New measurement standards and methods for photometry and radiometry

Abstract

Concordance of measurement units is a prerequisite for international trade, telecommunication and scientific co-operation. National Metrology Institutes (NMIs) develop, maintain and compare these measurement units to ensure world-wide consistency. This thesis presents new national measurement standards for luminance, spectral radiance and luminous flux developed at the Metrology Research Institute, which is the NMI for optical quantities in Finland. In addition, evaluation of photometer calibration methods is presented followed by a correction method for illuminance responsivity calibrations of commercial luxmeters.

The units of luminance and spectral radiance are realized using a characterized reference photometer, a spectroradiometer and an integrating-sphere light source. The realization of the unit of luminous flux (lumen) is based on a 1.65-m integrating sphere instead of a goniophotometer. This is one of the first realizations of lumen in the world using the absolute integrating sphere method originally introduced by the National Institute of Standards and Technology (USA). The measurement set-ups and the characterization measurements are presented with comprehensive uncertainty analysis for each unit. Based on the analyses, the expanded uncertainties (k = 2) of the units of luminance and luminous flux are calculated to be 0.36 % and 0.47 %, respectively. The measurement uncertainty for the luminous flux is one of the lowest values ever reported. The validity of the claimed uncertainty is tested by international comparison measurements.

Two methods to calibrate a photometer that measures light-emitting diode (LED) sources are evaluated. This pioneering study shows that narrow spectra and challenging intensity distributions of LED-based light sources require thorough characterization measurements for both the photometer and the LED source. The results from these measurements are presented and compared, followed by a recommendation concerning the preferred calibration method.

A diffuser that improves the angular response of a luxmeter also affects the effective measurement plane, leading to substantial errors in calibration if this plane is simply assumed to be at the outermost surface of the diffuser. For the first time, extensive research was carried out to determine the diffuser reference planes of commercial luxmeters. A number of test measurements with three diffusers of different shapes were conducted to find out that in reality the reference planes are located several millimeters inside the diffuser. At short distances, this causes systematic errors of several per cent in the measured illuminance values, greatly exceeding the calibration uncertainties. A correction method based on the true shift of the reference plane is presented to correct the erroneous calibration results.