The single backscattered light should be selectable by accepting the signal only from sufficiently late time. The calibration is applied as follows. In practice, however, there may be some important limitations. The entire mirror is illuminated with light from a very large solid angle. If the mirror is perfectly clean, a pixel should see only the diaphragm-size disk illuminated by light from its direction in the case of diffuse light.
Dust particles anywhere on the mirror, however, can scatter light to the pixel.
The diffuse light through the diaphragm does not all hit the mirror. A pixel can receive photons that scatter off the walls, ceiling, and floor. The camera diffusively reflects light from all directions and all parts of the mirror. Any pixel will see the illuminated camera reflected in the mirror. The first two contaminants can be estimated by placing an opaque disk between the pixel and mirror that just eclipses the diaphragm as seen from that pixel.