Optical Customisation of Touch Screens and Display Front ... AGM Presentations/3M.pdf · 1-...

3M Optical Systems Division

Society for Information DisplayMarch 2013

Rob Bennett3M Optical Systems Division Europe

Optical Customisation of Touch Screensand Display Front Surfaces


SummaryPerformance characteristics that can be altered :• contrast and readability• durability• privacy

• Contrast enhancement solutions:• Durability options

• Addendum: summary of 3M technologies relevant to touch screens


Contrast enhancement solutions:

• Anti-Glare Surfaces• Anti-Reflective Surfaces• Circular Polarisers• Louvre Films• Air gap elimination


Measuring Reflections

• When light reaches an interface between two media the following can happen:

– Transmitted, Reflected, Absorbed, or ScatteredIncident Light = T + R + A +S

• Reflection Opportunities– Change in refractive index between adjacent media– Reflection occurs at every optically dissimilar interface

of the light path– Results in a cumulative transmission loss – Results in a loss of contrast in high ambient light


Vaccum : 1.00

A i r : 1.0028

Water : 1.33

Si l i cone : 1.41

Glass : 1.43-1.74

PMMA : 1.49

Po lycarbonate : 1.58

PET : 1.60

Ti tan iu m Oxide : 2.40

OCA : 1.47

Transparent

Reflection

ReflectionnC

nB

nA

OCA

Air

PMMA

Reflectivity =(nC-nB)2/ (nC+nB)2

Reflectivity;PMMA/OCA 0.04%PMM/Air 3.8%

Common Refractive Indices

Air-solid interfaces create significant reflections


Adding Structures to the Front of a Display Adds Reflections

…………………………

R1R2

R3R4R5 PET film

ITO coatingAir GapSpacer dotsITO coatingGlass substrateAir GapPolariser

ResistiveTouch screen

LCD

R1

PolariserLCD

Black state of display is reducedContrast degrades

Readability is decreased

TransmissionReduced


Dealing with Reflections3 common mechanisms:

1. Scatter the reflection:- Anti-glare

2. Optically suppress or absorb the reflection: - Anti-reflection,- Circular Polariser- Louvres

3. Eliminate the refractive index mismatches- Eliminate air-gaps


Typical Front Surface TreatmentsAnti-Glare (AG) Mechanism

Low Diffusion High Diffusion

• Significant “Punch Through”

• Rapid Fall-off with Angle

• Low Image “blur”

• Minimal “Punch Through”

• Slow Fall-off with Angle

• High Image “blur”

Mechanism of AG Film

▶Anti-glare scatters incident light into many angles diffusion level can be optimized

▶Anti-glare removes reflected “images”, but creates a uniform gray-level reflection

▶Anti-glare improves contrast at angle of bright reflections, but reduces contrast at all other angles


Typical Front Surface TreatmentsAnti-Reflection (AR) Mechanism

▶ Path difference(△) between reflected light 1 and 2 equals 2nd cosθ (n =refraction index)

▶ Reflected light 1, 2 interfere destructively when path difference △ is λ/ 2

▶ Thickness condition to have Min. Reflectance at 580nm, d ~ 100nm

Mechanism of AR Film

d

θ

Reflective light 1

Reflective light 2

Incident light

n1n2

n0

n0 < n1 < n2 Thin Layer

DestructiveInterference

Incident Light

Reflective light 1

Reflective light 2


Anti-Reflection (AR) vs. Anti-Glare (AG/Matte)

Anti-Reflection (AR)

nGlass=1.44

nair=1.0

nGlass=1.44

nair=1.0

Anti-Glare (AG)

GlassAnti-Reflection

GlassAnti-Glare

Anti-Reflection Films reduce light reflections and increase

transmission from a display.

%R is less, %T increases

Anti-Glare (matte) diffuses the specular reflected light component.

%R is not reduced

DestructiveInterference


Appearance of Anti-Reflection vs. Anti-Glare

Antireflection(AR)

SpecularUntreated

Antiglare(AG)

Optical performance and durability vary depending upon treatment type

BlackestLeast durable

Good blackwhen not in a reflection path

Least black

Highestreflection:localised

Moderatereflection:Lamp reflection dispersed

Lowreflection:localised

Lamp createsglare peak


Example of a combined AR and AG structurevs original specular surface


Considerations Using Anti-Reflection Technology

Finger-printing• AR films work via the destructive inference of light• AR films finger-print since the deposit of grease is large compared to the active

AR structure (~100nm)• a low surface energy treatment (eg Scotchgard) renders finger-prints

far more removable, but not invisible

No additional air gaps• AR films must be applied on to the surfaces involved• Only the surface to which the AR treatment is applied has a lowered reflectivity

Durability• An AR structure is outermost on a film and is very thin • An AR film is less durable than a film designed solely for protection • The level of AR performance vs the level of durability is a trade off• the durability of the film is linked to the material to which it is applied


Circular Polarisers

37-40% effective displaytransmission

Reflection largely extinguished…if polarisation is maintained in the path to and from the display surface


Adding CP to a multilayer example

…………………………

R1R2

R3R4R5

Circular PolariserPET filmITO coatingAir GapSpacer dotsITO coatingGlass substrateAir GapPolariser


LCD

TransmissionReduced

R2,R3, R4 largely suppressed if polarisation is maintained through the touch screenR5 reduced.%T reduced. Typical transmission of a circular polariser: 37-40%


Design Considerations for a Circular Polariser • Polarisation-maintaining optical path

CP effect is reduced if surface of display de-polarises the reflectionor if the touch screen structure scatters the polarisation through material birefringence.

