Goniophotometry: new calibration method and instrument design

Abstract

In photometry the unit of luminous flux is realized as total luminous flux of reference lamps. Goniophotometry enables a fundamental realization by a direct derivation from the luminous intensity unit. The new calibration method uses one lamp as transfer standard for both units. Then, the ratio (unit steradian) of total luminous flux and luminous intensity (in a specified direction) is a characteristic property of the lamp. Its value is determined with a goniophotometer as weighted solid angle from normalized photocurrents and angular relations. Goniophotometric results are thus only affected by the uncertainty of the spatial integration, which makes the weighted solid angle a good measure to compare mechanics and method of goniophotometers. Finally, the luminous flux value is calculated as the product of the weighted solid angle, determined by goniophotometry, and the luminous intensity value assigned to the lamp. At the Physikalisch-Technische Bundesanstalt (PTB), a goniophotometer with a maximum diameter of 6 m is planned as a combination of service robots. One robot aligns the lamp centred in and directed to the instrument's coordinate system. The surrounding space is divided into two parts, each with a separate robot to move a detector continuously on a path with dynamically adjusted location, direction and speed, optimized to obtain minimum uncertainty of the goniophotometric measurement.