The exit pupil (ExP) is the image of the aperture stop formed by the optical elements that lie between the stop and the image plane. It serves as the common opening through which all rays leave the optical system, and its centre is located at the intersection of the chief rays in image space.

During collimator-based testing, the collimator objective Lcol and the imaging objective of the device under test Ldut combine to form a single imaging system, in which the target image produced by Lcol becomes the object for Ldut. The two objectives must be aligned on a common axis to avoid introducing aberrations, and the exit pupil of Lcol should be efficiently coupled with the entrance pupil (EnP) of Ldut. This condition is referred to as pupil matching.

Figure 1. Exit pupil of the collimator's collimating objective and entrance pupil of the DUT's imaging objective

Pupil matching is a fundamental requirement in optical system design. In a collimator, the exit pupil serves as the common opening through which all rays leave the system. Because the target inside the collimator has a finite size and is not an ideal point source, the output beam is divergent across the entire field of view.

When the pupils are mismatched, several problems can arise.

First, if the entrance pupil of the DUT is located behind the exit pupil of the collimator, a portion of the marginal rays emerging from the exit pupil will be blocked by the entrance pupil and fail to enter the DUT objective, resulting in additional energy loss. Even when the beam fully fills the entrance pupil at the centre of the field, it may fail to do so at the edges, causing vignetting and further non-uniformity in the image-plane illumination of the DUT. Placing the exit pupil and entrance pupil on the same plane minimises this energy loss.

Second, the aberrations of a DUT objective typically worsen as the aperture area increases. If the entrance pupil has a larger diameter than the exit pupil, the incident beam cannot fully fill the entrance pupil, forcing the DUT to operate at an effectively higher f-number than its design value. This artificially suppresses lens aberrations and produces spatial frequency response measurements that are erroneously high, diverging from the DUT's real-world performance. The exit pupil diameter should therefore be no smaller than that of the entrance pupil.

Figure 2. The exit pupil of the collimator objective should be larger in diameter than the entrance pupil of the DUT imaging objective, and the two should be coplanar.

In summary, when selecting a collimator, the exit pupil diameter should be larger than the entrance pupil diameter of the DUT so that the incident beam fully fills the entrance pupil with some margin to spare, and the two pupils should be placed on the same plane wherever possible.