# Converting Transmission to Absorbance (Optics)

1. Jul 27, 2010

### csnsc14320

I have taken data for the transmission vs. wavelength for several types of glasses in the IR. I want to convert this to absorbance so that I can generalize transmission to different thickness glasses.

I found an equation online that stated A = 2-Log(T%) (where Log is base 10). But I do not see how this considers thickness.

Does anyone know how I can translate knowing the transmission % for a specific thickness glass to knowing the absorbance/transmission to thicker glasses?

2. Jul 27, 2010

### Andy Resnick

There's a few simple formulas you can use. Starting with conservation of energy, a+t+r = 1, where 'a' is the fraction of absorbed light, 'r' the fraction of reflected light, and 't' the fraction of transmitted light.

If r = 0 (or the measurements are performed in such a way to allow r = 0), then a = 1-t. As you note, this can often be written in a way that accounts for a material's thickness. Beer's law is an approximation valid for 'weakly scattering' materials, and is given as:

I = I_0 *10^(-al), where 'a' is the *absorption coefficient* (not the same 'a' as above), and 'l' the thickness. ('t' = I/I_0)

http://en.wikipedia.org/wiki/Beer's_law

Now, you have the transmission as a percentage, but hopefully the wiki page will guide you along at this point.