This paper presents the common sources of errors on illuminance measurements caused by incorrect techniques applied in those tests, causing low confidence studies. Even though the NOM-025-STPS-1999, a compulsory standard in Mexico, gives the guidelines to evaluate lighting conditions on working areas, several methods used by technicians produce results that could be wrong, with serious consequences to workers and some troubles in legal aspects.
Since illuminance is the quantity to be measured, its value is strongly influenced by environmental conditions of the room to be inspected and they can change unexpectedly; also the lack of known of the metrological characteristics of measurement instruments increase significantly the errors on those measurements.
"Common errors of illuminance measurements of working areas", Proc. SPIE 6046, Fifth Symposium Optics in Industry, 60461Z (10 February 2006); doi: 10.1117/12.674611; https://doi.org/10.1117/12.674611