured for 2 luminous flux?luminous flux?

Any light forward of Any light forward of this plane should be this plane should be OK as a definition.OK as a definition.

Any light forward of Any light forward of this plane should be this plane should be OK as a definition.OK as a definition.

But this assumes the But this assumes the LED is a point LED is a point

source and we know source and we know this is incorrect.this is incorrect.

But this assumes the But this assumes the LED is a point LED is a point

source and we know source and we know this is incorrect.this is incorrect.


Forward-looking or 2 FluxANDAND LED holders can LED holders can affect results.affect results.

How far the LED extends into the

sphere can affect results

ANDAND many commercial many commercial products for products for 2 Flux ignore cosine collection ignore cosine collection at the detectorat the detector, giving , giving large errors.large errors.

ANDAND many commercial many commercial products for products for 2 Flux exclude an auxiliary lampexclude an auxiliary lamp, , giving large errors.giving large errors.

CIE committee TC2-45 is currently working on

recommended conditionsfor this type of measurement.

CIE committee TC2-45 is currently working on

recommended conditionsfor this type of measurement.


Here is an example of LEDs Here is an example of LEDs measured in measured in

““22” flux (without auxiliary lamp) ” flux (without auxiliary lamp) and total flux (with auxiliary lamp) and total flux (with auxiliary lamp)

conditions.conditions.

Here is an example of LEDs Here is an example of LEDs measured in measured in

““22” flux (without auxiliary lamp) ” flux (without auxiliary lamp) and total flux (with auxiliary lamp) and total flux (with auxiliary lamp)

conditions.conditions.

Comparing Fluxes

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

400 450 500 550 600 650 700 750

Wavelength [nm]

Re

lati

ve

Sp

ec

tra

l Ra

dia

nt

Flu

x [

W/n

m]

LED1 forward flux

LED1 total flux

LED2 forward flux

LED2 total flux

LED3 forward flux

LED3 total fluxThese These are clear are clear epoxy.epoxy.

These These are clear are clear epoxy.epoxy.

This is This is red red

epoxy.epoxy.

This is This is red red

epoxy.epoxy.


LEDs





How should illuminance/irradiance be measured?



Illuminance/Irradiance

• Illuminance and irradiance is the light falling onto an area of surface.

• The light can come from any direction and may be from multiple sources.

The The totaltotal light hitting light hitting the area must be the area must be measured.measured.


Illuminance/IrradianceWith LED With LED

packages, the packages, the pattern on a pattern on a

screen varies screen varies with distance.with distance.

With LED With LED packages, the packages, the pattern on a pattern on a

screen varies screen varies with distance.with distance.

Although it is not focused, we can Although it is not focused, we can clearly see the cup/die structure on clearly see the cup/die structure on

the screen.the screen.

Although it is not focused, we can Although it is not focused, we can clearly see the cup/die structure on clearly see the cup/die structure on

the screen.the screen.

cupcupcupcup

diediediedie

We can also see the light is not We can also see the light is not uniform at the surface, so results uniform at the surface, so results

depend on the size and position of the depend on the size and position of the measurement area.measurement area.

We can also see the light is not We can also see the light is not uniform at the surface, so results uniform at the surface, so results

depend on the size and position of the depend on the size and position of the measurement area.measurement area.


Illuminance/Irradiance

• Apart from noting that the illuminance depends on measurement area and position, we should note:– Illuminance is not really a property of a LED.

• The method of measurement is independent of the position, orientation or distance of the source(s).

– Single LEDs are rarely used in general lighting.• The illumination provided by an LED “lamp”, which contains

several elements, is likely to be more uniform.

• Chip LEDs give fairly uniform illuminance.– Little dependence on area or position.


LEDs





How should luminance/radiance be measured?



Luminance/RadianceThe LED The LED

emits light.emits light.The LED The LED

emits light.emits light.

The telescope The telescope refocuses it to refocuses it to give an image.give an image.

The telescope The telescope refocuses it to refocuses it to give an image.give an image.

An aperture then An aperture then isolates the part isolates the part of the image to of the image to be measured.be measured.

An aperture then An aperture then isolates the part isolates the part of the image to of the image to be measured.be measured.


Luminance /Radiance

• The size of the lens defines the solid collection angle.

