In chapter Optical Transitions, the theoretical quantities transition dipol moment µ_{mn}, matrix element R_{mn}, oszillator strength f_{mn} and Einstein's factors A_{mn} and B_{mn} for spontaneous and induced emission as well as B_{nm} for the phenomenon of absorption were presented. Now we aim on how to determine these quantities experimentally. First, we consider measurements where emission and absorption are resolved within time, then "ordinary" measurements of absorption. Treating emission like a firstorder decay, the reduction of an initial number of excited species within time is
N_{m}(0s) is the initial number of particles within a certain volume and dN_{m} / dt is the number of emitted photons per time. The life time τ of a state is defined as the time needed for a reduction by the factor of 1/e. Introducing τ yields the equation

Provided no processes like collisions with other particles or internal conversions occur, a measurment of the life time τ directly yields the transition probability A_{mn}. As the mentioned competitive processes reduce the life time, a higher transition probability is pretended. Having calculated A_{mn}, further theoretical quantities are within reach.
According to LambertBeer's law, light of an initial intensity I_{o} passes through a sample with an intensity I_{trans} when a beam travels along a path with length l where an absorbing substance is found. The concentration N this substance is here defined as the number of particles per volume. σ is introduced as the absorption coefficient for a defined wavelength.
Adequate units are cm^{2} for σ cm^{3} for N and cm for l. Using experimental data, we are now able to determine the absorption coefficient σ.
σ is dependent on the frequency ν. Through integration of σ over the frequencies &nu within an absorption peak, we obtain the total intensity for a transition n to m, which is connected with B_{nm}, the coefficient that reflects an atomic or molecular property.
The obtained equation allows to estimate experimental values from an theoretical approach and, vice versa, to determine a transition coefficient B_{nm} by using experimental data. The above mentioned integration of σ over frequencies leads to the integrated absorption coefficient σ_{o}.
In laboratory practice, the extinction ε is the preferred measure to characterize the absorption of light by a substance. It is given in units of