Air temperature measurement challenges in precision metrology

The measurement of air temperature is a requirement in many types of metrology. Despite being generally an auxiliary measurement, in some fields poor knowledge of air temperature represents a limiting uncertainty. Applications where this is the case include interferometric dimensional measurements, in which the air refractive index correction is required, and the determination of relative humidity. Yet despite its ubiquity and importance, relatively little attention has been paid to determining air temperature with low uncertainty. In this paper we discuss an under-appreciated systematic error in air temperature measurements: the diameter-dependence of the radiation correction. After a discussion of the typical magnitude of the effect we consider ways to mitigate the effect by the careful design of air temperature sensors. Finally we consider the development of non-contact air temperature measurements using acoustic techniques.