Hot metalwork from a blacksmith. The yellow-orange glow is the visible part of the thermal radiation emitted due to the high temperature. Everything else in the picture is glowing with thermal radiation as well, but less brightly and at longer wavelengths that the human eye cannot see. A far-infrared camera will show this radiation (See Thermography). wiki stranglesnakejpg|thumb|Thermographic image of a Snake held by a human]] Infrared Thermography, thermal imaging, or thermal video, is a type of

This diagram shows how the peak wavelength and total radiated amount vary with temperature. Although this plot shows relatively high temperatures, the same relationships hold true for any temperature down to absolute zero.

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the light emitted by a glowing incandescent light bulb. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of Radiators and convectors are types of Heat exchangers designed to transfer Thermal energy from one medium to another for the purpose of cooling Electric heating is any process in which electrical energy is converted to heat Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The incandescent light bulb, incandescent lamp or incandescent light globe is a source of electric Light that works by Incandescence, (a general Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. In Physics, a charged particle is a particle with an Electric charge. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny The emitted wave frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine black body is given by Planck’s law of radiation. Frequency is a measure of the number of occurrences of a repeating event per unit Time. In Physics, a black body is an object that absorbs all light that falls on it For a general introduction see Black body. In Physics, Planck's law describes the spectral radiance of Electromagnetic radiation Wien's law gives the most likely frequency of the emitted radiation, and the Stefan–Boltzmann law gives the heat intensity. The Stefan–Boltzmann law, also known as Stefan's law, states that the total Energy radiated per unit surface Area of a Black body in unit

Properties

There are three main properties that characterize thermal radiation:

• Thermal radiation, even at a single temperature, occurs at a wide range of frequencies. How much of each frequency is given by Planck’s law of radiation (for idealized materials). For a general introduction see Black body. In Physics, Planck's law describes the spectral radiance of Electromagnetic radiation This is shown by the curves in the diagram at the right.
• The main frequency (or color) of the emitted radiation increases as the temperature increases. For example, a red hot object radiates most in the long wavelengths of the visible band, which is why it appears red. If it heats up further, the main frequency shifts to the middle of the visible band, and the spread of frequencies mentioned in the first point make it appear white. We then say the object is white hot. This is Wien's displacement law. In the diagram the peak value for each curve moves to the left as the temperature increases.
• The total amount of radiation, of all frequencies, goes up very fast as the temperature rises (it grows as T⁴, where T is the absolute temperature of the body). An object at the temperature of a kitchen oven (about twice room temperature in absolute terms - 600 K vs. 300 K) radiates 16 times as much power per unit area. An object the temperature of the filament in an incandescent bulb (roughly 3000 K, or 10 times room temperature) radiates 10,000 times as much per unit area. Mathematically, the total power radiated rises as the fourth power of the absolute temperature, the Stefan–Boltzmann law. The Stefan–Boltzmann law, also known as Stefan's law, states that the total Energy radiated per unit surface Area of a Black body in unit In the plot, the area under each curve rises rapidly as the temperature increases.

Interchange of energy

Radiant heat panel for testing precisely quantified energy exposures at National Research Council, near Ottawa, Ontario, Canada. Ottawa (ˈɒtəwə or sometimes /ˈɒtəwɑː/ is the Capital of Canada and the country's fourth largest municipality. Ontario (ɒnˈtɛrioʊ is a province located in the central part of Canada, the largest by population and second largest after Quebec Country to "Dominion of Canada" or "Canadian Federation" or anything else please read the Talk Page

Thermal radiation is an important concept in thermodynamics as it is partially responsible for heat exchange between objects, as warmer bodies radiate more heat than colder ones. In Physics, thermodynamics (from the Greek θερμη therme meaning " Heat " and δυναμις dynamis meaning " In thermal physics, heat transfer is the passage of Thermal energy from a hot to a colder body In Physics, a physical body (sometimes called simply a body or even an object) is a collection of Masses taken to be one (Other factors are convection and conduction. Convection in the most general terms refers to the movement of molecules within Fluids (i Heat conduction or thermal conduction is the spontaneous transfer of thermal energy through matter from a region of higher Temperature to a region of lower ) The interplay of energy exchange is characterized by the following equation:

Here, represents spectral absorption factor, spectral reflection factor and spectral transmission factor. All these elements depend also on the wavelength . The spectral absorption factor is equal to the emissivity ; this relation is known as Kirchhoff's law of thermal radiation. The emissivity of a material (usually written \epsilon is the ratio of energy radiated by a particular material to energy radiated by a Black body at See also Kirchhoff's laws for other laws named after Kirchhoff. An object is called a black body if, for all frequencies, the following formula applies:

