At first, incandescent bulbs were made with a vacuum inside them. Air oxidizes the filament at high temperatures. Later, it was discovered that filling the bulb with an inert gas such as argon or an argon-nitrogen mixture slows down evaporation of the filament. Tungsten atoms evaporating from the filament can be bounced back to the filament by gas atoms.
The filament can be operated at a higher temperature with a fill gas than with a vacuum. This results in more efficient radiation of visible light. So why are some bulbs still made with a vacuum? The reason is that a fill gas conducts heat away from the filament. This conducted heat is energy that cannot be radiated by the filament and is lost, or wasted. This mechanism reduces the bulb’s efficiency of producing radiation. If this is not offset by the advantage of operating the filament at a higher temperature, then the bulb is more efficient with a vacuum.One property of thermal conduction from the filament to the gas is the strange fact that the amount of heat conducted is roughly proportional to the filament’s length, but does not vary much with the filament’s diameter. The reason this occurs is beyond the scope of this document. However, this means that bulbs with thin filaments and lower currents are more efficient with a vacuum, and higher current bulbs with thicker filaments are more efficient with a fill gas. The break-even point seems to be very roughly around 6-10 watts per centimeter of filament. (This can vary with filament temperature and other factors. The break-even point may be higher in larger bulbs where convection may increase heat removal from the filament by the gas.) Sometimes, premium fill gases such as krypton or xenon are used. These gases have larger atoms that are better at bouncing evaporated tungsten atoms back to the filament. These gases also conduct heat less than argon. Of these two gases, xenon is better, but more expensive. Either of these gases will significantly improve the life of the bulb, or result in some improvement in efficiency, or both. Often, the cost of these gases makes it uneconomical to use them.