### intensity

**(radiance)**
(power reaching a surface from a specific source)

**Intensity** in astronomy (commonly called radiance outside astronomy)
is a measure of electromagnetic radiation striking
a surface from a given solid angle, i.e., source.
Alternately, electromagnetic radiation from a surface
radiating within a solid angle can use the same measure.
A common unit is watt per steradian per square meter.

d²Φ
*L* = ————————
d*A*dΩcosθ
Φ
≈ ———————
*A*Ωcosθ

- L - intensity
- Φ - power in watts
- Ω - solid angle in steradians
- θ - angle between source and normal of the source

This is the common way the term *intensity* is used in astronomy.
In physics, the term is often used all electromagnetic
radiation striking a surface in watts per square meter.
Thus the term "radiance" to distinguish meanings.

**Specific intensity** or **spectral radiance** is the intensity
at a specific wavelength.

The **mean intensity** is the average intensity in all directions
from a surface (perhaps within a solid angle), i.e., integrating it
over the angle and dividing by 4π. It can be "specific", i.e.,
per wavelength. Using the mean is a useful way to simplify models.

(*measure,EMR,physics*)
http://en.wikipedia.org/wiki/Radiance

**Referenced by:**

Balmer jump

color index

continuous absorption

convolution

Eddington approximation

filter

foreground subtraction

fundamental plane

globular cluster (GC)

grating

HIRAX

imaging Fourier transform spectroscopy (IFTS)

imaging spectrometer

irradiance

Kramers opacity law

light cone

light curve

line blanketing

line shape function

opacity

optical depth (τ)

Planck function

radiance

radiative flux

reddening

Rosseland mean opacity

equation of radiative transfer (RTE)

Schuster-Schwarzschild model

SIMSTACK

source function (S)

spectrometer

spectrometry

stellar temperature determination

two-stream approximation

Very Small Array (VSA)

Wien's displacement law

index