What is the hottest filament I could find in an incandescent lamp?

Question

I read old light bulbs get really hot.

I read that the incandescent lamps get quite hot but I couldn’t find a filament that gets ‘the hottest’.

Is there a filament that gets the hottest?

Answer 1

In commercial bulbs run at recommended voltage, you can still get photoflood bulbs which draw 500W and have a color temperature of 3400K, which, in an incandescent bulb (without color filter) is also the temperature of the filament (3125°C),

Lifetime is rated at only 8 hours, so there is a direct trade-off for the higher filament temperature and resulting whiter light (the filament has a smooth black-body radiation spectrum).

Answer 2

The hotter the filament, the whiter the light, and the more efficiently it will run. Tungsten halogen bulbs run somewhat hotter and more efficiently than GLS bulbs, and so have replaced them on the market where a filament is still required.

The lifetime of a filament bulb is strongly related to its running temperature. In the GLS type bulb, it typically varies inversely as the 14th power of the applied voltage. This wikipedia entry on Lamp Rerating gives the following relationships

• lifetime \$V^{-12}\$ to \$V^{-16}\$
• colour temperature \$V^{0.42}\$
• luminous intensity \$V^{3.4}\$ to \$V^{3.5}\$

If you are doing a brief experiment and have no need for bulb longevity, you can increase the applied power and increase the filament temperature. You would need to sacrifice a few bulbs to verify the lifetime at your required colour temperature

You can estimate the filament temperature from the ratio of its resistance when running, to resistance when cold, using published tables of temperature coefficient of resistance for tungsten.

The melting point of tungsten is 3695 K, so @Sphero's identification of a lamp that runs at 3400 K and is rated to last 8 hours represents about the best you are going to get commercially.

The way the halogen cycle works to prolong filament life means that a proper high temperature 'photo flood' light will last longer than a standard tungsten halogen overrun to the same temperature. This is because the manufacturer would have added more halogen to the envelope, anticipating the higher filament boil-off rate at higher temperature. In the standard bulb, the halogen filling will become overwhelmed at the higher temperature, and fail to recycle the filament.