The Broca-Sulzer effect or Broca-Sulzer phenomenon is an experimental observation related to the psychophysics of vision. It has two parts, temporal and spatial. In the temporal effect, the perceived brightness of a single flash of light first increases with the flash duration, then reaches a maximum, and decreases for a longer pulse. The maximum is more pronounced and is observed at shorter durations for a stronger illumination; it is reached at approximately 0.1 for a 100 lux flash.[1]
Similarly, in the spatial Broca-Sulzer effect, the perceived brightness increases with increasing angular size of the flashing object until it reaches approximately 2.5 arcminutes, and then decreases for a larger object.[2]
The Broca-Sulzer effect was reported by André Broca and David Émile Sulzer in 1902.[1] It conflicted with the 1885 report by Adolphe-Moïse Bloch who believed that the perceived brightness monotonously increases with the flash duration.[3] Later research showed that while the Bloch's law may hold for some individuals and for some experimental conditions, the Broca-Sulzer effect is a more general phenomenon.[4]