The lumen (symbol: lm) 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 (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all light emitted, independent of the eye's ability to perceive it.

The lumen is defined in relation to the candela as

1 lm = 1 cd·sr

A full sphere has a solid angle of 4·π steradians,^[1] so a light source that uniformly radiates one candela in all directions has a total luminous flux of 1 cd·4π sr = 4π ≈ 12.57 lumens.^[2]

Explanation[link]

If a light source emits one candela of luminous intensity uniformly across a solid angle of one steradian, the total luminous flux emitted into that angle is one lumen (1 cd·1 sr = 1 lm). Alternatively, an isotropic one-candela light-source emits a total luminous flux of exactly 4π lumens. If the source were partially covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux—only 2π lumens. The luminous intensity would still be one candela in those directions that are not obscured.

The lumen can be thought of casually as a measure of the total "amount" of visible light in some defined beam or angle, or emitted from some source. The number of candelas or lumens from a source also depends on its spectrum, via the nominal response of the human eye as represented in the luminosity function.

The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 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. Mathematically, 1 lx = 1 lm/m².

A source radiating a power of one watt of light in the color for which the eye is most efficient (a wavelength of 555 nm, in the green region of the optical spectrum) has luminous flux of 683 lumens. So a lumen represents at least 1/683 watts of visible light power, depending on the spectral distribution.

Lighting[link]

Lamps used for lighting are commonly labeled with their light output in lumens; in many jurisdictions this is required by law.

A 23 watt compact fluorescent lamp emits about 1,500–1,600 lm.^[3][4]

On 1 September 2010, European Union legislation came into force mandating that lighting equipment must be labelled primarily in terms of lumens, instead of watts of electricity consumed.^[5] This change is a result of the EU's Eco-design Directive for Energy-using Products (EuP).^[6] For example, according to the European Union standard, an energy-efficient bulb that claims to be the equivalent of a 60 W tungsten bulb must have a minimum light output of 700-750 lumens.^[7]

Projector output[link]

ANSI lumens[link]

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.^[8] For marketing purposes, the luminous flux of projectors that have been tested according to this procedure may be quoted in "ANSI lumens", to distinguish them from those tested by other methods. ANSI lumen measurements are in general more accurate than the other measurement techniques used in the projector industry.^[9] This allows projectors to be more easily compared on the basis of their brightness specifications.

The method for measuring ANSI lumens is defined in the IT7.215 document which was created in 1992. First the projector is set up to display an image in a room at a temperature of 25 degrees Celsius. The brightness and contrast of the projector are adjusted so that on a full white field, it is possible to distinguish between a 5% screen area block of 95% peak white, and two identically sized 100% and 90% peak white boxes at the center of the white field. The light output is then measured on a full white field at nine specific locations around the screen and averaged. This average is then multiplied by the screen area to give the brightness of the projector in "ANSI lumens".^[10]

Peak lumens[link]

Peak lumens is a measure of light output normally used with CRT video projectors. The testing uses a test pattern typically at either 10 and 20 percent of the image area as white at the center of the screen, the rest as black. The light output is measured just in this center area. Limitations with CRT video projectors result in them producing greater brightness when just a fraction of the image content is at peak brightness. For example the Sony VPH-G70Q CRT video projector produces 1200 "peak" lumens but just 200 ANSI lumens.^[11]

SI photometry units[link]

See also[link]

• Brightness
• Lux
• Luminous efficacy
• Nit (unit)

References[link]

v t e SI units Base units Ampere · Candela · Kelvin · Kilogram · Metre · Mole · Second Derived units Becquerel · Coulomb · Degree Celsius · Farad · Gray · Henry · Hertz · Joule · Katal · Lumen · Lux · Newton · Ohm · Pascal · Radian · Siemens · Sievert · Steradian · Tesla · Volt · Watt · Weber Accepted for use with SI Dalton (Atomic mass unit) · Astronomical unit · Day · Decibel · Degree Celsius · Degree of arc · Electronvolt · Hectare · Hour · Litre · Minute · Minute of arc · Neper · Second of arc · Tonne Atomic units · Natural units See also SI prefixes · Systems of measurement · Conversion of units · New SI definitions · History of the metric system Book:International System of Units  · Category:SI base units

Lumen can mean:

• Lumen (unit), the SI unit of luminous flux
• Lumen (anatomy), the cavity or channel within a tubular structure
• Thylakoid lumen, the inner membrane space of the chloroplast
• Phenobarbital (trade name)
• 141 Lumen, an asteroid discovered by the French astronomer Paul Henry in 1875
• Lumen (band), Russian alternative rock band
• Lumen Pierce, a fictional character played by Julia Stiles from the television series Dexter
• Lumen (branding agency), a design and branding company headquartered in Milan, Italy

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

Look up unit in Wiktionary, the free dictionary.

Unit may refer to:

Mathematics[link]

• Unit (ring theory), an element that is invertible with respect to ring multiplication
• 1 (number), the unit number
• Unit vector, a vector with length equal to 1

Science and technology[link]

• Units (software), a software program for unit conversion
• Units of measurement, a standardized quantity of measurement of a physical quantity
• Enzyme unit
• Unit operation, a basic step in a chemical plant process
• Functional unit, a component of a computer system such as the CPU
• Unit of blood, a measurement in blood transfusion equal to one pint or 450 ml
• Unit of alcohol, 10 millilitres of pure ethanol in the United Kingdom
• Rack unit, a measure to describe the height of rack-mounted computer equipment
• Geological unit or rock unit, a volume of identifiable rock or ice
• In statistics:
  • Unit of observation, a type of entity (e.g. person, house, neighborhood) specifying the level at which data is collected
  • Unit of analysis, a type of entity (e.g. person, house, neighborhood) specifying the level at which data is analyzed
  • Statistical unit, a single data point, or collection of data points grouped as a single entity, on which statistical analysis is performed

Entertainment and the arts[link]

• UNIT, a fictional military organization in the science fiction television series Doctor Who
• Unit (album), by the Australian band Regurgitator
• Unit (art collective), a British artists' collective
• G-Unit, a hip-hop music group
• The Units, a synthpunk band
• The Unit, an American television series
• Second unit, a team that shoots footage which is of lesser importance for the final motion picture
• Unit of action, a discrete piece of action (or beat) in a theatrical presentation
• UNIT Post Production, a British company
• The Tony Rice Unit, an American bluegrass group

Other uses[link]

• Unit (housing), a self-contained suite of rooms within a set of similar dwellings
• Unit of account, a monetary unit of measurement
• An apartment
• Multiple unit, a passenger train where each carriage has it own motors
• Military unit
• Strategic business unit
• In real time strategy games, an individual (or squad, depending on the game) infantry or vehicle
• Work unit or danwei (单位), a place of employment in China

See also[link]

• UNITA
• Subunit (disambiguation)

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

Low Profile was originally a duo of two aspiring rappers and a DJ coming out of L.A. The groups membership consisted of WC and DJ Aladdin, who went on to pursue a career in visual art. The duo made its debut with Rhyme Syndicate Records, on a compilation album produced by Ice-T and Afrika Islam, before becoming a duo on Profile Records.

The group was a short-lived but influential West Coast hip hop duo. Together, they released one album, We're in This Together before breaking up. They also appeared on the Rhyme Syndicate compilation Rhyme Syndicate Comin' Through. DJ Aladdin began working with Ice T and WC formed a group called WC and the Maad Circle which included a then-unknown rapper named Coolio.

Discography[link]

• Hip Hop I Crave (Single) (1987), Nu School Records
• We're in This Together (1989), Priority Records

References[link]

External links[link]

• Low Profile at Discogs.