In a practical situation and room-temperature setting, objects lose considerable energy due to thermal radiation. However, the energy lost by emitting infrared heat is regained by absorbing the heat of surrounding objects. Infrared ( IR) radiation is Electromagnetic radiation whose Wavelength is longer than that of Visible light, but shorter than that of For example, a human being, roughly 2 square meter in area, and about 307 kelvins in temperature, continuously radiates about 1000 watts. The kelvin (symbol K) is a unit increment of Temperature and is one of the seven SI base units The Kelvin scale is a thermodynamic However, if people are indoors, in a room of 296 K, they receive back about 900 watts from the wall, ceiling, and other surroundings, so the net loss is only about 100 watts. Clothes (having poorer thermal conductivity than human skin, therefore reducing the speed of heat loss from the human body to surrounding environment) reduce this loss still further.

If objects appear white (reflective in the visual spectrum), they are not necessarily equally reflective (and thus non-emissive) in the thermal infrared; e. g. most household radiators are painted white despite the fact that they have to be good thermal radiators. Acrylic and urethane based white paints have 93% blackbody radiation efficiency at room temperature (meaning the term "black body" does not always correspond to the visually perceived color of an object).

Calculation of radiative heat transfer between groups of object, including a 'cavity' or 'surroundings' requires solution of a set of simultaneous equations using the Radiosity method. In Mathematics simultaneous equations are a set of Equations containing multiple variables Radiosity is a Global illumination Algorithm used in 3D computer graphics rendering. In these calculations, the geometrical configuration of the problem is distilled to a set of numbers called view factors, which give the proportion of radiation leaving any given surface that hits another specific surface. In Radiative heat transfer, a view factor F_{A \rarr B} is the proportion of all that radiation which leaves surface A and strikes surface B These calculations are important in the fields of solar thermal energy, boiler and furnace design and raytraced computer graphics. Solar thermal energy ( STE) is a technology for harnessing Solar energy for Thermal energy ( Heat) A boiler is a closed vessel in which Water or other Fluid is heated A furnace is a device used for Heating The name derives from Latin fornax, Oven. In Computer graphics, ray tracing is a technique for generating an image by tracing the path of Light through pixels in an Image plane

Formula

Thermal radiation power of a black body per unit of area, unit of solid angle and unit of frequency ν is given by

This formula mathematically follows from calculation of spectral distribution of energy in quantized electromagnetic field which is in complete thermal equilibrium with the radiating object. Area is a Quantity expressing the two- Dimensional size of a defined part of a Surface, typically a region bounded by a closed Curve. The solid angle, Ω, is the angle in three-dimensional space that an object Subtends at a point Frequency is a measure of the number of occurrences of a repeating event per unit Time. In Thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium Mechanical equilibrium, and

Integrating the above equation over ν the power output given by the Stefan–Boltzmann law is obtained, as:

Further, the wavelength , for which the emission intensity is highest, is given by Wien's Law as:

For surfaces which are not black bodies, one has to consider the (generally frequency dependent) emissivity correction factor ε(υ). The Stefan–Boltzmann law, also known as Stefan's law, states that the total Energy radiated per unit surface Area of a Black body in unit This correction factor has to be multiplied with the radiation spectrum formula before integration. The resulting formula for the power output can be written in a way that contains a temperature dependent correction factor which is (somewhat confusingly) often called ε as well:

Constants

Definitions of constants used in the above equations:

	Planck's constant	6. The Planck constant (denoted h\ is a Physical constant used to describe the sizes of quanta. 626 0693(11)×10-34 J·s = 4. 135 667 43(35)×10-15 eV·s
	Wien's displacement constant	2. 897 7685(51)×10–3 m·K
	Boltzmann constant	1. Bridge from macroscopic to microscopic physics Boltzmann's constant k is a bridge between Macroscopic and microscopic physics 380 6505(24)×10−23 J·K-1 = 8. 617 343(15)×10−5 eV·K-1
	Stefan–Boltzmann constant	5. The Stefan–Boltzmann constant (also Stefan's constant) a Physical constant denoted by the Greek letter σ, is the Constant of proportionality 670 400(40)×10−8 W·m-2·K-4
	Speed of light	299,792,458 m·s-1

Variables

Definitions of variables, with example values:

	Temperature	Average surface temperature on Earth = 288 K
	Surface area	Acuboid = 2ab + 2bc + 2ac; Acylinder = 2π·r(h + r); Asphere = 4π·r2

See also

• Thermography
• Infrared photography

External links

• Free Black Body Emission Calculator
• Heat Transfer
• Thermal Radiation
• Atmospheric Radiation