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Q&A: Controlling a lens system to be telecentric

Question
I am working on an optical model where I want the system to be telecentric in both object and image space. How do I do this?

Synopsis
Controlling a lens system to be telecentric

Solution
When a system is telecentric in both object and image space, it is called “doubly telecentric”. OSLO actually handles the two conditions separately.

Telecentric in object space:
To make a system telecentric in object space you need to “aim” rays at an infinite entrance pupil. There are 2 ways to do this:

  1. Use Telecentric Ray Aiming mode. This mode essentially ignores the current or “manual” placement of the Aperture Stop and places the Entrance Pupil at infinity.
  2. Use a dummy surface in object space (i.e. surface 1) and place this surface at a very large distance from the object (i.e. 100,000 units). Make this surface the Aperture Stop surface and then “undo” the thickness (maybe using a minus-thickness pickup) before moving to the next surface.

Telecentric in image space:
Since the ray has already passed through the aperture stop, there is no opportunity to apply a generalized ray aiming approach to control the position of the exit pupil. Instead, the slope of individual rays need to targeted (i.e. added as operands in the optimization error function). I would recommend setting the K and L direction cosines to zero for a reference ray at a couple of field points (at the image surface).

Use the direction cosine operands:
nvk(fpt, ray, wvn, srf, cfg) K direction cosine of surface normal
nvl(fpt, ray, wvn, srf, cfg) L direction cosine of surface normal

Example ray definition:
Ray Type = “Reference”, FX = 0.0, FY = 0.0

Example field point definitions:
FBX = 0.0, FBY = 0.0
FBX = 0.0, FBY = 0.7
FBX = 0.0, FBY = 1.0

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