3M Optical Systems Division Society for Information Display March 2013 Rob Bennett 3M Optical Systems Division Europe Optical Customisation of Touch Screens and Display Front Surfaces
3M Optical Systems Division
Society for Information DisplayMarch 2013
Rob Bennett3M Optical Systems Division Europe
Optical Customisation of Touch Screensand Display Front Surfaces
3M Optical Systems Division
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
3M Optical Systems Division
Contrast enhancement solutions:
• Anti-Glare Surfaces• Anti-Reflective Surfaces• Circular Polarisers• Louvre Films• Air gap elimination
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
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
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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
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
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
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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
3M Optical Systems Division
Example of a combined AR and AG structurevs original specular surface
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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
3M Optical Systems Division
Circular Polarisers
37-40% effective displaytransmission
Reflection largely extinguished…if polarisation is maintained in the path to and from the display surface
3M Optical Systems Division
Adding CP to a multilayer example
…………………………
R1R2
R3R4R5
Circular PolariserPET filmITO coatingAir GapSpacer dotsITO coatingGlass substrateAir GapPolariser
ResistiveTouch screen
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%
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
Eliminating Air Gaps
…………………………
R1R2
R3R4R5 PET film
ITO coatingAir GapSpacer dotsITO coatingGlass substrateAir GapPolariser
ResistiveTouch screen
LCD
TransmissionReduced
…………………………
R1R2
R3R4R5 PET film
ITO coatingAir GapSpacer dotsITO coatingGlass substrateOptically Clear AdhesivePolariser
ResistiveTouch screen
LCD
Transmission increased ~ 8% vs original design
Reflections R4 and R5 largely disappear.
3M Optical Systems Division
Example of gap filling
Reflection = 4%
Touch Panel or Cover Lens
LCDCEF
Air GapCEF/OCA
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
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
3M Optical Systems Division
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
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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 onuntreated surface
Wipe with tissue
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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
3M Optical Systems Division
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.
3M Optical Systems Division
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 ?
3M Optical Systems Division
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 Optical Systems Division
3M DisplaySurface Products
Addendum
3M Optical Systems Division
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
3M Optical Systems Division
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