Optical imaging detects wavelengths in the optical, ultraviolet, or infrared spectrum to image whole specimens. For example, specific genes, cells, or organisms can be “tagged” with a gene encoding one of the luciferase enzymes that enable some bacteria, insects, and animals to glow. When the tagged entity is active, it glows. The emitted light corresponds to the number and location of the tagged entities. This information enables scientists to non-invasively observe the spread of disease or the effects of a drug throughout the system.