PTB News 3/2006 (118 kB) PTB News 3/2006, English edition, Issue December 2006With an extensively equipped photometer bench system for the characterisation of both light sources and photometers, distances up to 40 m can now be achieved for the measurement of directed photometric quantities. Thus, the facility – which is unique worldwide – also makes it possible to calibrate even extremely strong light sources with only a small associated measurement uncertainty.
A photometer bench serves to align specific light sources, detectors and/or reference photometers horizontally in relation to each other, and to vary the distance between them by means of different carriages. PTB thus fulfils the main tasks for the realisation, maintenance and dissemination of the SI base unit for luminous intensity, candela, as well as for the derivation of other units of directed photometrical quantities.
The new photometer bench system is composed of three different photometer benches aligned in a row. They can be used singly or together, so that measuring distances up to 40 metre become possible. This system is unique worldwide (normally, even in national institutes, photometer benches up to only 6 metre are in use). At PTB it is now possible to vary (by means of distance modifications) the illuminance over more than three orders of magnitude. Especially for extremely strong lamps, which require a larger distance for their light to appear point-like and provide a more homogeneous illumination, the achievable associated measurement uncertainty could be significantly reduced: at daylight-like radiation by one percent, and for less bright lamps still by approx. one per mil. New monitoring systems for detecting the stability of the luminous intensity, as well as the camera-aided adjustment, contribute significantly to this improvement. Furthermore, an automatic photometer carousel makes it possible to carry out several measurements simultaneously so that the time needed for many of the measurement tasks is reduced by half.
Due to this novel facility, new possibilities for calibration are opening up in four fields: first, for large-scale novel light sources as standard lamps for luminous intensity; second, for luxmeters with illuminances up to 20 klx (illuminant A) or 100 klx (daylight with an HMI lamp); third, for luminance meters with luminances up to 10 kcd/m2; and fourth, for luminance standards considering the dependence on inhomogeneity and direction.
The photometrical bench system will also be available for scientists from other institutes and countries. In addition, one of the essential international tasks of the near future will be to conduct a key comparison of the unit of luminous intensity within the scope of EUROMET, with PTB as pilot laboratory. For the comparison, in which 19 European countries will take part, measurements will be carried out on more than 120 standard lamps of luminous intensity.