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1. TO add to what Ian said, here's a nice graph from Schott showing the IR transmittance of one of their IR absorbing glass:
http://www.us.schott.com/borofloat/e...ighted_text=IR

Keep in mind the graph is in transmittance. If it was in absorbance, then you could more easily see that a sheet twice as thick had twice the absorbance.

Ray - Your questions #2 and #3 are more easily seen when using absorbance. That's why most all lab tests that use spectrophotometery use absorbance rather than transmittance, as the math can be done by using simple addition or subtraction.

Keep in mind the Beer-Lambert Law (aka Beer's Law):
http://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law
It tells us that if the filter (or cuvette) thickness is doubled, then the absorbance is doubled.

2. Absorbance = -log (percent transmittance/100)

The abs of a 50% transmittance filter is 0.30.

The abs of a 25% transmittance filter is 0.60.

Adding two 50% transmittance filters, each with an absorbance of 0.30, gives a filter with an absorbance of 0.60.

That is, 0.30 + 0.30 = 0.60, or the same as a 25% transmittance filter.

3. Finally, look into the Schott KG series of IR absorbing filters. Here's specs on the KG1
http://www.optical-filters.com/kg1.html

4. Ian, Kirk,

Thanks!

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