Applied Photometry: Review of Retrospective and Reality Part 1: Candela and Watt, from Incandescent Lamp to LED

Photometry is the science of measuring light, which began with a visual comparison of the light of stars, the moon, and the sun with the light of a candle. Today it is a part of optical radiometry, inextricably linked with physical radiation detectors, taking into account the fact that light is actinic radiation, and physical detectors must be actinic in the case of integral measurements. At the same time, optical radiometry has introduced and introducing new technologies into spectral radiometric measurement methods, which, in turn, make it possible to solve the problems of measuring the parameters of actinic radiation by objective physical methods of spectral radiometry, taking into account the weight functions of actinic radiation. The choice between integral and spectral methods is determined by: – The required accuracy of the measurement task to be solved; – Knowledge of the weight functions of actinic radiation; – The quality of correction of radiation receivers for the weight functions of actinic radiation; – Methods for calibrating radiation receivers; – Actinic coefficients of optical radiation receivers (correction coefficients) and the accuracy of their determination; – Technical capabilities of spectral equipment; – Methods and means of spectrometers calibration; – Software for spectral measuring instruments, – Comparative analysis of the accuracy (uncertainty) of measurements by the spectral and integral methods in each individual case. The replacement of traditional sources of general lighting and signaling by radiation sources with LEDs, which have a spectral distribution of light emitting characteristics different from traditional light sources, has raised a number of issues in the metrological support of optical radiometry and applied photometry related to the oneness of the reproduction of units of energy and photometric quantities based on Watt.