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 us ed for irradiance measurements of UVB and UVC have been studied. Radiometer s with 24 diffuser/filter combinations were assessed using a deuterium sour ce, and three different diffuser/filter designs were assessed using a monoc hromatic source. The directional responses of the radiometers have been cal culated and expressed in terms of figures of merit similar to those describ ed for (photopic) illuminance meters in BS 667 and CIE 69. Those radiometer s that performed best for the measurement of both small and extended source s of UVB and UVC had raised PTFE diffusers. We conclude that UV radiometers with a directional response error f(2) < 10% are readily available commerc ially, and that it would be appropriate for future ultraviolet standards to set an upper limit of 5% on f(2) This would ensure that the overall uncert ainty in irradiance measurements of extended ultraviolet sources is not dom inated by the error in the directional response of the radiometer.