Also spelled roundel. Circular glass color media used in compartmentalized striplights or borderlights, sometimes called "Xrays".
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.
While some may refer to them as lenses, rondels have no magnifying power, although are available in a variety of finishes: smooth, stippled, and 50° spread.
An exception is the Corona PAR3 and PAR4 fixtures by L&E: