Intro: Giant Fresnel Lens Deathray: an Experiment in Optics
So you don't have access to your own rail gun or military space laser....but never fear, we'll use the 1000 Watts/m2 of free sunlight in your backyard! But how?
....A 13 square foot magnifying glass!
Seriously. A solid glass lens that size would be silly, but instead we can use a 4 foot wide Fresnel lens. You know, those clear, flat things with the ridges, you find them on overhead projectors and rear windows on some buses? The idea is pretty simple: a Fresnel lens is just a normal curved lens chopped into thousands of little rings, but just as effective.
This instructable (my second) chronicles my progress over the last month or so on this Fresnel deathray. Each step was figured out in real time, but the general idea is this: once you have your giant Fresnel lens, all that remains is to build a frame to keep it straight, and hold perpendicular to the sun. While you can stop here and enjoy the blinding energy of the nickel-sized spot you get at the focus, I went further and attempted to collimate the light into a straight beam. I ordered a focusing lens online and constructed a scaffold to hold it in place, but ultimately found the Fresnel lens to have imperfections standing in the way of proper functionality.
Disclaimer: This device is extremely dangerous, and will INSTANTLY set things on fire! It's extremely cool, but I'm not responsible for anything that happens if you decide to ignite yourself, your house, the forest, or anything else. Also, if you decide to skip the eye protection step, I hope you like braille.
Step 1: Acquire the Lens
They can be had online, but only for substantial piles of cash (from $80-$150 on Ebay), which is why few people ever enjoy these devices. Traditionally, the actual lens is by far the biggest cost in a project like this, with lumber and hardware being almost nothing if you already have the tools. And now, I will impart to you the ultimate source of FREE giant Fresnel lenses:
...Rear Projection TVs.
Every rear projection TV uses a Fresnel lens the exact size of the screen to focus the image. The screen has several layers:
- Outer cover (optional) - Some TVs have a clear layer on the very outside....keep it, it could be useful in another project.
- Lenticular lens - This is the hideous outer screen with 1000s of vertical lines. The purpose of the lines is to spread each pixel outward so you can see the screen from the side. It will probably rip apart as you separate the layers.
- Fresnel lens - this is the innermost layer - clear with millions of circular ridges on one side. The crown jewel of the TV.
1. Craigslist! Go to the free section on your local Craigslist community, and you'll probably find dozens of massive, usually broken projection TVs being given away. Say Billy has a TV from about 10 years ago, and when it breaks, Billy decide to upgrade to a newer technology. Big-screen TVs usually weigh 200-400 pounds, so all Billy wants is someone to make it disappear. If you have a truck and at least one strong friend, this is a great option especially if you don't like option 2.
2. The Dump. If your local dump recycles TVs, you may be fortunate enough to find a pile of TVs sitting around there. My dump doesn't allow scavenging, so we just made sure there was no one around, and helped ourselves to the front parts of TVs and scored 3 giant lenses.
Once you have your TV screen, peel the layers apart (you may need to cut some tape along the top) and extract the precious Fresnel. Admire your plunder, and dispose/recycle the TV carcass.
Step 2: Build a Frame
Note: the ridges on one side of the lens are extremely delicate and scratch effortlessly. A few scratches won't affect performance, but look terrible. Try not to drag the lens against anything.
- At least 15 feet of lumber - I recommend 1x2" boards
- Plywood or misc. scrap wood
- 20-40 wood screws
- Power drill
- Tape measure & pencil
The goal here is to secure the lens in a frame. The most elegant way to do this is to cut a groove down the length of each piece of wood, so the lens fits into the slot. By lowering the saw blade on my table saw so it only stuck up about half an inch, I was able to cut perfect grooves down the boards.
Cutting the Frame
Once you have your grooved beams, you'll need to cut them just long enough to come together with the lens nested into the grooves. Make sure the grooves are all on the inside, and after measuring exactly how long each side should be, cut the sides at 45° angles so the corners look nice. I used a miter to get the precise angles here.
Once the frame pieces are positioned around the lens you can pull everything together. We sandwiched each corner between two pieces of plywood and put screws through all 3 layers, but there are lots of options for this part.
It's a little complicated, and the method varies depending on what tools you have available.If you don't have a table saw, there are other ways to make a groove, or you could trap the lens with multiple boards. If you have any kind of workshop you should be able to rig up something. I don't advise screwing directly into the lens though, because it might crack.
