How to Identify your miniature bulb:

You will need to identify the base of the bulb, the glass size and shape, the voltage, amperage, and wattage of your bulb. You will need a ruler that measures milimeters and possibly a magnifying glass.

Part I: The Base
The base is the most critical part of the bulb. If the base is improperly identified the bulb will not work. It will not fit into your lamp/fixture. Start with the base!

Screw Bases
Screw bases are designated with an "E" number, i.e. E12. The "E" refers to Edison and the number refers to the diameter of the base in Milimeters. An E12 would measure 12mm across the base. An E5 is referred to as a Midget Screw base, an E10 is a Miniature Screw Base, an E11 is known as a Miniature Candelabra, an E12 is known as a Candelabra Screw Base and an E14 is known as a European Screw Base.

Midget Screw Base E5 Miniature Screw Base E10 E12 Candelabra Screw Base E14 European Screw

Bayonet Bases
Bayonet Bases are inserted and twisted in place. They are designated with a "BA", "BAY", "BAZ" or "BAU", a number, and a letter, i.e. BA15D. The number refers to the diameter of the base measured in millimeters while the letter refers to the number of contact points on the bottom of the bulb. A "S" indicates a single contact point while a "D" indicates a dual contact point.

Pin orientation on Bayonet bulbs Pin orientation on Bayonet bulbs

Viewed from the bottom note how the side pins are 180 degrees apart on BA, and BAY bayonet bases in Figure 1 while BAZ and BAU bases are offset differently as shown in Figure 2.

Pin orientation on bayonet base bulbs Pin orientation on Bayonet Bulbs

Viewed from the side note how the side pins are parallel on BA and BAU bases shown in Figure 3 while BAY and BAZ bases are staggered as shown in Figure 4.

BA9S Base
BA15D bay15d BA15D ba15s

Miniature Flange Bases
Miniature Flange based bulbs are most commonly used in flashlights. You will note a Flanged collar at the top of the base at the point where the glass connects to the base. These are known as P13.5S bases. The actual base of the bulb measures 9mm and the flanged collar measures 13.5mm.

Miniature Flange

Midget Flange Bases
Midget Flange bulbs are smaller than miniature flange bulbs and the flange is located at the bottom of the base rather than the top. Note that the flange is a larger diameter than the rest of the base. Midget Flange Bases are also known as a SX6S base. The diameter of the base is slightly more than 6mm and the Flange portion of the base is slightly more than 7mm in diameter.

Midget Flange Bulb

Also Avaliable are Sub-Midget Flange Base bulbs also known as SX4S Bases. The diameter of the base is 4mm while the diameter of the flange is 4.6mm.

#22 MINIATURE BULB SX4S BASE - 14 Volt 0.04 Amp T-1, Sub-Midget Flange (SX4s) Base, C-2V Filament Design, 0.38" Maximum Overall Length, 16,000 Average Rated Hours

Midget Groove Bases
Midget Groove Bases have a groove at the bottom of the base. The diameter of the base remains constant except for the indentation of the groove. The Midget Groove Base is also known as a S5.7S Base and measures 5.7mm in diameter.

Midget Groove Base

Festoon Bulbs
Festoon bulbs look like a double ended fuse but are actually a bulb. They are found with a "Double End Cap" base and SV7mm and a SV8.5mm bases. The SV7mm bases measure 7mm in diameter and the SV8.5 measure 8.5mm in diameter. SV bases can be both flat and pointed.


Prefocus Base
A Prefocus base allows a precise alignment of the bulb to provide an accurate focusing of the light for projectors, automobile headlights, etc. Pre-Focus bases are found with single and double contact points and have a distinctive ring.

Prefocus Base

Axial and Right-angle Prefocus Base used in automotive headlight bulbs are somewhat different. The base of the bulb is a female receptacle that inserts on the headlight connector.

Axial Prefocus Right Angle Prefocus
Axial and Right Angle PreFocus Base

Wedge Based Bulbs
Wedge Base bulbs simply push into their sockets. They come in several sizes and are easy to misidentify. Careful measurment is important!

plastic wedge
Plastic Wedge Base

T1-1/2 Glass Wedge Base Bulb
T1-3/4 Glass Wedge Base Bulb

As we can see there is only a slight difference in width between a T1-1/2 and a T1-3/4 bulb so measuring correctly is important! T1-1/2 bulbs are more commonly associated with European Fixtures and most have an "E" designation: i.e. E73 while T1-3/4 are more often used in America and have a number designation: i.e. #56. If the width of the glass is clearly more than 5mm but less than 6mm you probably have a T1-3/4 Wedge Base Bulb.

