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General Electric Light Meter used in photography to measure light values in foot-candles.
United States customary units
illuminance
fc
1 fc in ...... is equal to ...
1 lm⁄ft2
10.76 lux (lm⁄m2)
1.076×10−3 phots
A foot-candle (sometimes foot candle; abbreviated fc, lm/ft2, or sometimes ft-c) is a non-SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in parts of the world where United States customary units are used, mainly the United States. Nearly all of the world uses the corresponding SI derived unit lux, defined as one lumen per square meter.
The foot-candle is defined as the illuminance of the inside surface of a one-foot-radius sphere with a point source of one candela at its center. Alternatively, it can be defined as the illuminance of one lumen on a one-square foot surface with a uniform distribution. Given the relation between candela and lumen, the two definitions listed are identical, with the second one potentially being easier to relate to in some everyday situations.
One foot-candle is equal to approximately 10.76 lux. In many practical applications, as when measuring room illumination, it is often not needed to measure illuminance more accurately than ±10%; in these situations it is sufficient to think of one foot-candle as about ten lux.
Quantity
Qv
lumen second
lm⋅s
T J
The lumen second is sometimes called the talbot.
Φv
lumen (= candela steradian)
lm (= cd⋅sr)
J
Luminous energy per unit time
Iv
candela (= lumen per steradian)
cd (= lm/sr)
J
Luminous flux per unit solid angle
Lv
candela per square metre
cd/m2 (= lm/(sr⋅m2))
L−2J
Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.
Ev
lux (= lumen per square metre)
lx (= lm/m2)
L−2J
Luminous flux incident on a surface
Mv
lumen per square metre
lm/m2
L−2J
Luminous flux emitted from a surface
Hv
lux second
lx⋅s
L−2T J
Time-integrated illuminance
ωv
lumen second per cubic metre
lm⋅s/m3
L−3T J
K
lumen per watt
lm/W
M−1L−2T3J
Ratio of luminous flux to radiant flux
η
lumen per watt
lm/W
M−1L−2T3J
Ratio of luminous flux to power consumption
V
1
Luminous efficacy normalized by the maximum possible efficacy
See also: SI · Photometry · Radiometry
^ The symbols in this column denote dimensions; \"L\", \"T\" and \"J\" are for length, time and luminous intensity respectively, not the symbols for the units litre, tesla and joule.
^ Standards organizations recommend that photometric quantities be denoted with a subscript \"v\" (for \"visual\") to avoid confusion with radiometric or photon quantities. For example: USA Standard Letter Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^ a b c Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.
