<title>The Photometry Of Lighting Installations</title>

Abstract

The paper briefly traces the development of illumination photometers from Preece's crude greasespot photometer to the present day selenium and silicon photocell photometers, with cosine and colour correction and digital displays. The accuracy and calibration of modern photometers is discussed. In addition to the variation of illuminance values with time there is also the variation in space and the need to determine the average illuminance. The origin of the I.E.S. recommended method for measuring average illuminance is explained and its limitations discussed. There is great interest in the measurement of Contrast Rendering Factor which is a measure of the visibility of a task in a given lighting installation. Blackwell in the U.S.A. and the Building Research Establishment have developed visibility meters which are intended for use in measuring CRF. The need to consider the lighting of the space within the room envelope has resulted in two alternative parameters being suggested. These are scalar illuminance and cylindrical illuminance. Scalar illuminance is the average illuminance on an infinitesimal sphere and an instrument has been developed to measure scalar illuminance. Cylindrical illuminance is the mean vertical illuminance and this may be measured by averaging a series of readings or by using a cylindrical integrator. Luminance measurements are made only infrequently, but they can be important for research work. Typical lUminance meters are described.