[8] Biophotons may be detected with photomultipliers or by means of an ultra low noise CCD camera to produce an image, using an exposure time of typically 15 minutes for plant materials.
[16] Such reactions can lead to the formation of triplet excited species, which release photons upon returning to a lower energy level in a process analogous to phosphorescence.
[23] In the 1970s Fritz-Albert Popp and his research group at the University of Marburg (Germany) showed that the spectral distribution of the emission fell over a wide range of wavelengths, from 200 to 750 nm.
[citation needed] The difficulty of teasing out the effects of any supposed biophotons amid the other numerous chemical interactions between cells makes it difficult to devise a testable hypothesis.
[25] The hypothesis of cellular communication by biophotons was highly criticised for failing to explain how could cells detect photonic signals several orders of magnitude weaker than the natural background illumination.