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The lumen (symbol: lm) is the unit of luminous flux, a measure of the total quantity of visible light emitted by a source per unit of time, in the International System of Units (SI). 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, this weighting is standardized by the CIE and ISO.[2] One lux is one lumen per square metre.
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 1,000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1,000 lux. The same 1,000 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.
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.[16] 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.[17] 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 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".[18]
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.[19]
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.
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The Lumen 850 Duo offers versatility, high lumen output, and a wide feature set ready for any adventure. This light features UltrAspire 3D Lighting which casts shadows across uneven surfaces giving you a better understanding of your terrain. The Lumen 850 Duo comes with a headlamp strap allowing the light to clip in and double as a headlamp. Equipped with 3 regular modes, high, medium, low and a red light mode allows complete control over your lumen output. The large power button makes toggling through the setting intuitive and also includes a battery indicator light that when toggled on quickly illuminates green, gold, or red indicating battery level. It can be adjusted up or down in the bracket to your preferred lighting angle. The belt is built with an ErgoFit Waist design that sits comfortably on the body. Open cell foam placed behind the light bracket makes this light comfortable and helps to wick and dissipate moisture. This waist light includes one of our UltrAspire rechargeable 18650 batteries and features a USB Type-C charging port for a very quick charge. Includes a USB Type-C charging cable.
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