Ircon, Inc., a leading producer of industrial radiation thermometers, line scanners and quantitative thermal imagers, in its training programs for many years used to teach something they called the RAT Theory.
Reflectance, Absorbtance and Transmittance, or the coefficients of them, abbreviated as R, A &T must sum to 100%, or R + A + T=1.
An easy way for newcomers to Infrared radiation thermometry to remember a very important concept.
The associated concept is that Absorbtance=Emittance, or A=E. Or the RAT theory could be written as R+E+T=1 and renamed the RET Theory.
So, while not as easily recalled, the RET Theory name just didn’t catch on as easily as the RAT Theory.
(BTW, whenever I tried to teach some basics of Radiation Thermometry, I used to call it the TAR Theory because I thought it might “stick” better- it didn’t – RAT wins by a landslide every time.)
All this is a lead in to the wonderful resources by the folks at LabSphere for those who want to know or learn how to measure emittance or absorbtance through the roundabout way of measuring reflectance and transmittance first and then doing a bit of math.
They have a readily downloadable 26 page PDF document entitled “A Guide to Integrating Sphere Radiometry and Photometry”.
It explains far more than the RAT or RET or TAR theories about optical radiation metrology.
I think it and many of their online aids are well worth a read.