The Fourier transform of a profile is calculated when right clicking on one of the Fourier functions in a profile window. The 1D Fourier window has strong tools for analyzing periodic structures and diagnosing noise or vibration problems. To achieve the highest accuracy it will be an advantage to apply the Fourier X 16 (Requires the Calibration Module) function of the Profile context menu.
The 1D Fourier window has its own context menu activated by the right mouse click:
Cursor On; When right clicking on Cursor On or pressing 'C' on the keyboard the number of cursor pairs will change between zero, 1 and 2 pairs.
The cursors can be used to measure a periodic distance (pitch) by moving a cursor to the first harmonic peak. Then the corresponding wavelength is calculated and written in the text field of the window and next the first cursor, see example below.
The MX/M1 fields are used for testing the harmonic numbers; SPIP automatically divides the wavelength of the different cursors with the wavelength of curser M1. In this example, M1 points to the first harmonic, i.e., the pitch, M2, M3 and M4 are pointing to the 2nd, 3rd and 4th harmonics respectively, which is confirmed by the M2/M1, M3/M1, and M4/M1 fields. The higher harmonics can be used for getting a statically estimate of the pitch by multiplying the wavelength values by their harmonic numbers.
Put Cursors on Peaks (Requires the Calibration Module)
Click this function and the four highest peaks will be detected and indicated by the cursors. When using this automated technique the peak positions will be calculated at sup-sample level by parabolic fits, so that the wavelength calculations shown in the graph will have a much higher accuracy.
Auto Update Cursors on Peaks (Requires the Calibration Module)
This option will cause the Peaks to be detected (at sub-sample level and indicated each time the Fourier transform changes.
dB Scaling; The dB scaling can be used to enhance the weaker details of the Fourier transform. The maximum Fourier value will be set to 0 dB.
Log Log Scaling
When the profile window contains an amplitude spectrum the fractal dimension can be evaluated by the Log Log function. The fractal dimension D is defined as
D = (6 + s)/2,
where s is the (negative) slope of the Log Log plot [John Russ]. The fractal dimension can also be calculated for all directions of an image as part of a roughness analysis.
Freeze Axes; when performing comparisons it can be useful to keep the axis frozen. Like wise it can be an advantage to duplicate a window before entering new data.