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Shop Exclusively at Lumens at Lumens.com. Guaranteed low prices on modern lighting, fans, furniture and decor + free shipping on orders over $75! Lumen brings together the talent, experience, infrastructure and capabilities of CenturyLink, Level 3 and 25+ other technology companies to create a new kind of company—one designed specifically to address the dynamic data and application needs of the 4th Industrial Revolution. Shop Exclusively at Lumens at Lumens.com. Guaranteed low prices on modern lighting, fans, furniture and decor + free shipping on orders over $75! Question: Which is more likely: (1) To find a boreal owl nest with 3 eggs, or (2) To find a boreal owl nest with 4 eggs. Answer: Both of these events are equally likely. P(3 eggs) = P(4 eggs) = 0.25. There is a 25% chance that if you find a boreal owl nest, it will have 3 eggs. You are equally likely to find a boreal owl nest with 4 eggs.
The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time. Luminous flux differs from power (radiant flux) in that radiant flux includes all electromagnetic waves emitted, while luminous flux is weighted according to a model (a 'luminosity function') of the human eye's sensitivity to various wavelengths. Lumens are related to lux in that one lux is one lumen per square metre.
The lumen is defined in relation to the candela as
A full sphere has a solid angle of 4π 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 cd⋅sr ≈ 12.57 lumens.[2]
Explanation[edit]
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 partly 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[edit]
A standard LED lamp capable of producing 470 lumens. It consumes about one sixth the energy of an incandescent light bulb producing the same light.
Lamps used for lighting are commonly labelled with their light output in lumens; in many jurisdictions this is required by law.
A 23 W spiral compact fluorescent lamp emits about 1,400–1,600 lm.[3][4] Many compact fluorescent lamps and other alternative light sources are labelled as being equivalent to an incandescent bulb with a specific power. Below is a table that shows typical luminous flux for common incandescent bulbs and their equivalents.
On 1 September 2010, European Union legislation came into force mandating that lighting equipment must be labelled primarily in terms of luminous flux (lm), instead of electric power (W).[11] This change is a result of the EU's Eco-design Directive for Energy-using Products (EuP).[12] 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 lm.[13]
Projector output[edit]ANSI lumens[edit]
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.[14] 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.[15] This allows projectors to be more easily compared on the basis of their brightness specifications.
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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 °C (77 °F). 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'.[16]
Peak lumens[edit]
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.[17]
Color light output[edit]
Brightness (white light output) measures the total amount of light projected in lumens. The color brightness specification Color Light Output measures red, green, and blue each on a nine-point grid, using the same approach as that used to measure brightness.
SI photometry units[edit]
SI photometry quantities
See also[edit]Notes and references[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Lumen_(unit)&oldid=982726612'
Solid AnglesOne of the key concepts to understanding the relationships between measurementgeometries is that of the solid angle, or steradian. A sphere contains4p steradians. A steradian is definedas the solid angle which, having its vertex at the center of the sphere,cuts off a spherical surface area equal to the square of the radius ofthe sphere. For example, a one steradian section of a one meter radiussphere subtends a spherical surface area of one square meter.The sphere shown in cross section in figure 7.1 illustrates the concept. A cone with a solid angle of one steradian has been removed from the sphere. This removed cone is shown in figure 7.2. The solid angle, W,in steradians, is equal to the spherical surface area, A, divided by thesquare of the radius, r.
Most radiometric measurements do not require an accurate calculationof the spherical surface area to convert between units. Flat areaestimates can be substituted for spherical area when the solid angle isless than 0.03 steradians, resulting in an error of less than one percent. This roughly translates to a distance at least 5 times greater than thelargest dimension of the detector. In general, if you follow the“five times rule” for approximating a point source,you can safely estimate using planar surface area.
Radiant and Luminous FluxRadiant flux is a measure of radiometric power. Flux, expressed inwatts, is a measure of the rate of energy flow, in joules per second. Since photon energy is inversely proportional to wavelength, ultravioletphotons are more powerful than visible or infrared.
Luminous flux is a measure of the power of visible light. Photopicflux, expressed in lumens, is weighted to match the responsivity of thehuman eye, which is most sensitive to yellow-green. Scotopic fluxis weighted to the sensitivity of the human eye in the dark adapted state.
