Digital Camera Light Intensity

The intensity of the light (amount of light emitted, transmitted or reflected per unit of time) is the characteristic we measure with the photometer; through the measuring of different zones of the frame we can rate the general brightness and contrast.

Absolute Intensity (Photometry)
The intensity of light is affected by three factors:

The intensity of light emitted by the source;
The intensity of light that falls on a surface ( incident light);
The intensity of light reflected from a surface ( reflected light).

The intensity of emission (measured in lumen ) expresses the amount of light emitted per unit of time. In an illumination devise, the emission intensity depends on the power of the lamp (in watts) and its output, meaning, the greater or lesser proportion of electric energy transformed to light. The intensity of incident light (measured in lux ) on a point expresses the quantity of light that reaches a point per unit of time; depends on the distance of that point to the source, in an relationship inversely proportional to it's square. This means that the intensity of the incident light on a point is reduced to a fourth part if we double the distance that separates it from the source, and to a ninth part is we triple it, etcetera.

The intensity of light reflected by a surface ( brightness ) depends on: 1) the angle it forms with the direction of the light: the more slanted the incident light is regarding the surface, the lower the reflected intensity of each point; 2) from the greatest to the smallest proportion of reflected light by that surface ( reflection coefficient or reflectance ). The reflected light s always a fraction of the incident light and depends on the nature of the surface: for example, snow reflects 95% of the light that falls on it (reflection coefficient of 95), white paper is 70-80%, black velvet is 1%, etc. Surfaces with medium brightness tones (not too light nor too dark surfaces) can be considered of the same reflectance as a neutral gray surface with a coefficient of 18%.