|MadSci Network: Physics|
Your question is an interesting one because light bulbs are all around us and we don't often think much about people testing them before they are actually sold. Luckily, I used to work for a company in which a Division manufactured a large number of lighting products, so that I can respond pretty reasonably. First, let me tell you that most light bulbs are NOT tested for efficiency at all -- and probably aren't tested for any characteristics. That's because the experience of the manufacturers and the stability of their manufacturing processes is such that, unless something goes wrong on the assembly line that will make a large number of lights defective, the lights will generally work must fine. To see if something is going wrong, the manufacturers will generally pull a random sample of a few (almost always more than 6 units, but rarely more than a fraction of a percent of a production run) and subject them to testing. Efficiency testing is pretty easy. They use a photocell detector (probably made of silicon) to measure the light output from a bulb. They use a voltmeter and ammeter to determine the electrical input used to drive the bulb. The electrical power input in watts is just the volts X amps. The efficiency is the optical power (measured by the photocell detector) divided by the electrical power input. This is usually measured for several thousand hours of operation or until the light output falls below a specified value. If too many of the sample bulbs fail to meet the desired efficiency over lifetime, then the whole lot from which the samples were taken is usually scrapped. Alternatively, they might be sold at very low prices to other vendors who would market them as discount bulbs -- but without the reputable vendors trademark or warrantee. The main factors which affect the performance of a lightbulb included the following: 1. Filament thickness. Since light bulbs usually generate light by heating a tungsten filament which glows, it's logical that they might boil off a little of the filament material as the light operates. Making the filament thicker can make the lamp last longer, but requires considerable skill because the tungsten metal is very brittle and hard to hand in significant thicknesses. 2. Operating power. Since light bulbs may gegrade by boiling off filament material, it makes sense that by operating them a lower powers -- which is possible using dimmer switches or simply selecting bulbs of lower powers -- the life of a given bulb may be extended. Manufacturers are often reluctant to use radically different filament designs among similar products, so tht you can check to see that -- among similar products of varying wattage -- the lower wattage units can last much longer than the higher wattage ones. All of these considerations apply only to conventional filament-type light bulbs -- the sort that Thomas Edison helped to pioneer. Other types -- fluorescent fixtures used in many public buildings, sodium arc lamps, and alkali halide high brightness bulbs -- use different physical principals to generate their special output radiation. In general, the lifetime of such units is much longer than the conventional filament bulds, but they are also much more expensive. Rather than try to go into a detailed explanation here, it might be wise for you to contact a technical representative of a lighting firm in your country -- they're generally very responsive and happy to answer any reasonable questions. Hope this provides you with the desired information! Steve Guch
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