Units Conversion: Power
RADIANT FLUX:
1 W (watt)
LUMINOUS FLUX:
= 683.0 lm at 555 nm 1 J (joule)
= 1700.0 scotopic lm at 507 nm
= 1 W*s (watt * second)
= 107 erg = 0.2388 gram * calories
1 lm (lumen)
= 1.464 x 10-3 W at 555 nm 1 lm*s (lumen * seconds)
= 1/(4p) candela (only if isotropic)
= 1 talbot (T)
= 1.464 x 10-3 joules at 555 nm
Spectroradiometry is the calibrated analysis of lightfrom radiant sources, e.g. the sun, lamps and other light sources.
Photometry involves measurement of radiation visible to the human eye.
Irradiance and Illuminance:Irradiance is a measure of radiometric flux per unit area, or flux density.Irradiance is typically expressed in W/cm 1 0 Math2 (watts per squarecentimeter) or W/m2 (watts per square meter).
Illuminance is a measure of photometric fluxper unit area, or visible flux density.
Illuminance is typically expressed in lux (lumens per square meter) or foot-candles (lumens per square foot).
So, 1 steradian has a projected area of 1 square meter at a distanceof 1 meter. Therefore, a 1 candela (1 lm/sr) light source will similarlyproduce 1 lumen per square foot at a distance of 1 foot, and 1 lumen persquare meter at 1 meter. Note that as the beam of light projects fartherfrom the source, it expands, becoming less dense. In fig. 7.4, for example,the light expanded from 1 lm/ft2 at 1 foot to 0.0929 lm/ft2(1 lux) at 3.28 feet (1 m).
Cosine LawIrradiance measurements should be made facing the source, if possible. The irradiance will vary with respect to the cosine of the angle betweenthe optical axis and the normal to the detector.Calculating Source DistanceLenses will distort the position of a point source. You can solvefor the virtual origin of a source by measuring irradiance at two pointsand solving for the offset distance, X, using the Inverse Square Law:
Figure 7.5 illustrates a typical setup to determine the location ofan LED’s virtual point source (which is behind the LED due to the built-inlens). Two irradiance measurements at known distances from a referencepoint are all that is needed to calculate the offset to the virtual pointsource.
Units Conversion: Flux Density
IRRADIANCE:
1 W/cm2 (watts per square centimeter)
ILLUMINANCE:
= 104 W/m2 (watts per square meter)
= 6.83 x 106 lux at 555 nm = 14.33 gram*calories/cm2/minute
1 lm/m2 (lumens per square meter)
= 1 lux (lx)
= 10-4 lm/cm2 = 10-4 phot (ph) = 9.290 x 10-2 lm/ft2 = 9.290 x 10-2 foot-candles (fc) Radiance and Luminance:Radiance is a measure of the flux density per unit solid viewing angle,expressed in W/cm2/sr. Radiance is independent of distancefor an extended area source, because the sampled area increases with distance,cancelling inverse square losses.
The radiance, L, of a diffuse (Lambertian) surface is related to theradiant exitance (flux density), M, of a surface by the relationship:
Some luminance units (apostilbs, lamberts, and foot-lamberts) alreadycontain p in the denominator, allowing simplerconversion to illuminance units.
Example:
Suppose a diffuse surface with a reflectivity, r,of 85% is exposed to an illuminance, E, of 100.0 lux (lm/m2)at the plane of the surface. What would be the luminance, L, of thatsurface, in cd/m2?
Solution:
1.) Calculate the luminous exitance of the surface:
M = E * rM = 100.0 * 0.85 = 85.0 lm/m2
2.) Calculate the luminance of the surface:
L = M / p
L = 85.0 / p = 27.1 lm/m2/sr= 27.1 cd/m2 Irradiance From An Extended Source:The irradiance, E, at any distance from a uniform extended area source,is related to the radiance, L, of the source by the following relationship,which depends only on the subtended central viewing angle, q,of the radiance detector:
So, for an extended source with a radiance of 1 W/cm2/sr,and a detector with a viewing angle of 3°, the irradiance at any distancewould be 2.15 x 10-3 W/cm2. This assumes, ofcourse, that the source extends beyond the viewing angle of the detectorinput optics.
