luminous efficacy

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Luminous efficacy

There are three ways this term can be used: (1) The luminous efficacy of a source of light is the quotient of the total luminous flux emitted divided by the total lamp power input. Light is visually evaluated radiant energy. Luminous flux is the time rate of flow of light. Luminous efficacy is expressed in lumens per watt. (2) The luminous efficacy of radiant power is the quotient of the total luminous flux emitted divided by the total radiant power emitted. This is always somewhat larger for a particular lamp than the previous measure, since not all the input power is transformed into radiant power. (3) The spectral luminous efficacy of radiant power is the quotient of the luminous flux at a given wavelength of light divided by the radiant power at that wavelength. A plot of this quotient versus wavelength displays the spectral response of the human visual system. It is, of course, zero for all wavelengths outside the range from 380 to 760 nanometers. It rises to a maximum near the center of this range. Both the value and the wavelength of this maximum depend on the degree of dark adaptation present. However, an accepted value of 683 lumens per watt maximum at 555 nanometers represents a standard observer in a light-adapted condition. See Luminous efficiency, Luminous flux, Photometry

luminous efficacy

[′lü·mə·nəs ‚ef·ə·kə·sē]

(optics)

The ratio of the total luminous flux in lumens emitted by a light source over all wavelengths to the total radiant flux in watts. Formerly known as luminous efficiency.

The ratio of the total luminous flux emitted by a light source to the power input of the source; expressed in lumens per watt.

luminous efficacy

The ratio of the total emitted luminous flux, in lumens, to the total electric power consumption in watts.

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To compare across these sectors, and because electricity is now and likely in the future the dominant source of energy for lighting, we list in Table 1 luminous efficacies in units of lm/[W.sub.e], calculated as if electricity were the initial energy source.

Note that for the vehicle sector the range of luminous efficacies is not very great, varying from the [nu][psi]veh= 18 lm/[W.sub.e] typical of tungsten incandescent bulbs to the [[nu].sub.[psi][veh]=24 lm/[[W.sub.e] of tungsten-halogen incandescent bulbs (Denton 2004, p.

Finally, given the luminous efficacies and per-capita consumptions of light of the various sectors for a particular nation or group of nations, an aggregate luminous efficacy for all the sectors combined is calculated by averaging the inverse luminous efficacies of each sector weighted by the fraction of light consumed per capita by that sector,

Note that in general the more recent data points have higher luminous efficacies and lower costs of energy.

As for all other estimates, we use estimates of GDP based on Maddison's work (GGDC 2007), EIA estimates for average price of energy, and light-consumption-weighted inverse luminous efficacies, all listed in Table 1.

The world's appetite for light: empirical data and trends spanning three centuries and six continents