Optical Customisation of Touch Screens and Display Front Surfaces

Transcription

1 Optical Customisation of Touch Screens and Display Front Surfaces Society for Information Display March 2013 Rob Bennett Europe

2 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

3 Contrast enhancement solutions: Anti-Glare Surfaces Anti-Reflective Surfaces Circular Polarisers Louvre Films Air gap elimination

4 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 Results in a loss of contrast in high ambient light

5 Common Refractive Indices Vaccum : 1.00 Air : Water : 1.33 Silicone : 1.41 Glass : PMMA : 1.49 Polycarbonate : 1.58 PET : 1.60 Titanium Oxide : 2.40 OCA : 1.47 n A n B n C Transparent Reflection Reflection Reflectivity =(n C -n B ) 2 / (n C +n B ) 2 Reflectivity; PMMA/OCA 0.04% PMM/Air 3.8% OCA Air PMMA Air-solid interfaces create significant reflections

6 Adding Structures to the Front of a Display Adds Reflections R1 LCD Polariser R1 R2R3 Transmission Reduced Resistive Touch screen LCD R4 R5 Black state of display is reduced Contrast degrades Readability is decreased PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Air Gap Polariser

7 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 - Eliminate air-gaps

8 Typical Front Surface Treatments Anti-Glare (AG) Mechanism Mechanism of AG Film Significant Punch Through Rapid Fall-off with Angle Low Image blur Minimal Punch Through Slow Fall-off with Angle High Image blur Low Diffusion High Diffusion 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

9 Typical Front Surface Treatments Anti-Reflection (AR) Mechanism Mechanism of AR Film Incident light θ Reflective light 1 Reflective light 2 Incident Light Reflective light 1 Reflective light 2 n 0 n 1 n 2 d Destructive Interference n 0 < n 1 < n 2 Thin Layer 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

10 Anti-Reflection (AR) vs. Anti-Glare (AG/Matte) Anti-Reflection (AR) Anti-Glare (AG) Destructive Interference n air =1.0 n air =1.0 n Glass =1.44 Anti-Reflection Glass n Glass =1.44 Anti-Glare Glass 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

11 Appearance of Anti-Reflection vs. Anti-Glare Antireflection (AR) Specular Untreated Antiglare (AG) Lamp creates glare peak Low reflection: localised Highest reflection: localised Moderate reflection: Lamp reflection dispersed Blackest Least durable Good black when not in a reflection path Least black Optical performance and durability vary depending upon treatment type

12 Example of a combined AR and AG structure vs original specular surface

13 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

14 Circular Polarisers Reflection largely extinguished if polarisation is maintained in the path to and from the display surface 37-40% effective display transmission

15 Adding CP to a multilayer example Resistive Touch screen LCD R1 R2R3 R4 R5 Transmission Reduced Circular Polariser PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Air Gap Polariser R2,R3, R4 largely suppressed if polarisation is maintained through the touch screen R5 reduced. %T reduced. Typical transmission of a circular polariser: 37-40%

16 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

17 Impact of adding micro-louvres Micro-louvres are typically added to a screen system to provide privacy There is an additional effect: off-axis ambient light is largely absorbed. R1 R2 R3 Transmission Reduced Touch screen LCD lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll Off-axis rays R2 and R3 are largely suppressed. Typical transmission of micro-louvres: 65%

18 Eliminating Air Gaps R1 R2R3 Transmission Reduced Resistive Touch screen LCD R4 R5 PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Air Gap Polariser R1 R2R3 Transmission increased ~ 8% vs original design Resistive Touch screen LCD R4 R5 Reflections R4 and R5 largely disappear. PET film ITO coating Air Gap Spacer dots ITO coating Glass substrate Optically Clear Adhesive Polariser

19 Example of gap filling Reflection = 4% Air Gap Touch Panel or Cover Lens CEF LCD CEF/OCA

20 OCA and Refractive Index Vaccum : 1.00 Air : Water : 1.33 Silicone : 1.41 Glass : PMMA : 1.49 Polycarbonate : 1.58 PET : 1.60 Titanium Oxide : 2.40 OCA : 1.47 n A n B n C Transparent Reflection Reflection Reflectivity =(n C -n B ) 2 / (n C +n B ) 2 Reflectivity; PMMA/OCA 0.04% PMM/Air 3.8% OCA Air PMMA

21 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

22 Increasing durability R1 LCD Polariser 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

23 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

24 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

25 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 Scotchgard surface Pen repellency Level 1 (complete bead up) Pen repellency Level 2 (partial bead up) Pen repellency Level 3 (no bead up)

26 Hexadecane Contact Angle The more the hexadecane droplet beads up, the better the oil resistance 61 degrees 52 degrees 21 degrees <10 degrees Scotchgard surface Typical silicone surface Typical untreated surface

27 Easy Cleaning Performance Wipe with tissue Pen mark remains on untreated surface Pen mark beads up on Scotchgard surface Pen mark is easily wiped away on Scotchgard surface

28 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 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

29 Key requirements Sacrificial Front Surface 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.

30 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?

31 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?

32 Addendum 3M Display Surface Products

33 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

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