Humans are visually oriented creatures, trusting vision over all other senses and when adrenaline kicks in your body generally ignores all the other senses. For this reason we are significantly disadvantaged in the dark. To help make up for this disadvantage we need portable light, the flashlight. Flashlights have come a long ways over the years from dim, heavy, barely useful light to powerful lightweight Luxeon LEDs. Each type of light has its advantages and disadvantages, but regardless of which one you have this is a critical piece of gear to include in a bug out bag.
Lighting Element Types
The lighting element or "bulb" is the actual generator of light and the type that is used is the largest factor in determining the lights performance.
Incandescents have been around for a long time, they are cheap, reliable, and provide a slightly yellow tinted light. Incandescents will burn through bulbs and batteries pretty quickly and have relatively poor visible range.
Traditional LEDs have a life of 100,000 hours (or eleven years of continuous operation), they have excellent battery life, and are much more durable than incandescents. White LEDs range from near true white to heavily blue tinted with newer technologies creating truer whites. LEDs are also available in other colors making them useful for avoiding attention or helping to preserve night vision.
Luxeon LEDs have about half the traditional LEDs, but make up for it with a truer white and much brighter light.
Xenons are the most powerful elements commonly available. They don't last nearly as long as LEDs concerning batteries or bulb life, but they are extremely bright.
Flashlights are only as effective as their power sources. The brightest portable lights (around 1 million candlepower) use very powerful batteries such as rechargeable 12 volt nickle-hydride (NiH2) or nickle cadmium (NiCad) storage batteries. Some use non-rechargeable lithium or alkaline batteries. For emergencies there are flashlights which have their own dynamo chargers attached to the flashlight. Alternatively one can use solar powered chargers for rechargeable batteries.