The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. 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 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 luminous intensity would still be one candela in those directions that are not obscured. If the source were partly covered by an ideal absorbing hemisphere, that system would radiate half as much luminous flux-only 2π lumens. Alternatively, an isotropic one-candela light-source emits a total luminous flux of exactly 4π lumens. 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♱ sr = 1 lm). Lumens are related to lux in that one lux is one lumen per square metre. 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. 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. The old name "candle" is still sometimes used, as in foot-candle and the modern definition of candlepower. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured. A common wax candle emits light with a luminous intensity of roughly one candela. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function (a model of the sensitivity of the human eye to different wavelengths). The candela (/kænˈdɛlə/ or /kænˈdiːlə/ symbol: cd) is the base unit of luminous intensity in the International System of Units (SI) that is, luminous power per unit solid angle emitted by a point light source in a particular direction. The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, Kcd, to be 683 when expressed in the unit lm W –1, which is equal to cd sr W –1, or cd sr kg –1 m –2 s 3, where the kilogram, metre and second are defined in terms of h, c and ΔνCs. The luminous intensity I v in candela (cd) is equal to the luminous flux Φ v in lumens (lm),ĭivided by 2 times pi times 1 minus cosine of half the apex angle θ in degrees (º): The solid angle Ω in steradians (sr) is equal to 2 times pi times 1 minus cosine of half the You can also use the following lumen-to-candlepower and vice versa conversion calculator to convert one quantity to another.For uniform, isotropic light source, the luminous intensity I v in candela (cd) is equalĭivided by the solid angle Ω in steradians (sr): Thus, to convert from candlepower to lumen, you can multiply the candlepower measurement with 12.57, and to convert from lumen to candlepower, you can simply divide the lumen measurement by 12.57.įor example, a 40-watt lamp rated at 500 lumen has a candlepower of (500 ÷ 12.57) = 39.8 cd. While the formula to convert candlepower to lumen is given as, Hence, the formula to covert lumen to candlepower can be obtained as following, Therefore, a light source which uniformly radiates an intensity of one candela in all directions is given by following, The solid angle in a full sphere is equal to 4 Π steradians. Thus, a lumen can be expressed mathematically as, From its definition, a source of light emitting one candela of luminous intensity uniformly over a solid angle of one steradian is said to be emitting one lumen. However, by applying mathematics, a close-enough approximation can be obtained.įor all practical purposes, candlepower equates directly to candela. How to Convert Lumen to Candlepower and Vice Versaįrom the above discussion, it is clear that lumen and candlepower measure two different things, and therefore, accurately converting one to another is not possible.
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