2. Basic Illumination
(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.
E = illuminance of a surface, lm/m2 or lux
F = luminous flux incident on the
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
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.
B = luminance of a surface, Cd/m2
E = illuminance of that surface, lux
r = luminous factor (sometimes called
reflectance factor), of the surface.