Figure 1. The conventional double-tier cage arrangement for monkeys makes adequate, uniform illumination impossible. A flashlight has to be used to read the chest tatoos and to assess the well-being of animals housed in the relatively dark lower-row cages. A flashlight also has to be used to check the cleanliness of the interior of the lower-row cages, and to check the contents of their drop pans.

Viktor Reinhardt, Ph.D., D.V.M. "Lighting Conditions For Laboratory Monkeys: Are They Adequate?" Animal Welfare Information Center Newsletter, 8(2):3-6, Summer 1997.