I am trying to design an optical system to produce stereo images of an object with a single camera. To do this, I am going to utilize three mirrors: a reflector at each stereo angle and a knife-edge mirror to combine the two images for the camera detector. To maximize the field of view at the camera, I want to collimate my image before the stereo mirrors and focus the collimated image to the detector after the two images have been combined.
My issue is that I'm not sure what the proper set up of lenses would be to accomplish this. From what I have learned of optics, I believe that a plano-convex lens paired with a plano-concave lens would be able to collimate the image. I could then use another plano-convex lens to focus my collimated image to the detector. Would this combination of lenses accomplish what I think it can?
You don't necessarily need the plano-concave (negative) lens to collimate the light from the object. The object just needs to be placed at the focus of a positive lens. This will image the object at infinity, which is collimated.
A couple things to consider:
1) If your object is 3D, I would suggest you use a slow (long focal length) lens to extend the depth of focus. If you use a fast lens, there is a possibility that light emitted from the front and back of your object will not be collimated.
2) If you're illuminating your object with broadband light for color imagery, I would suggest you use achromatic lenses.
Thank you Milo Hyde for your response to my question! Thank you for the tip regarding achromatic lenses as well. I do have a follow-up question to add to this regarding focusing the collimated image onto the detector. From what I understand, the final focused image would be located at the focus or focal point of the focusing lens. Is this correct?
Also, I image that the size of the final image is dependent on the focal length of both the collimating and the focusing lenses. Is the magnification ratio simply equal to the ratio of the two focal lengths? For example, if I collimate the image with a focal length of 100 mm and focus the image with a focal length of 50 mm, is the final image 1/2 the size of the object?
Yes, an image of your object will be formed at the focus of your focusing lens. This occurs because the object is at infinity. What you are effectively building is a telescope (or a microscope), which in its simplest form consists of two positive lenses.
Yes, on the second question. The transverse magnification is the focusing lens focal length divided by the collimating lens focal length. The image will be inverted. The angular magnification is the inverse of the transverse magnification. For telescopes, most people quote the angular magnification, as telescopes make objects that "appear" small in the sky look much larger. Of course, the objects in the sky are, in reality, very large, so a telescope actually demagnifies.
That makes sense. So in my application, I'll be capturing two images with a single detector. If I reflect two collimated images such that they are parallel (as shown in the attached image), can they be focused with a single lens? My intuition tells me that this would not work, and I would end up with both images occupying the same space on my detector. Is my thinking correct here? In other words, is it best to focus the collimated image before the "joining" central mirror?
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