halogen/krypton bulbs

[elliotb@xxxxxxxxxxxxx (Elliot M Burke )]

George Themelis writes:

>>Out of curiousity, what is the practical difference between a halogen bulb 
>>and a Krypton bulb? Or how do they compare? (brightness, colour, life etc).

>My bulb importer also carries krypton bulbs but tells me that the halogen
>bulbs are brighter and last longer...

>In the USA, Radio Shack has a screw-base krypton bulb (SK222, I think) that
>is, excuse my French, a piece of junk!  I was all-excited when I discovered
>it and was promoting it because it is brighter than the No. 245, until I 
>realized that the life span is extremely short and many of the bulbs are
>defective to start with...

>Trust me, you cannot do better in terms of price vs. performance than the
>halogen screw-base bulb that I carry.  I am a perfection-seeking, active
>and very demanding stereo photographer... I only carry and promote what I
>like, use and trust. 

Incandescent bulbs are a bit of black magic, but here's a little of the physics:
The brightness is dependant only on the temperature and tightness of wind of the 
filament.  Wattage doesn't matter, it only makes the filament bigger, not brighter, 
giving more light but not more light/area ( brightness ).  The upper limit of 
brightness is set by the melting point of tungsten ( 3680K ).  The closer to that point 
the filament is run, the shorter the life, the tungsten evaporates.  The purpose of 
halogen is to recycle the tungsten: it reacts with tungsten vapor near the filament, 
and breaks down at the filament to redeposit it there.  Thus life can be greatly 
extended at high temperature, not much so a lower temperature.
Efficiency is another issue.  Energy is lost by conduction through the gas.  The 
(stable) gas with lowest thermal conductivity is Xenon, which is extremely expensive 
and rare ( and causes mild anathesia when inhaled ).  Krypton is next.  Bulbs are filed 
with mixtures of xenon, krypton, nitrogen, and a halogen ( bromine or iodine ).
If you don't care about efficiency, some money can be saved by leaving out the 
expensive gases.
Any lamp can be run at any temperature up to the melting point of tungsten; the life 
will vary between different design.  There is a power law relationship between voltage 
and life, something like 
                           life=voltage^^-5.6  

If you have a bulb you like, don't hesitate to change the voltage to change brightness 
or life, be aware that these are mutually exclusive.  Is there interest in seeing the 
entire set of bulb life/brightness equations?


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