Background: The spectral sensitivity of the pupillary mechanism is reported to be greater in the blue part of the visible spectrum compared to the sensitivity of the visual system as defined by V(lambda) for a two-degree field. This means that blue light gives rise to smaller pupils compared to those that occur with light of other colours of the same luminance. This has been interpreted as indicating that there may be a rod input to pupil response even at photopic levels of illumination. An alternative explanation is that the smaller pupil size for blue light is an artefact arising from the use of the CIE V(lambda) function which is based on the spectral sensitivity of the eye to a two-degree field. In most investigations of pupil response, the adapting luminous field is much larger than two degrees.
Method: The size of the static pupil was measured using an entoptic method when the eye was adapted to a large 'Ganzfeld' field of wavelengths of 624, 580, 521, 467 and 429 nm and luminances ranging from mesopic to photopic.
Results: The pupil is smaller for light of shorter wavelengths compared to that for light of longer wavelengths of the same luminance. This effect disappears at photopic luminances when luminance is calculated using the V(10 degrees ) (lambda) function instead of the V(2 degrees ) (lambda) function but is still evident at mesopic levels. When pupil size is plotted against equivalent luminance for a 10-degree field V(10 degrees ) (lambda, L(eq)) pupil size is independent of wavelength.
Conclusion: The apparent enhanced sensitivity of the pupil to blue light at photopic levels is an artefact arising from the inappropriate use of the V(2 degrees ) (lambda) for the measurement of luminance when the adapting field in pupil response measurements is a large field. Pupil size is a simple linear function of the log of equivalent luminance calculated for large fields for adapting luminances from photopic to mesopic and is independent of the wavelength of the adapting field for the whole range of adapting luminances.