Once your frame is done, you can move the lens around safely. Now BEFORE you go out and start burning stuff, I must urge you to wear the strongest sun glasses you can find, glacier goggles are better, but nothing short of welding goggles are really going to protect your eyes.
The light spots these lenses can produce are literally as bright as the sun.
On that note, be extremely careful where you put this lens. If it's sunny out, the thing shouldn't even be left outside...you never know when it might decide to focus and set your house on fire! Once again I am not liable for anything, including forest fires, so use your head.
Step 3: Eye Protection!!!
This is such a good idea, I decided to make it a whole step. Let me tell you - throughout this project you will spend a lot of time playing with this device, and you'll want to look at the focus a lot to see the results.
However, you will find that after a few seconds, spots linger in your vision when you look away. The center of your retina will become more and more desensitized until it starts taking permanent damage. Then you won't be able to see anything.
When you're looking at this spot, it may not seem so bright. That is because your eyes are already being desensitized. Thus, you have to wear at very least some dark sunglasses. With welding goggles, you can't really see anything except the focal point, so I recommend glacier glasses (used in mountain climbing so you aren't blinded by the sun reflecting off of ice).
Step 4: Measure the Focal Length
- 2 (or more) laser pointers
- A level
- Some flat ground
- A T-square
- A tape measure
- A large, rigid screen
To set up our parallel beams of light, put your two laser pointers on either side of a book or something so that they're parallel. The goal is for the lasers to be perpendicular to the lens, so make sure they're on a level surface. Turn them on and aim the whole setup straight at the lens.
Meanwhile, have someone hold the lens straight up, using a T-square to make the lens perfectly vertical. You'll get two weird diffraction patterns on the wall behind the lens.
Finding the Focus
Now, with your tape measure extending out from the base of the lens, hold your screen up so the two lasers hit it. Move it back and forth until the two spots converge. When they do, see how far from the lens your screen is.
This may sound confusing, but the pictures should help. I recommend trying several times, maybe moving the lasers around, so you can see whether your results are consistent. For my lens, the focal length was about 40 inches (about 100cm) which is average for especially large Fresnels.
Step 5: Acquire Focusing Lens
Benefits of Creating a Beam:
- Objects don't have to be right at the focal point to burst into flames!
- The beam can be further manipulated - magnified, reflected, put through a prism, whatever floats your optical boat.
- Ridiculously intense light beams are like lasers - they're awesome.
In optics, the strength of a lens is measured by its focal length (stronger lenses have shorter ones). To cancel the converging effect of the Fresnel lens, we need to either diverge the light before it gets to the focal point (use a diverging lens with a negative focal length) or converge it after the light spreads out beyond the focal point (using a converging lenses like a magnifying glass).
When two lenses are far apart, it's useful to think of light in terms of geometry and angles: the focusing lens has to be strong enough that its focal length is small so that the light spreading out from the Fresnel's focal point is completely captured by the second lens.
From basic geometry, we know that the second lens has to have at a ratio ratio of diameter to focal length at least as big as the Fresnel lens in order to capture all the light. This means if the second lens has a focal length fB, it has to have a diameter of at least
dB = fB (dA / fA)
where dA and fA are the diameter and focal length of your Fresnel (use the larger width since the Fresnel is not a circle).
With a strong enough lens (the one I got had a focal length of 35mm), you put the lens 35mm (or whatever) past the Fresnel's focal length. The light will then be bent inward, forming a beam. Of course, this will only be approximate, so you'll have to move the lens back and forth until you find the correct distance.
An excellent resource for basic optics is this Optics Applet I've found. You can't really use it to get real-world numbers, but it's very handy for planning and understanding how lenses interact. Place a "beam" on the x-axis, then a couple lenses (you can adjust the focal lengths by dragging the little white squares).
You can find lenses in lots of random places online, and the closer the focal length is to your measurement, the better. Also, bigger lenses are preferable because giant Fresnels typically don't create a very small focus spot (between 1 and 2 inches wide) so you'll need at least a 2" wide lens to capture all the light.
Where I bought my lens:
There are other places I'm sure, especially educational sites and the like...but it may be hard to find the exact lens you need. I should also mention that you want a glass lens, plastic simply won't do for this intensity of light.
