Kodachrome original was launched by Kodak in 1935. The first practical fluorescent lighting was developed by a group of General Electric scientists researching an improved and practical fluorescent lamp. Under pressure from many competing companies the team designed the first practical and viable fluorescent lamp (U.S. Patent No. 2,259,040) that was first sold in 1938.

So the only available light source for viewboxes back then would probably have been incandescent.