Generally speaking, a Background illuminator is to ensure best details in diagnosis from the cornea to the retina.
There are two major reasons why an imaging slit lamp should have a quality background illumination system:
1. Overview images: for this the diffuser will work but using the background illumination and slit beam (diffused) will produce a shadow free illumination with natural colors and two light reflexes.
2. Slit images: the background illumination is required if the view through the slit lamp is to be reproduced on screen.