Describing the transmission of a filter by a complex transmission function implies a point-to-point relationship between the signals just in front of and just behind the filter. In general, this relationship is valid only for very thin filters. A set of two sufficient conditions is derived that allows a thin-filter description of pure phase filters with a thickness large compared to the wavelength of the incident light. Only filters that consist of two adjacent layers with varying thicknesses and constant refractive indices are considered.