The effect of glass neutral density filter on illuminance measurement error

Abstract

We present an analysis of the illuminance measurement results when a glass neutral density filter (ND Filter) inserted between light source and photometer. ND filters are widely used as an alternative solution in illuminance calibration systems to perform lower illuminance level which caused by inadequate length of the photometric bench. However, there are some variables that must be considered when inserting ND filters in illuminance measurement, one of which is the perceived color temperature of the light source which is influenced by the non-flatness profile of the filter’s spectral. This research was conducted by calculating correction factor for each ND filters, which contribute to illuminance error. Six different ND filters in the range of 1% to 49% are used in this experiment. Scanning the spectral distribution of filter are done to analyzing the change of color temperature of the light source as it passes through an ND filter, due to the lamp must correspond to Illuminant A. This research shows the illuminance measurement error can be up to 20% proportionality to the transmittance as the effect of using ND filters. The results of this study are expected to provide recommendations for the calibration laboratory, especially in illuminance meter calibration.