• The measurement area corresponds to the aperture at the image of the telescope.

• The source MUST be bigger than the measurement area.

Solid Solid Collection Collection

angleangle

Solid Solid Collection Collection

angleangle

Measurement Measurement areaarea

Measurement Measurement areaarea

SourceSourceSourceSource


Luminance /Radiance

• Two main types of telescope exist for this application1. Reflex Telescopes

The reflex mirror lets the The reflex mirror lets the user see what is being user see what is being

measuredmeasured

The reflex mirror lets the The reflex mirror lets the user see what is being user see what is being

measuredmeasured

……is focussed by the is focussed by the telescope.telescope.

……is focussed by the is focussed by the telescope.telescope.

The sectional drawing shows The sectional drawing shows what happens inside the what happens inside the

solid housing.solid housing.

The sectional drawing shows The sectional drawing shows what happens inside the what happens inside the

solid housing.solid housing.

If the mirror is flipped If the mirror is flipped out of the way…out of the way…

If the mirror is flipped If the mirror is flipped out of the way…out of the way…

Light from the Light from the source…source…

Light from the Light from the source…source…


Luminance /Radiance

• Two main types of telescope exist for this application1. Reflex Telescopes

The image is directed The image is directed onto the aperture for onto the aperture for

measurement .measurement .

The image is directed The image is directed onto the aperture for onto the aperture for

measurement .measurement .


Luminance /Radiance

• Two main types of telescope exist for this application2. Direct Viewing Telescopes

ObjectObjectObjectObject

Image appears with a Image appears with a “missing” circular area“missing” circular area

(the aperture)(the aperture)

Image appears with a Image appears with a “missing” circular area“missing” circular area

(the aperture)(the aperture)The “missing” The “missing”

portion is sent to the portion is sent to the detector.detector.

The “missing” The “missing” portion is sent to the portion is sent to the

detector.detector.

The mirror and The mirror and aperture are combined aperture are combined

so the area being so the area being measured is viewed measured is viewed

directly.directly.

The mirror and The mirror and aperture are combined aperture are combined

so the area being so the area being measured is viewed measured is viewed

directly.directly.


Luminance /Radiance

• Relatively inexpensive

• If the viewing optics and aperture are not perfectly equivalent it gives:– Alignment errors

– Parallax errors

• No cross-checks which aperture is being used

• Aperture in image plane

• Costs more• Since the image and

aperture are viewed together there are:– No alignment errors– No parallax errors

• The size of the aperture is seen with the image

• Aperture at an angle to the image plane

Reflex TelescopeReflex TelescopeReflex TelescopeReflex Telescope Direct Viewing TelescopeDirect Viewing TelescopeDirect Viewing TelescopeDirect Viewing Telescope


Luminance /Radiance

• For large, uniform, Lambertian sources, luminance measurements are generally:– Insensitive to focus of the telescope– Insensitive to position of the measurement area– Insensitive to rotation of the telescope axis– Insensitive to lens or measurement area size– Insensitive to the source/telescope distance

• For single LED packages, luminance measurements are just the opposite:– They are extremely sensitive to everything


Luminance /Radiance

• Chip LEDs are easy to measure, provided a small enough aperture is available.

• Package LEDs are very difficult to measure:– Lenses create a co-dependence of measurement

collection angle and measurement area.• Almost any value can be obtained, depending on the

conditions of measurement.


Luminance /Radiance

• There are no recommendations for measurement of luminance of LED packages.

• Currently, the following are being discussed:– Measure the chip before it is packaged.– Cut and polish the package to give a flat exit surface.– Measure the Condition A averaged LED intensity and

divide the result by the chip emission area (excluding any contact areas).

• This gives the “effective” luminance, rather than true luminance, but has the advantage of being easy and consistent with other types of measurement.


Conclusions

• Chip LEDs are relatively easy to measure.• Packaged LEDs can prove difficult to

measure.• When comparing results, make sure the

same measurement conditions are used.• Where possible, use recommended

conditions.• Use well designed measurement equipment.

Optronic Laboratories, Inc. LED Measurement ALI 2003 LED Measurement Dr. Richard Young Optronic...

Documents

Transcript of Optronic Laboratories, Inc. LED Measurement ALI 2003 LED Measurement Dr. Richard Young Optronic...