ABSTRACT
LUMENS AND LAMP LIFE One common practice for comparing light sources relates to the actual amount of light or lumen output that is produced by a lamp. This is simply determined by the amount of work that the lamp completes as it converts electrical energy into light. The most common way of measuring this is by rating a lamp according to its wattage. However, it should be noted that although lumen output generally corresponds to wattage, the principle doesn’t always hold true; some lamps are more efficient than others. As a rule though, it can be said that lamps of a higher wattage usually appear brighter and generally produce more light than lamps of comparatively lower wattages. We also consider how long a lamp will last before burning out as another criteria before selecting a lamp for a given task. We measure this in lamp life. Simply put, lamp life is a means of determining the average number of hours that a lamp will burn before failing. It is based on trials in which a number of lamps are tested until either they actually fail or their light output drops to below 80%. Obviously, if you are specifying exterior spotlights for a church steeple, where the lamps are going to be hard to replace, it would be more practical to use lamps with higher lamp lives. Another general rule relates to the fact that lower wattage lamps tend to have longer lives than those with higher wattages. Therefore, it can also be concluded that, in general, brighter lamps tend to have shorter lamp lives. Some special-duty stage and studio lamps have a lamp life of as few as 50 hours while most theatrical lamps can average several hundred hours. Household lamps on the other hand will be rated for up to a 1,000 hours, while fluorescent lamps typically have lamp lives that can extend to approximately 10,000 hours. Some extended-service lamps, such as the familiar mercury street or utility lamp, can have lamp lives up to 24,000 hours or more. In addition to lamp life, it is also important to consider how quickly the light output from a lamp drops off over time. This relates to the efficiency of the lamp and is called lumen maintenance. Some lamps or light bulbs progressively cloud or darken over time, which can have a serious impact on the amount of light actually being produced by the lamp. As time progresses, this effect on lumen output becomes greater and greater. Other lamps stay essentially clear throughout their lives and maintain a more consistent light output as they age.
