Foot-candle
Encyclopedia
A foot-candle is a non-SI
unit of illuminance
or light
intensity widely used in photography
, film
, television
, conservation lighting, and the lighting
industry. The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away."
The unit is defined as the amount of illumination the inside surface of a 1-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 1-square foot
surface of which there is a uniformly distributed flux
of one lumen
. This can be thought of as the amount of light that actually falls on a given surface. The foot-candle is equal to one lumen per square foot.
The SI derived unit
of illuminance is the lux
. One footcandle is equal to approximately 10.764 lux, although in the lighting industry, typically this is approximated as 1 footcandle being equal to 10 lux.
In the lighting industry, footcandles are a common unit of measurement used to calculate adequate lighting levels of workspaces in buildings or outdoor spaces. Footcandles 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 footcandles 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. Full, unobstructed sunlight
has an intensity of approximately 10,000 fc. An overcast day will produce an intensity of around 1,000 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
.
. The footcandle (or lumen per square foot) is a non-SI
unit of illuminance. Like the BTU, it is still in fairly common use in the United States, particularly in construction-related engineering and in building codes. Because lux and footcandles are different units of the same quantity, it is perfectly valid to convert footcandles to lux and vice versa.
The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away." As natural as this sounds, this style of name is now frowned upon, because the dimensional formula for the unit is not foot · candela, but lumens per square foot. Some sources do however note that the "lux" can be thought of as a "metre-candle" (i.e. the illuminance cast on a surface by a one-candela source one meter away). A source that is farther away casts less illumination than one that is close, so one lux is less illuminance than one footcandle. Since illuminance follows the inverse-square law, and since one foot = 0.3048 m, one lux = 0.30482 footcandle ≈ 1/10.764 footcandle.
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 footcandle as about ten lux.
Small candles molded in the shape of a foot were distributed by General Electric during the 1960s as advertising novelties. These visual puns promoted GE's line of fluorescent lighting and were an intentional reference to the unit of light; the sole of the "foot" had a label reading "GE makes the difference in light!" These were functional candles, had wick
s, and could be lit. They were not, of course, "standard candles" nor did they deliver any calibrated amount of illuminance.
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
unit of illuminance
Illuminance
In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per...
or light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
intensity widely used in photography
Photography
Photography is the art, science and practice of creating durable images by recording light or other electromagnetic radiation, either electronically by means of an image sensor or chemically by means of a light-sensitive material such as photographic film...
, film
Film
A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
, television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
, conservation lighting, and the lighting
Lighting
Lighting or illumination is the deliberate application of light to achieve some practical or aesthetic effect. Lighting includes the use of both artificial light sources such as lamps and light fixtures, as well as natural illumination by capturing daylight...
industry. The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away."
The unit is defined as the amount of illumination the inside surface of a 1-foot radius sphere
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
would be receiving if there were a uniform point source of one candela
Candela
The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...
in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a 1-square foot
Square foot
The square foot is an imperial unit and U.S. customary unit of area, used mainly in the United States, Canada, United Kingdom, Hong Kong, Bangladesh, India, Pakistan and Afghanistan. It is defined as the area of a square with sides of 1 foot in length...
surface of which there is a uniformly distributed flux
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
of one lumen
Lumen (unit)
The lumen is the SI derived unit of luminous flux, a measure of the total "amount" of visible light emitted by a source. Luminous flux differs from power in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux...
. This can be thought of as the amount of light that actually falls on a given surface. The foot-candle is equal to one lumen per square foot.
The SI derived unit
SI derived unit
The International System of Units specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre , and of density is...
of illuminance is the lux
Lux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
. One footcandle is equal to approximately 10.764 lux, although in the lighting industry, typically this is approximated as 1 footcandle being equal to 10 lux.
In the lighting industry, footcandles are a common unit of measurement used to calculate adequate lighting levels of workspaces in buildings or outdoor spaces. Footcandles 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 footcandles 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
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
of plants, horticulturalists
Horticulture
Horticulture is the industry and science of plant cultivation including the process of preparing soil for the planting of seeds, tubers, or cuttings. Horticulturists work and conduct research in the disciplines of plant propagation and cultivation, crop production, plant breeding and genetic...
often measure and discuss optimum intensity for various plants in foot-candles. Full, unobstructed sunlight
Sunlight
Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...
has an intensity of approximately 10,000 fc. An overcast day will produce an intensity of around 1,000 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
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
.
Footcandle versus lux
One footcandle ≈ 10.764 luxLux
The lux is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface...
. The footcandle (or lumen per square foot) is a non-SI
Si
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...
unit of illuminance. Like the BTU, it is still in fairly common use in the United States, particularly in construction-related engineering and in building codes. Because lux and footcandles are different units of the same quantity, it is perfectly valid to convert footcandles to lux and vice versa.
The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away." As natural as this sounds, this style of name is now frowned upon, because the dimensional formula for the unit is not foot · candela, but lumens per square foot. Some sources do however note that the "lux" can be thought of as a "metre-candle" (i.e. the illuminance cast on a surface by a one-candela source one meter away). A source that is farther away casts less illumination than one that is close, so one lux is less illuminance than one footcandle. Since illuminance follows the inverse-square law, and since one foot = 0.3048 m, one lux = 0.30482 footcandle ≈ 1/10.764 footcandle.
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 footcandle as about ten lux.
Advertising novelties
Candle wick
A candle wick is a string, cord, or wooden object that holds the flame of a candle. A candle wick works by capillary action, drawing the fuel to the flame. When the liquid fuel, typically melted candle wax, reaches the flame it then vaporizes and combusts. The candle wick influences how the...
s, and could be lit. They were not, of course, "standard candles" nor did they deliver any calibrated amount of illuminance.