Posts

Thermal Management for Touch Screen Displays

I can’t seem to find a straight answer on this. What are the differences (if any) in anti-reflective, anti-fingerprint, anti-smudge, and anti-glare?? Every piece of information I find ends up as a sales pitch. Can you clarify things for me?

Touch guy thought that the new fad of taking and sharing selfies meant everyone wanted to see themselves (a lot), so he wonders why there are so many requests for anti-reflective/anti-fingerprint films (AR/AF) on his products. He is so out-of-touch (haha). Ok, enough cracking myself up.

What customers want when they ask for AR coatings is to reduce the ability to see oneself when looking at the display, especially when you are outside. Ultimately, AR coatings make the display easier to see.

In legacy touch products, glare reduction was done using anti-glare (AG) coatings. AG treatments are made by either coating the touch surface with silica “bumps” or lightly etching the glass. The result is that some of the light hitting the touch surface is diffused or scattered, and not reflected back at the user. This was good enough for 25 years of touch technology.

Then along comes Apple and their beautiful high resolution displays to upset the cart. To preserve that super display image, polished glass was used on the iPhone instead of the traditional AG treatment. It is kind of OK for mobile devices because you can tilt the surface to reduce reflections, but less easy to “fix” on bigger devices such as pads.

So the market is asking for something that reduces reflections but preserves that pretty little picture on the display. This has traditionally been done using anti-reflection treatments. Reflection reduction can be achieved in a number of ways, including moth-eye treatments, circular polarizers, eliminating the reflective surface, or that which I will talk about, multi-layer thin film interference coatings.

AR thin film treatments work by “trapping” the light. OK, English majors, stay up with me here….if you took a physics class you know that most energy can be represented by a wave of peaks and valleys (sine pattern). In your physics class you probably also played with a 3 by 5 card with slits, or (as touch guy did) a wave generator made of Popsicle sticks which showed that light can “cancel” itself. Angstroms wide thin films, made up of multiple layers of transparent materials, are put down (on the touch surface) to a precise thickness relative to a visible wave length (550nm) so that light (from your smiling face) passes into the thin film, reflects on the touch surface, and then is reflected back on itself by the thin film, and, as if by magic, the reflection is gone (canceled). Another benefit of AR coatings is that because the light is not reflected, much more light from the display passes through the AR coating so the display is actually brighter. (OK, physics majors, I know this is not a compete explanation, but I might have lost most readers by now)

Touch Guy finds that customers want less than 1% of the light (your pretty face) to be reflected, and that is pretty commonly (though still not cheaply) achieved. If you have enough money (and don’t mind parting with a chunk of it), you can get an AR stack to have less than 0.1% reflection. It used to be that only a few companies could make production quantities of AR coated material but the demand for this stuff has dramatically reduced the cost and increased the number of suppliers, both on plastic and glass.

There are two problems with AR coatings on touch screens. The finger print problem has been fixed with anti-finger print (AF) coatings. Recalling the description above, you know that the thin film coating must be the precise thickness of a light wave (say ¼ of the width) to work. Along comes your dirty, oily finger tip, laden with hand cream, to touch the screen. Boom, the oil you left on the AR coating has changed the thickness of the coating which reduces the ability to trap the light and most often shows up as a finger print. AF coatings work by resisting the ability of the oil on your finger to “stick” to the AR coating; these are called hydrophobic (fear of water) coatings. Of course, for the AR coating to work, the AF coating must be built into the precise thickness of the thin-film stack. Touch Guy is not impressed with the after market spray-on AF coatings over AR stacks. Not impressed at all.

AR problem number two is that it wears off…think about it, angstroms thick material in an abrasive and chemically active environment, and you have limited time before its anti-reflection properties go away, especially on frequently touched areas. There seem to be two solutions…the first is that the AF coating on the AR stack is made of (secret) tough material that will wear-off your finger tip (just kidding) before it gives up. The other is to use thicker, but less effective (2% reflection) organic AR coatings, that seem to have better resistance to finger wear.

Another way around the AR wear problem is to eliminate the reflective surfaces in the first place. Touch International is a big supplier of optically bonded (touch panel to display) products which increased the ruggedness of the display while eliminating the internal reflections. An AG coating and an optically bonded p-cap touch panel eliminates the wear, and anti-finger print issues, with only a minor loss in display sharpness. And the new p-cap+ product has a secret method of AR reduction.

Until next time,
Touch Guy