- #1
CaptainZappo
- 92
- 0
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.
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.
