Maximum Overall Length
Light Centre Length
Figure 2.5. Other bulb dimensions.

Only diameter is quoted in the GE Spectrum catalogue for incandescent lamps.

Bulb Finish
The most common types of bulb finish are clear and pearl. The pearl finish is an internal frosting of the glass of the bulb. There are also colour finishes and opal which is a fine white powder coating which produces a more even brightness than pearl where diffusion of the light is important. Metalized coatings are used for various reflector lamps.

Lumens Produced
Lumen rating is a measurement of how much light a lamp produces. Lumen output decreases with the use of the lamp; output, therefore, is listed as approximate average lumens, meaning the average through life. In general, the higher the wattage rating of an incandescent lamp, the greater the lumen output. The lamp lumen rating may seldom be used for lamp identification, but it is most important in lamp selection.



Figure 2.6. Typical average lumen ratings.

Lamp Life
Lamps are rated as to their average life in hours. Average life rating means that 50% of a large number of lamps of a given specification will have burned out by that time and 50% will still be operational. Typical average lives for incandescent lamps range from 1000 to 3000 hours depending upon the type.