MadSci Network: Physics

Re: What are the experiments done to test the properties of the light bulb?

Date: Sun Jan 31 22:02:27 1999
Posted By: Steve Guch, Post-doc/Fellow, Physics (Electro-Optics/Lasers), Litton Systems, Inc., Laser Systems Division
Area of science: Physics
ID: 917434910.Ph

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

  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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