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Luminous efficacy can be normalized by the maximum possible luminous efficacy to a dimensionless quantity called luminous efficiency.The distinction between efficacy and efficiency is not always carefully maintained in published sources, so it is not uncommon to see "efficiencies" expressed in lumens per watt, or "efficacies" expressed as a percentage.
The luminous efficacy of lamps is the number of lumens emitted for each watt of electric power used. ... 650 800 800 800 910 950 Luminous efficacy (lm/W) 14.3 14.42 ...
The lumen (symbol: lm) is the unit of luminous flux, a measure of the perceived power of visible light emitted by a source, in the International System of Units (SI). Luminous flux differs from power ( radiant flux ), which encompasses all electromagnetic waves emitted, including non-visible ones such as thermal radiation ( infrared ).
Φ v is the luminous flux, in lumens; Φ e,λ is the spectral radiant flux, in watts per nanometre; y (λ), also known as V(λ), is the luminosity function, dimensionless; λ is the wavelength, in nanometres. Formally, the integral is the inner product of the luminosity function with the spectral power distribution. [2]
I e is the radiant intensity in watts per steradian (W/sr), y ¯ ( λ ) {\textstyle {\overline {y}}(\lambda )} is the standard luminosity function . If more than one wavelength is present (as is usually the case), one must sum or integrate over the spectrum of wavelengths present to get the luminous intensity:
Radiant intensity, a radiometric quantity measured in watts per steradian (W/sr) Luminous intensity, a photometric quantity measured in lumens per steradian (lm/sr), or candela (cd) Irradiance, a radiometric quantity, measured in watts per square meter (W/m 2) Intensity (physics), the name for irradiance used in other branches of physics (W/m 2)
Watts are units of radiant flux while lumens are units of luminous flux. A comparison of the watt and the lumen illustrates the distinction between radiometric and photometric units. The watt is a unit of power. We are accustomed to thinking of light bulbs in terms of power in watts.
"Candlepower" is largely an obsolete term. However, people still sometimes use it to describe the luminous intensity of high powered flashlights and spotlights.