Perceived light intensity
Luminous intensity is a property of the light source and does not depend on the distance of an observer. It quantifies the part of the luminous flux (unit: lumen) emitted in a given direction (per solid angle). The spectral perception capability of the human eye is taken into account. For example, the luminous intensity of an infrared radiation source is zero because it is invisible to the human eye. The brightness of a light source as perceived by the eye does not always match the physical luminous intensity. The contrast with the surroundings influences the physiological perception. A light source with a small surface area is perceived as brighter (or more dazzling) than a light source with the same physical luminous intensity but a larger surface area. This effect can be observed, for example, with car headlights of different sizes or with the rising or setting of the moon or sun.