Phytochromes are red and far-red photoreceptors widespread among life, which bind a bilin chromophore molecule. They photoswitch between two main or parent photostates: the Pr state, which absorbs red light, and the Pfr state, which absorbs far-red light. Apart from the photoconversion, there is another process called dark conversion. The phytochrome photocycle can be easily followed by ultraviolet (UV)–visible or visible spectroscopy. Through the acquisition and the later data analysis of serial UV–visible absorption spectra in either unpurified or purified protein samples, many important aspects of the phytochrome photocycle can be learned.