ABSTRACT
After the introduction of the quantum character of light by Planck in 1900, the Utrecht physiologist Zwaardemaker was in 1909 probably the first to express the available values of the minimum energy for light perception in quanta. This chapter provides the development of the quantum concept in vision as it is revealed particularly by psychophysical experiments with human observers. It aims to evaluate the status of the quantum concept and discuss how it has been and is a fruitful guideline for research on the limits of visual perception at low luminance levels. The hypothesis was based upon experimental findings which were already known for a long time and which showed that the threshold energy depends upon the spatial and temporal extent of the test flash. The contrast threshold of both increment flashes and spatio-temporal gratings showed a square-root relation with respect to the mean luminance.
