On the meaning(fullness) of the intensity unit 'suns' in light induced degradation experiments

In many studies on light induced degradation (LID) phenomena, light intensity is specified in the unit 'suns'. However, the actual meaning of this expression is rather undefined at least if a light source with its spectrum deviating from sun's spectrum is used, e.g., a halogen incandescent lamp featuring a distinctly red shifted black body spectrum. Within this contribution it is shown that the different spectrum can imply different photon fluxes depending on the interpretation of the unit 'sun'. Furthermore, it is shown that also the quantitative determination of intensity can yield different photon fluxes if the sensitivity of the detector is not taken into account. Finally, it is shown that also the optical properties of sample and setup yield different absorbable photon fluxes or generation. All three effects should be taken into account when describing and comparing LID studies.