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Hello!

Me and my professor are re-simulating a catadioptric capsule endoscope imaging system design created by Mr. Roy Wang and Prof. Qiyin Fang. The design has a separate file for forward and side view raytracing. The forward view looks as following:

Forward view layout

And the side view as following:

Side view layout

The last four lenses are are shared with the forward and side views. My question is about the ray aiming option provided in Zemax. The figures above are obtained with the “Real” ray aiming option and “Float by Stop Size” aperture option. If the ray aiming is turned off, the simulation for the radial view does not even converge:

Side view simulated without ray aiming

For the forward view, the simulation without ray aiming does converge, but the results (spot sizes, MTF) differ significantly from those obtained by Mr. Wang and Prof. Fang. Based on the documentation, I did not thoroughly understood the pupil aberration and circumstanses when the ray aiming option should be used. If it will always produce more accurate results, why not use the ray aiming option all the time? How can one determine when the ray aiming is actually needed and how to choose between paraxial and real ray aiming? These questions are partially addressed in the documentation, but as an inexperienced optics designer, I would be grateful for a clarification.

The second question is a more theoretical than a practical one. We are using this existing design as a basis for our own capsule endoscopy imaging system design and thus trying to figure out the purposes of different lenses and mirrors. The three last lenses, (plano-convex and the doublet) form a structure called Kellner eyepiece. According to my understanding, eyepieces are typically used as the final element in systems such as microscopes or telescopes for producing a distant virtual image that can be viewed with a relaxed eye. So my question is, why would one use an eyepiece here to produce a real image on an imaging sensor? The raytrace diagram of the eyepiece part is shown below (ray aiming turned on):

Raytracing diagram of the Kellner eyepiece

 

Best,

Pyry Kiviharju

Ray aiming accounts for the aberrations of the optics between the entrance pupil and the stop surface. To see if you need it, look at Analyze...Aberrations...Pupil Aberration, and turn ray aiming on if it is more than a few percent. The System Check utility in the Setup ribbon will also advise you on this.

Just use Paraxial Ray Aiming with the Cache turned on. The other settings are remnants of earlier days when the algorithm needed more manual setup than it does now.

  • Mark

I can address your second question. The term eyepiece is partly historical because it was the cluster closest to the eye and was used to do some additional magnifying or whatever was needed. I think the term has just stuck. In your system, the rays from each field only converge after the eyepiece, so it is clearly needed to get the real image that the sensor will then record. No doubt an eyepiece intended for human eyes will be a bit different than one intended for direct use with a sensor, but the terminology is very general here. I looked up the Kellner eyepiece and indeed it looks like it was created to take diverging light rays after a focus and collimate them for re-focus by the eye, but that doesn't have to be its use, as your diagram shows. It is providing the final positive focus to create the real image at the sensor plane.

So in short, and eyepiece is still a very general grouping of lenses, and it can easily be repurposed if it is found suitable as is the case here.

To add to Kevin’s comment, eyepieces are often times designed in reverse, from the long to the short conjugate (see note below about the sign of distortion).  The eye pupil is assumed to be the stop, nominally taken to have a diameter of 5 mm, located some 10-20 mm away from the nearest glass surface in order to provide suitable eye relief -- so it’s obvious that an eyepiece is fundamentally designed to work with the human eye.  However, as Kevin notes, it’s perfectly fine to use the eyepiece as a focusing element as long as its properties satisfy the needs of the application at hand.

Here is what Bentley and Olson say about eyepieces in their SPIE Field Guide to Lens Design:

 

Thank you all for your advice, it is extremely valuable for us and I believe we can now get started.

-Pyry Kiviharju

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