Wednesday, January 27, 2010

Light Transmission of Clear Plexiglass Sheets

I learned something VERY interesting the other day. How CLEAR is Plexiglass sheet?

We always say - "It's water clear" and I have heard of people saying it transmits 100% of the visible light. But the specs say 92% light transmission. Why? Where is the other 8%? You can look down an 8 foot sheet of Plexiglass and read a business card. Perfectly clear.

Well it turns out that our eyes cannot detect the 8% anyway. But where did it go?
Turns out, 4% is REFLECTED off the front surface, and another 4% is reflected off the inside surface of the back. OK - I get it.

But wait - there's more. What about the UV or Ultra Violet resistance of Plexiglas?
Plexiglas is naturally UV RESISTANT. Most manufacturers offer a 10 year limited warranty: Here's what the makers of Plexiglas offer:

Pursuant to the terms and limitations set forth herein, Altuglas International warrants that for a period of ten (10) years from the Date of Purchase as set forth below (“Warranty Period”), flat, colorless, smooth PLEXIGLAS® G Acrylic Sheet manufactured by Altuglas International will not undergo a loss of light transmission in excess of three percent (3%).

Like I said - you cannot detect 8%, much less 3%.

As far as UV transmission, regular Plexiglas and other brands of acrylic sheet start letting UV through at 420nm - the danger zone is in the higher wavelengths of 400nm to 300nm. The UV blocking grades stop ALL of this UV light at 420 and higher (a lower nm number) at a price....

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