In the field of heat transfer

Heat transfer

Heat transfer is a discipline of thermal engineering that concerns the exchange of thermal energy from one physical system to another. Heat transfer is classified into various mechanisms, such as heat conduction, convection, thermal radiation, and phase-change transfer...

, intensity of radiation is a measure of the distribution of radiant heat flux per unit area and solid angle

Solid angle

The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...

, in a particular direction, defined according to


where

• is the infinitesimal source area
• is the outgoing heat transfer from the area
• is the solid angle
  Solid angle
  The solid angle, Ω, is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large that object appears to an observer looking from that point...
  
   subtended by the infinitesimal 'target' (or 'aperture') area
• is the angle between the source area normal vector and the line-of-sight between the source and the target areas.

Typical units of intensity are W·m^-2·sr^-1.

Intensity can sometimes be called radiance

Radiance

Radiance and spectral radiance are radiometric measures that describe the amount of radiation such as light or radiant heat that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction. They are used to characterize both emission from...

, especially in other fields of study.

The emissive power of a surface can be determined by integrating the intensity of emitted radiation over a hemisphere surrounding the surface:


For diffuse emitters, the emitted radiation intensity is the same in all directions, with the result that


The factor (which really should have the units of steradian

Steradian

The steradian is the SI unit of solid angle. It is used to describe two-dimensional angular spans in three-dimensional space, analogous to the way in which the radian describes angles in a plane...

s) is a result of the fact that intensity is defined to exclude the effect of reduced view factor at large values ; note that the solid angle corresponding to a hemisphere is equal to steradians.

Spectral intensity

Specific radiative intensity

Specific intensity is a fundamental quantity used in theoretical physics that fully describes the field of classical electromagnetic radiation of any kind, including thermal radiation and light. It is a description based on radiometry rather than Maxwellian electromagnetic fields or photon...

is the corresponding spectral measurement of intensity; in other words, the intensity as a function of wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

.