Foot-candle

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.[1] Nearly all of the world uses the corresponding SI derived unit lux, defined as one lumen per square meter.

Foot-candle
General Electric Light Meter used in photography to measure light values in foot-candles.
General information
Unit systemUnited States customary units
Unit ofilluminance
Symbolfc
Conversions
1 fc in ...... is equal to ...
   US   1 lm⁄ft2
   SI   10.76 lux (lm⁄m2)
   CGS   1.076×10−3 phots

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.

Use

In the US lighting industry, foot-candles are a common unit of measurement used by architects to calculate adequate lighting levels. Foot-candles are also commonly used in the museum and gallery fields in the US, where lighting levels must be carefully controlled to conserve light-sensitive objects such as prints, photographs, and paintings, the colors of which fade when exposed to bright light for a lengthy period.

In the motion picture cinematography field in the US, incident light meters are used to measure the number of foot-candles present, which are used to calculate the intensity of motion picture lights, allowing cinematographers to set up proper lighting-contrast ratios when filming.

Since light intensity is the primary factor in the photosynthesis of plants, US horticulturalists often measure and discuss optimum intensity for various plants in foot-candles.

Examples

Full, unobstructed sunlight has an intensity of approximately 10,000 fc.[2] An overcast day will produce an intensity of around 100 fc. The intensity of light near a window can range from 100 to 5,000 fc, depending on the orientation of the window, time of year and latitude.

Indoor lighting for residences seeks to provide 5-40 fc for general spaces and 70-90 fc for work spaces.[3]

Lighting requirements for commercial spaces range from 5 fc for storage spaces to 200 fc for visually intensive work.[2]

SI photometry units

SI photometry quantities
Quantity Unit Dimensions
[nb 1]		 Notes
Name Symbol[nb 2] Name Symbol
Luminous energy Qv[nb 3] lumen second lm⋅s T J The lumen second is sometimes called the talbot.
Luminous flux, luminous power Φv[nb 3] lumen (= candela steradian) lm (= cd⋅sr) J Luminous energy per unit time
Luminous intensity Iv candela (= lumen per steradian) cd (= lm/sr) J Luminous flux per unit solid angle
Luminance 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.
Illuminance Ev lux (= lumen per square metre) lx (= lm/m2) L−2J Luminous flux incident on a surface
Luminous exitance, luminous emittance Mv lumen per square metre lm/m2 L−2J Luminous flux emitted from a surface
Luminous exposure Hv lux second lx⋅s L−2T J Time-integrated illuminance
Luminous energy density ωv lumen second per cubic metre lm⋅s/m3 L−3T J
Luminous efficacy (of radiation) K lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to radiant flux
Luminous efficacy (of a source) η[nb 3] lumen per watt lm/W M−1L−2T3J Ratio of luminous flux to power consumption
Luminous efficiency, luminous coefficient V 1 Luminous efficacy normalized by the maximum possible efficacy
See also: SI · Photometry · Radiometry
  1. 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.
  2. 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
  3. Alternative symbols sometimes seen: W for luminous energy, P or F for luminous flux, and ρ for luminous efficacy of a source.

See also

References
  1. "Lux Meters (Light Meters) Information". Retrieved November 27, 2019.
  2. "How Much Light Is Enough? Footcandle Recommendations". Retrieved 2018-04-18.
  3. "How much light do I need?". Retrieved 2018-04-18.
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