Units Conversion: Radiance & Luminance
RADIANCE:
1 W/cm2/sr (watts per sq. cm per steradian)
= 6.83 x 106 lm/m2/sr at 555 nm
= 683 cd/cm2 at 555 nm
LUMINANCE:
1 lm/m2/sr (lumens per sq. cm per steradian)
= 1 candela/m2 (cd/m2)
= 1 nit = 10-4 lm/cm2/sr = 10-4 cd/cm2 = 10-4 stilb (sb) = 9.290 x 10-2 cd/ft2 = 9.290 x 10-2 lm/ft2/sr = p apostilbs (asb) = p cd/p/m2 = p x 10-4 lamberts (L) = p x 10-4 cd /p/cm2 = 2.919 x 10-1 foot-lamberts (fL) = 2.919 x 10-1 lm/p/ft2/sr Radiant and Luminous Intensity:Radiant Intensity is a measure of radiometric power per unit solid angle,expressed in watts per steradian. Similarly, luminous intensity isa measure of visible power per solid angle, expressed in candela (lumensper steradian). Intensity is related to irradiance by the inversesquare law, shown below in an alternate form:
If you are wondering how the units cancel to get flux/sr from flux/areatimes distance squared, remember that steradians are a dimensionless quantity. The solid angle equals the area divided by the square of the radius, sod2=A/W, and substitution yields:
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The biggest source of confusion regarding intensity measurements involvesthe difference between Mean Spherical Candela and Beam Candela, both ofwhich use the candela unit (lumens per steradian). Mean sphericalmeasurements are made in an integrating sphere, and represent the totaloutput in lumens divided by 4p sr in a sphere. Thus, a one candela isotropic lamp produces one lumen per steradian.
Beam candela, on the other hand, samples a very narrow angle and is onlyrepresentative of the lumens per steradian at the peak intensity of thebeam. This measurement is frequently misleading, since the samplingangle need not be defined.
Suppose that two LED’s each emit 0.1 lm total in a narrow beam: Onehas a 10° solid angle and the other a 5° angle. The10° LED has an intensity of 4.2 cd, and the 5° LED an intensityof 16.7 cd. They both output the same total amount of light, however-- 0.1 lm.
A flashlight with a million candela beam sounds very bright, but ifits beam is only as wide as a laser beam, then it won’t be of much use. Be wary of specifications given in beam candela, because they often misrepresentthe total output power of a lamp.
1 Equals 0Units Conversion: Intensity
RADIANT INTENSITY:
1 W/sr (watts per steradian)
= 12.566 watts (isotropic)
= 4*p W = 683 candela at 555 nm
LUMINOUS INTENSITY:
1 lm/sr (lumens per steradian)
= 1 candela (cd)
= 4*p lumens (isotropic) = 1.464 x 10-3 watts/sr at 555 nm Converting Between GeometriesConverting between geometry-based measurement units is difficult, and shouldonly be attempted when it is impossible to measure in the actual desiredunits. You must be aware of what each of the measurement geometriesimplicitly assumes before you can convert. The example below showsthe conversion between lux (lumens per square meter) and lumens.Example:
You measure 22.0 lux from a light bulb at a distance of 3.162 meters. How much light, in lumens, is the bulb producing? Assume that theclear enveloped lamp is an isotropic point source, with the exception thatthe base blocks a 30° solid angle.
Solution:
1.) Calculate the irradiance at 1.0 meter:
E1 = (d2 / d1)2 * E2 2.) Convert from lm/m2 to lm/sr at 1.0 m:
E1.0 m = (3.162 / 1.0)2 * 22.0 = 220 lm/m2
220 lm/m2 * 1 m2/sr = 220 lm/sr
3.) Calculate the solid angle of the lamp:
W = A / r2 = 2ph/ r = 2p[1 - cos(a/ 2)]
W = 2p[1 - cos(330/ 2)] = 12.35 sr
4.) Calculate the total lumen output:
220 lm/sr * 12.35 sr = 2717 lm
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