Understanding of light emission is a keystone in mastering classical and quantum optics. We review and compare approaches used in these two fields for description of light emission to point out and clarify common misconceptions.
We present a critical review of the works related to the statisticalproperties of biological ultra-weak photon emission (UPE) and in particular toits coherence properties. Starting with a brief description of the concept ofcoherence in quantum and classical physics, we then review models used toanalyze photon distributions obtained from measurements of UPE. Moreover,we review experiments focused on statistical properties of UPE and try toassess them from the point of view of current understanding of physics andbiophysics. A critical study of the results and their interpretations leads us toconclude that there is no undoubted proof of the coherence of UPE. We highlightparticular problems of past research with respect to data interpretationor hypothesis building when looking for coherent light sources or emission anddiscuss the application of standard quantum optical methods for assessing thecoherence of an optical field. Since emerging studies show that not only coherenceproperties but also fractal and chaotic properties of UPE time seriessignals can be analyzed, we outline briefly these fractal and chaotic propertiesof UPE time series presenting them as a possible new avenue for UPE signalanalysis.