A foot-candle (sometimes foot candle; abbreviated fc, lm/ft², or sometimes ft-c) is a non- SI unit of

illuminance In photometry (optics), photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate ...

light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

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 United States customary units form a system of measurement units commonly used in the United States and most U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units that ...

are used, mainly the United States. Nearly all of the world uses the corresponding SI derived unit

lux The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the irradiance, as perceived by ...

, 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 The candela (symbol: cd) is the unit of luminous intensity in the International System of Units (SI). It measures luminous power per unit solid angle emitted by a light source in a particular direction. Luminous intensity is analogous to radi ...

at its center. Alternatively, it can be defined as the illuminance of one lumen on a one-

square foot The square foot (; abbreviated sq ft, sf, or ft2; also denoted by '2 and ⏍) is an imperial unit and U.S. customary unit (non- SI, non-metric) of area, used mainly in the United States, Canada, the United Kingdom, Bangladesh, India, Nepal, Pa ...

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

. 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 An architect is a person who plans, designs, and oversees the construction of buildings. To practice architecture means to provide services in connection with the design of buildings and the space within the site surrounding the buildings that h ...

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 Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

of plants, US

horticulturalists Horticulture (from ) is the art and science of growing fruits, vegetables, flowers, trees, shrubs and ornamental plants. Horticulture is commonly associated with the more professional and technical aspects of plant cultivation on a smaller and mo ...

often measure and discuss optimum intensity for various plants in foot-candles.

Examples

Full, unobstructed

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

has an intensity of up to 930 fc. 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 In geography, latitude is a geographic coordinate system, geographic coordinate that specifies the north-south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from −90° at t ...

. Indoor

lighting Lighting or illumination is the deliberate use of light to achieve practical or aesthetic effects. Lighting includes the use of both artificial light sources like lamps and light fixtures, as well as natural illumination by capturing daylight. ...

for residences seeks to provide 5-40 fc for general spaces and 70-90 fc for work spaces. Lighting requirements for commercial spaces range from 5 fc for storage spaces to 200 fc for visually intensive work.

SI photometry units

References

External links

Example chart of foot-candles and corresponding activities
{{DEFAULTSORT:Foot-Candle Imperial units Units of illuminance Customary units of measurement in the United States

Use

Examples

SI photometry units

See also

References

External links