Luminous intensity
Measure of perceived power per unit solid angle.
Luminous intensity is a photometric 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. Its SI unit is the candela (cd), an SI base unit. It is a fundamental quantity in photometry, the science of measuring visible light as perceived by human eyes. The human eye is not equally sensitive to all wavelengths of visible light; under bright (photopic) conditions, it is most sensitive to yellow-green light at a wavelength of 555 nm. Light at other wavelengths, even with the same objective radiant power, will produce a lower luminous intensity. This response is defined by the luminosity function, a standard curve established by the International Commission on Illumination (CIE) and standardized with the ISO. Luminous intensity differs from luminous flux, which is the total perceived power emitted in all directions; intensity is the perceived power per unit solid angle. For example, a lamp emitting one lumen of flux focused into a one-steradian beam yields one candela, while focusing the same flux into half a steradian yields two candela, producing a narrower, brighter beam with unchanged total flux. It also differs from radiant intensity, the objective physical measure used in radiometry. Artificial light sources are typically measured using a goniophotometer with a photometer or spectroradiometer. The candela is defined operationally: a monochromatic green light source with a frequency of 540 THz and a radiant intensity of 1/683 watt per steradian in a given direction emits one candela in that direction. Historically, prior to the candela, various units were used, such as the English candlepower (based on a spermaceti candle of defined weight and burn rate), the Hefnerkerze (based on a Hefner lamp), and the Violle (the first unit independent of a specific lamp). All were superseded by the candela. For monochromatic light, luminous intensity is the product of radiant intensity and the standard luminosity function; for multiple wavelengths, the contributions are summed or integrated across the spectrum.
- field
- Photometry
- unit
- Candela (cd)
- based_on
- Luminosity function V(λ) established by the CIE
- related_to
- Luminous flux, radiant intensity
Lore & Background
Luminous intensity measures the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on a standardized model of human eye sensitivity known as the luminosity function. Its SI unit is the candela, an SI base unit. The human eye, when adapted for bright conditions, is most sensitive to yellow-green light at 555 nanometers; light of the same radiant intensity at other wavelengths produces a lower luminous intensity. The response curve of the eye is a defined standard function established by the International Commission on Illumination (CIE) and standardized with the ISO. Luminous intensity is measured using a goniophotometer equipped with a photometer or spectroradiometer. It should not be confused with luminous flux, which is the total perceived power emitted in all directions. If a lamp has a 1 lumen bulb and optics focus the light evenly into a 1 steradian beam, the beam has a luminous intensity of 1 candela. Concentrating the beam into 1/2 steradian yields 2 candela, producing a narrower, brighter beam with unchanged luminous flux. Luminous intensity also differs from radiant intensity, the corresponding objective physical quantity in radiometry. Historically, before the candela was defined, various units were used, such as the English candlepower (based on a pure spermaceti candle of specified weight and burn rate), the Hefnerkerze in Germany, Austria, and Scandinavia, and the Violle, the first unit independent of a particular lamp. All were superseded by the candela. The candela is operationally defined: a monochromatic green light source emitting at 540 THz with a radiant intensity of 1/683 watt per steradian produces one candela in that direction.
Reader's Guide
The frequency used in the definition of the candela corresponds to the peak of the human eye’s photopic response. If a 1 candela source emitted uniformly in all directions, the total radiant flux would be about 18.40 mW. A typical modern candle produces roughly one candela while releasing heat at about 80 W. Prior to the candela, various units were used, such as the English candlepower (based on a spermaceti candle weighing one sixth of a pound and burning at 120 grains per hour), the Hefnerkerze (based on a Hefner lamp), and the Violle (the first unit not dependent on a particular lamp). All were superseded by the candela. For multiple wavelengths, integration over the spectrum is required. Luminous intensity is distinct from luminous flux, which is total perceived power in all directions; focusing the same flux into a narrower solid angle increases intensity. It also differs from radiant intensity, the objective radiometric quantity. Measurement typically uses a goniophotometer with a photometer or spectroradiometer. The human eye, adapted for bright conditions, is most sensitive to yellow-green light at 555 nm, and the standardized luminosity function defines perceived brightness across wavelengths.
Did You Know?
- The candela is an SI base unit.
- A typical modern candle produces very roughly one candela while releasing heat at roughly 80 W.
- Prior to the candela, units included candlepower, Hefnerkerze, and the Violle.
Frequently Asked Questions
Who is Luminous intensity?
Luminous intensity is a photometric quantity that tells you how much visible light a source pushes out in one particular direction, weighted by the human eye's sensitivity at each wavelength. It sits as a foundational pillar of photometry and is measured in the SI base unit called the candela (cd).
What are Luminous intensity's powers/role?
Its core job is to convert raw optical power per unit solid angle into a number that reflects what a human observer actually perceives in a given direction. The weighting comes from the CIE luminosity function V(λ), which encodes the eye's spectral sensitivity.
Why is Luminous intensity important?
It is a fundamental photometric quantity because it is the bridge between physical electromagnetic output and human visual perception in a specific direction. Without a standardized definition of luminous intensity, comparing how bright different sources appear to the eye would have no common reference.
What are Luminous intensity's connections to other quantities?
It is the directional cousin of luminous flux (total perceived power integrated over all directions) and the perceptual counterpart of radiant intensity (unweighted physical power per solid angle). The CIE luminosity function V(λ) is the shared thread that ties all three together.
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