Optical Customisation of Touch Screens and Display Front Surfaces

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Society for Information Display

March 2013

Rob Bennett

3M Optical Systems Division Europe

Optical Customisation of Touch Screens

and Display Front Surfaces

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Summary

Performance characteristics that can be altered :

contrast and readability

durability

privacy

Contrast enhancement solutions:

Durability options

Addendum: summary of 3M technologies relevant

to touch screens

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Contrast enhancement solutions:

Anti-Glare Surfaces

Anti-Reflective Surfaces

Circular Polarisers

Louvre Films

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Measuring Reflections

When light reaches an interface between two media the

following can happen:

Transmitted, Reflected, Absorbed, or Scattered

Incident 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

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V a ccum : 1.00 A i r : 1.0028 Wa ter : 1.33 Si l i cone : 1.41 Gl a s s : 1.43-1.74 PMMA : 1.49 P o lyc ar bo n at e : 1.58 PET : 1.60 Ti tan i u m Oxi de : 2.40 OCA : 1.47

Transparent

Reflection

Reflection

n

C

n

B

n

A

OCA

Air

PMMA

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

Reflectivity;

PMMA/OCA 0.04%

PMM/Air 3.8%

Common Refractive Indices

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Adding Structures to the Front of a

Display Adds Reflections

………

R1 R2 R3 R4 R5 PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Air Gap Polariser Resistive Touch screen LCD R1 Polariser LCD

Black state of display is reduced Contrast degrades

Readability is decreased Transmission

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Dealing with Reflections

3 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

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Typical Front Surface Treatments

Anti-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 Æ diffusion level can be optimized

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

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Typical Front Surface Treatments

Anti-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 n1 n2 n0 n0 < n1 < n2 Thin Layer Destructive Interference Incident Light Reflective light 1 Reflective light 2

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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) Glass Anti-Reflection Glass Anti-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

Destructive Interference

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Appearance of Anti-Reflection vs. Anti-Glare

Antireflection

(AR)

Specular

Untreated

Antiglare

(AG)

Optical performance and durability vary depending upon treatment type

Blackest Least durable Good black when not in a reflection path Least black Highest reflection: localised Moderate reflection: Lamp reflection dispersed Low reflection: localised Lamp creates glare peak

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Example of a combined AR and AG structure

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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

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Circular Polarisers

37-40% effective display transmission Reflection largely

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Adding CP to a multilayer example

………

R1 R2 R3 R4 R5 Circular Polariser PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Air Gap Polariser Resistive Touch screen LCD Transmission Reduced

R2,R3, R4 largely suppressed if polarisation is maintained through the touch screen R5 reduced.

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Design Considerations for a Circular Polariser

Polarisation-maintaining optical path

CP effect is reduced if surface of display de-polarises the reflection

or 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

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Impact of adding micro-louvres

lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll R1 R2 R3 Touch screen LCD

Micro-louvres are typically added to a screen system to provide privacy There 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%

Transmission Reduced

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Eliminating Air Gaps

………

R1 R2 R3 R4 R5 PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Air Gap Polariser Resistive Touch screen LCD Transmission Reduced

………

R1 R2 R3 R4 R5 PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate

Optically Clear Adhesive Polariser Resistive Touch screen LCD Transmission increased ~ 8% vs original design

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Example of gap filling

Reflection = 4%

Touch Panel or Cover Lens

LCD CEF

Air Gap

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V a ccum : 1.00 A i r : 1.0028 Wa ter : 1.33 Si l i cone : 1.41 Gl a s s : 1.43-1.74 PMMA : 1.49 P o lyc ar bo n at e : 1.58 PET : 1.60 Ti tan i u m Oxi de : 2.40 OCA : 1.47

Transparent

Reflection

Reflection

n

C

n

B

n

A

OCA

Air

PMMA

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

Reflectivity;

PMMA/OCA 0.04%

PMM/Air 3.8%

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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 ?

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Increasing durability

Touch enabled plastic outer surfaces will experience wear

and tear in use.

Measures to increase their resistance to damage:

1.

Hardcoat

2.

Low surface energy coating

3.

Make the front surface sacrificial

R1

Polariser

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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

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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

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Measuring Surface Energy:

Water Contact Angle

The more the water droplet beads up, the better the cleanability and

pen bead up

120 degrees

98 degrees

85 degrees

52 degrees

Pen repellency – Level 1 (complete bead up)

Pen repellency –

Level 2

(partial bead up)

Pen repellency –

Level 3

Scotchgard™surface

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Hexadecane Contact Angle

The more the hexadecane droplet beads up, the

better the oil resistance

61 degrees

52 degrees

21 degrees

<10 degrees

Typical silicone

surface

Typical untreated

surface

Scotchgard™ surface

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Easy Cleaning Performance

Pen mark beads up on Scotchgard™ surface Pen mark is easily wiped away on Scotchgard™ surface Pen mark remains on untreated surface Wipe with tissue

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Durable Quick Clean - DQC

Scratch 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 Æ 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.

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Sacrificial Front Surface

Key requirements

1- Sacrificial protective film

2- Optically clear removable adhesive

easy to apply

stays in place while in use

easy to remove

Since dust creates air bubbles, optimum application

condition would be clean.

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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 ?

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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 ?

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3M Display

Surface Products

Addendum

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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 rbennett@mmm.com

European Market Development Manager 3M Optical Systems Division

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