Irradian Limited
L203 Photometer
12
(CIE FOV-112/101 probe fitted)
Luminance (lumens per steradian per unit area) or brightness is the
photometric measurement of the response from viewing an illuminated
object or self luminous source.
Distance d
Area A
Unit solid angle, steradian = A/d²
Uniform Source
Luminance can be expressed in candela (lumens per steradian) per unit
area, cd.m
-2
(also know as nit) or cd.cm
-2
or cd.ft
-2
.
Using the expression below, luminance, L, can be calculated by
measuring the illuminance, E, on a surface and knowing the reflectance,
ρ
, of the surface.
L = ( E
*
ρ
) /
π
where
π
is expressed in steradians
= (x
*
ρ
) lm. m
-2
/
π
= (x
*
ρ
/
π
) lm. str
-1
. m
-2
= (x
*
ρ
/
π
) cd.m
-2
e.g.. Calculate the luminance of a surface with a diffuse reflectance of
ρ
= 0.9 when illuminated at a level of 100 lux.
L = ( 100
*
0.9 / 3.14) cd.m
-2
= 28.7 cd.m
-2
Luminance can also be expressed as footLambert, fL. This unit
incorporates the
π
value and thus provides a simple relationship between
the illuminance of a surface of reflectance
ρ
and the luminance of the
surface.
1 footLambert = 1
*
ρ
footcandles