Thanks for contributing to Parf Edhellen!
Do you feel like the attested word calar ‘lamp’ or kalatta ‘lamp’ > calath. As for raeth, I don’t think it actually means the kind of sphere you are looking for, the full gloss “extent, reach; region, sphere, district, scope” from a √RAHA “stretch forward”, which suggests to me that raeth has nothing to do with roundness in particular.
Gilruin , Gilruin
The Sindarin word for lightbulb. It is derived from "calad" (meaning light, specifically inner light as opposed to "galad" (reflected light)) and "raeth" (meaning sphere).