The noise equivalent temperature difference (NETD) is one of the most convenient measures used in describing performance of infrared systems as thermographs and radiometers. The NETD expression is also contained as a kernel within any minimum resolvable temperature difference (MRT) and minimum detectable temperature difference (MDT), so the conclusions to be reached relative to NETD apply to MRT and MDT as well. For reasons of convenience, some assumptions have been made in defining (measuring and deriving relation for) the NETD. However, for a variety of practical purposes in the field, these assumptions are not satisfied. Consequently the conventional laboratory NETD is applicable under certain favorable laboratory conditions. Therefore the typical, laboratory NETD expressions found in the literature cannot be simply applied for infrared systems under field conditions. In this paper the practical, field NETD expression is derived. It incorporates none of the assumptions which have been used in defining and deriving the laboratory NETD expression. Therefore, the given expression can be applied to assessment of infrared systems capabilities under field conditions.