If the 1 candela source emitted uniformly in all directions, the total radiant flux would be about 18.40 mW, since there are 4π steradians in a sphere. The frequency of light used in the definition corresponds to a wavelength in a vacuum of 555 nm, which is near the peak of the eye's response to light. By definition, if one constructs a light source that emits monochromatic green light with a frequency of 540 THz, and that has a radiant intensity of 1/683 watts per steradian in a given direction, that light source will emit one candela in the specified direction. Like other SI base units, the candela has an operational definition-it is defined by the description of a physical process that will produce one candela of luminous intensity. Luminous intensity is also not the same as the radiant intensity, the corresponding objective physical quantity used in the measurement science of radiometry. The resulting beam is narrower and brighter, though its luminous flux remains unchanged. If the optics were changed to concentrate the beam into 1/2 steradian then the source would have a luminous intensity of 2 candela. 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 another photometric unit, luminous flux, which is the total perceived power emitted in all directions. Luminous efficacy normalized by the maximum possible efficacyĬomparison of photometric and radiometric quantities Luminous efficiency, luminous coefficient Ratio of luminous flux to power consumption The candela per square metre is sometimes called the nit. Luminous flux per unit solid angle per unit projected source area. The lumen second is sometimes called the talbot. Luminous intensity of artificial light sources is typically measured using and a goniophotometer outfitted with a photometer or a spectroradiometer. The curve which represents the response of the human eye to light is a defined standard function y(λ) or V(λ) established by the Commission Internationale de l'Éclairage (CIE) and standardized in collaboration with the ISO. Light with the same radiant intensity at other wavelengths has a lower luminous intensity. When adapted for bright conditions ( photopic vision), the eye is most sensitive to yellow-green light at 555 nm. The human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. Photometry deals with the measurement of visible light as perceived by human eyes. The SI unit of luminous intensity is the candela (cd), an SI base unit. The candela per square metre is sometimes called the nit.In photometry, 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 standardized model of the sensitivity of the human eye. 10 −4 stilbs (the CGS unit of luminance).

One candela per square metre is equal to: Comparison with other units of luminance HDR displays range from 450 to above 1600 cd/m 2. Most consumer desktop liquid crystal displays have luminances of 200 to 300 cd/m 2. Typically, monitors calibrated for SDR broadcast or studio color grading should have a brightness of 100 cd/m 2. The sRGB spec for monitors targets 80 cd/m 2. Īs a measure of light emitted per unit area, this unit is frequently used to specify the brightness of a display device. The term nit is believed to come from the Latin word nitēre, 'to shine'. The nit (symbol: nt) is a non-SI name also used for this unit (1 nt = 1 cd/m 2). The unit is based on the candela, the SI unit of luminous intensity, and the square metre, the SI unit of area. The candela per square metre (symbol: cd/m 2) is the unit of luminance in the International System of Units (SI). For the unit of information entropy, see Nat (unit).