Luminous intensity (photometry)
Amount of visible light power emitted by a source in a specific direction per unit solid angle, weighted for human eye sensitivity. SI base unit: candela; used in lighting, displays and standards.
Luminous intensity is a photometric quantity that describes how much visible light a source emits in a particular direction per unit solid angle. It differs from pure radiometric power because it is weighted to reflect the spectral sensitivity of the average human eye: contributions at different wavelengths are adjusted by a standardized luminosity function so that perceived brightness, not sheer energy, is represented. See the spectral weighting commonly called the photopic response: wavelength weighting.
Definition and units
By definition, luminous intensity is luminous flux per unit solid angle. Its SI base unit is the candela (cd), formally defined in terms of a monochromatic source and a precise radiant intensity. In practice the photometric quantity is related to radiometric power (radiant power) through the eye's sensitivity curve. The candela is one of the SI base units and is established by international agreement: see the SI system for context SI base unit.
Characteristics and measurement
Luminous intensity is directional: it is measured per steradian, so two sources with the same total emitted light can have very different intensities if their light is concentrated into narrower beams. Instruments used to measure or describe luminous intensity include photometers, goniophotometers and integrating spheres. Measurements account for angular distribution (beam pattern) and spectral weighting to produce values meaningful for human vision.
Applications and distinctions
- Lighting design and standards: specifying lamp and fixture outputs, beam distributions and safety lighting.
- Display and automotive engineering: headlamp intensity, projector brightness and contrast rely on directional luminous intensity metrics.
- Distinction from related terms: luminous flux (lumens) measures total visible power; illuminance (lux) measures flux per unit area; radiant intensity is the radiometric counterpart that does not include eye sensitivity.
Because human vision changes with adaptation level, different weighting functions are used for low-light (scotopic) or intermediate (mesopic) conditions, which can lead to different practical evaluations of brightness. Luminous intensity remains a central, practical concept in photometry because it combines directionality with human perception to inform lighting design, regulation and equipment specification.
Questions and answers
Q: What is luminous intensity?
A: Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per solid angle unit.
Q: How is luminous intensity measured?
A: Luminous intensity is measured based on the luminosity function, a standardized model of the sensitivity of the human eye.
Q: What is the standardized model of the sensitivity of the human eye called?
A: The standardized model of the sensitivity of the human eye is called the luminosity function.
Q: What is the SI unit of luminous intensity?
A: The SI unit of luminous intensity is the candela (cd).
Q: Is the candela an SI base unit?
A: Yes, the candela is an SI base unit.
Q: Can luminous intensity be used to determine brightness of a light source?
A: Yes, luminous intensity can be used to determine the brightness of a light source as it measures the power emitted per solid angle unit.
Q: Why is luminous intensity important in photometry?
A: Luminous intensity is important in photometry as it allows for accurate measurement of the brightness of a light source, taking into account the sensitivity of the human eye.
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Author
AlegsaOnline.com Luminous intensity (photometry) Leandro Alegsa
URL: https://en.alegsaonline.com/art/59904