Thermonicolet Omnic Software User's Guide 6.1 (PDF) - Charles E ...

The photoacoustic value (PAS) at a frequency is defined by the equation
PAS = 100(I S /I B )
where I S is the intensity of energy emitted as thermal waves by the sample and I B
is the intensity of energy emitted by the background material (usually carbon
black). (Numerically, this is identical to % transmittance.)
% Reflectance
Select % Reflectance to convert spectra collected using a reflection technique to %
reflectance units (see the following example. These units show the amount of
infrared energy reflected from the sample. Some common reflection techniques
include diffuse reflection (DR) and specular reflection (SR).
Percent reflectance at a frequency is defined by the equation
%R = (I S /I B )*100
where I S is the intensity of infrared energy reflected from the sample and I B is the
intensity of infrared energy passing through the reflection accessory with the
sample replaced by a reference sample, usually a mirror for SR, powdered
potassium bromide (KBr) or chloride (KCl) for DR, or simply the open crystal for
ATR. I B is called the background.
Log (1/R)
Select Log (1/R) to convert reflection spectra to log (1/R) units (see the following
example). These units show the amount of infrared energy absorbed by the sample
in a reflection experiment. You can convert spectra collected using the specular
reflection (SR) and diffuse reflection (DR) techniques. Displaying these spectra in
log (1/R) units is useful for identifying the technique used to obtain the data.
190 Thermo Nicolet