Spatial Frequencies of the Fourier Transform

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Discussion Overview

The discussion revolves around the concept of spatial frequencies in the context of the Fourier Transform, particularly focusing on how to derive these frequencies from a spatial function and the implications of sampling in numerical simulations. It includes technical aspects of frequency resolution and the relationship between spatial frequencies and wavenumbers.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant explains that the Fourier Transform converts a spatial function ##f(x, y)## into a frequency function ##F(p, q)##, where ##p## and ##q## represent spatial frequencies.
  • Another participant states that the frequency resolution in the frequency domain is given by the formula ##2\pi/N\Delta x##, where ##N## is the number of samples and ##\Delta x## is the sampling interval.
  • A participant questions their understanding of the frequency resolution formula, seeking clarification on the definitions of ##N## and ##\Delta x##.
  • A subsequent reply confirms the participant's understanding of the terms involved in the frequency resolution calculation.
  • A participant inquires about how to determine the boundaries of the frequency domain, leading to the mention of the Nyquist frequency.
  • Another participant introduces the term "wavenumber" as a common alternative to "spatial frequency," prompting further questions about its definition and context.
  • A reply provides a definition of wavenumber, relating it to the wavelength ##\lambda##, and explains the analogy to frequency definitions.

Areas of Agreement / Disagreement

Participants generally agree on the definitions and relationships between spatial frequencies, sampling, and wavenumbers, but there are ongoing questions and clarifications regarding specific terms and their implications.

Contextual Notes

Some participants express uncertainty about the definitions and implications of terms like Nyquist frequency and wavenumber, indicating that further clarification may be needed.

ecastro
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The Fourier Transform transforms a function of space into a function of frequency. Considering a function ##f\left(x, y\right)##, the Fourier Transform of such a function is ##\mathcal{F}\left\{f\left(x, y\right)\right\} = F\left(p, q\right)##, where ##p## and ##q## are the spatial frequencies.

In numerical simulations, the function ##f## can easily be transformed by using an algorithm (built-in in the software). However, I am concerned on acquiring the spatial frequencies ##p## and ##q##. Is there a way to do it?

Thank you in advance.
 
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If the function in space domain is sampled ##N## times with sampling interval ##\Delta x##, then the frequency resolution in the frequency domain is given by ##2\pi/N\Delta x##.
 
blue_leaf77 said:
If the function in space domain is sampled ##N## times with sampling interval ##\Delta x##, then the frequency resolution in the frequency domain is given by ##2\pi/N\Delta x##.
I don't know if I got this right. Is the value of ##N## the number of points, ##\Delta x## is the interval between two adjacent points, and the interval between two points in the frequency domain is ##\frac{2 \pi}{N \Delta x}##?
 
ecastro said:
I don't know if I got this right. Is the value of ##N## the number of points, ##\Delta x## is the interval between two adjacent points, and the interval between two points in the frequency domain is ##\frac{2 \pi}{N \Delta x}##?
Yes, exactly.
 
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Thank you.

I have another question, how will I know the boundaries (maximum and minimum values) of the frequency domain?
 
I'll also throw in that a "spatial frequency" is more commonly called a "wavenumber".
 
boneh3ad said:
I'll also throw in that a "spatial frequency" is more commonly called a "wavenumber".

The wavenumber of what? What is ##\lambda## in this case?
 
ecastro said:
The wavenumber of what? What is ##\lambda## in this case?
The wavenumber of "whatever the spatial frequency belongs to."

The usual way of defining the wavenumber is exactly analogous that for frequency:
##k = 2\pi/\lambda## in radians per length, or ##\nu = 1/\lambda## if the units are cycles per length.
 

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