Figure 1
Illustration of how widefield OCT(A) increases susceptibility to axial alignment error and thereby exacerbates vignetting. (A) With the correct working distance, the OCT beam pivot (yellow dot) is coincident with the pupil plane. (B) With an incorrect working distance, the OCT beam pivot is offset (here by a distance d) from the pupil plane. When the OCT beam is parallel (solid rays) to the optic axis of the eye, there is no vignetting in either situation. Moreover, with the correct working distance, there is no vignetting even when OCT beam scanned to a different position on the retina (dashed rays); this is because although the scanning changes the beam angle, it does not translate the beam at the pupil plane. However, with an incorrect working distance, when the OCT beam is scanned, the beam changes both its angle and transverse position at the pupil plane, the latter of which results in vignetting (red area). Since the amount of translation increases with increasing scan angles, larger OCTA fields-of-view increase the likelihood of vignetting, and make correct instrument alignment increasingly important. Methods for correct instrument alignment are discussed in Part IV.
