Here's an interesting paper about the application of multiple techniques for the study of murals and graffiti:
http://www.ijcs.uaic.ro/public/IJCS-15-03_Cosentino.pdf
Although it's not mentioned in the paper, one of the authors (A. Cosentino) describes an Arduino distance meter to check the position of the speedlight while capturing RTIs of the murals:
http://chsopensource.org/reflectance-transformation-imaging-rti-with-arduino/