View source for Foot-candle

{{Short description|Unit of illuminance}}
{{More citations needed|date=March 2008}}
{{use mdy dates | cs1-dates=ly|date=December 2024}}

{{Infobox unit
| name         = Foot-candle
| image        = File:GELightmeter.JPG
| caption      = General Electric Light Meter used in photography to measure light values in foot-candles.
| standard     = [[United States customary units]]
| quantity     = [[illuminance]]
| symbol       = fc
| symbol2      = 
| namedafter   = 
| extralabel   = 
| extradata    = 
| units1        = [[United States customary units|US]]
| inunits1      = 1 {{frac|[[lumen (unit)|lm]]|[[ft^2|ft<sup>2</sup>]]}}
| units2       = [[SI derived unit|SI]]
| inunits2     = 10.764 [[lux]] ({{frac|[[lumen (unit)|lm]]|[[m^2|m<sup>2</sup>]]}})
| units3       = [[Centimetre–gram–second system of units|CGS]]
| inunits3     = {{val|1.076|e=-3|u=[[phot|phots]]}}
}}

A '''foot-candle''' (sometimes '''foot candle'''; abbreviated '''fc''', '''lm/ft<sup>2</sup>''', or sometimes '''ft-c''') is a non-[[SI]] unit of [[illuminance]] or [[light]] intensity. The foot-candle is defined as one [[Lumen (unit)|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.<ref>{{cite web|url=https://www.globalspec.com/learnmore/optics_optical_components/optoelectronics/lux_meters_light_meters|access-date=November 27, 2019|title=Lux Meters (Light Meters) Information}}</ref> Nearly all of the world uses the corresponding [[International system of units|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 (unit)|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 10.764 [[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 [[architect]]s 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 [[Horticulture|horticulturalists]] often measure and discuss optimum intensity for various plants in foot-candles.

==Examples==
Full, unobstructed [[sunlight]] has an intensity of up to 930 fc.<ref name="Larson"/><ref>{{Cite web| title = NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity or field of science| access-date = 2024-12-12| url = https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b9#LIGHT |date=September 9, 2024  |publisher=National Institute of Standards and Technology}}</ref>  An overcast day will produce an intensity of around 100 fc. The intensity of light near a window can range from 100 to 460 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.<ref>{{cite web |url=http://www.maximlighting.com/how-much-light |title=How much light do I need? |access-date=2018-04-18}}</ref>

Lighting requirements for commercial spaces range from 5 fc for storage spaces to 200 fc for visually intensive work.<ref name="Larson">{{cite web |url=https://www.larsonelectronics.com/blog/2016/11/07/white-papers/much-light-enough-footcandle-recommendations/ |title=How Much Light Is Enough? Footcandle Recommendations |access-date=2018-04-18}}</ref>

==SI photometry units==
{{SI_light_units}}

==See also==
*[[Photometry (optics)]] for more on the measurement of light

==References==
{{Reflist}}

==External links==
*[https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxhc3NpZ25tZW50MWJ1aWxkaW5ncHJvZ3JhbXxneDo3NTRlMGMxYzk5MjJlZWM0 Example chart of foot-candles and corresponding activities]

{{United States Customary Units}}
{{Imperial units}}

{{DEFAULTSORT:Foot-Candle}}
[[Category:Imperial units]]
[[Category:Units of illuminance]]
[[Category:Customary units of measurement in the United States]]