Fundamental Lighting Definitions
What is a foot-candle [fc]
A foot-candle (abbreviated fc, lm/ft2, or sometimes ft-c) is a non-SI unit of illuminance or light intensity.
Define Luminous Intensity, I-v
Candela, Lumen / Steradian [cd]. Luminous power per unit solid angle. 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.
Define Luminance, L-v
Cd / m², Luminous power per unit solid angle per unit projected source area. Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction.
What is a candela? [cd]
The candela is the SI base unit of luminous intensity; that is, luminous power per unit solid angle emitted by a point light source in a particular direction.
What is a Lumen? [lm]
The lumen is the SI derived unit of luminous flux, a measure of the total "amount" of visible light emitted by a source
Define Luminous Flux, Φ-v
Lumen = Cd*str, Luminous energy per unit time. In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light), in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.
What is luminous energy, Q-v?
Lumen-second, lm*s. In photometry, luminous energy is the perceived energy of light. This is sometimes called the quantity of light.
What is luminous efficacy, η?
Luminous efficacy, Lm/W, is a measure of how well a light source produces visible light. It is the ratio of luminous flux to power.
Define Illuminance, E-v
Lux = Lumens / m², Luminous power incident on a surface. Illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception.
Define Luminous Exitance/Emittance, M-v
Lux, Luminous power emitted from a surface. In photometry, illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of how much the incident light illuminates the surface, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface.
What is luminous exposure, H-v?
Lux-second, lx*s. Illuminance * exposure time.
What is a lux? [lx]
The lux, Lm/m² is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area.
