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How to Increase the Range of Your LED Flashlight... Cheaply, Quickly, and Temporarily

... without wiring, soldering, or getting your hands dirty
Copyright © 2014 Andrew B. Davidson. All Rights Reserved.
Last Updated: April 2014
LED flashlights are great technology. You can learn more about LED flashlights on several websites, for example The LED Museum, the Candle Power Forums and FlashlightReviews.com.
One problem with LED flashlights, however, is that they often lack the range (or "throw") of regular incandescent lights. However, with a simple, temporary, and nearly free trick, you can greatly increase the range of your LED flashlight.
The trick is: Hold a Fresnel lens [Amazon.com aff. link] in front of the flashlight!
You will be amazed how a cheap 7" x 10" (18cm x 25cm) Fresnel lens can focus the light coming from your flashlight so that it can illuminate objects far, far beyond its normal range. However, please be careful when focusing light — see the Warning section below.

What Is a Fresnel Lens?

A Fresnel lens is a type of lens that is much thinner than regular lenses. Cheap consumer Fresnel lenses are typically made out of thin plastic (e.g. 0.5mm thick). The lens consists of concentric rings etched into the plastic that focus the light like a regular lens. Learn more about Fresnel lenses at Wikipedia.
7in x 10in Fresnel Lens
7" x 10" Fresnel lens

Where To Get a Fresnel Lens

It is possible to buy Fresnel lenses very cheaply. A typical price at a drugstore may be US$3 for a 7" x 10" lens. But don't pay much more than that... some lenses are being sold online for $15-$20, which is a ripoff for our purposes! Perhaps those expensive lenses are optically high-grade or something, but we don't need accurate image reproduction. A cheap surplus fresnel lens will work just fine. The page-sized ones are generally marketed as a "Full Page Magnifier". These are normally used to magnify small type so people with poor vision can read, so you can often find them in the eye care or vision care section of drugstores. Small credit-card-sized Fresnel lenses (about 2" x 3", or 5cm x 7.6cm) also work to an extent and are a very handy thing to keep in your wallet for emergencies. As of April 2014, Fresnel lenses are available cheaply (for about $10) at Amazon.com.

How to Do It

To do it, simply hold the Fresnel lens directly in front of the LED flashlight, about 3 to 12 inches (7-30cm) away. The lens should be held perpendicular to the flashlight beam. The precise distance to hold the lens depends mainly on the distance to the object you are attempting to illuminate. By moving the lens in and out (i.e. closer to, or farther from the flashlight), you can find the "sweet spot" and achieve excellent focus on far away objects. By focusing the beam onto a far away object, the LED light is concentrated onto that object instead of spreading out into dim uselessness.
Illustration of how to use a Fresnel lens

The End Result

The end result is that you can illuminate objects that would otherwise be far out of range.

What are the disadvantages of this technique?

The main disadvantage is that by focusing the light to a single point, you are reducing the wide spread of the illumination. That is, you are making an area much brighter than before, but you are illuminating a smaller area than before. But that's the whole point of "focusing" anyway. Another disadvantage is that a fresnel lens is required, and it can be a little bulky to carry around. Also, the technique requires two hands unless you build some sort of mount for the lens. And, multi-LED flashlights will focus into multiple focal points (one for each LED), so this technique is perhaps not quite as powerful for such flashlights.

Advanced Techniques

After a little practice, you may notice that you can change the size of the focal point by moving the lens in and out. When the focal point is made larger, the light is less concentrated and thus dimmer. However, with practice, you can quickly find the exact combination of size and intensity that you desire.

Warning: Do Not Stare into Light! Do not aim at people!

Please be careful when focusing light with a Fresnel lens! You can literally start a fire when using a Fresnel lens to focus the rays of the sun. Larger Fresnel lenses can literally melt aluminum and asphalt using only solar power. While an LED flashlight has a lot less power than the sun, you could still possibly cause eye damage by focusing light from more powerful flashlights. Please use extreme caution and educate yourself before using Fresnel lenses to focus anything more powerful than an LED flashlight. Do not aim the light at people! You assume full responsibility for anything you do with a Fresnel lens, so educate yourself about their power first. Also, be careful aiming the focused light at mirrors as well. It's not always easy to guess where the focused light will be reflected!

Does it have to be a Fresnel lens?

No. A regular (non-Fresnel) lens of the same size, focal length, etc. will work the same way. But Fresnel lenses are lighter, cheaper, thinner, and much less fragile than regular lenses. They are also much easier to find, being available in most drugstores.

Does it have to be an LED flashlight?

LED flashlights tend to work better because the light emitter area is physically smaller. But, incandescent flashlights and other light sources (like the sun) can be focused similarly. Remember, please use extreme caution when using more powerful lights (especially the sun). Focusing light onto a small area will greatly increases its intensity and can cause eye damage if caution is not taken. Do not aim the light at people!

Comments?

If you have an comments or suggestions on this technique, you can email me at AndrewDavidson {at} AndrewDavidson {dot} com, or use my Feedback Form if you prefer.