A technique positioning a diffuse illuminator behind the object to give a silhouette. This technique is normally used if 'through holes' or shapes are of interest, or the object is generally opaque with darker/brighter variations in the areas of interest.
The images show how the fill level of a bottle is being checked. The first image was taken under normal domestic lighting while the second image was captured using a high intensity backlight to create a clear image of the liquid surface which shows up as a dark line in the camera image that is then very easily detected by the software.
Another common use for backlights is part presence verification. In the images the component has been transformed into a binary image that is much easier to examine for simple defects such as bent or missing connectors or for major damage.