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From Transwiki

Parabolic Aluminized Reflector (PAR) lamps have an internal reflective coating made of vapor deposited aluminum. Standard sizes are PAR16, PAR20, PAR30, PAR38, PAR36, PAR56 and PAR64. The diameter in inches of any PAR lamp is simply the PAR number divided by 8.

Changing the filament locations and/or lenticular patterns of the bulb face provides a way to provide a variety of different light dispersions.

There are now LED lamps in a PAR38 package.