Light Source Spectral Power Effect on Luminance Measurements in Underground Coal Mines

Lighting regulations for luminance in U. S. coal mines are verified in the field by using a luminance photometer calibrated to a light source that uses a tungsten-filament light. Significant measurement errors can exist when measuring light sources that are dissimilar to the light source used to calibrate the photometer. This paper quantifies the measurement errors when measuring these dissimilar light sources commonly used in a U.S. underground coal mine-an LED, a CFL with a clear cover, a CFL with an amber cover, and a tungsten halogen. The impact of photometer quality was also evaluated. Three different luminance measuring instruments of high, medium, and low quality were compared-a PR-650, an LS-100, and a PMEX, respectively. The PMEX was under evaluation for measuring luminance compliance in U.S. underground coal mines. The PR-650 was used as the referent to which the other photometers were compared. The PMEX percent error consistently exceeded -15% for each light source and varied the most for the amber CFL with a -26.5% error. The LS-100 closely matched the luminance measurement for the LED and halogen; however, it had a percent error of -10.4% for the amber CFL. These results show that the spectral content of a light source and the photometer quality can greatly impact the accuracy of luminance measurement.