Radiometry

thumb|Albedo change in Greenland: The map shows the difference between the amount of sunlight [[Greenland reflected in the summer of 2011 versus the amount it reflected between 2000 and 2006. Some areas reflect close to 20 percent less light than a decade ago.]] Albedo ( ; ) is the fraction of sunlight that is diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation). Surface albedo is defined as the ratio of radiosity Je to the irradiance Ee (flux per u

thumb|alt=Image of spiderweb bolometer for measurements of the cosmic microwave background radiation.|Spiderweb bolometer for measurements of the cosmic microwave background radiation. Image credit: NASA/JPL-Caltech.

the determination of light intensities of astronomical bodies, including stars

intensity of sunlight

thumb|Table-top spectrophotometer thumb|Beckman IR-1 Spectrophotometer, thumb|right | Beckman Model DB Spectrophotometer (a double beam model), 1960 thumb|Hand-held spectrophotometer used in graphic industry

thumb|upright=1.5|Comparison of photometric and radiometric quantities Radiometry is a set of techniques for measuring electromagnetic radiation, including visible light. Radiometric techniques in optics characterize the distribution of the radiation's power in space, as opposed to photometric techniques, which characterize the light's interaction with the human eye.

thumb|210px|An example of a Crookes radiometer. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. A radiometer is an instrument for measuring radiometric quantities such as radiant flux (power), irradiance, or radiance. Definitions typically limit radiometry to optical radiation, but some definitions include other kinds of electromagnetic radiation. Radiometers typically measure infrared radiation, visible radiation (light), ultraviolet radiation, or some combination of these.

thumb|upright|An optical pyrometer thumb|upright|A sailor checking the temperature of a ventilation system

thumb|right|Blacksmiths work [[iron when it is hot enough to emit plainly visible thermal radiation.]] The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. Thermal radiation is electromagnetic radiation that most commonly includes both visible radiation (light) and infrared radiation, which is not visible to human eyes. A portion of the thermal radiation from very hot objects (see photograph) is easily visible to the eye.

power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy

thumb|255px|An actinometer instrument from the 1800s designed by John Herschel and used to estimate the temperature of the Sun's surface.

energy carried by waves or particles

description in optics of the angular dependency of the radiant intensity of a radiant surface

alt=ISO 9060:2018 Class A Pyrheliometer|thumb|ISO 9060:2018 Class A, fast response & spectrally flat, pyrheliometer thumb|1913 Balloon Borne Pyrheliometer thumb|Typical pyrheliometer, for measurement of direct solar radiation

thumb|Chromaticity Diagram of the LCH color space

In radiometry, irradiance is the radiant flux received by a surface per unit area. The SI unit of irradiance is the watt per square metre (symbol W⋅m−2 or W/m2). The CGS unit erg per square centimetre per second (erg⋅cm−2⋅s−1) is often used in astronomy. Irradiance is often called intensity, but this term is avoided in radiometry where such usage leads to confusion with radiant intensity. In astrophysics, irradiance is called radiant flux.

device that rotates under influence of light

power carried by electromagnetic waves

thumb|An Ellipsometer at Laboratory for Analysis and Architecture of Systems|LAAS-CNRS in Toulouse, France Ellipsometry is an optical technique for investigating the dielectric properties (complex refractive index or dielectric function) of thin films. Ellipsometry measures the change of polarization upon reflection or transmission and compares it to a model.

physical quantity

In radiometry, radiance is the radiant flux emitted, reflected, transmitted or received by a given surface, per unit solid angle per unit projected area. Radiance is used to characterize diffuse emission and reflection of electromagnetic radiation, and to quantify emission of neutrinos and other particles. The SI unit of radiance is the watt per steradian per square metre (). It is a directional quantity: the radiance of a surface depends on the direction from which it is being observed.

thumb|Spectral reflectance curves for aluminium (Al), [[silver (Ag), and gold (Au) metal mirrors at normal incidence]]

albedo subtype

brightening of a rough surface, when illuminated from directly behind the observer

albedo subtype

function of four real variables that defines how light is reflected at an opaque surface

energy transfer through a medium through emission and absorption of electromagnetic radiation

areal density of exiting radiant flux

angle between incident and reflected light in astronomy

incident radiant energy per area

Radiometric device

thickness of an attenuating layer which reduces the value of a quantity to half of its initial value

model for determining radiant energy reflected off diffuse surfaces

measure for the exponential reduction of a quantity along a path due to absorption and scattering

radiometer that measures energy in the one millimetre to meter wavelengths

instrument for measuring the net radiation on Earth's surface

thumb| Simplified representation of a bolometric pixel A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Infrared radiation with wavelengths between 7.5–14 μm strikes the detector material, heating it, and thus changing its electrical resistance. This resistance change is measured and processed into temperatures which can be used to create an image. Unlike other types of infrared detecting equipment, microbolometers do not require cooling.

spaceborne scientific measuring instrument to collect data on global thermal emissions

thumb|right|450px|Example of a pyrgeometer

physical quantity in radiometry

term

physical quantity

power per unit area per unit wavelength of a wavelength concentration, important for a substance's transmittance, reflectivity, and absorbance

thumb|upright=1.0|right|ECOSTRESS radiometer

quotient of the linear photon attenuation coefficient and the mass density

type of cryogenic energy sensor or cryogenic particle detector