Notes:

One of the great difficulties in working with the eye, that doesn't occur with man-made optical systems, is that we cannot easily get access to the optical image formed by the eye in order to measure it. Therefore we have to go about our business a bit more indirectly, as illustrated on this slide.

Normally we think of light rays entering the eye and forming an image on the retina. If the object is a distant point of light, like a star, then these rays of light are parallel when they enter the eye and get refracted to form a perfect point image on the retina.

Another way to say the same thing is that a point source emits waves of light in the same way that a pebble dropped into a still pond generates circular waves that propagate outwards. If the point source is far enough away, by the time the rays enter the eye they will be essentially flat, in the shape of a plane. Since rays of light are just lines we draw perpendicular to wavefronts, saying that the waves are planar is the same as saying that the rays are parallel. Once inside the eye the waves change shape to become spherical as they come to focus at a point.

Now imagine the process in reverse. Suppose some of the light from a point on the retinal image reflects back out of the eye. If the eye is perfect, the emerging wavefront will be a plane wave and the rays will be parallel to each other. However, if the eye is aberrated then the emerging wavefront will be distorted away from the perfect shape of a plane wave.

Naturally the reflected light will be very dim, but anyone who has ever had their flash photographs ruined by a red spot covering the normally black pupil of their subject's eye will appreciate that it is possible to capture photographically the light reflected back out of the eye if the flash is bright enough. A system for using this reflected light to analyze the wavefront and its aberrations is shown in the next slide.