 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Foot-candle" – news ·newspapers ·books ·scholar ·JSTOR(March 2008) (Learn how and when to remove this message) |
| Foot-candle | |
|---|---|
 General Electric Light Meter used in photography to measure light values in foot-candles. | |
| General information | |
| Unit system | United States customary units |
| Unit of | illuminance |
| Symbol | fc |
| Conversions | |
| 1 fcin ... | ... is equal to ... |
| US | 1lm⁄ft2 |
| SI | 10.764lux (lm⁄m2) |
| CGS | 1.076×10−3 phots |
Afoot-candle (sometimesfoot candle; abbreviatedfc,lm/ft2, or sometimesft-c) is a non-SI unit ofilluminance orlight intensity. The foot-candle is defined as onelumen per square foot. This unit is commonly used in lighting layouts in parts of the world whereUnited States customary units are used, mainly the United States.[1] Nearly all of the world uses the correspondingSI derived unitlux, 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 onecandela at its center. Alternatively, it can be defined as the illuminance of onelumen 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 10.764lux. 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.
In the US lighting industry, foot-candles are a common unit of measurement used byarchitects 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 thephotosynthesis of plants, UShorticulturalists often measure and discuss optimum intensity for various plants in foot-candles.
Full, unobstructedsunlight has an intensity of up to 930 fc.[2][3] 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 andlatitude.
Indoorlighting for residences seeks to provide 5-40 fc for general spaces and 70-90 fc for work spaces.[4]
Lighting requirements for commercial spaces range from 5 fc for storage spaces to 200 fc for visually intensive work.[2]
| Quantity | Unit | Dimension [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 thetalbot. |
| Luminous flux, luminous power | Φv[nb 3] | lumen (= candelasteradian) | lm (= cd⋅sr) | J | Luminous energy per unit time |
| Luminous intensity | Iv | candela (= lumen per steradian) | cd (= lm/sr) | J | Luminous flux per unitsolid angle |
| Luminance | Lv | candela per square metre | cd/m2 (= lm/(sr⋅m2)) | L−2⋅J | Luminous flux per unit solid angle per unitprojected source area. The candela per square metre is sometimes called thenit. |
| Illuminance | Ev | lux (= lumen per square metre) | lx (= lm/m2) | L−2⋅J | Luminous fluxincident on a surface |
| Luminous exitance, luminous emittance | Mv | lumen per square metre | lm/m2 | L−2⋅J | Luminous fluxemitted from a surface |
| Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | Time-integrated illuminance |
| Luminous energy density | ωv | lumen second per cubic metre | lm⋅s/m3 | L−3⋅T⋅J | |
| Luminous efficacy (of radiation) | K | lumen perwatt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux toradiant flux |
| Luminous efficacy (of a source) | η[nb 3] | lumen perwatt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to power consumption |
| Luminous efficiency, luminous coefficient | V | 1 | Luminous efficacy normalized by the maximum possible efficacy | ||
| See also: | |||||