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

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

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

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 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'.

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

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

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 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)).

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).

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.

Defines the efficiency of light under well-lit conditions, tailored to the human eye's cone cell response. Measured relative to a typical daylight response curve.