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J17 Instruction Manual
B-1
AppendixĂB:
Photometry and
Radiometry Reference
This appendix summarizes convenient relationships
and formulas used in photometry and radiometry.
Photometric Relationships
In photometry, the spectral sensitivity of the sensor is
matched to the average human eye, with a peak at
555Ănm.
H An isotropic light source (a source that emits light
uniformly in all directions) of 12.6 lumens is 1 canĆ
dela (1 candle power).
H A 1 candela source at a distance of 1 meter proĆ
vides an illuminance of 1 lux on a surface, regardĆ
less of the reflectance of the surface.
H A 1 candela source at a distance of 1 foot provides
an illuminance of 1 footcandle on a surface, regardĆ
less of the reflectance of the surface
H Moving a light source further from a surface reĆ
duces the illuminance proportionally with the
square of the distance (inverse square law). For
example, moving a 1 candela source from 1 foot to
2 feet will reduce the illuminance to ¼ footcandle.
H A perfectly white, diffuse surface illuminated by
1 footcandle has a surface luminance of 1 footlamĆ
bert.
J17 Instruction Manual
B-1
AppendixĂB:
Photometry and
Radiometry Reference
This appendix summarizes convenient relationships
and formulas used in photometry and radiometry.
Photometric Relationships
In photometry, the spectral sensitivity of the sensor is
matched to the average human eye, with a peak at
555Ănm.
H An isotropic light source (a source that emits light
uniformly in all directions) of 12.6 lumens is 1 canĆ
dela (1 candle power).
H A 1 candela source at a distance of 1 meter proĆ
vides an illuminance of 1 lux on a surface, regardĆ
less of the reflectance of the surface.
H A 1 candela source at a distance of 1 foot provides
an illuminance of 1 footcandle on a surface, regardĆ
less of the reflectance of the surface
H Moving a light source further from a surface reĆ
duces the illuminance proportionally with the
square of the distance (inverse square law). For
example, moving a 1 candela source from 1 foot to
2 feet will reduce the illuminance to ¼ footcandle.
H A perfectly white, diffuse surface illuminated by
1 footcandle has a surface luminance of 1 footlamĆ
bert.