AS LIGHT ENERGY Longest wavelengths = red Shortest wavelengths = blue Property of light: white light total spectrum, black absent of light Light bends and splits- oil in water, rainbow Light measured in FOOT CANDLES (US) or LUX (most other places) 1footcandle (FC)= 10.08 Lux 1 fc is the amount of light from a standard candle from 1 ft away 1 lux is the amount of light from a standard candle from 1 meter away When film is exposed for 1 second to 1 lux = 1 lex-sec What is a footcandle?
Foot-candle
A foot-candle (sometimes designated footcandle; abbreviated fc, lm/ft, or sometimes ftc) is a non-SI (international standard) unit of illuminance or light intensity widely used in photography, film, television, and the lighting industry. The unit is defined as the amount of illumination the inside surface an imaginary 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 1square 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 10.76 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. 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. Foot-candles can be easily measured and calculated with the use of a (manual) camera equipped with a built-in light meter. With the film speed set to ASA 25 and the shutter speed set to 1/60th of a second, focus on a sheet of white paper placed in the area where intensity is to be measured. Adjust the f-stop for proper exposure. Each f-stop has an approximate corresponding foot-candle reading (see the table below).
Equivalences
Other measures of light intensity are the Lux and EV (exposure value) Lux = 2.5 x 2^EV , 1 foot-candle = about 10.764 LUX; EV 1 roughly equals to 0.46 foot candle or 5 Lux. Note that EV is a linear progession while foot candle and Lux are not. F-STOP FOOT-CANDLES 2 2.8 4 5.6 8 11 16 100 200 370 750 1,500 2,800 5,000
Explanation
Lux is a derived unit based on lumen, and lumen is a derived unit based on candela. One lux is equal to one lumen per square metre, where 4 lumens is the total luminous flux of a light source of one candela of luminous intensity. Illuminance Abbr. 0.00005 lux 50 lx Starlight 1 lux Moonlight Example
10 lux 400 lux 400 lux 1000 lux 1000 lux 10000 lux 32000 lux 100000 lux
Candle one foot away A brightly lit office Sunrise or sunset on a clear day. 1 klx Typical TV studio lighting Level capable of producing small shifts in the human biological 1 klx clock[1] Level capable of resynchronizing human the biological clock to a 10 klx new schedule[1] 32 klx Sunlight on an average day (min.) 100 klx Sunlight on an average day (max.)
Trivia: Unicode has a symbol for "lx": (I ), but this is just a legacy code to accommodate old code pages in certain Asian languages, and it is not recommended for use in any language today.
Lux versus lumen
The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same level of lux requires a greater number of lumens.
Lux versus footcandle
One footcandle = 10.76 lux. The footcandle is a non-SI unit of illuminance. Like the BTU, it is obsolete but 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 candela/ft2. Some sources do however note that the "lux" can be thought of as a "meter-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 meter = 3.281 feet, one lux = 1/3.282 footcandle = 1/10.76 footcand...