1. Natural light outdoors

1.1 Natural light sources

The photoreceptors in the retina are sensitive to radiation in the visible range. At normal temperatures, surrounding bodies have no visible emissions of their own; they simply re-emit the visible light that illuminates them, by reflection, diffusion or diffraction. This light can come from a variety of sources, both man-made and natural: the stars and the sky,

Although starlight is directly perceptible to the human eye, it is not sufficient to illuminate the earth's environment. Light from the moon is sometimes sufficient to cast a shadow, i.e. create a perceptible difference in illumination between directly illuminated faces and others. However, the levels involved, while enabling orientation and movement, are not sufficient for...