All light bulbs get hot when in use – it’s the nature of how they work. With the exception of LEDs and fluorescent bulbs, light bulbs work on the principle of resistance. Electric current is directed through the bulb. The filament is designed to resist the flow of electrons. This resistance creates heat, and the filament glows. Different types of filaments (and different gases in the bulb itself) glow more brightly than others. How hot do your headlight and taillight bulbs get?
The question of type
There’s no single answer here. It greatly depends on the type of bulb you’re using. A standard halogen headlight bulb can reach several hundred degrees during operation, and the headlight lens itself might be over 100 degrees. HID lights can reach very, very high temperatures (far in excess of what a halogen can achieve). Xenon plasma bulbs also reach immensely high temperatures.
Taillight bulbs are a bit different from headlights. The lights don’t need to be as bright, and the red lens helps brighten the light produced by the filament. The bulbs work on the same principle, but different wattages, filaments and gasses are used here. However, taillight bulbs can still get quite warm during operation. They can be uncomfortable to touch after being used, but they don’t reach the 100 – 300 degree temperature range common for even low-end headlights.
If you’ll be replacing your headlight or taillight bulbs, exercise caution. If the lights have been in use, let them cool completely before attempting to replace the bulb, or a serious burn could result.