Updated 05-XI-2011
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

Cap Nomenclature

Because of the large variety of different cap types a comprehensive system of nomenclature has been developed by the International Electrotechnical Commission. It commences with a single letter to denote the general type of cap, which may be followed by additional letters to denote a sub-category. The second part is a number which denotes the shell diameter (or pin spacing) in millimetres. The third part of the code is a small letter to indicate the number of contact plates (or pins). Special modifications to the basic design may be indicated by a character following a hyphen. These three or four parts are generally sufficient to define the cap as far as the end user is concerned, for compatibility purposes. However to provide a full definition certain other dimensions may be necessary, and these follow a forward slash (such as, for example, the length of the cap). The list of all general lighting cap types is maintained in the international standard IEC 60061.

First Part - Cap Type
B Bayonet Collar E Edison Screw F Single Pin Type G Multiple Pin Type
K Cable Connections P Pre-focussed R Recessed Contact(s) S Shell-type
T Telephone Slide W Wedge Base X Special Type

Second Part - Shell Diameter or Pin Spacing
This denotes the principal dimension, which is typically either the shell diameter, or the pin spacing for type G caps. Dimensions are given in millimetres.

Third Part - Number of Contact Plates or Pins
s Single d Double t Triple q Quadruple

Common Examples
B15s Bayonet Collar, 15mm diameter, single contact
B15d Bayonet Collar, 15mm diameter, double contact
B22d Bayonet Collar, 22mm diameter, double contact
B22d-3 Bayonet Collar, 22mm diameter, double contact, 3 retention prongs
BA15s Bayonet Automotive (short prongs), 15mm diameter, single contact
E27s Edison Screw, 27mm diameter, single contact
E39d Edison Screw, 39mm diameter, double contact
G6.35 2-pin base with 6.35mm pin spacing
GY5.3 2-pin base with flattened pins, 5.3mm spacing
G17q 4-pin base with 17mm pin spacing
R7s Recessed Contact, 7mm diameter, single contact
R17d Recessed Contact, 17mm diameter, double contact
P28s Pre-focussed design, 28mm diameter, single contact
PG22d Pre-focussed 2-pin design, 22mm diameter, double contact
Fa4 Single Pin base, smooth pin surface, 4mm diameter
S15s Shell type base, 15mm diameter, single contact
SFc10-4 Shell type base, 10mm diameter, Single pin protrusion with screw thread size M4
W2.1x9.5d Wedge base, 2.1 x 9.5mm, double contact