One of the most important things to know if you're looking to buy a LED light bulb is the shape. When it comes to light bulbs, bulb shape designations have two parts made up of letters and a number. The letters describe the bulb type while the number is an indication of the actual size where a higher number represents a larger bulb size. These numbers are used to reflect the diameter of the bulb in multiples of 1/8th of an inch so if the number were 8 it would be an inch, 16 would be two inches and so on. LED bulbs can be broken down into four primary categories of shapes:
A-Shape, Reflector, Decorative and Specialty
A-Shape LED light bulbs are the most common shape available and are commonly referred to as standard shape LED bulbs. The A stands for arbitrary and when shopping you may see the shape referred to as A19. This is because it has an arbitrary shape and a maximum diameter of 2 3/8th inches (19/8 = 2 3/8th). This shape is commonly used for LED bulbs that replace 40 or 60-watt incandescent bulbs. LED replacements for 75 watts or higher typically have an A21 shape. You may also find some LED bulbs which use the smaller A15 or A17 shapes.
Reflector LED light bulbs are most commonly referred to as flood or spot light bulbs due to their reflective coating which is used to create a wide or narrow beam angle of light. Reflector LED bulbs have the letter R in their bulb shape designation. You'll find the most common types listed below:
MR = Mirrored Reflector
BR = Bulged Reflector
PAR = Parabolic Aluminized Reflector
AR = Aluminum Reflector
PAR shaped bulbs are usually used outside in security lights while R and BR are used indoors.
Decorative LED light bulbs are smaller than A-Shape LED bulbs and come in different shapes, Bullet Candelabra, Flame (or Bent Tip) Candelabra and Globe.
B = Bullet(Please note that these are also referred to as Torpedo or Chandelier LED bulbs)
CA= Flame Tip(Also referred to as Bent Tip)
G = Globe
Specialty LED light bulbs contain several less popular shapes since they are only used for limited specific applications. The bulb shapes below reflect the most searched for in this group.
S = Straight-Sided - Typically used in signs and patio string lights
T = Tubular - Typically used to replace small halogen lights in automotive, instrument or landscape lighting
While this doesn't cover ever single LED bulb shape out there it covers a large majority of what's available.
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1) Bulb Shape