Luminous efficacy

The light yield (english luminous efficacy) is the quotient of the luminous flux emitted by a lamp and the power consumed by her performance and is usually given in the SI derived unit lumens per watt.

Symbols and formulas

The symbol of the luminous efficiency is η, the luminous flux Φ, the power P.

Or

Details

The light output is a connection between the physical quantity (electrical) power and the luminous flux physiological quantity, which depends on an empirically derived spectral sensitivity of the human eye.

By the definition of the luminous flux can not appear brighter than 683 lumens, a radiation source of 1 watt radiant power at a wavelength of 555 nm (green ) for the light -adapted eye. For a dark- adapted eye, the values ​​are at 510 nm ( blue-green) and 1725 lumens. 555 nm and 510 nm, the respective sensitivity maxima of the eye.

For other wavelengths or wavelength ranges, the empirical luminosity curve of the eye must be considered.

The eye takes a light source that radiates uniformly distributed in the range of 380-740 nm, as white light true, but only with a reduced efficiency compared to green by about 30%. Instead of 683 lm of brightness impression is in such a light source then only about 200 lm.

With

- Light sensitivity curve of the eye,

- Wavelength of light.

In the narrow range of 410-705 nm, the efficiency is 36 % and the brightness impression 240 lm:

A black body is an inefficient light source. The right figure shows the radiation power in function of wavelength for various temperatures. The human eye can perceive only a part of the spectrum. At 2000 K accounts for only about 0.2 % of the radiation power on the visible range ( see table). Even at the optimum temperature of 7000 K contribute less than 15 % of the total radiation at the lighting.

The correlation between the weighted by the spectral luminous radiation power (measured in watts ) and the brightness impression (measured in lumens ) produces the photometric radiation equivalent.

In a thermal radiator, the percentage efficiency of the radiation in the visible range to the produced radiation power is strongly dependent on temperature ( see diagram). That is the reason why the light output of incandescent strongly increases with the filament temperature and drops sharply at low voltage.

In the picture, the relative brightness of a light bulb and the filament temperature as a function of the operating voltage is shown. The lower curve is based on the photopic luminosity curve of the eye. It can be seen that the light yield doubled with a 20 % increase in the operating voltage is about, with the lifetime, however, drastically reduced.

Not for thermal emitters, such as gas discharge lamps, the light output is not as efficient physiological, but directly as the light output in lumens per watt ( lm / W) is specified.

For guidance on the purchase of bulbs of the EU energy label energy rating provides information about the respective luminous efficiency of incandescent lamps, fluorescent lamps, halogen lamps. The energy efficiency class A stands for products with high luminous efficiency.

Examples

The item ' light sources ' contains an extensive table with examples of the light yield of several light sources.

Indications:

Overview of basic light sizes

• Measure of lighting technology
• Lighting technologies