Protans and PAPI: Recognition of a two colour code by persons with defective colour vision

In some countries there is controversy as to whether normal or near normal colour vision is necessary for pilots of aircraft. It has been argued that information displayed by aviation colour codes is always displayed redundantly and that in modern aviation there is less dependence on information acquired visually from outside the aircraft. One aviation signal system that displays information solely by colour coding is the Precision Approach Path Indicator (PAPI) which signals that an approach to land is too high or too low by means of the number of red and white signals. A Farnsworth lantern was modified to simulate PAPI signals viewed from one nautical mile under conditions close to 1.5 km visibility. Normal observers were able to identify the red and white signals and distinguish them from blank signals without difficulty except that some subjects made a few errors confusing a yellowish white (simulating the colour of a white PAPI signal attenuated by the atmosphere) with red. Most colour vision defective observers made errors confusing red and white, and protan observers frequently failed to see the red signal, calling it a blank. Although a few protans could identify the signal colours without error it is concluded that the majority of colour vision defective observers are not able to identify a red/white signal system reliably despite the code being a simple two category code and the fact that brightness differences between the red and white signals were an available cue. The results are surprising since past reports suggest that colour vision defectives are able to recognise red with only a few errors so that a two colour red/white code might have been expected to be within their capacity. The number of red = white and white = red errors made by deutans was also surprising.