> > Isn't it deposited metal from the filament that blackens the glass? > > No. > > Therei is no filament. The Rubidium lamps are excited with a UHF field which is external to the bulb. I didn't know that: How interesting! Doesn't it take a greate amount of RF power to excite cold Rb? FYI: There's a HP 5065 on eBay: 190060656439