How hot is the beam of a Fresnel Lens? (page 3)
GREENPOWERSCIENCE.COM
continued.....
"Larger masses for lower temperature metals and glass testing."
For lower temperature metals like zinc, a mass of roughly 14 grams (equal to about 6 pennies zinc type) is used to determine melt speeds. Larger masses of zinc can easily be melted up to about 200 grams. Glass is usually tested with dark soda glass, like a beer bottle fragment sized to 1" x 1/4". This size is large enough to fit the focal point and small enough not to experience thermal shock and shatter. All the temperatures are based on an average of 80°F ambient temperature. Colder climates slow reaction times down a bit but wind is a more significant factor. For example: If it requires 12 seconds to melt a piece of glass with a specific lens with an 80°F ambient temperature, the same test would take 16 seconds at 32°F. That is not a big difference. Compare this to a 10 mile per hour wind at 80°F and melt times can take up to one minute. Shielding from wind is strongly recommended and even if it is a cold day outside, small scale metal forging is still possible.
"GPS testing test scale"
Here is an example of technical data from a 55" Spot Fresnel Lens with explanations listed below each category.
Some information is repeated from previous pages:
* SIZE-----------------43H X 33W inches
55" is a corner to corner measurement like all square screens.
* POWER EST.-----------9.4 This is an in-house number on a scale from 1-10
We now are up to 12 as the newer lenses exceed the performance of our previously most powerful "10" lenses. This number will not be of much help to you but it gives us an easy reference for power vs max temperature. They are not easy to calculate. This will be explained in future writings. Fo now, the higher this number, the stronger the lens power.
* BEAM-----------------SPOT
Spot is a tight beam and Linear is spread over a larger area. A large linear lens can be more powerful than a smaller spot lenses and will have a higher in-house number. Even though the smaller dimension spot lens starts a fire quicker, the larger lens may have more power. This is not always the case and the reason for the in-house number. Spot lenses generally have better optics, clearer material composition and result in much more power of equal dimensions.
* BEAM SIZE MAX POWER--.4 INCH (2.7" OUTER AREA 900 °F)
The first number .4 is a bit less than 1'2 of an inch in diameter or about 1 cm. Because this is an optical lens and the bright spot is nothing more than an image of the sun, this lens takes 1200 square inches of sunlight and concentrates it to under 1/2 of an inch. Since four (4) half inch squares fit into a square inch this is 1200 x 4 or 4800:1 ratio. Some would claim "4800 suns" but IT DOES NOT WORK THAT WAY. It is actually much less (future writing or video). The 2.7 inch outer area is mild optical aberration that scatters across the target area. This is a for of wasted energy but because it is close enough to the target, it adds heat. All Fresnel Lenses experience this and it is the result of spherical reproduction on a flat plain. The smaller this area is, the better the optical quality of the lens is. This size is near perfect optics for such a large lens. Smaller lenses should have a smaller outer area as they collect less light, have a shorter focal length, and are easier to manufacture. Larger lenses require more optical precision. This is the reason telescope mirrors double in cost with each additional inch in diameter.
* FOCAL LENGTH---------31 INCHES
This is the distance the optimal focal point is from the surface of the lens. This may vary a bit in afternoon to evening based on a few factors.
* WEIGHT --------------Aprox. 14 LBS.
Weight of the framed lens.
* MAX TEMP. CLIMB------2110 °F IR THERMOMETER
Using a $400 infrared thermometer, steel, and a few other similar materials, temperature readings are recorded from various exposure times. This is the climb temperature listed above. It give you an idea of the rapid climb rate or roughly what the beam temperature's potential is assuming it were possible to measure it. This is roughly the instant temperature on micro surfaces.
* MAX COLLECTION-------2300 °F MATERIAL EST...........
Using a $400 infrared thermometer, cement and a few other similar materials are tested for a 3-5 minute period. This is the build temperature. By using high temperature thermally insulated materials, maximum open air temperatures are recorded.
* MATERIALS TESTED
* WATER------------12oz. BOILS 65 sec.
Water boiled in a dark colored vessel with average insulation. These times are reduced with a vacuum tube water vessel. Water cannot be scaled up. For example: 24 ounces of water does not boil in 130 seconds. Because it is a thermally conductive mass and depending on the vessel used, 2x the water volume can require 4x the exposure time. Likewise, 6 ounces or 1/2 the amount of water would boil in 25-30 seconds or less than 1/2 of the 65 second estimate for 12 ounces.
* WOOD-------------FLAME .1 SEC.
Regular wood with a dark spot to focus on, flash point times.
* ZINC-------------MELTS .5OZ 14 GRAMS - 9 SEC. * 3.1 GRAMS - 5 SEC.
Zinc on cement no crucible.
* GLASS------------MELTS 1" X 1/4" BROWN GLASS 12 SEC.
Glass on cement no crucible.
* CONCRETE---------GLOW 14 SEC. EXPOSURE, MELT 28 SEC.
Cement block glow times and meet times. The build temperatures are recored in using the same method but exposure times are increased well beyond the melting point.
"14 years of solar testing and UV resistant testing."
These are all the best methods of testing the power of a Fresnel Lens or Parabolic Mirror. They have worked well over the years and verified by many of our clients including a majority of major university science labs and professors.
This is the long answer to the short question "How hot is that beam of light?"
Dan Rojas
GreenPowerScience.com
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Smaller amounts of metal can superheat blasting right through a sheet of plaster. This test blasted past 3000 degrees in just a few seconds.
Using a skid steer loader to act as a sun tracker with a Large Fresnel Lens.