I was buying light bulbs for a load bank for work and it occurred to me...what determines the size of the glass that goes around the filament?

It seems somewhat arbitrary since it's a vacuum so it doesn't seem like heat conduction would be a factor, but most seem to go for a pear shape that seems rather excessive if you're just trying to merge the diameters required to accommodate the socket at the end of the throat and the filament.

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    \$\begingroup\$ Most light bulbs dont use vaccums. Most anything big enough to be 'pear shaped' uses an unreactive gas inside. Krypton, Nitrogen, Argon. \$\endgroup\$ – ambitiose_sed_ineptum Feb 21 at 20:19
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    \$\begingroup\$ What Phil G said. || MOST of the input energy leaves as heat via the envelope - whether radiant or convective. Cooling is a function of envelope area, air flow & magic. A very small envelope (as seen in eg automotive halogen headlamp bulbs) necessarily results in extremely high temperatures. These are tolerable with quartz envelopes in confined spaces without likelihood of inflammable material or charrable skin, but not so much so in domestic use. \$\endgroup\$ – Russell McMahon Feb 21 at 21:58
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    \$\begingroup\$ Instead of buying light bulbs, buy cheapie heater-fans. Watt for watt dissipated, the savings is in the sockets. Instead of a $2 socket per 100W bulb, you need a 50 cent socket per two 1500W heaters. \$\endgroup\$ – Harper - Reinstate Monica Feb 21 at 23:03
  • \$\begingroup\$ @Harper-ReinstateMonica Those are more inductive, no? \$\endgroup\$ – DKNguyen Feb 21 at 23:03
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    \$\begingroup\$ @DKNguyen I forgot to mention longevity - they will last many thousands of hours, 20 years if you get good ones. No, plain old heaters are not inductive - except for the fans, which is ~1% of total power. The 99% is pure resistive. \$\endgroup\$ – Harper - Reinstate Monica Feb 21 at 23:06

The vast majority of the energy dissipated by the filament is heat (about 96-97%) and this makes its way to the glass envelope by radiation, and some conduction - most bulbs do have some gas in them, generally a noble gas that limits the evaporation of the filament. Small envelopes have, obviously, a smaller surface to dissipate the heat, and hence the bulb runs far hotter, causing the materials used for the base and holder to need to be more heat tolerant - and generally needing the bulb to be contained in a housing that avoids the risk of flammable materials coming into contact.

High efficiency domestic bulbs have a small, but hot-running envelope, containing krypton or xenon, inside a larger protective envelope that will run cooler.

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