T3D Re: Med. Prefocis base (TECH-3D digest 544)

[bobh@xxxxxxxxxxxxxxxxxx]

RE: Older projection lamps.  Rob wants to know what med. 
prefocus base meant. I think I described it as having two tabs or 
ears for keying into socket. Its name grew like Topsy, i.e. ordinary 
household lamps have a MEDIUM screw base. (As opposed to 
Miniature, Small or Large). The projection lamp is same diameter 
without the threads (screw) and prefocus means the filament is at 
the right height and centering to match the intended optical system 
of the projectors that use it.
 About measuring lamps found in the field..we used to have trouble 
with that..e.g. a lamp bulb has a tolerance as to bulb height overall. 
On small exciter lamps used in the sound head of a motion picture 
projector..some engineers grabed a lamp and made the socket and 
lamp house just fit it. Alas they got a lamp that was on the short 
side, so most exciter lamps of the nominal type wouldn't fit. Since 
they had already done the tooling and produced projecters, the 
Lamp maker had to add yet another lamp to the catalog. Some old 
catalogs I had did have drawings of the different filament types but 
even if near to scale not showing dimensions. One oddity was that 
in lamp engineering up to 1955 anyway, bulbs and bases were 
measured in inches..and internal parts like filaments and stems  
and supports in mm. So Rob maybe you should call Headquarters 
of some of the makers and ask for their techical information. 
Sylvania now owned by Osram of Germany, and Westinghouse 
Lamp as mentioned before by Norelco. (North American Phillps). 
  I think Sylvania is Owensboro, KY headquartered. Phillips is 
probably New York State..I doubt that our Bloomfield, NJ 
headquarters still exists other than as a lamp plant if at all.
 (All this so we can find lamps substitutes for our stereo projectors!)
BobH 
Bell and Howell movie projector lamps had a B&H ring base with a 
long vertical key parallel to the bulb. Fortunately, this base did not 
get into the TDC line (Three Dimension Company a sub of B&H 
formed to do stereo at NY World's fair of 1939-40.)
Bob Howard (bobh@xxxxxxxxxxxxxxxxxx)
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