Wednesday, October 28, 2009

Transmission - Anti Reflectives and Anti Glare

The term transmission is often used when specifying Polycarbonate or other clear plastics.  The terms anti-reflective and anti-glare are also used, often without a clear understanding of the meaning.
Polycarbonate sheet made from a high quality resin has a refractive index of 1.585  
This number means that light travels in Polycarbonate at 1 /1.585 or about 2/3 of the speed of light in a vacuum.

When light passes from one substance to another substance with a different refractive index two effects occur.  Firstly the light changes direction slightly and secondly some of the light is reflected.
The amount of light that is reflected can be calculated using the Fresnell Equations:
R = [ (h0 - h1) / (h0 + h1) ]2
Where  R is the amount of light reflected and h0 and h1 are refractive indices of the two materials.

If the refractive index of air (1.001) and polycarbonate (1.585) are used, the reflection on the surface is calculated to be 5.1%
However it should be remembered that there are two surfaces giving a total reflection of 10.2%; this is the reason why high quality Polycarbonate sheet has around 89% transmission as the remaining 10.2% of the light is reflected.
In display applications it is important to both increase transmission and reduce reflection. Increasing transmission allows a brighter display for a given backlight.  Reducing reflection makes the display easier to see for the user, particularly in bright sunlight.

There are two solutions to reduce the reflection from the surface back to the user.  The first is to use an anti-glare coating.  This reduces the light that is reflected back to the user by scattering the light, much like a matte surface.  Unfortunately this method also reduces the light passing through the sheet and the transmission can often be reduced to 80% or lower.

The second and better method is to use an anti-reflective coating.  With an anti-reflective coating the reflection can be reduced to 0.75% on each surface giving a total of 1.5% reflection. With an anti-reflective coating, the total transmission of a Polycarbonate sheet can be raised to 98.5%.  Anti-reflective coatings allow the goals of increased transmission and reduced reflection to be achieved.

HighLine Polycarbonate LLC produces Polycarbonate sheets with a range of anti-reflective coatings.  These can be combined with transparent conductive ITO layers for the display industry.



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