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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.
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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.
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