The ratio of the total luminous flux to the radiant flux is called the luminous efficacy. Light outside the visible band does not contribute. The luminous flux is a weighted sum of the power at all wavelengths in the visible band. The luminous flux accounts for the sensitivity of the eye by weighting the power at each wavelength with the luminosity function, which represents the eye's response to different wavelengths. In other systems of units, luminous flux may have units of power. One lumen is defined as the luminous flux of light produced by a light source that emits one candela of luminous intensity over a solid angle of one steradian.ġ lm = 1 cd × 1 sr The SI unit of luminous flux is the lumen (lm). It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. In photometry, luminous flux or luminous power is the measure of the perceived power of light. Integrating sphere used for measuring the luminous flux of a light source. The photopic includes the CIE 1931 standard (solid), the Judd-Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). The resulting beam is narrower and brighter, though its luminous flux remains unchanged.Photopic (black line) and scotopic (green line) luminosity functions. If the optics were changed to concentrate the beam into 0.5 steradians then the source would have a luminous intensity of 2 candelas. If a lamp has a 1 lumen bulb and the optics of the lamp are set up to focus the light evenly into a 1 steradian beam, then the beam would have a luminous intensity of 1 candela. Luminous intensity is the perceived power per unit solid angle.
Luminous intensity should not be confused with luminous flux, which is the total perceived power emitted in all directions. However, when calculating radiant intensity, the contributions of each frequency of electromagnetic radiation are treated equally, but for luminous intensity, the contributions of each frequency of light are weighted according to the standard luminosity function.Įssentially, luminous intensity is a measure of the visible component of a radiant intensity. Luminous intensity is analogous to radiant intensity. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye.
The human eye, when adapted for bright conditions, is most sensitive near this frequency. The frequency of electromagnetic radiation chosen for the definition of the candela is in the green region of the visible spectrum. The definition does not specify the luminosity function for weighting radiation at other frequencies. Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardised model of the sensitivity of the human eye across the visible range of the electromagnetic spectrum. In effect it is the ratio of the luminous flux to radiant intensity. Luminous efficacy is a measure of the brightness of a light source, as perceived by the human eye, relative to the light source’s radiant power. The effect of this definition is that one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10 12 hertz and that has a radiant intensity in that direction of 1⁄ 683 W sr -1. Inverting this relation gives an exact expression for the candela in terms of the defining constants h, Δ ν Cs and K cd : The definition of the candela implies the exact relation K cd = 683 kg –1 m –2 s 3 cd sr for monochromatic radiation of frequency ν = 540 × 10 12 Hz. One candela is equal to one lumen per steradian. The unit lm W ‑1 is equal to cd sr W −1, or kg −1 m −2 s 3 cd sr, where the kilogram, metre and second are defined in terms of h, c and Δ ν Cs. The candela, symbol cd, is the SI base unit of luminous intensity in a given direction.īy definition, one candela is equal to one lumen per steradian, where the lumen is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10 12 Hz, K cd, to be 683 when expressed in the unit lm W ‑1.
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