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This article is about units and quantities of light.
People sometimes ask why there need to be so many different units of light measurement, or ask for conversions between units that can't be converted (lumens and candelas, for example). We are familiar with the idea that the adjective "heavy" can refer to weight or density, which are fundamentally different things. Similarly, the adjective "bright" can refer to a lamp which delivers a high luminous flux (measured in lumens), or to a lamp which concentrates the luminous flux it has into a very narrow beam (candelas). Because of the ways in which light can propagate through three-dimensional space, spread out, become concentrated, reflect off shiny or matte surfaces, and because light consists of many different wavelengths, the number of fundamentally different kinds of kinds of light measurement that can be made is large, and so are the numbers of quantities and units that represent them.
Photometric and radiometric quantities
There are two parallel systems of quantities known as photometric and radiometric quantities. Every quantity in one system has an analogous unit in the other system. Some examples of parallel quantities include:
- Luminance (photometric) and radiance (radiometric)
- Luminous flux (photometric) and radiant flux (radiometric)
- Luminous intensity (photometric) and radiant intensity (radiometric)
This article is concerned with photometric quantities and units.
Photometric quantities are concerned with light as perceived by the human eye. In photometric quantities, every wavelength is weighted according to how visible it is. For example, infrared light is invisible to the eye, so infrared radiation is not included in any photometric measurements. A 1000 watt space heater may put out a great deal of radiant flux (1000 watts, in fact), but as a light source it puts out very few lumens (because most of the energy is in the infrared, leaving only a dim red glow in the visible).
Watts (radiant flux) vs. lumens (luminous flux)
A comparison of the watt and the lumen illustrates the distinction between radiometric and photometric units.
The watt is a unit of energy. We are accustomed to thinking of light bulbs in terms of power in watts. But power is not a measure of the amount of light output. It tells you how quickly the bulb will increase your electric bill, not how effective it will be in lighting your home. Because incandescent bulbs sold for "general service" all have fairly similar characteristics, power is a guide to light output, but only a rough one.
Watts can also be a measure of output. In a radiometric sense, an incandescent light bulb is about 80% efficient; 20% of the energy is lost (e.g. by conduction through the lamp base) The remainder is emitted as radiation. Thus, a 60 watt light bulb emits a total radiant flux of about 45 watts.
Incandescent bulbs are, in fact, sometimes used as heat sources, (as in a chick incubator), but usually they are used for the purpose of providing light. As such, they are very inefficient, because most of the radiant energy they emit is invisible infrared. There are compact fluorescent bulbs that say on their package that they "provide the light of a 60 watt bulb" while consuming only 15 watts.
The lumen is the photometric unit of light output. Although most consumers still think of light in terms of power consumed by the bulb, in the U. S. it has been a trade requirement for several decades that light bulb packaging give the output in lumens. The package of a 60 watt incandescent bulb indicates that it provides about 900 lumens, as does the package of the 15 watt compact fluorescent.
The lumen is defined as one candela per steradian; while the candela, a base SI unit, is defined as the luminous intensity of a source of monochromatic radiation, of frequency 540 terahertz, and a radiant intensity of 1/683 watts per steradian. (540 THz corresponds to about 555 nanometres, the wavelength, in the green, to which the human eye is most sensitive. The number 1/683 was chosen to make the candela about equal to the standard candle, the unit which it superseded).
Combining these definitions, we see that 1/683 watt of 555 nanometre green light provides one lumen.
The relation between watts and lumens is not just a simple scaling factor. We know this already, because the 60 watt incandescent bulb and the 15 watt compact fluorescent both provide 900 lumens.
The definition tells us that 1 watt of pure green 555 nm light is "worth" 683 lumens. It does not say anything about other wavelengths. Because lumens are photometric units, their relationship to watts depends on the wavelength according to how visible the wavelength is. Infrared and ultraviolet radiation, for example, are invisible and do not count. One watt of infrared radiation (which is where most of the radiation from an incandescent bulb falls) is worth zero lumens. Within the visible spectrum, wavelengths are light are weighted according to a function called the "photopic spectral luminous efficiency." According to this function, 700 nm red light is only about 4% as efficient as 555 nm green light. Thus, one watt of 700 nm red light is "worth" only 27 lumens.
SI light units
SI radiometry units
Non-SI light units
Non-SI radiometry units
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