It does matter what shape a light bulb is. The shape of the light bulb is important because it affects how the light is cast. Some bulb shapes provide more focused light while others give off a diffused glow in all directions. Plus, light bulb shape can impact the light fixture’s design value. For instance, flame-shaped candle bulbs can work well in a chandelier. Here’s what you need to know about light bulb shape codes. Most standard screw-base light bulbs—such as the GE 60 watt incandescent light bulb—are A bulbs. A-19 and A-21 light bulbs, which are most 60 watt and 75 watt light bulbs respectively, are two common examples of A bulbs. BR denotes a bulged reflector bulb. BR bulbs typically have two funnel-shaped reflector areas, one behind the other, to help shape and focus the light. They’re narrow at the bottom and then flair out for a wider top. A BT code can indicate a bulged tube or chimney light bulb—essentially just wide tubes sometimes with an even wider middle. It can also stand for a blunt tip light bulb—the ones that are flame-shaped with a rounded end. In addition, BT light bulbs can stand for bent tip, which is the exact opposite of a blunt tip. To minimize the confusion, many companies just spell out blunt tip or bent tip on the package rather than relying on the code. CIR tells you it’s a circular or circline lamp. In other words, it’s a fluorescent tube made in a circle. T-12 and T-8 fluorescent tubes are two fairly common examples of bulbs in this shape. Another is the T-10 tubular incandescent light bulb that is commonly used in the gallery light fixtures that mount to a picture’s frame. Variations on the R shape include BR—bulged reflector bulbs, PAR bulbs—which have a parabolic shape, and MR—mirrored reflector bulbs. Two familiar examples are T-12 and T-8 fluorescent tubes. The older T-12 tubes are 1 1/2 inches in diameter while the newer T-8 tubes are 1 inch in diameter. A common A-19 light bulb is 2 3/8 inches in diameter, and an R-30 flood light is 3 3/4 inches across its widest part.
