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Physical characteristics of artificial illumination

1. The artificial illumination (same as natural) is characterized by:

- light intensity (I)- the light source capacity, measured in candles (Cd). It’s a light intensity, that generates the monochrome radiation of the 540∙10¹² Hz frequency in certain direction, with radiant intensity in that direction of 1/683Wt/steradian;

- light flow (luminous flux) (F) - the density of light, measured in lumens (lm) - light flow, radiated by the individual source with intensity of 1 cd in the solid angle of 1 steradian. The solid (spatial) angle is the cone, which, if its top is considered to be in the center of the sphere, cuts the surface, equaling to the squared radius of that sphere from it;

- illuminance (E)- the amount of light falling on a surface (surface density of the light flow) , where S is the illuminated surface area, m². The illuminance is measured in luxes. 1 lux is the illuminance of the 1 m² surface, illuminated by the light flow of 1 lumen;

- brightness (B) –light intensity, at which the light is radiated or reflected from the surface in certain direction. , where is the angle between the light direction and the perpendicular to the surface.

The unit of brightness is cd/m² - the brightness of the surface with the area of 1m², radiating or reflecting the light with the intensity of 1cd;

- reflection coefficient (β) - the ratio of the reflected light flow (F_ref) to the light flow received by the surface (F_rec). It is calculated using the following formula .

The β value is 0.9 for fresh snow, 0.7 – for white paper and 0.35 – for untanned skin.

- optical transmission coefficient (τ) is the light flow, which goes through the (F_through.) medium, divided by the light flow, which falls on that medium .

This coefficient allows assessing the quality and the cleanness of the window glass and the glass parts of different lighting fixtures.

- luminosity (M) – surface density of the light flow, expressed in lm, which is radiated from the 1m² surface (lm/m²).