Quote Originally Posted by BetterSense View Post
I built a spotmeter and it totally works, but it's too IR sensitive. I knew the absorption spectrum of the silicon sensor was biased toward IR but I figured it might work anyway since lots cameras use Si sensors, plus I hoped that my glass lens would absorb some IR and of course it will be OOF on the measurement plane compared to visible light. However grass makes the sensor respond much more than a sheet of white paper even though the paper is much whiter. So clearly I have terminal IR problems at this point.
Not clear. It may be that the IR refected from a "white" target may, or may not, be different than that from a green target. The central frequency of "white" (~ ...oh squint your eyes - more or less) is probably somewhere close to the central frequency of "green"... and a measurement of the radiated eneregy would indicate overall brightness - rather than energy at a specific (or random) blend of color.

Try the same with a RED and a BLUE card. Still rough, but possibly closer - there really is no way to tell how much IR is being tossed around.

Run-of-the mill window glasses do not have much of an effect on Infra Red radiation. The other end of the spectrum - Ultra Violet - IS absorbed rather efficiently - that is why it is difficult to get a sun tan sitting in front of an ordinary window. Other materails, plasics, quartzes are used in te manufacture of UV lamps.

Your best bet would be to visit the fiter manufacturer's catalogs - B&W in particular publishes spectral transmission graphs of their filters, and choose from there.