Calibration of the Nuclear Associates 07-621 precision photometer

The spectral response of the Nuclear Associates 07-621 photometer has been measured over an extended range (400-990 nm). The measurements reveal that the photometer shows large deviations from the standard CIE photopic response. In particular it has considerable excess response in the red (650-780 nm) and in the near infra red (780-990 nm). Although the instrument is claimed to have a photometric accuracy of +/- 7% this is only realised when measuring sources, such as tungsten lamps, which are rich in the red and near infra red regions of the spectrum. When used with sources, such as fluorescent lamps, which contain little radiation in the range 650-1200 nm it underestimates the light output by approximately 20%. As a consequence this photometer can only be used in radiology QA for such tasks as measuring view box luminance if the appropriate correction factor is used. This factor should be determined at the outset by comparing the Nuclear Associates 07-621 with a high quality photometer using the same type of light that is emitted by the boxes.