Sunday, August 1, 2010

The Quality of Polycarbonate and Light Transmission

As we explained in a previous blog post, we would typically expect the light transmission of 0.118" one side hard-coated Polycarbonate to be in the range of 90% [with 5.1% reflectance on the uncoated side, 4% reflectance on the coated side and a little internal loss of light transmission due to the internal structure of the Polycarbonate itself]. As the thickness of the Polycarbonate increases, we would expect the internal loss of light transmission to also increase a little.

We were recently asked by a customer to apply an anti-reflective coating to the uncoated side of 0.236" one side hard-coated Polycarbonate. The Polycarbonate was provided by the customer and had been produced by another manufacturer. By applying the anti-reflective coating we were expecting to reduce the reflection on the uncoated side from 5.1% to around 1.0%. We were therefore expecting to increase the overall light transmission from 89-90% to around 94%.

After we had coated the material with the anti-reflective we discovered, to our surprise, that we were only getting a light transmission of 89%. The application of the anti-reflective coating appeared to have failed. We examined our coating process and found no obvious problems. We then decided to test the light transmission of the material before we applied the anti-reflective coating. To our surprise we found that the light transmission was only 84-85% instead of the 90% that we expected. The problem was with the quality of the competitors Polycarbonate and not the anti-reflective coating.

We then measured the reflection on both surfaces and calculated the internal loss of light transmission across the entire visible spectrum. We then repeated this process with our own 0.236" Polycarbonate. We then plotted our the internal loss of light transmission for both materials over the visible spectrum. This plot can be seen in the diagram at the top of the page (for a better view, click on the picture).

The results were shocking.
Over the range of 450-500 nm, our material had an internal loss of light transmission of 2% and the competitors had a loss of 5%
Over the range of 525-575 nm, our material had an internal loss of light transmission of 4% and the competitors had a loss of 7%
Over the range of 650- 750 nm our material had an internal loss of light transmission of 1% and the competitors had a loss of 7%

The end result was that the customer would have been better off buying our HighLine Polycarbonate without an anti-reflective rather than applying an expensive anti-reflective to the competitors material. In the end the customer decided to use our Polycarbonate with an anti-reflective and achieved a light transmission of over 94%.

The lesson to be learned from this recent experience is that not all Polycarbonate sheet is equal. The Polycarbonate sheet from this competitor, who is a major international supplier of Polycarbonate sheet, clearly had a much lower light transmission across the visible spectrum than the Polycarbonate sheet from HighLine Polycarbonate. This lower transmission is caused by inferior resin, use of regrind and the commodity production methods used by some of the large producers. In the vast majority of applications, particularly commodity applications, this loss of light transmission is not important. However, in some quality and high-tech applications, a 6% light transmission loss in the 650-750nm range can be critical. Any application requiring an anti-reflective coating should seriously consider the quality of the base Polycarbonate and should be extremely cautious about buying an off the shelf product from a distributor. Polycarbonate sheet for high quality applications should always be bought directly from the manufacturer so that you can have the material produced specifically for the required application.
All of HighLine Polycarbonate's material is designed for high quality optical applications. If you are using another supplier's material it would be wise to ask for them to provide the light transmission curve for the actual lot number of the sheet you will be receiving. We were certainly surprised by the poor quality of some of the material that is being sold as high quality product.

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