Step 6: Lens Scaffold
- 4 1x1 stakes
- Scrap plywood
- Drill, countersink if available
- 2x4 plank
- 2" hole saw (or larger)
- Several right-angle brackets
Odds are you're going to do this your own way if you try it, so I won't go into too much detail about the construction. I assembled the sides first (minus the 2x4s) by cutting the 1x1 stakes with a miter saw to get the necessary angles, then cut plywood gussets to hold these together. We used 2 right-angle brackets (inside corners) to attach these gussets to the plywood crosspiece that will eventually hold the lenses.
Note: A very important thing here is the orientation of the Fresnel lens. I found out the hard way that when the flat side of the lens is facing the sun, it doesn't work right (but well enough that you might not notice). So make sure the ridges are facing out, away from the scaffold - that means they'll be facing down if you build this with the lens on the ground, as I did.
After the sides are completed, two long plywood gussets secure them onto the side of the frame. Since we want the whole device to rotate about its center of gravity (somewhere between the Fresnel and the small lenses), we need a strong beam that passes through that point (hence the 2x4s in the diagram), so we screwed the 2x4s onto the necessary gussets, providing a substantial increase in strength.
Finding the Center of Gravity
To find the center of gravity of this whole scaffold (it will be along the centerline of the 2x4), you and a friend each grab one of the 2x4s and see where the thing balances. You'll want to choose a point closer to the Fresnel (so the Fresnel wants to hang down) because the lens assembly hasn't been installed yet. Finally, drill 1/4" or 5/8" holes (depending on the carriage bolt in the next step) through the points you choose.
Note: when using wood screws in the small wooden stakes, you definitely want to pre-drill/countersink holes, because wood this thin is very easy to crack.
Step 7: Support Base
- One 8 foot 2x4
- Around 8 feet of 2x6 board
- 8 medium lag bolts
- 2 big lag bolts
- 2 carriage bolts and nuts
- 6 washers
- 2 wood spacers (use the lightening holes you cut out for the scaffold)
You can see the basic design from the diagram. The planks are held together with large lag bolts - to use these, drill clearance holes through the first part (as wide as the part of the bolt without threads) and a pilot hole through the second part (not as wide, so the threads can bite into the wood). Then you screw the bolts in with a ratchet. It's very tight, and very strong. A few of these should hold each part together.
You'll want to measure the lens scaffold first, then slightly overestimate the width for the base so you can get it between the two supports easily (the spacers will take up the rest of the width).
Mount the Lens Scaffold
We want to put a couple holes at the top of each support, and insert a suitable collection of a spacer, washers, a nut and carriage bolt (see the diagram).
Once the pivot is together, you can use a wrench to tighten the bolts and lock the scaffold in position.
Step 8: Lens Mount
- 2" PVC expansion joint
- Your favorite epoxy
- Miter saw or hack saw
- Sand paper (60, 150)
The easiest way to set up the optics here is to mount the main focusing lens on the end of a tube around 2 inches wide. This will do exactly what this instructable does - collimate the light into a smaller beam. In a sense, the entire device is already doing this with the sun's parallel rays, but we want the smallest beam possible.
Up until now, I was troubled and lost as to what I would use for this part. What was needed was essentially two tubes inside each other, the inner one allowed to telescope in and out easily, but be able to stay put. Then, while wandering the aisles of Home Depot I found the perfect part: a PVC expansion joint for 2" conduit pipe. It consists of two pipes, the inner one having two o-rings and a lot of silicon lube , allowing it to slide in and out of the outer pipe beautifully. It also happened to be a perfect fit for my 57mm focusing lens.
Preparing the Tube
This was fairly straight forward - the inner tube had a rim sticking out past the ridge where the lens wanted to sit, so I made quick work of it with a miter saw (a hack saw would work equally well, just take it slow and rotate the tube as you're cutting). After a quick sand, the tube was ready for the lens.
I rifled through the adhesives toolbox, found something appropriate for both glass and plastic (Duco Cement) and glued down the lens. A day later someone knocked the tube over and the lens popped off, so I decided to use epoxy to seal the lens in. This worked better (the specific epoxy isn't that important, just pile it up around the sides of the lens to keep it in).
Note: Since diverging light is entering this lens, we want the least curved side of the lens (assuming your lens isn't symmetrical) facing out so the angle of incidence is lower, minimizing loss of light by reflection. Imagine a stone skipping off a pond versus a stone dropping straight down (which is what we want in this case).
Step 9: Installing the Lens Mount
One morning I woke up and found the sun actually out. The past week had been mostly overcast , with almost no sunlight hitting the deck where this project sat. So I ran out, put on my glacier glasses and pulled the tarp off the device.
