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The measure of the luminous flux emitted by a source in a particular direction per unit solid angle, measured in candela (cd).

The measure of the luminous intensity per unit area of light traveling in a given direction. Measured in candela per square meter (cd/m^2).

The quantity of radiation emitted per unit solid angle per unit projected area of the source, measured in watts per square meter per steradian (W/(m^2⋅sr)).

The total luminous flux incident on a surface, per unit area. Measured in lux (lx) or foot-candles.

Defines the efficiency of light under low-light levels, where human vision is dominated by rod cells in the eye. Measured relative to a standard response curve.

The total luminous flux emitted from a surface per unit area. Measured in lumens per square meter (lm/m^2).

Refers to the total amount of visible light emitted by a source, regardless of direction. Not tied to a single unit, often expressed in terms of luminous energy.

The SI base unit of luminous intensity; defines the luminous power emitted by a light source in a specific direction. Equivalent to one lumen per steradian (1 cd = 1 lm/sr).

Power per unit solid angle emitted by a source in a particular direction, measured in watts per steradian (W/sr). Often used in radiometry.

The measure of the perceived power of light. Measured in lumens (lm).

Function describes how well the power of light at different wavelengths is perceived by the human eye, no direct unit but a relative scale, often given as a function of wavelength V(λ).

The total amount of light received per unit area at a surface. Measured in lux seconds (lx⋅s).

The ratio of luminous flux to power, indicating the efficiency of a light source in terms of light produced. Measured in lumens per watt (lm/W).

The quantity of visible light energy emitted by a source over time. Measured in lumens times seconds (lm⋅s), often termed 'lumen seconds'.