2.    Basic Illumination Concept

(a) Luminous flux, F

 Luminous flux is the rate at which light energy flows from the source. It is measured in 'lumen', abbreviated as lm The units 'lumen' and 'watt' have the same dimension as they both represent energy per second. However, it is incorrect to convert photometric quantities directly into energy quantities or vice-versa, as the luminous effect of radiant energy depends on its wavelength.

(b) Luminous Intensity, I

 The concept of a unit of brightness of a source to allow for spatial distribution of light energy, which is measured in 'candela', abbreviated as 'Cd'.

where, w = solid angle, in steradian (st)

Figure 2 Luminous Intensity of Source

(c) Illuminance, E

 Illuminance is the result of illumination on a lighted surface and is defined as the luminous flux falling on unit area of the surface under consideration.

where,

E = illuminance of a surface, lm/m2 or lux

F = luminous flux incident on the surface, lumen

A = area of the surface, m2

(d) Luminance of a Light source, L

 The luminance of a light source, L ,in a given direction is defined as the luminous intensity per unit projected surface area of the light source in that direction, i.e.

The unit for luminance of a light source is Cd/m2.

(e) Luminance of an Illuminated surface. B

 The luminance of an illuminated surface in a given direction is defined as the luminous intensity per unit area coming from the surface in the particular direction.

where,

B = luminance of a surface, Cd/m2

E = illuminance of that surface, lux

r = luminous factor (sometimes called reflectance factor), of the  surface.