In psychophysics, why are log luminance rather than absolute luminance values reported? Are there any papers which justify converting into log luminance? For example papers showing humans being sensitive to changes in log luminance rather than luminance per se?
In general, subjective sensation increases linearly with the the log of physical intensity, which is described by Fechner's law. 

We are sensitive to small variations when light is dim, but we need large differences in intensity under conditions of high luminance (Weber's law). This is a linear relation, but taken together with Fechner these findings are described as the widely-known Weber-Fechner's law, justifying the log scales used. 

Another well-known example is the decibel scale in hearing, which is also a log scale.

Any basic textbook on perception or psychophysics will explain this and a reference is added below.

Reference
- Masin et al., J History Behav Sci (2009); 45(1): 56–65