Also spelled roundel. Circular glass color media used in compartmentalized striplights or borderlights, sometimes called "X-rays".
Kopp Glass was/is probably the most common manufacturer.
Colors are limited to Primary Red/Blu/Grn, Amber, and Clear. Diameter varies from about 3" to 8"; the most common is 5-5/8" which fit the Altman R40 striplight. Kliegl Bros., Hub Electric Company, and Major Equipment Company also produced fixtures that used roundels.
While some may refer to them as lenses, rondels have no magnifying power, although are available in a variety of finishes: smooth, stippled (diffusion), and 50° spread.
An exception is the Corona PAR3 and PAR4 fixtures by L&E: