The Framework of Photochemistry State Diagram

Indications that there was something more than what absorption spectra indicated came from various sources, first of all from luminescence studies, for the simple reason that instruments, and primarily the human eye, are more sensitive to a weak emission (detected against a dark background) than to a weak absorption (where the transmitted light is almost identical to the blank). 

Beginning from normal emission, that light was emitted under illumination was early reported (see Sect. 1.2) and was distinguished from other optical phenomena, e.g., reflection or diffraction. In a very important work, Stokes observed that [1] when exposed to daylight, certain materials, solid or liquid, exhibited a peculiar emission. Thus, by looking at the sample in a direction such as the ordinary reflection was avoided, a different emission emanating in all directions could be 

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