Radiance

For other uses, see Radiance (disambiguation).

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. In Optics, radiometry is the field that studies the Measurement of Electromagnetic radiation, including Visible light. The solid angle, Ω, is the angle in three-dimensional space that an object Subtends at a point They are used to characterize both emission from diffuse sources and reflection from diffuse surfaces. Diffuse reflection is the reflection of Light from an uneven or granular surface such that an incident ray is seemingly reflected at a number of angles The SI unit of radiance is watts per steradian per square metre (W·sr^-1·m^-2). The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space M^2 redirects here For other uses see M². CM2 redirects here

1 Description
2 Definition
3 Intensity
4 See also
5 External links

Description

Radiance characterizes total emission or reflection, while spectral radiance characterizes the light at a single wavelength or frequency. The radiance is equal to the sum (or integral) of all the spectral radiances from a surface. Spectral radiance has SI units W·sr^-1·m^-3 when measured per unit wavelength, and W·sr^-1·m^-2·Hz^-1 when measured per unit frequency interval.

Radiance is useful because it indicates how much of the power emitted by an emitting or reflecting surface will be received by an optical system looking at the surface from some angle of view. In this case, the solid angle of interest is the solid angle subtended by the optical system's entrance pupil. In an optical system the entrance pupil is a virtual aperture that defines the area at the entrance of the system that can accept light Since the eye is an optical system, radiance and its cousin luminance are good indicators of how bright an object will appear. Eyes are organs that detect Light, and send signals along the Optic nerve to the visual areas of the brain Luminance is a photometric measure of the density of Luminous intensity in a given direction For this reason, radiance and luminance are both sometimes called "brightness". This usage is now discouraged — see Brightness for a discussion. Brightness is an attribute of Visual perception in which a source appears to emit or reflect a given amount of Light. The nonstandard usage of "brightness" for "radiance" persists in some fields, notably laser physics. Laser science or laser physics is a branch of Optics that describes the theory and practice of Lasers Laser science is principally concerned with

The radiance divided by the index of refraction squared is invariant in geometric optics. In Mathematics and Theoretical physics, an invariant is a property of a system which remains unchanged under some transformation. This means that for an ideal optical system in air, the radiance at the output is the same as the input radiance. This is sometimes called conservation of radiance. For real, passive, optical systems, the output radiance is at most equal to the input, unless the index of refraction changes. As an example, if you form a demagnified image with a lens, the optical power is concentrated into a smaller area, so the irradiance is higher at the image. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit The light at the image plane, however, fills a larger solid angle so the radiance comes out to be the same assuming there is no loss at the lens.

Definition

Radiance is defined by

$L = \frac{\mathrm{d}^2 \Phi}{\mathrm{d}A\,\mathrm{d}{\Omega} \cos \theta} \approx \frac{\Phi}{\Omega A \cos \theta}$

where

the approximation holds for small A and Ω,

L is the radiance (W·m^-2·sr^-1),

Φ is the radiant flux or power (W),

θ is the angle between the surface normal and the specified direction,

A is the area of the source (m²), and

Ω

is the solid angle (sr). The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. M^2 redirects here For other uses see M². CM2 redirects here The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space In Radiometry, radiant flux or radiant power is the measure of the total power of Electromagnetic radiation (including Infrared, The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. In Geometry and Trigonometry, an angle (in full plane angle) is the figure formed by two rays sharing a common Endpoint, called Area is a Quantity expressing the two- Dimensional size of a defined part of a Surface, typically a region bounded by a closed Curve. M^2 redirects here For other uses see M². CM2 redirects here The solid angle, Ω, is the angle in three-dimensional space that an object Subtends at a point The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space

The spectral radiance (radiance per unit wavelength) is written L_λ and the radiance per unit frequency is written L_ν.

Intensity

Radiance is sometimes, confusingly, called intensity. This usage is not common in physics or optics, but is sometimes encountered in other fields (notably astrophysics and astronomy). Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. Astrophysics is the branch of Astronomy that deals with the Physics of the Universe, including the physical properties ( Luminosity, Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study Intensity has many other meanings in physics, with the most common being power per unit area. In Physics, intensity is a measure of the time-averaged Energy Flux.

External links

International Lighting in Controlled Environments Workshop

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SI radiometry units
Quantity	Symbol	SI unit	Abbr. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit Luminance is a photometric measure of the density of Luminous intensity in a given direction See also Lambert's cosine law If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the apparent brightness of the surface to an observer Etendue or étendue is a property of an optical system, which characterizes how "spread out" the Light is in area and angle	Notes
Radiant energy	Q	joule	J	energy
Radiant flux	Φ	watt	W	radiant energy per unit time, also called radiant power
Radiant intensity	I	watt per steradian	W·sr⁻¹	power per unit solid angle
Radiance	L	watt per steradian per square metre	W·sr⁻¹·m⁻²	power per unit solid angle per unit projected source area. Radiant energy is the Energy of Electromagnetic waves The quantity of radiant energy may be calculated by integrating Radiant flux (or power The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity The joule (written in lower case ˈdʒuːl or /ˈdʒaʊl/ (symbol J) is the SI unit of Energy measuring heat, Electricity In Physics and other Sciences energy (from the Greek grc ἐνέργεια - Energeia, "activity operation" from grc ἐνεργός In Radiometry, radiant flux or radiant power is the measure of the total power of Electromagnetic radiation (including Infrared, The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. In Radiometry, radiant intensity is a measure of the The watt (symbol W) is the SI derived unit of power, equal to one Joule of energy per Second. The steradian (symbol sr) is the SI unit of Solid angle. It is used to describe two-dimensional angular spans in three- Dimensional space M^2 redirects here For other uses see M². CM2 redirects here Sometimes confusingly called "intensity".
Irradiance	E, I	watt per square metre	W·m⁻²	power incident on a surface. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit Sometimes confusingly called "intensity". In Physics, intensity is a measure of the time-averaged Energy Flux.
Radiant exitance / Radiant emittance	M	watt per square metre	W·m⁻²	power emitted from a surface. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit
Radiosity	J or J_λ	watt per square metre	W·m⁻²	emitted plus reflected power leaving a surface
Spectral radiance	L_λ or L_ν	watt per steradian per metre³ or watt per steradian per square metre per hertz	W·sr⁻¹·m⁻³ or W·sr⁻¹·m⁻²·Hz⁻¹	commonly measured in W·sr⁻¹·m⁻²·nm⁻¹
Spectral irradiance	E_λ or E_ν	watt per metre³ or watt per square metre per hertz	W·m⁻³ or W·m⁻²·Hz⁻¹	commonly measured in W·m⁻²·nm⁻¹

In the fields of Optics and Heat transfer, radiosity is the total emitted and reflected radiation leaving a surface 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 The metre or meter is a unit of Length. It is the basic unit of Length in the Metric system and in the International The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. Irradiance, radiant emittance, and radiant exitance are Radiometry terms for the power of Electromagnetic radiation at a surface per unit

Dictionary

-noun

the quality of being radiant, shining, bright or splendid
(physics) the flux of radiation emitted per unit solid angle in a given direction by a unit area of a source

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