An optical alignment where the lower edge of a lens is tilted away from the focal plane or the viewer. While common in specific corrective eyewear adjustments, in projection and luminaire optics, an unintended retroscopic tilt can introduce severe astigmatism and focal distortion across the beam field.
Heard on the job
The mounting bracket got bent in transit, giving the primary lens a retroscopic tilt that threw the whole beam out of whack.