A timeline-based sampling method applied in National Institute of Metrology primary standard goniophotometer

The SI derived unit lumen is a unit of total luminous flux, describing a total quantity of visible light emitted from a light source. The realization of lumen unit is implemented by the facility called goniophotometer in National Institute of Metrology China (NIM). The goniophotometer is a facility measuring the luminous intensity of a light source at various angles, and the total luminous flux is calculated by integrating the spatial distribution of luminous intensity. Because of sampling characteristic of the goniophotometer, the spatial distribution of luminous intensity is a discrete curve. In our previous method, the goniophotometer measure the luminous intensity value at polar angle step size of 1°, 2° or 5°. For measurement of LED-light sources, which are likely oriented light sources, the spatial distribution of light is not homogeneous. Carefulness must be taken because the choice of angle sampling interval is going to be a considerable uncertainty component. Recently, a timeline-based sampling method is developed and applied in the primary standard goniophotometer in NIM, to realize the lumen unit, as well as measuring the LED light sources. The control computer is not to record the luminous intensity at exact integer angle, but to record the luminous intensity and its related time, and to record the angle and its related time. These two record threads are running respectively and simultaneously during the whole testing process. An interpolation is used to calculate out the luminous intensity and the angle at the same timeline. Thus, the spatial distribution of luminous intensity curve has much more effective data than that from the previous method. It is an improvement to help to get a more accurate result. The repetitive fluctuation is evaluated to be 0.025%, a much lower level than that of the previous method.