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Fresnel lens need help with focusing light

  1. May 30, 2007 #1
    Hi, I'm in a bit of a dilemma, where my limited knowledge of optics has failed me. Here's the scenario:

    I have acquired a large Fresnel lens (34" x 45"), as used in rear projection TVs. My Dad and I built a frame on which the lens was mounted. Supports have also been installed diagonally to minimize distortion of the lens, as it is quite flexible. Around 2:00 PM today, I took the lens outside and tried to form a nice focal point. I did manage to an aluminum can in less than 15 seconds, reaching a maximum temperature of 1411 degrees Fahrenheit. Unfortunately, the focal point was not as tight as I would've liked. Although there was a distinct point, which was roughly the size of a quarter, there was still a lot of light dispersed around the focal point. I am trying to figure out a way to gather more, preferably all, light into the focal point.

    After doing some reading online, I found an article by a person with nearly the exact same problem. He did a calculation and reported that only ~10% of the total energy gathered by the lens was collected at the focal point. That article can be found at the following link: http://www-personal.umich.edu/~bclee/lens.html

    As a way of trying to solve this problem, I have purchased a magnifying glass and a couple of smaller Fresnel lenses (one is roughly 10" x 6" and the other is the size of a credit card). Using the magnifying glass and smaller Fresnel lenses, I experimented, but could never gather the light into a more defined point. I reasoned that using the magnifying glass and/or smaller Fresnel lenses would not enable me to gather the light into a more defined point because all the light rays are not coming in at the same angle, as they are before they hit the large Fresnel lens.

    As an aside, I built a device similar to this for my 8th grade science fair project. I used a piece of plywood, to which I attached 600 small mirrors, each with a face area of 1 square inch. Today, I went back to my notes and learned that I recorded a temperature of 2,100 degrees Fahrenheit during the month of February. This raises an obvious concern for the efficiency of the Fresnel lens with which I am currently experimenting. The 8th grade project effectively gathered 600 square inches of solar energy and focused the light onto a 1 square inch spot. The Fresnel lens has nearly two and half times that surface area (1530 square inches), but isn't reaching nearly the same temperature, even with the advantage of being tested during the summer.

    If anyone has any suggestions as to how I can more efficiently use this Fresnel lens, please let me know. I would greatly appreciate it.
     
  2. jcsd
  3. May 30, 2007 #2
    After an evening of deliberation and experimentation, I've began to entertain the idea of a conical mirror (with a small diameter opening at the bottom). I am thinking along the lines of placing the wide end of the conical mirror upwards, toward the Fresnel lens, in such a way that it gathers the light surrounding the focal point. My question is, will the light simply bounce around inside, eventually coming back out the top, or will the light in fact make it out the bottom and be focused on a small point? I've drawn a few geometric diagrams, but cannot definitively prove to myself either case.

    Any further insight is welcomed.
     
  4. May 31, 2007 #3
    After thinking about it further, I've convinced myself that a conical mirror will not work because the light will end up coming back out the top of the mirror, rather than exiting at the bottom.

    I can't seem to find a solution to this problem...
     
  5. Jul 30, 2007 #4
  6. Aug 31, 2007 #5
    This Information might help

    Captain-

    Check out Fresnel Technologies as they specialize in making Fresnel lenses @

    http://www.fresneltech.com/

    One thing to remember out Fresnel lenses is the installation is actually crucial to the achieved focal point. If you browse into one of the application notes they have on lenses it illustrates this point (i.e. grooves installed wrong way will actually diverge some of the light and not receive as an exact focal point). From your explanation this might be your problem, and a simple fix. Not sure if you fixed this problem or not, but thought I would throw in my two cents....
     
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