We have developed a method to measure triplet optical densities OD_r(λ) when considerable photodecomposition is present. The method is demonstrated on Coumarin 120. By focusing an UV cw laser beam on a sample with the aid of a lens, high triplet optical densities OD_T(λ) can be recorded. However, the excitation of organic compounds with light causes varying degrees of photodecomposition. In order to take photodecomposition into account, one records triplet optical-density values as well as the accumulation of absorbing photoproducts as a function of time. Turning off the cw laser excitation, one records the accumulation of photoproducts only. Separating the two processes, one can determine how the triplet optical density OD_T declines as a function of time t. The obtained curve can be expressed with an equation that appears to decline exponentially with time. This allows one to extrapolate back to t = 0 and recover OD_T when there was no photodecomposition. The extrapolated OD_T values are used to obtain triplet-extinction coefficients ∊_T(λ) by McClure’s method of Coumarin 120.

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