Luminous efficacy

<p>Luminous efficacy is a measure of how well a light source produces visible light. It is the ratio of <a href="/facts/Luminous_flux/Sfya3RXh">luminous flux</a> to <a href="/facts/Power_(physics)/mkaEyRnn">power</a>, measured in <a href="/facts/Lumen_(unit)/DbdghwnN">lumens</a> per <a href="/facts/Watt/VK8oqRrm">watt</a> in the <a href="/facts/International_System_of_Units/S7YLglNz">International System of Units</a> (SI). Depending on context, the power can be either the <a href="/facts/Radiant_flux/ak2d0btz">radiant flux</a> of the source's output, or it can be the total power (electric power, chemical energy, or others) consumed by the source.
Which sense of the term is intended must usually be inferred from the context, and is sometimes unclear. The former sense is sometimes called luminous efficacy of radiation, and the latter luminous efficacy of a light source or overall luminous efficacy.
</p><p>Not all wavelengths of light are equally visible, or equally effective at stimulating human vision, due to the <a href="/facts/Spectral_sensitivity/F4V3Kkww">spectral sensitivity</a> of the <a href="/facts/Human_eye/a24ua4fy">human eye</a>; radiation in the <a href="/facts/Infrared/44XdJNcc">infrared</a> and <a href="/facts/Ultraviolet/kncBUqn5">ultraviolet</a> parts of the spectrum is useless for illumination. The luminous efficacy of a source is the product of how well it converts energy to electromagnetic radiation, and how well the emitted radiation is detected by the human eye.
</p>

Luminous efficacy open-in-new

Luminous efficacy