A simple way to aim at the sun is to rotate the device until its shadows are parallel to the supports on the ground (if the ground is flat). This means the sun is directly forward. Then all you have to do is rotate it so the lens is closer to the sun, and an intense spot of light should form on the lens scaffold.
Even in the middle of winter at this latitude, a 1-inch charred spot formed in a few seconds. It wasn't exactly in the center of the plywood beam, meaning the device wasn't facing perfectly towards the sun.
I didn't expect the light spot on the plywood to be so small. This meant that the focus was right on the plywood - farther than I expected. And since the lens assembly can only extend forward (towards the Fresnel), I had to recess the tube past the plywood. We accomplished this with a primitive housing made of 2x4 beams and plywood sides.
The 2x4s were ripped to a width slightly less than that of the lens tube, so the plywood sides squeezed the tube in place. If you decide to mount the lens this way, be careful not to accidentally crack the tube. But even better, think of a better way to attach it, and make the lens scaffold stick out at least 4 inches past the focal point.
Step 10: Testing
Despite it being December 22nd, winter solstice, the shortest day of the year, I proceeded to test out the completed lens system. But even with the least possible sunlight to work with (several hours before noon, at 37o latitude, we got a very satisfactory spot of blinding energy at the focus. When the device was aimed so that this spot fell on the secondary lens....nothing happened.
Despite moving the lens tube back and forth through the focal point, no beam of light formed beyond the lens mount. To find out why the light wasn't cooperating, we decided to do a beam visualization by blowing dust to reflect the light. We first used flour, but then switched to water mist (from a sprayer) since it's not as messy.
The light funnels into a highly concentrated point, as expected, but then basically fizzes out. If your Fresnel deathray is doing this, most likely the Fresnel lens is backwards and flat side is facing the sun, rather than the ridged side. Getting this right is essential to getting a good beam profile (which we'll see in the next step).
Since the secondary lens is convex, i.e. it bends light inward, the incoming light has to be diverging in order to form a straight beam. Since the light from the Fresnel seems to disperse randomly past the focal point, almost no light even entered the secondary lens. Other Fresnel lens devices on the internet demonstrate good beam shapes, such as in this picture.
Step 11: OK - Let's Burn Something!
Inspired by similar Fresnel experiments floating around the net, I decided to try melting a penny. On winter solstice, I found that a zinc penny melts within a minute when held in the focus. Solid copper pennies (from 1982 or earlier) wouldn't melt, but probably would during summer. Copper's melting point is almost 2000oF compared to Zinc's 790oF. See the first row of images for these tests.
With the Fresnel lens oriented correctly, I had another crack at melting those coins. The following video and the second row of images shows my results. MUHAHAHAHA!!!
Note: Copper's melting point is about 2000oF, but Nickel's is 2600o. So it's highly possible that only the copper in the coin (75% copper, 25% nickel) melted, resulting in the mutilated pitted surface.
Step 12: Conclusion
Clearly, a giant Fresnel lens with an area of ~1.2m2 is a powerful asset. Assuming the maximum available solar energy hitting the ground is around 1000W/m2, this lens could theoretically concentrate 1200W of power into a square centimeter. Of course, at this latitude and time of year, around half of the maximum sunlight is available so this would make an excellent summer project. But even during winter, the fact that I could easily melt solid copper and make a nickel red hot is pretty damn cool.
There are a good number of websites about the joys of giant Fresnels, namely:
- "Now We're Cooking with Light"
- "Random Destructive Acts via Focused Solar Radiation"
- "How to Melt a Penny"
- Howstuffworks Article
- Wikipedia Article
Perhaps the most valuable thing you can get out of this instructable is the source for these giant lenses. There are loads of of them heading for landfills, or recycling, or god knows what else, so reclaim these things and put them to use!
Note: You may think, as I did, "Gee, I bet I could make a super efficient solar panel with one of these". But according to this discussion board that isn't a very good idea, and could ruin your expensive solar panel. You could certainly power a small heat engine like this stirling engine though, by trapping all the light in a black container thermally connected to the boiler. A company working on this technology, but using reflectors instead of lenses, is Stirling Energy Systems.
Thanks to everyone for your comments and suggestions.
Special thanks to foobaz utne for solving my problem with the Fresnel lens focusing properly.
I hope you enjoyed this project, and I will either update this if I further develop it, or post other solar-related projects in the future.
jkimball made it!