Incandescent/Halogen Lamps 2 of 15
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The common household bulb shape is referred to as "A" bulb e.g. A-19 or A-21. The number refers to the diameter of the bulb (at its widest point) measured in 1/8 ths of an inch, i.e. A-19 is 19/8 ths of an inch or 2 3/8 inch in diameter.
Reflector lamps
"Reflector lamps" or R-lamps, used as spotlights and floodlights,
have built-in reflectors to focus the light into a beam.
Other reflector lamps, such as parabolic aluminized reflector (PAR) and
bulged reflector (BR) lamps, have similar built in reflectors but slightly
different shapes. PAR 38 is a Parabolic Aluminized Reflector 38 eighths
of an inch in diameter.
Other Bulb Shapes
Straight sided (S) and tubular (T) shapes are designed for use in areas
where space may be limited. Still other bulb shapes are designed to be
decorative. .
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