To expand a little on electures good info.
This 1/8 of inch system is also used with other lamps.
A PAR 38 is a Parabolic reflector lamp 38 - 1/8"s across.
A R40 is a simple reflector lamp 40 - 1/8"s across.
Just recently I ran into T-5 high output lamps on dimming ballasts, had to replace the ballast, not cheap.
