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Q&A: Vignetting

Question
“Which tools within TracePro are you aware of for the detection of vignetting? What is your strategy?

One (clumsy) way which comes to my mind would be the following:

  1. check that the object plane (which is to be imaged) is really imaged by the system to the plane of observation by applying a point source in the object plane on the optical axis using TracePro’s ” Grid ray trace” with the option ” Beam orientation method: Diverge from point (Fans)”. This ray trace should result in an image point in the plane of observation.
  2. Apply a surface source in the object plane. The extent of the surface source has to match the required FOV of the imaging system. After ray tracing, observe the shape of the source image in the plane of observation.

Synopsis
Vignetting

Solution
“The method you have outlined will work for analyzing vignetting using TracePro. It will require tracing a great many rays to get a smooth distribution in the observation plane but will give you a continuous distribution as a result. You should define importance sampling from the source to the optical system to improve the sampling.

I suggest an alternative: define a grid source diverging from a point that overfills the “entrance” to the optical system. For a conventional imaging system, you would overfill the aperture stop. To develop a graph of vignetting point by point, select a sequence of points in the object plane for which you want to analyze vignetting. Using the “Beam orientation method: Diverge from point (Fans)” feature, enter the coordinates of each object point and do a ray-trace. You may need to move the grid coordinates as you move the “diverge from” point to be sure that the optical system remains overfilled with rays. You should also keep constant the distance from the “diverge from” point to the grid so that the object f-number remains constant. For each new point, record the total flux reaching the observation plane. Repeat this procedure for each of the points you have selected. Each new flux value becomes a new point on the vignetting curve. If you choose many points, you may wish to write a macro program to automate the process.

There will be noise due to the finite number of rays traced. As always, you can reduce this noise by tracing more rays. “

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