Collimated backlight illumination
This technique produces a collimated beam of light, and is sometimes referred to as telecentric illumination. This form of backlighting is useful for measurement tasks that require a homogeneous illumination with no scattered light. It produces a distinct silhouette even when imaging transparent items.
When using a standard backlight, stray light can sometimes contaminate the edges of an object which produces an indistinct outline that is difficult to measure accurately.