An artifact in measurements of in vivo light-induced absorbance changes

The effects of scattered actinic radiation on photomultipliers (Hamamatsu R-562) were investigated. Using cotton-wool to model dense biological preparations, it was found that the scattered actinic radiation received by the photomultiplier gives rise to phytochrome-like signals. This demonstrated the necessity to shield the photomultiplier from scattered actinic light for sensitive measurements with light-scattering preparations.