![]() ![]() 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 For uniform, isotropic light source, the luminous intensity I v in candela (cd) is equalĭivided by the solid angle Ω in steradians (sr): ![]()
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