Wednesday, April 20, 2011

Infrared and Ultraviolet Transmission in Plexiglass Acrylic and Makrolon Polycarbonate Sheet

Well, this explains everything. I finally got my hands on the charts to show UV, Visible, and IR light transmissions in Plexiglass sheet. And, I learned something new today: IR or InfraRed light varies in transmission logarithmically depending on the thickness of the Plexiglas sheet. And, it's not a smooth curve. There are areas of filtered transmission. This explains why the IR remote on my entertainment center works like a champ on the TV and DVR but sucks on the audio amplifier (which has a different IR band). (I am using frost acrylic) (update 8/11 - replaced the panels with the IRT acrylic - it's opaque black but the IR bands are all working for all the equipment!!)

Before sharing the charts, let's think about UV and IR and visible "light".
1. You cannot SEE this type of "light".
2. You can feel the IR - it warms you up.
3. You can't see or feel UV, but your skin will fry if exposed too long, and paper, artwork, fabrics, and other materials will degenerate from exposure.
4. IR is heat, and we can capture heat images in a CCD or IR film - way cool to see.
5. IR can penetrate opaque material. UV cannot.
6. Visible light transmission through clear Plexiglas is 92%. You cannot detect the 8% loss - it looks clearer than water. You lose 4% reflected off the front surface and 4% off the INSIDE of the back surface. If I put a business card at the end of a 4" thick piece of Plexi 8 ft long, you could read it perfectly.

OK - the charts - check this out!

Your UV and Visible Light chart - you can see the natural drop of UV filtering from standard Plexiglas, and then the extra filtering of the UVF and UVT grades. Plexiglas MC is your standard general purpose extruded  sheet, and Plexiglas G is cell-cast acrylic sheet. Plexiglas UF5 is the standard extruded "MC" with a UV filtering additive. UF3 is the version from "G" cell-cast acrylic.

Now the IR light transmission for CLEAR Plexiglas:

According to the makers of Plexiglas®:

Colorless Plexiglas® sheet sheet transmits most of the invisible near-infrared energy in the 700 to 2,800 nanometer region, but it also absorbs certain bands as shown. The curves for 0.118 inch and 0.944 inch thick colorless Plexiglas® sheet show that near-infrared transmittance depends on thickness, decreasing logarithmically as thickness increases.

At infrared wavelengths longer than 2,800 nanometers and as long as 25,000 nanometers, and in thicknesses greater than 0.118 inch, colorless Plexiglas® sheet is entirely opaque. At thicknesses less than 0.118 inch, Plexiglas® sheet transmits small amounts of infrared energy at certain wavelengths within this region. All standard formulations of colorless Plexiglas® sheet have the same general infrared transmittance characteristics.

Sensitive instruments confirm that weathering produces no change in the infrared transmittance characteristics of Plexiglas® sheet.

But wait, there's more! Here's the special grade of Infra-Red Transmitting sheet. It is BLACK in color, but allows the IR to transmit through. Very cool for spy related video equipment. Available in 1/8" and limited amounts of 1/16" thick.

1146 is the IRT and 199-0 is opaque black. They look identical but obviously are not....

But wait.... there's more!
Here's the chart for POLYCARBONATE sheet - known as Lexan, Tuffak, Makrolon etc.  Note that polycarbonate goes opaque in the UV spectrum and has good IR transmission.