When checking vegetables and fruits with a light box, the item to be checked is placed close to or on the light box. The light box is especially useful in checking for insects which are of the same color as the vegetable or fruit, or for insects which are within the thickness of the vegetable or fruit, and not on its surface. Though insects in such locations are difficult to detect normally, they do become visible when they are placed over the light box. In such cases, the presence of the insect shows up as a dark spot upon the vegetable or fruit.
A light box may be purchased at hardware stores that cater to observant Jews or at photography or art supply stores. One may also check vegetables and fruits in natural light, by holding them up to the window. One should not, however, hold them up to the sun, since the brightness of the sun makes checking difficult.