Spelunkers won't get very far into a cave if they can't see where they're going. Even if they could have reached their underground objective, it would be nearly impossible finding the way out. There is a total absence of light within a cave and this is why lights are essential caving equipment. Serious cavers choose a good, reliable unit and complements this with at least two backups.
Most societies and organizations recommend that each person must have at least three independent sources of light. The three light source system doesn't mean you will necessarily use all at the same time or one after another but rather for you to have two backups if the main one fails. Not having a light deep inside a cave could be a matter of life and death so you must choose reliable light sources. These must be water proof and be durable enough to withstand rough use.
There used to be a time when carbide lamps were widely used outdoors and by miners and cavers. Battery powered electric lights have since taken over now that these have become more reliable and economical. LED lamps are particularly getting more popular because of the amount of light it can deliver not to mention its being lightweight and energy efficient.
The main light source must be mounted on the helmet so you can see wherever you turn your head and both hands are free for climbing or crawling.
The second source of light is typically a flashlight though another helmet mountable light is even better. The third source of light is usually another flashlight. Maglites are recommended by most cavers.
There are other sources of light but with LED lamps and flashlights being small, light and inexpensive, these should be considered better than all the others. However, in a real emergency, these alternatives can be life savers.
Candles give out light but are unreliable. Together with matches, these are quite vulnerable to getting wet. While these can be used in a pinch, and can be a heat source, candles shouldn't be considered among the three light sources.
Glow sticks are another source of light but again, these should only be considered as a backup to your three light source system.
In a really tight situation, don't forget other items you may have with you that can give out light such as your cellular phone, or the LCD screen of a GPS Device.
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