Given a sample absorption spectrum(Figure 1), one can see that it has
a characteristic peak height representing an intensity as well
as a width the center of which usually represents an energy of absorption,
in wavenumbers, nanometers, MHz, or whatever. A sample cell is illustrated
in Figure 2. The incident light, 
, enters a cell of path length, l,
and is absorbed by some species with a concentration, c, and extinction
coefficient or absorptivity, a. The amount of light passing through the
sample, I, is a function of the intensity of the incoming light,
concentration of absorbing substance, path length of the cell, and the
absorptivity of the substance.
This expression for absorptivity is only for one wavelength, when in
reality there is absorption over a range of energies such that an
integrated absorptivity should be calculated. This is what psi calculates
when determining intensities, integrated absorptivities
which reflect the strength of an absorption.
If we substitute in a path length of 1 cm, a concentration in terms
of 
, and the integral as a function of energy in 
,
our units become:
Another quantity often determined is 
:
where 
is the center of the absorbance peak.