Marginal and Chief Rays in an optical system

The picture is from an MIT lecture but the concepts are explained in many optical texts. The chief/principal rays go through the center of the aperture stop, hit the edge of the field stop, and the angle between the two outside chief rays gives the angular field of view.

My question is: what happens to a ray that starts outside the field of view, but does not go through the center of the aperture stop / pupils? As in, it starts outside the entrance window but is neither a marginal nor a chief ray. I assume it would get stopped somewhere, but how and where?

  • 1
    $\begingroup$ the other rays hit the stops; in a microscope these are typically blackened in order to suppress reflections. $\endgroup$ Apr 9 '16 at 23:29
  • $\begingroup$ We are including so called "optical baffles" in technical optics, so that stray light that does not participate in the actual imaging process doesn't get reflected into the exit window (which is usually the photodetector). When that happens the image will have ghost images and a foggy appearance, which destroys the signal to noise ratio of the image. Baffle design is both important and non-trivial, especially in diffraction limited optics, where the diffraction on the apertures and the baffle can't be neglected. $\endgroup$
    – CuriousOne
    Apr 10 '16 at 0:48
  • $\begingroup$ Interesting. @PeterDiehr, that's what I thought but is there any theory to explain which stop such a ray would hit? On a similar note and something I thought of overnight, if one moves the location of the aperture stop, does that mean it moves the chief ray and therefore all the image locations, including the field stop? That seems odd - the image location being depended on the aperture stop. $\endgroup$
    – Mark Ucka
    Apr 10 '16 at 7:23
  • $\begingroup$ @MarkUcka: briefly, note the axis of symmetry, the dotted line, passing thru the centers of each lens and aperture. Thus you only need to trace two rays to solve the system; symmetry takes care of the rest. Trace a few other rays thru a simpler system to get a feel for it. Some texts construct a proof. $\endgroup$ Apr 10 '16 at 11:35
  • $\begingroup$ @MarkUcka: of course the actual design process is much more complicated, and requires many steps. For example, the bending of the rays at each lens has to be determined, and of course the system must actually accomplish its design goals. For example, see this microscope objective design solution. $\endgroup$ Apr 10 '16 at 12:27

Chief rays are stopped by the FIELD STOP and marginal rays are stopped by the APERTURE STOP, or pupil. There is only one aperture stop and one true field stop in an optical system. The detector is the final and true field stop and the designated aperture stop is the one true STOP. Images of the aperture stop are called pupils and images of the field stop are called glare stops (or field stops). The entrance pupil is the image of the aperture stop as viewed from object space and the exit pupil is the image of the aperture stop as viewed from image space. Internal field or glare stops will occur at intermediate image planes within the optical train. The aperture stop will determine the F/# (along with the focal length) and the field stop will determine the overall field of view. I hope that helps. Pupils and images are one of the most difficult things to master but once you do master them, they are very powerful concepts. They are actually Fourier transforms of each other.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.