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 measurements indicate the total power of all electromagnetic waves emitted,
independent of the eye's ability to perceive it. A lux is one lumen per square meter.
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,
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.
Explanation 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. 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 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
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. On September 1, 2010, European Union legislation
came into force mandating that lighting equipment must be labelled primarily in terms of lumens,
instead of watts of electric power consumed. This change is a result of the EU's Eco-design
Directive for Energy-using Products. 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.
Projector output ANSI lumens
The light output of 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.
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. 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". Peak lumens
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.
Color light output Brightness 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. Experts advise projector buyers to make certain that white light output
and color light output are equal for optimum viewing experiences.
SI photometry units
See also Brightness
Lux Luminous efficacy
Nit Notes and references