
Using the Huygens OTF pot
The Huygens calculator can plot the point-spread function (PSF), the line-spread function (LSF) or the optical transfer function (OTF), as calculated using the Huygens-Fresnel wavelet theory.
The Huygens calculator window shows a plot of LSF or MTF, and a thumbnail (small image) of the PSF. The thumbnail is useful for troubleshooting.

To create the OTF plot window:
- Select from the menu Analysis > OTF > Huygens > vs spatial frequency
The OTF plot
The OTF is displayed as a function of the spatial frequency. It is the fourier transform of the LSF. To calculate the fourier transform, the number of points in each direction must be a power of two. The x scan is plotted as a solid curve. The y scan is plotted as a dashed curve. The curve color is that of the field (see the Sources window).
Options panel
Rays tab
- Field
- Using the Field popup menu, select the field for which you wish to calculate. Select All to calculate for all fields (this can be very time-consuming).
- Wave
- Using the Wave popup menu, select the wave for which you wish to calculate. At present only the monochromatic diffraction OTF may be calculated.
- Pattern
- For the Huygens calculator, the pattern of rays used to sample the pupil may be specified indpendently of the grid used to sample the image plane. Using the Pattern popup menu, select the pupil pattern to use. If the Random pattern is selected, a randomize (Roll) button will appear and the toolbar Randomize icon will be enabled.
- Number of points per side
- The image plane is sampled using a rectangular N by N grid of points. N is the "number of points per side". The points are spaced by "delta" which is the next parameter. If you plot the OTF, the number of spatial frequencies at which the OTF is calculated is N/2 (i.e., up to the Nyquist frequency). These N spatial frequencies are uniformly distributed from zero to the physical limit (the spatial frequency at which the contrast must becomes zero by diffraction theory).
- Image plane sampling delta
- The spacing of the points on the image plane sampling grid. If you change this quantity the width will be recalculated.
- Image plane sampling width
- The width of the sampling grid, equal to the product of image delta and the number of points per side, N. If you change this quantity delta will be recalculated.
Plot tab
- Plot LSF
- If checked, the LSF is plotted. If unchecked the OTF is plotted.
- Units
- Allows selection from a list of length units.
- Plot OTF
- If checked, the OTF is plotted. If unchecked the LSF is plotted.
- Plot diffraction limit
- Plot the diffraction limit curve: the LSF or OTF of an unaberrated system with a circular pupil. n.b., assumes Fraunhofer diffraction.
- Plot real and imaginary parts
- If checked, the real and imaginary parts of the OTF are plotted, as well as its magnitude (MTF).
- Maximum spatial frequency
- The maximum spatial frequency displayed on the plot. Note that the Huygens calculation allows calculation at spatial frequencies beyond the physical range.
- Plot physical range
- Sets the maximum spatial frequency to the physical limit: the spatial frequency that is the reciprocal of the product of the wavelength and the working focal ratio.
- Divide by focal length
- Divide the image space spatial frequency (cycles per unit length) by the focal length of the lens, so that it becomes an angular frequency (cycles per unit angle).
- Spline smoothing
- Interpolates in the calculated data using a spline, to give a smoother looking curve.
PSF tab
- Draw Airy disk
- If checked, the Airy disk is drawn. n.b., this enables image resampling.
- Draw rays
- If checked, the geometrical rays are superimposed on the PSF image. n.b., this enables image resampling.
- Resample image
- Resamples image to center the pattern and make the image a specified minimum resolution, in case you wish to copy it out or save it.
- False-color image
- Colors instead of intensity shading.
- Enhance faint features
- Uses a nonlinear intensity map to amplify weak features.
- Negative image
- Reverse shading for high and low intensity.
Using the thumbnail for troubleshooting
Aliasing |
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Multiple intensity peaks appear when the image analysis patch is too wide for the number of rays traced. What you see is the diffraction pattern from the grid of wavelet sources in the pupil. This will manifest as spurious oscillations in the MTF. Either increase the number of traced rays, or decrease the width of the analysis patch. |
Clipping |
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If the intensity is visible right to the edge, there is information missing that will result in an invalid MTF. Increase the width of the analysis patch. |