Incandescent Lamp Caps
The second most important characteristic is the cap. The lamp cap must match the socket in which the lamp will be used. Most incandescent lamps have a bayonet or screw cap. Both types of cap come in a range of sizes.

Figure 2.1. Typical incandescent lamp caps.

Two other types of cap used for incandescent lamps are end prong caps for various sizes of PAR (Parabolic Aluminised Reflector) lamps and single contact on Striplights.

Figure 2.2. Other incandescent caps.

Wattage Ratings
The third lamp characteristic is wattage rating. Watt is a measure of the electrical power consumed by the lamp. Typically, the higher the wattage rating, the greater the lumen output, and the greater the operating cost. Most luminaires are marked as to the maximum wattage lamp that should be used. This is to ensure that the temperature limitations for the socket and wiring are not exceeded.

Voltage Rating
Voltage is also important. Lamp Voltage should match the voltage rating of the circuit on which it operates. Most UK household circuits are rated at 240V, although this was "harmonised" with Europe at 230V as from 1 Jan 1995. Beware, that although the rating may become 230V, it is likely to still be 240V that is supplied. Commercial and industrial applications also use 240V for lighting, sometimes referred to as single phase.

Bulb Shapes
Incandescent lamp bulbs are made in many shapes. This is one of the most easily recognised characteristics of a lamp. Many shapes are given easy-to-remember names.