Photometry is science of measurement of light, in terms of the perceived brightness to human eye. It is different from radiometry that is the science of measurement of radiant energy (comprising light) in terms of absolute power; rather, in photometry, radiant power at every wavelength is weighted by luminosity function (also known as visual sensitivity function) which models human brightness sensitivity. Classically, this weighting function is photopic sensitivity function, though scotopic function - and others - may also be applied in same way.
Photometry and eye:
Human eye is not evenly sensitive to every wavelengths of visible light. Photometry tries to account for this by weighting calculated power at every wavelength with the factor which signifies how sensitive eye is at that wavelength. Standardized model of eye's response to light as function of wavelength is provided by luminosity function. Note that eye has different responses as the function of wavelength when it is adapted to light situation (photopic vision) and dark situation (scotopic vision). Photometry is normally based on eye's photopic response, and so photometric measurements may not exactly indicate perceived brightness of sources in dim lighting situation where colors are not visible, like under just moonlight or starlight.
Several different units of measure are utilized for photometric measurements. People at times ask why there is require being several different units, or asking for conversions between units which cannot be converted. The adjective bright can refer to light source that delivers the high luminous flux (measured in lumens), or to light source that concentrates luminous flux it has into very narrow beam (candelas), or to light source which is seen against the dark background. Light comprises of several different wavelengths, number of basically different types of light measurement which can be made is large, and so are numbers of quantities and units which represent them.
Photometric versus radiometric quantities:
There are two parallel systems of quantities called as photometric and radiometric quantities. Each quantity in one system has the analogous quantity in other system. Examples of parallel quantities:
In photometric quantities wavelength is weighted according to how sensitive human eye is to it, whereas radiometric quantities utilize unweighted absolute power. For instance eye reacts much more strongly to green light than to red, thus green source will have greater luminous flux than red source with similar radiant flux would. Radiant energy outside visible spectrum doesn't contribute to photometric quantities at all, thus for instance 1000 watt space heater may put out huge deal of radiant flux (1000 watts, in fact), however as light source it puts out very few lumens.
Watts versus lumens:
Watts are units of radiant flux whereas lumens are units of luminous flux. The comparison of watt and lumen shows distinction between radiometric and photometric units. Watt is unit of power. Watts can also be direct measure of output. In the radiometric sense, incandescent light bulb is approx 80% efficient: 20% of energy is lost (example by conduction through lamp base). Remainder is emitted as radiation, generally in infrared. Therefore, 60 watt light bulb emits entire radiant flux of approx 45 watts. Incandescent bulbs are, at times utilized as heat sources, but generally they are utilized for purpose of giving light. As such, they are very ineffective, because most of the radiant energy they produce is invisible infrared. Lumen is explained as amount of light provided in one steradian by point source of one candela strength; while candela, base SI unit, is stated as luminous intensity of source of monochromatic radiation, of frequency 540 terahertz, and radiant intensity of 1/683 watts per steradian. (540 THz corresponds to approx 555 nanometres, wavelength, in green, to which human eye is most sensitive. Number 1/683 was chosen to make the candela about equal to the standard candle, the unit which it superseded). Merging these definitions, conclude that 1/683 watt of 555 nanometre green light gives one lumen.
Photometric measurement methods:
Photometric measurement is based on photodetectors, devices which make and electric signal when exposed to light. Easy applications of the technology comprise switching luminaires on and off based on ambient light situation, and light meters, utilized to determine total amount of light incident on point.
More complex forms of photometric measurement are utilized often within lighting industry. Spherical photometers can be utilized to compute directional luminous flux made by lamps, and comprise of the large-diameter globe with the lamp mounted at its center. The photocell rotates about lamp in three axes, estimating output of lamp from all sides. Luminaires are tested by using goniophotometers and rotating mirror photometers that keep photocell stationary at the enough distance that luminaire can be considered a point source. Rotating mirror photometers utilize motorized system of mirrors to reflect light coming from luminaire in all directions to distant photocell; goniophotometers utilize rotating 2-axis table to change orientation of luminaire with respect to photocell.
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