lumen: Definition and Much More from Answers.com
- ️Wed Dec 20 2006
A unit of measurement of the amount of brightness that comes from a light source. Lumens define "luminous flux," which is energy within the range of frequencies we perceive as light. For example, a wax candle generates 13 lumens; a 100 watt bulb generates 1,200 lumens.
Lumen Ratings for Data Projectors
When choosing a data projector, the lumen rating is an important specification. In a darkened room, 500 lumens may be ample; however, in a conference room with normal lighting, 1,000 lumens would be better. In a room with daylight, 2,000 lumens is preferred. The standard lumen rating of a data projector is the average of photometer readings at several points on a full white image on the screen. See ANSI lumen, candela and spectrum.
[Etymology: Lat: ‘light’] luminous flux. Symbol lm. SI, Metric-m.k.s. 1948 Having been proposed as lumen in 1896 and long accepted
[Moon P. J. Opt. Soc. Amer. Vol. 32, 348-62 (1942)], then accepted into the SI in 1948 as new lumen, identically candela·steradian (cd·sr). The following are among the coherent derived units:
• lm·m-2 = lux for luminous exitance, illuminance;
• lm·W-1 for luminous efficacy;
• lm·s for quantity of light;
• lm·h for quantity of light, used, similarly to kW·h, in illumination.
The lumen is the luminous flux emitted from a point-source of uniform intensity of 1 candela into unit solid angle; the total spherical intensity of such a source is therefore 4π lm = 12.566 37~ lm. The power of such a flux depends on the wavelength of the light, 540 THz being the standard for the candela.
History
The term appears to have been introduced before 1900, and adopted officially in France in 1919, applying to the candle in its successive forms, before being adopted internationally (as new lumen) in 1946 with the ‘new candle’; see candela for the history of the candle.
| 1946 | CIPM: ‘4. New lumen (unit of luminous flux). - The new lumen is the luminous flux emitted in unit solid angle (steradian) by a uniform point source having a luminous intensity of 1 new candle.’ |
| 1948 | 9th CGPM ‘lumen’ adopted in place of new lumen.see note below |
[Le Système International d'Unités (Sèvres, France: Bureau International de Poids et Mesures, 1985)]
For other uses of the word, see Lumen.
The lumen (symbol: lm) is the SI unit of luminous flux, a measure of the perceived power of light. Luminous flux differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.
Explanation
If a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen. Alternatively, an isotropic one-candela light source emits a total luminous flux of exactly 4π lumens. The lumen can be thought of casually as a measure of the total "amount" of visible light emitted.
A standard 100 watt incandescent light bulb emits approximately 1700 lumens in North America and around 1300 lumens in 220V areas of the world. See luminous efficacy for the specific efficiency of various types of electric light sources.
ANSI lumens
The light output of projectors (including video projectors) is typically measured in lumens. A standardized procedure for testing projectors has been established by the American National Standards Institute, which involves averaging together several measurements taken at different positions.[1] For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in "ANSI lumens", to distinguish devices that have been so tested from those tested by other methods. ANSI lumen measurements are in general more accurate than the other measurement techniques used in the projector industry.[2] This allows projectors to be more easily compared on the basis of their brightness specifications.
SI photometry units
SI photometry units| Quantity | Symbol | SI unit | Abbr. | Notes |
|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lm·s | units are sometimes called talbots |
| Luminous flux | F | lumen (= cd·sr) | lm | also called luminous power |
| Luminous intensity | Iv | candela (= lm/sr) | cd | an SI base unit |
| Luminance | Lv | candela per square metre | cd·m–2 | units are sometimes called nits |
| Illuminance | Ev | lux (= lm·m–2) | lx | Used for light incident on a surface |
| Luminous emittance | Mv | lux (= lm·m–2) | lx | Used for light emitted from a surface |
| Luminous efficacy | - - - - | lumen per watt | lm/W | ratio of luminous flux to radiant flux; maximum possible is 683.002 |
| Units-interrelation diagram (full page) |
See also
- Brightness
- Lux
- Lm is also the sign for the Malta Lira currency along side the ₤
References
- ^ ANSI lumen article. PC Magazine Encyclopedia. Retrieved on 2006-12-20.
- ^ Projector Guide. CPILive.net (February 2004). Retrieved on 2006-12-20.
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