Luminous energy

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In photometry, luminous energy is the perceived energy of light. This is sometimes also called the quantity of light.

Luminous energy is not the same as the radiant energy, the corresponding objective physical quantity. This is because the human eye can only see light in the visible spectrum and has different sensitivities to light of different wavelengths within the spectrum. When adapted for bright conditions (photopic vision), the eye is most sensitive to light at a wavelength of 555 nm. Light with the same power at longer or shorter wavelengths has a lower luminous energy.

[edit] Units

The SI unit of luminous energy is the lumen second, which is unofficially known as the Talbot in honor of William Henry Fox Talbot.

In other systems of units, luminous energy may be expressed in basic units of energy.

[edit] See also

• Radiant energy

SI photometry units

Quantity	Symbol	SI unit	Abbr.	Notes
Luminous energy	Qv	lumen second	lm·s	units are sometimes called talbots
Luminous flux	F	lumen (= cd·sr)	lm	also called luminous power
Luminous intensity	Iv	candela (= lm/sr)	cd	an SI base unit
Luminance	Lv	candela per square metre	cd/m2	units are sometimes called nits
Illuminance	Ev	lux (= lm/m2)	lx	Used for light incident on a surface
Luminous emittance	Mv	lux (= lm/m2)	lx	Used for light emitted from a surface
Luminous efficacy		lumen per watt	lm/W	ratio of luminous flux to radiant flux; maximum possible is 683.002 lm/W
SI • Photometry