ANSI and ISO standards specify that to measure ANSI/ISO standard diffuse transmission density the photographic emulsion must be in contact with the opal glass diffuser of the densitometer. This paper shows that deviations from in contact can cause measurement errors which are not linear and depend on the light scattering and reflecting properties of the material being measured. This problem may be encountered with automatic transport densitometers, which usually have a separation (air gap) between the diffuser and the material being measured. The error introduced by a separation of less than 0.2 mm is insignificant for many purposes. -- AATA