### Spectral Density

(strength of power or energy as a function of frequency)

**Spectral Density** is a term used in signal processing and
Time Series Analysis. (Similar terms are used to describe
the distribution of Electromagnetic Radiation or other Frequency-related
phenomena.)

**Power Spectral Density** (**PSD**) is the distribution of power
as described by the Fourier Transform of the auto-correlation
function of a signal. It shows the strength of the variations
in power as a function of frequency.

**Energy Spectral Density** (**ESD**) is similar,
taken over a specific interval of time.

The terms *energy* and *power* are, in a sense, "borrowed", used
because one is an amount and the other a rate of the same concept
over time, analogous to how the terms are used for physical energy.

The terms can be used for analysis of a continuous signal, at least
in theory, but modern electronics and data collection is generally
digital and records and analyzes a discrete signal formed by sampling,
taking periodic samples from an incoming signal (such as a voltage
output from a Radio Telescope antenna over time), thus time
series analysis. In the discrete case, the analysis is on a
sequence of numbers, using computer algorithms, e.g., for the Fourier
analysis, using the Fast Fourier Transform
(calculating the **Discrete Fourier Transform**).

(*statistics,analysis,function*)
http://en.wikipedia.org/wiki/Spectral_density

http://www.fysik.lu.se/fileadmin/fysikportalen/UDIF/Bilder/Signal_analysis_in_astronomy.pdf

http://adsabs.harvard.edu/full/2002ASPC..278..123H

http://user.it.uu.se/~ps/SAS-new.pdf

**Referenced by:**

Antenna Temperature

Magnetic Energy Spectrum

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