Negative-studier's aid

Wikipedia Word definitions in Wikipedia
A lightbox is a translucent surface illuminated from behind, used for situations where a shape laid upon the surface needs to be seen with high contrast.

Wiktionary Word definitions in Wiktionary
n. 1 A box containing a source of light and a white, translucent cover, used to view photographic transparencies. 2 A box containing fluorescent tubes or LED technology, used to treat people with seasonal affective disorder (SAD) and circadian rhythm disorders. ...