Photopic (black) and scotopic [1] (green) luminosity functions. The photopic includes the CIE 1931 standard [2] (solid), the Judd-Vos 1978 modified data [3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [4] (dotted). The horizontal axis is wavelength in nm.

Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of light in terms of absolute power; rather, in photometry, the radiant power at each wavelength is weighted by the luminosity function that models human brightness sensitivity. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... In telecommunication and physics, radiometry is the science of radiation measurement. ... The luminosity function is a standard function established by the Commission Internationale de lÉclairage to account for the variable sensitivity of the human eye to radiation at different wavelengths. ...

Photometry and the eye

The human eye is not equally sensitive to all wavelengths of light. Photometry attempts to account for this by weighting the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength. The standardized model of the eye's response to light as a function of wavelength is given by the luminosity function. Note that the eye has different responses as a function of wavelength when it is adapted to light conditions (photopic vision) and dark conditions (scotopic vision). Photometry is based on the eye's photopic response, and so photometric measurements will not accurately indicate the perceived brightness of sources in dim lighting conditions. // A human eye. ... The wavelength is the distance between repeating units of a wave pattern. ... The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ... The luminosity function is a standard function established by the Commission Internationale de lÉclairage to account for the variable sensitivity of the human eye to radiation at different wavelengths. ... Photopic vision is the vision of the light-adapted eye; in many animals, color vision, mediated by cone cells. ... Scotopic vision is the monochromatic vision of the eye in dim light. ...

Photometric quantities

Many different units of measure are used for photometric measurements. People sometimes ask why there need to be so many different units, 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 light measurement that can be made is large, and so are the numbers of quantities and units that represent them. The lumen (symbol: lm) is the SI unit of luminous flux. ... Photopic (black) and scotopic [1] (green) luminosity functions. ...

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Look up si, Si, SI in Wiktionary, the free dictionary. ... luminous energy : visible radiant energy, visible light ; units are lumen*seconds or Talbots (T = lm*s) ; lumen = candela*steradian ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Look up second in Wiktionary, the free dictionary. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Look up second in Wiktionary, the free dictionary. ... In photometry the Talbot (T) is a nonstandard unit of luminous energy. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... Photopic (black) and scotopic [1] (green) luminosity functions. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... This article is about the unit of measure. ... Illuminance is the total luminous flux incident per unit area. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... In photometry, illuminance is the total luminous flux incident on a surface, per unit area. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... A square metre (US spelling: square meter) is by definition the area enclosed by a square with sides each 1 metre long. ... The lux (symbol: lx) is the SI derived unit of illuminance or illumination. ... Luminous efficacy is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is usable for human vision. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... The lumen (symbol: lm) is the SI unit of luminous flux. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... Luminous flux or luminous power is the measure of the perceived power of light. ...

Photometric versus radiometric quantities

There are two parallel systems of quantities known as photometric and radiometric quantities. Every quantity in one system has an analogous quantity 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)

In photometric quantities every wavelength is weighted according to how visible it is, while radiometric quantities use unweighted absolute power. For example, the eye responds much more strongly to green light than to red, so a green source will have higher luminous flux than a red source with the same radiant flux would. Light outside the visible spectrum does not contribute to photometric quantities at all, so for example 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). Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. ... Luminous flux is a measure of the energy emitted by a light source in all directions. ... Luminous flux or luminous power is the measure of the perceived power of light. ... Luminous intensity is a measure of the energy emitted by a light source in a particular direction. ... In physics, intensity is a measure of the time-averaged energy flux. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ...

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Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. ...

Watts (radiant flux) versus 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 power. 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 amount of light given into one steradian by a point source of one candela strength; 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). The steradian (ste from Greek stereos, solid) is the SI derived unit of solid angle, and the 3-dimensional equivalent of the radian. ... Look up point source in Wiktionary, the free dictionary. ... A nanometre (American spelling: nanometer, symbol nm) is a unit of length in the metric system, equal to one thousand-millionth of a metre, which is the current SI base unit of length. ...

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 of 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.

Photometric measurement techniques

Photometric measurement is based on photodetectors, devices (of several types) that produce an electric signal when exposed to light. Simple applications of this technology include switching luminaires on and off based on ambient light conditions, and light meters, used to measure the total amount of light incident on a point. Photosensors or photodetectors appear in several varieties: Photoresistors or Light Dependant Resistors (LDR) which change resistance when illuminated Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated Photodiodes which can operate in photovoltaic mode or photoconductive mode Phototubes containing a photocathode which emits...

More complex forms of photometric measurement are used frequently within the lighting industry. Spherical photometers are used to measure the directional luminous flux produced by lamps, and consist of a large-diameter globe with a lamp mounted at its center. A photocell rotates about the lamp in three axes, measuring the output of the lamp from all sides. A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. ...

Luminaires (known to laypersons simply as light fixtures) are tested using goniophotometers and rotating mirror photometers, which keep the photocell stationary at a sufficient distance that the luminaire can be considered a point source. Rotating mirror photometers use a motorized system of mirrors to reflect light emanating from the luminaire in all directions to the distant photocell; goniophotometers use a rotating 2-axis table to change the orientation of the luminaire with respect to the photocell. In either case, luminous intensity is tabulated from this data and used in lighting design.

Non-SI photometry units

Luminance

• Footlambert
• Millilambert
• Stilb

The unit of measurement of brightness (in the USA). ... Stilb can refer to: Mullet (haircut) Stilb (unit), a CGS unit of luminance. ...

Illuminance

• Foot-candle
• Phot

A foot-candle (sometimes designated footcandle; abbreviated fc, lm/ft², or sometimes ft-c) is a non-SI unit of illuminance or light intensity. ... This page is a candidate to be moved to Wiktionary. ...

See also

• Reflectivity
• List of light sources
• Spectrometer
• Radiometer

In optics, reflectivity is the reflectance (the ratio of reflected power to incident power, generally expressed in decibels or percentage) at the surface of a material so thick that the reflectance does not change with increasing thickness; , the intrinsic reflectance of the surface, irrespective of other parameters such as the... A Standard Household Light bulb This page is a list of sources of light. ... A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum. ... A Radiometer is a device used to measure the radiant flux or power in Electromagnetic radiation. ...

External links

• Radiometry and photometry FAQ Professor Jim Palmer's Radiometry FAQ page (University of Arizona).