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.
- 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 is distinct from 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. If the beam is concentrated into 1/2 steradian, the luminous intensity becomes 2 candela, resulting in a narrower, brighter beam with unchanged luminous flux. Luminous intensity also differs from radiant intensity, the corresponding objective physical quantity in radiometry.
Reader's Guide
This frequency 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), 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 All were superseded by the candela.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.
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.
More in Optics And Photonics 1-24
Elsewhere in the Optics And Photonics universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record