A study of the directional response of ultraviolet radiometers: I. Practical evaluation and implications for ultraviolet measurement standards

The directional responses of a range of ultraviolet radiometers commonly used for irradiance measurements of UVB and UVC have been studied. Radiometers with 24 diffuser/filter combinations were assessed using a deuterium source, and three different diffuser/filter designs were assessed using a monochromatic source. The directional responses of the radiometers have been calculated and expressed in terms of figures of merit similar to those described for (photopic) illuminance meters in BS 667 and CIE 69. Those radiometers that performed best for the measurement of both small and extended sources of UVB and UVC had raised PTFE diffusers. We conclude that UV radiometers with a directional response error f2 < 10% are readily available commercially, and that it would be appropriate for future ultraviolet standards to set an upper limit of 5% on f2. This would ensure that the overall uncertainty in irradiance measurements of extended ultraviolet sources is not dominated by the error in the directional response of the radiometer.