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To begin with, you need to pay enough attention to the battery type and battery life of LED flashlight. It is widely accepted that LED flashlight is more efficient than traditional incandescent flashlight. Although LED flashlight is an energy-saving electrical device, it does not mean that LED flashlight has an exceptionally long battery life. As a matter of fact, the battery life of the particular LED flashlight is affected by many factors which include the battery type, the number of LED's, the amount of light given off, the rated power, etc. Generally speaking, there are two kinds of batteries: alkaline battery and lithium battery. Compared with alkaline battery, lithium battery is more expensive but it lasts longer and performs better in low temperatures. Of course, apart from battery type, other factors also have a big impact on the battery life of LED flashlight.
Secondly, you need to understand lumen and pay much attention to the lumens of LED flashlight. We know that light is usually measured by the special measurement unit of lumen. But lumens only tell a small part of the truth since lumens are measured at the centre of the beam. Thus, it is easy to understand why two different LED flashlights with the same lumen show quite different results. You may find that a keychain flashlight with one LED gives out quite bright light to the centre of the beam but fades quickly at the edge of light while a LED flashlight with several LED's may give out the same amount of light to the centre of beam but it stills projects quite bright light to the outer circle of light. In other words, these two LED flashlights have the same lumen but one may give out more light than the other. Therefore, you do not need to take lumens of LED flashlight seriously since lumens do not tell you how much light it will give out in total.
Finally, you need to play enough emphasis on the effective projection distance of LED flashlight. Just like a coin has two sides, LED flashlight also has some drawbacks. One of the most drawbacks is that LED flashlight can not throw light as far as one incandescent flashlight. However, most consumers are not annoyed by this factor since most of them use LED flashlights to see things relatively close to them. Of course, if you are especially critical about the effective projection distance, you need to note it whenever you are about to choose one LED flashlight.