Astrophysics (Index) About

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.

```       ∂²Φ
L = ————————
∂A∂Ωcosθ

Φ
≈ ———————
AΩcosθ
```
• L - intensity
• Φ - power in watts
• Ω - solid angle in steradians
• θ - angle between source and normal of the source

This is the common use of the term intensity in astronomy, but in physics, the term is often used all electromagnetic radiation striking a surface in watts per square meter. Thus the Physics 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 also be specific, i.e., per wavelength. The mean intensity can be useful in simplifying models.

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

Referenced by pages:
Balmer jump (BJ)
brightness temperature (TB)
color index
color-magnitude diagram (CMD)
computational astrophysics
continuous absorption
convolution
Eddington approximation
filter
flux density
foreground subtraction
fundamental plane
globular cluster (GC)
grating
HIRAX
imaging Fourier transform spectroscopy (IFTS)
imaging spectrometer
irradiance
Kramers opacity law
light curve
limb
line blanketing
line shape function
Narrabri Stellar Intensity Interferometer (NSII)
opacity (κ)
optical depth (τ)
radiance
radiative flux
reddening
Rosseland mean opacity
equation of radiative transfer (RTE)
SIMSTACK
source function (S)
specific intensity
spectrometer
spectrometry
stellar temperature determination
Stokes parameters
surface brightness profile
Sunyaev-Zel'dovich effect (SZ effect)
temperature
two-stream approximation
Very Small Array (VSA)
Wien's displacement law

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