The incandescent lamp made of tungsten wire in bulb as a filament can produce a continuous spectrum and is used in the visible spectral region of 400-780nm. In the spectrophotometer as a visible light source. Its spectral effective area can extend to 3μm, so it is also used as a light source in the near-infrared region.

In order to increase the service life of the bulb, the length of the tungsten wire in bulb can be lengthened. Let the heat energy be distributed in a longer space, but it will reduce the temperature of the filament, which will naturally reduce the sublimation and prolong the life of the bulb. However, because the temperature is also reduced, the luminous efficiency of generating visible light is also reduced. The bulb becomes dimmer, requiring a higher wattage bulb to produce the same brightness.

Although it saves some money by increasing the service life of the bulb, it is worth considering whether the wasted electricity is worth it. Therefore, long-life bulbs are usually more cost-effective to use in places where it is not easy to replace the bulb. If the length of the filament is longer, the temperature will be lower, and even only a dim red light will be emitted, but the heat (infrared rays) emitted can still be clearly felt.