T2-1/4 Glass Wedge Base bulb
T3-1/4 Glass Wedge Base Bulb
T5 Glass Wedge Base Bulb
T6 Glass Wedge Base Bulb

Printed Circuit Base
Similar to a plastic wedge base bulb is the Printed Circuit Base. The base of the Printed Circuit Base is much wider than the Glass Wedge Base.

Printed Circuit Base Printed Circuit Base

Slide Based Bulbs
Also known as a Telephone Slide Base these bulbs were originally used in telephone switch boards.

Slide Base 1

Note that the metal portion of the base does not extend up and around the plastic portion of the base as compared to Slide Base 2 and 3.

Slide Base 2
Slide Base 3

Note the notch on the side of the Slide Base 3 as compared to the otherwise identical Slide Base 2 Bulb.

Slide Base 5
Slide Base 6

Note that illustration for Base 5 and 6 show the bulb oriented from a different angle than Bsse 1, 2 and 3. The plastic portion of the base viewed from one side is in a "V" shape and viewed from the other side is flat.

Bi Pin Base
Bi pin Bases differ by the distance between the pins and usually have a "G" prefex. A bi pin base where the center of the pins are spaced 4mm apart is refered to as a G4 Base. Unfortunately the smaller Bi Pin Bases may not be refered to by their G designation and are refered to as a Bi Pin Base. It may be more useful to identify the Glass size in order to identify the bulb.

Bi Pin Base

Wire Terminal Base
Wire terminals are flexable wire leads that can be bent or otherwise adjusted to fit a socket or fixture. As with the Bi Pin base it may be more useful to identify your bulb by the glass dimensions.

Wire Terminal Base

Part II: Voltage and Amperage

Next in importance is voltage identification. Voltage is electrical potential while amperage is the flow or current of that electrical potential.  Typically voltage and amperage is stamped on the base of the bulb.  Wattage is calculated by multiplying voltage and amperage.  If your bulb is stamped with 12v and .1A you may multiply 12*.1=1.2 Watts.

There is some margin of error with voltage but ideally it should be as exact as possible. It is always better to order a slightly higher voltage than a slightly lower voltage. Typically bulbs are designed for a slightly higher voltage than the anticipated power source. For example, household bulbs are generally rated at 120 or 130 volts while the voltage coming out of your wall socket is only 110 Volts. A 130 volt bulb will have a heavier filament than a 110 volt bulb and will last much longer. Similarly automobiles have a 12 volt battery yet many automotive bulbs are rated at 12.8 Volts.

In summary if you cannot find a bulb with the exact voltage stamped on your bulb you will be OK with a bulb with a slightly different voltage.  A bulb rated with a slightly higher voltage will not be quite as bright but last longer or a bulb with a slightly lower rated voltage will burn brighter but not last as long.  Using too low of a voltage bulb i.e. a 24 volt bulb in a 110 volt lamp will result in the bulb burning out almost instantly while using a 120 volt bulb in a 24 volt lamp will result in a very, very dim bulb that will last a very, very long time.

Part III: Glass Size and Shape
The glass portion of your bulb has a letter and number designation i.e. A19. The "A" is the designation for the common lightbulb while the number refers to the diameter of the glass in 1/8 inches. 19/8=2.38 so a household lightbulb should be aproximatley 2 1/3" in diameter. The shape and size of the glass is only critical in circrumstances where the bulb is in a tight space otherwise it is possible to substitute a different size and shape of glass as long as space permits providing the base, voltage and amperage are correct.

Shape A: The Standard lightbulb is too large to be considered a miniature bulb.

A Shape
A Shape Bulb

Shape B: "B" bulbs are bullet shaped and most are pointed although there are some that the tip is rounded.

B Shape Glass
B Shape Glass

Shape C: "C" bulbs are Conical shaped.

C Shape Glass
C Shape Glass

Shape G: "G" bulbs are round like a globe.

G Shape Bulb
G Shape Glass

Shape R: "R" bulbs are reflector bulbs and look like flood lights. The front of the bulb may have a frosted, translucent coating but the sides of the bulb have a reflective opaque coating that directs a beam of light forward.

R Shape Bulbs
R Shape Glass

Shape RP: Reflector Pear Shaped. RP bulbs may or may not have a reflective coating.

RP Glass Shape
RP Shape Glass

Shape S: "S" shaped bulbs were originally used in signs and marquees. Shaped glass is similar to an A shaped glass only with a less pronounced taper. Some of the smaller S glass bulbs may appear to be similar to a T Glass with a slight taper.

S Shape Glass S Shape Glass
S Shape Glass

Shape T: "T" or tubular bulbs are shaped like a test tube. There is no Taper to the bulbs although the tip of the bulb may be flat, rounded, or pointed. The middle of the bulb is a constant diameter.

T Shape Glass
T Shape Glass

Shape TL: Tubular shape with Lense in the crown.

TL Shape Glass





























































































































































































































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