Measuring a Light Bulb - There are four ways of measuring a bulbs physical dimensions:
1) Bulb shape: bulb shapes vary, from pear shapes to funnel shapes to tube shapes. Each has a designation. Examples include: PAR for parabolic aluminized reflector or T for tube. Many household halogen lights are a MR16, the MR is for ‘metallic reflector’ or ‘mirrored reflector’.
2) Bulb diameter: bulb diameter is measured in 1/8 inches increments at its maximum point of diameter. A PAR30 is 30 one-eighths of an inch in diameter or 3 3/4 inches. A T8 is 8 one-eighths of an inch in diameter or 1 inch. MR16 is 16 one-eighths or 2 inches in diameter.
3) Bulb length: bulb length is measured in inches as the Maximum Overall Length (MOL). For example, a 4 foot fluorescent tube has an MOL of 48 (inches).
>4) Bulb base type: Bases are measured in millimeters. A typical household screw-in base, called a medium base, is actually an E26 (E is for Edison Screw-in). It is 26 millimeters in diameter. An E12 is a candelabra base and is a 12 millimeter diameter Edison screw-in base. For pinned bases like a florescent tube or a pinned MR16 halogen bulb, the measurement is from the center of one pin to the center of the other pin.