Thermo Scientific MIRAN 205B Series, Instruction Manual, Page: 136 / 214

Chapter 13 Infrared Spectroscopy Tutorial
13-16
Figure 13-15. Concentration vs. Absorbance
Relation of Transmittance to Absorbance Units
Since both transmittance (T = I
0
/I) and Absorbance (A = log
10
I
0
/I) are derived from the
same proportion of intensity of incident vs. transmitted energy, we can substitute to solve
for the value of A in terms of T. This relationship is A = log
10
(1 / T). Therefore the
absorbance value at 100 % Transmittance is 0.
Summary

% Transmittance and Absorbance Units are the standard measures for
representing the intensity of absorption of IR energy.

The most accurate calibration readings with the SapphIRe Analyzer are obtained
at full scale between 0.2 and 0.6 AU, which represents a 67% - 25% transmittance
of IR energy.
 AUs are derived from % Transmittance, a ratio of intensity of IR radiation
entering and leaving the sample cell.
THE BEER-LAMBERT LAW
The Beer-Lambert Law (usually referred to as Beer's Law) describes the relationship
between absorption and concentration. It states that the amount of energy absorbed at a
certain wavelength is directly proportional to the concentration of the sample and to the
pathlength over which the energy travels through the sample. A = kCL where:
A is the absorbance of the sample gas. This number represents the amount of IR
energy absorbed by the gas molecules in the sample cell.