| 词条 | Foot-candle |
| 释义 |
| name = Foot-candle | image = File:GELightmeter.JPG | caption = General Electric Light Meter used in photography to measure light values in foot candles. | standard = SAE units | quantity = illuminance | symbol = fc | symbol2 = | namedafter = | extralabel = | extradata = | units1 = SI derived unit | inunits1 = 10.76 lux | units2 = CGS units | inunits2 = {{val|1.076|e=-3|u=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. One foot-candle represents "the illuminance cast on a surface by a one-candela source one foot away."{{cn|reason=Where is this quote from or why is this quoted?|date=August 2018}} This unit is commonly used in lighting layouts in parts of the world where SAE units are used. The unit foot-candle is defined as the amount of illumination the inside surface of a one-foot-radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a one-square foot surface of which there is a uniformly distributed flux of one lumen. Thus one foot-candle is equal to one lumen per square foot or approximately 10.764 lux.[1] In practical applications, as when measuring room illumination, it is very difficult to measure illuminance more accurately than ±10%, and for many purposes it is quite sufficient to think of one foot-candle as about ten lux as is typically done in the lighting industry. UseIn the lighting industry, foot-candles are a common unit of measurement used to calculate adequate lighting levels of workspaces in buildings or outdoor spaces. Foot-candles are also commonly used in the museum and gallery fields, 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, 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, horticulturalists often measure and discuss optimum intensity for various plants in foot-candles. ExamplesFull, 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.[2] Lighting requirements for commercial spaces range from 5 fc for storage spaces to 200 fc for visually intensive work.[3] Notes1. ^One lumen per square foot is equal to 0.3048−2 lumens per square meter. Since illuminance follows the inverse-square law a source that is farther away casts less illumination than one that is close, so one lux is less illuminance than one foot-candle. 2. ^{{cite web |url=http://www.maximlighting.com/how-much-light |title=How much light do I need? |access-date=2018-04-18}} 3. ^1 {{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}} See also
External links
4 : Imperial units|Units of illuminance|Photometry|Customary units of measurement in the United States |
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