• Temperature:• Iodine polarisers can degrade if exposed to repeated temperatures in

excess of 80 degree C• Iodine is relatively loosely attached to a PVA lattice: it is freed by

temperature: Material ceases to act as a polariser.

• Polarisation Alignment: User wearing sunglasses• Non- polarised displays (CRT, OLED) – CP transmission axis should

be vertical to enable maximised performance • Polarised displays: eg LCD => some angular adjustment may be required to minimise colour variation


Impact of adding micro-louvres

lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll

R1R2

R3

Touch screen

LCD

Micro-louvres are typically added to a screen system to provide privacyThere is an additional effect: off-axis ambient light is largely absorbed.

Off-axis rays R2 and R3 are largely suppressed.Typical transmission of micro-louvres: 65%

TransmissionReduced


Eliminating Air Gaps

…………………………

R1R2

R3R4R5 PET film

ITO coatingAir GapSpacer dotsITO coatingGlass substrateAir GapPolariser


LCD

TransmissionReduced

…………………………

R1R2

R3R4R5 PET film

ITO coatingAir GapSpacer dotsITO coatingGlass substrateOptically Clear AdhesivePolariser


LCD

Transmission increased ~ 8% vs original design

Reflections R4 and R5 largely disappear.


Example of gap filling

Reflection = 4%

Touch Panel or Cover Lens

LCDCEF

Air GapCEF/OCA


Vaccum : 1.00

A i r : 1.0028

Water : 1.33

Si l i cone : 1.41

Glass : 1.43-1.74

PMMA : 1.49

Po lycarbonate : 1.58

PET : 1.60

Ti tan iu m Oxide : 2.40

OCA : 1.47

Transparent

Reflection

ReflectionnC

nB

nA

OCA

Air

PMMA

Reflectivity =(nC-nB)2/ (nC+nB)2

Reflectivity;PMMA/OCA 0.04%PMM/Air 3.8%

OCA and Refractive Index


Design considerations for Air Gap Elimination

1. Rigid to rigid lamination

2. Out-gassing

3.Re-workability in assembly

4. Repair: can the touch screen be removed from the display if one component fails ?

5. Spacer gap in a resistive touch screen cannot be filled


Increasing durability

Touch enabled plastic outer surfaces will experience wearand tear in use.

Measures to increase their resistance to damage:1. Hardcoat2. Low surface energy coating3. Make the front surface sacrificial

R1

PolariserLCD


Hardcoat

• Benefit is self explanatory

• The challenge is to apply in a film version with sufficient durability and flexibility

• Typical measurement method is pencil hardness…

• A steel wool scratch resistance test can be more meaningful


Low Surface Energy Coating

Making a front surface easier to clean:

• Reduces the impact of scratching in cleaning

• Lower surface energy typically lowers surface friction


Measuring Surface Energy:Water Contact Angle

• The more the water droplet beads up, the better the cleanability and pen bead up

120 degrees98 degrees

85 degrees52 degrees

Pen repellency – Level 1(complete bead up)

Pen repellency –Level 2

(partial bead up)

Pen repellency –Level 3

(no bead up)

Scotchgard™surface


Hexadecane Contact Angle

• The more the hexadecane droplet beads up, the better the oil resistance

61 degrees52 degrees 21 degrees <10 degrees

Typical silicone surface

Typical untreated surface

Scotchgard™ surface


Easy Cleaning Performance

Pen mark beads up on

Scotchgard™ surface

Pen mark is easily wiped

away on Scotchgard™

surface

Pen mark remains onuntreated surface

Wipe with tissue


Durable Quick Clean - DQCScratch Resistance technology

• Durability Testing: "Steel Wool Test" – 1kg weight of #0000 steel wool – 25 passes

• Cover Lens Result– High Scratching

• Haze went from 1.33 46.8• Display readability significantly decreased• Scattering surface “blurs” display image

• DQC typically showed no scratches at 25 passes and can even undergo 50 passes with little to no scratching.

• DQC combines a 4H hardcoat with a low surface energy treatment


Sacrificial Front Surface

Key requirements

1- Sacrificial protective film2- Optically clear removable adhesive

easy to applystays in place while in useeasy to remove

Since dust creates air bubbles, optimum applicationcondition would be clean.


Considerations: adding films to a touch screen

• Does the technology still operate with a film placed on the front surface ?

• Can it support a front surface film as long as it is not conductive ?

• Does adding a film significantly affect the response and accuracy ?

• Will adding a front surface film achieve the desired design effect ? If contract enhancement is the goal, will sufficient reflections be suppressed ?


Starting with the design needs in mind: Once a device is assembled, only the first surface is readily adaptable.

Readability:If readability in high ambient light is a requirement,consider an approach with minimal reflections from the outset.

Quality expectation:• Retro-fit by a consumer (eg self-applied iPhone protection) will tolerate

a lower QA threshold than an adapted OEM product going to a professional application

eg displays going into the military, industrial etc.

• If high quality is required...where in the supply chain can a ‘retrofit’ option best be added ?


3M DisplaySurface Products

Addendum


Primary areas where 3M technology can adapt touch screens:

- hard coat films- low reflection and anti-reflection films- finger print fading film - micro louvres- infra-red reflection (with visible transmission)- optically clear adhesives for gap filling

Contact [email protected] Market Development Manager3M Optical Systems Division

mailto:[email protected]


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