This topic is a follow up of previous blog on the 28th October 2009 – “Transmission – Anti Reflectives and Anti Glare”. The previous blog gives an introduction Refractive Index and Reflection.
One question that we are often asked is why the Light Transmission of our abrasion resistant coated polycarbonate sheet (90%) is higher than the Light Transmission of our uncoated polycarbonate sheet (88%)?
This question is asked because there is a belief that the coating should reduce the “optical properties” of the sheet. Some people even believe that we must be using a “purer” base sheet for our coated product.
The answer to the question is related to the reflection of light. As we discussed in our previous blog post, light is reflected from uncoated sheet on the front surface and the back surface.
Uncoated sheet.
At the front surface, the light passes from the air (with a refractive index of 1.00) to the Polycarbonate (with a refractive index of 1.585). Using the Fresnell Equations, the reflection can be calculated as 5.1%. [See the previous post for details of the Fresnell equations].
At the back surface, the light passes from the Polycarbonate (with a refractive index of 1.585) to the air (with a refractive index of 1.00). The reflection from this surface is also 5.1%.
The total reflection is 10.2% giving a light transmission of 89.8%. Typically we report a light transmission of 88% to be conservative.
Coated sheet.
In the case of one side coated sheet we introduce another layer – the coating. The coating material typically has a refractive index of 1.49. With this information we can calculate the transmission of the coated sheet.
At the front surface, the light passes from the air (with a refractive index of 1.00) to the coating (with a refractive index of 1.49). The reflection from this surface can be calculated as 3.9%
The light then passes from the coating (refractive index of 1.49) to the Polycarbonate (refractive index of 1.585). The reflection from this surface can be calculated as 0.1%.
At the back surface, the light passes from the Polycarbonate (with a refractive index of 1.585) to the air (with a refractive index of 1.00). The reflection from this surface is again 5.1%.
The total reflection is 9.1% giving a light transmission of 90.9%. Typically we report a light transmission of 90% to be conservative.
It can be seen that adding a coating actually increases the Light Transmission of the Polycarbonate sheet. The application of a layer with a Refractive Index between that of air and Polycarbonate is actually the theoretical basis of advanced reflective coatings.
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