Effects of Light Spectrum 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 the Standard Illuminant A light source. Significant measurement errors can exist when measuring light sources that are dissimilar to light sources used to calibrate the photometer. This article quantifies the measurement errors when measuring these dissimilar light sources commonly used in U.S. underground coal mines-a light-emitting diode (LED), a compact fluorescent light (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, LS-100, and 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 error ranged from -17.0% to -26.5% with the highest error for the amber CFL. 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. After the initial experiment, Mine Safety and Health Administration (MSHA) made improvements to the PMEX resulting in the PMEX-MSHA. The experiment was replicated using the new photometer and the newer PR-670. After repeating the experiment, the measurement errors ranged from -16% to -19% for the PMEX-MSHA, thus indicating an improvement over the PMEX. These results show that the spectral content of a light source and the photometer quality can greatly impact the accuracy of luminance measurement.