The subject of pharmaceutical photostability testing often raises many questions for several reasons. Predominantly governed by ICH Guideline Q1B for small molecules, or ICH Q5C for biotechnology products, the standards are not particularly well written and often cause confusion. Q1B, in particular, provides options to use two non-equivalent light sources and additional variants. The standard also mixes radiometric and photometric light measurements for the ultraviolet and visible light dosages. Allen Zielnik at Atlas Material Testing Technology LLC explains the guideline options available, and provides some practical guidance.

https://international-pharma.com/wp-content/uploads/2013/06/1-Photostability-Testing-Shedding-Light-on-a-Not-Well-Understood-Guideline.pdf