It is equal to retinal illuminance produced by a surface whose luminance is one nit when the apparent area of the entrance pupil of the eye is 1 square milimeter.
This complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, and providing an unvignetted field of view of about 11.
All points which lie on a projection line (i.e., a line-of-sight), intersecting with the entrance pupil of the camera, are projected onto a common image point.