Updated 30-XI-2011
Product Overview
Cap Nomenclature
Bulb Nomenclature
Filament Nomenclature
Operating Principle
Gas Filling Effects
Filament Coiling Effects
Vacuum vs Gas-Filled
Gas Filling Types
Burning Position
Voltage Variation Effects
Starting Characteristics
Lamp Life
End of Life & Fusing
Premature Failure
Lamp Designs
Carbon Filament
Tantalum Filament
Osmium Filament
Tungsten Filament
Advanced Filament
Infra-Red Recycling

Bulb Nomenclature

Each glass bulb is classified with a unique code which determines its size and shape. The shape of the bulb is denoted by a letter (or letters), and this is followed by a number which gives the maximum diameter. In most countries of the world the diameter is stated in millimetres. However in the Americas the number represents the maximum diameter in eighths of an inch (e.g. an A19 bulb has a diameter of 19 eighths of an inch, which is 2.375" or 60mm). The imperial measures are sometimes also encountered even in metric countries for certain bulb shapes, particilarly for PAR and MR reflector types, and for fluorescent tubes.

Figure I3 - Glass Bulb Shapes

First Part - Main Bulb Shape
A Arbitrary A bulb having a spherical end section that is joined to the neck by a radius that a) has a centre outside the bulb, b) has a magnitude greater than that of the spherical section, and c) is a tangent to both the neck and the curve of the spherical end section. These bulbs do not have any significant straight portion between the spherical end section and the transitional radius into the neck.
B Bulged A bulb in which the curve making up the major portion of the side of the bulb has a radius greater than half the bulb diameter and a centre in the plane of the maximum diameter. This design also applies where two radii are used, one for the lower part and a larger one for the upper part, as in a candle shaped bulb. Not to be confused with type E.
C Candle/Cone A bulb consisting of a conical or near-conical end section which is joined to the neck by an approximately hemispherical section. If the end section is not conical the curve making up the major portion of the side of the bulb has a centre below the plane of the maximum diameter.
D Drop Bulb having a frustrated conical end section which, in the case of the DK Doorknob shape, is joined to the neck by a radius with a centre outside the bulb and magnitude greater than the major diameter. Also available with a conical mid-section (DC).
E Ellipsoidal A bulb similar to type B, but having the sides formed by a section of an ellipse.
F Flame A bulb resembling the shape of a flame having irregular flutes on the sides.
G Globe A bulb of essentially spherical shape.
K Krypton A bulb which is similar in shape to an M bulb, except there is a conical transitional section between the major diameter and the neck rather than a curved section. Designed for Krypton GLS lamps - the smaller volume bulb contains less of the expensive krypton gas.
M Mushroom A bulb having a spherically shaped end section blended, above the major diameter, to a smaller radius curve centred on the major diameter, which blends with a transition curve of approximately the same radius for joining to the neck.
P Pear A bulb having a spherical end section and a conical mid-section, the sides of which are a tangent to the curve of the spherical section.
R Reflector A bulb that includes a parabolic or ellipsoidal section below the major diameter designed to receive a reflector coating, so as to direct a beam of light.
S Straight Sided A bulb having a spherically shaped end section, a conical lower section, and an intermediate curve joining the two. Commonly known as a Pygmy bulb in the smaller diameters.
T Tubular A bulb that is mostly cylindrical in form.

Second Part - Modifiers
A Angular top, e.g. a wick on a candle bulb, or an exhaustion tip on old lamps
C Conical section above or below the maximum diameter.
D Dimple pointing inwards or outwards.
F Flutes which twist around the outside and taper towards the tip of the bulb.
K Used only after the letter D to indicate a doorknob shaped bulb.
L Lens end.
R A bulb having a section below the major diameter to receive a reflective coating.
S Tubular neck section below the bulb and above the approximate base line.
T Tubular neck top section above the major diameter.

There are some special bulb shapes which do not fit into the above coding system, such as HX (hexagonal candle), NR (narrow reflector), CP (crystalline pear), MR (Multi-Mirror Facetted Reflector) and PAR (Parabolic Aluminised Reflector).