Giant Fresnel Lens Deathray: An Experiment in Optics

Step 5 Acquire Focusing Lens

Now that you know the focal length of your Fresnel, it's time to get a diverging lens to bend the light into a beam. This will go right at the focal point, so you get as small a beam as possible.

Benefits of Creating a Beam:

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• 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.

Optics Refresher

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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).

Diagram 1

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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 f_B, it has to have a diameter of at least

d_B = f_B (d_A / f_A)

where d_A and f_A are the diameter and focal length of your Fresnel (use the larger width since the Fresnel is not a circle).

Diagram 2

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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.

Optics Applet

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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).

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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:
Surplus Shed
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.

Comments

[davoww]

How about using a light tunnel to align the beam after the fresnel lens.
DLP tvs have them in their light engine. It is basically two lenses at each end of a square tube that has mirrors lining the inside.
Would that work?
And can anyone tell me what is the difference between fresnel & lenticular lens?
 

[DrSimons]

I've thought about using some kind of mirror tube. It would work to funnel the light down the tube, but it would diverge like crazy at the other end. I think the problem with a glass lens is with such a high intensity of light, a ton of it is reflected away or comes in at the wrong angle due to dispersion.

P.S. a fresnel lens is a series of concentric rings, which has a single focal point. Lenticular lenses are a 1 dimensional pattern like a bunch of rows of crops. It's pretty much useless except for image screens, such as projection TVs and certain holograms.

[joat_mon_99]

When taking apart the rear projection TV to get the fresnel lens, you should notice a projector array as well, be sure to remove that, or at least the lenses themselves - the lenses are usually black plastic tubes with 3 lenses inside (in some cases you may have to break the lens housing to get them out. In the ones I've got, the two end lenses are plastic, and are not suitable for use in this project, inside the casing, the middle lens is glass, mine is about 3inches in diameter. Pop that out and it can be used to further focus the fresnel beam. In may case there were also steel mounting brackets between the bottom of the lens assembly and the CRT projection tubes. (also, when you take these off, you will likely have to drain the optical fluid from on top of the CRT - this is ethylene glycol, a coolant with the interesting property of having the same refractive index as glass. I recommend draining it into a clean container and saving it for other uses.) If you remove and save those, they make a nice frame to mount your glass lens to your fresnel deathray project.

[sherp]

why won't the focusing lens be burned by the light beam of the fresnel? won't it be melted by the laser beam?

[matroska]

Nop, it's glass. And even if the melting point of glass is extremely high, the light is not 'hitting' the glass, it's passing through. Meaning, the glass is pretty much not absorbing any heat.

[SKULLOK]

In addition, I would like to say that they use the mirror array method in making the new "extreme" ultraviolet concentration needed to make the quad core extreme computer chips, as lenses alone diffused too much light, and didn't get a high enough concentration for the wavelength and frequency they needed. You should try it!

[SKULLOK]

There is no melting point for glass, it's not a solid. Glass is just a very slow moving liquid, and adding heat to it only speeds it up, so, as there is no melting point, it is very difficult to tell when a heat is actually causing the viscosity to fall significantly enough for the form to be changed significantly (which may not be visible, but may still cause negative effects). It also depends on the purity of the glass, and the lens convexity (the more convex the lens, the more concentrated the beam, as many people know). However, if one would want a more effective system, you wouldn't want to use one large lens, but rather an array, and lenses tend not to be nearly as effective as mirrors, as lenses diffuse the light due to impurities. If you set up an array of mirrors, you can concentrate a much higher amount/power of light for a lot less time/effort, as small mirrors are a lot easier to work with than 13 square feet of back pain... (people don't realize how heavy glass is. It may seem weak, but what you need to understand, is that glass is basically transparent ROCK... you try lifting a 13 square foot boulder and tell me how well you can position it to get the most light from the sun)

[matroska]

Haha ok! I agree. Well I learned something. I really thought that glass was a real solid (kinda incredible to think it's still a liquid too), and so that if light didn't pass through it, it must have been absorbed somehow. I just deducted that light absorbed was transferred as thermal energy. Thanks for pointing out

[tanmanknex]

yeah, and actually, if you go to 100 year old houses, the glass is thicker at bottom than at top!

[Ev]

Not true. Recent research has found that glass in centuries old cathedrals were designed to have thicker glass at the bottom to be strong enough to hold the thinner glass above. They found that glass does not flow as much as believed.

[steampirate]

This is true but anything will "flow" if left somewhere for long periods of time. Take for instance a steel rod. If you put a steel rod over a board for a hundred years it will be bent. The glass is thicker on the bottoms of windows in old buildings because it was designed that way. Back in the good old days one side of a piece of glass was thicker than the other. Construction workers put the thicker side down for structural integrity. Sometimes they made mistakes and you can find windows that are thicker on the top!

[sherp]

very interesting. so could you focus the beam on a mirror to deflect the energy?

[Easymac79]

That's what i was wondering with my question above, it is hitting the mirror, not passing through it, because a mirror works by reflecting the light that HIT it. and because that reflective material is usually metal, it will absorb the light. and in addition, it may not reflect the light to the side, the focal point may just end on the metalic surface of the mirror.

[matroska]

The metal that reflects the lights in the mirror doesn't absorb it (much, because it does a bit; everything does); the mirror is a (very) polished surface and thus all the light will be relfected.

In fact, I think I remember reading that the maximum percentage of light reflection with the BEST mirrors on earth is about .. 80%? I'm not sure at all, but I do know that total internal reflection does reflect light at 100% for sure. Hence the use of prims (prism use internal total reflection in telescopes and all) in many optics, mostly telescopes because it's often used in astronomy, and to minimize the loss of light.

But yeah, if it reflects say 60%, the other 40% HAS to be absorbed in some way, and that's why I think a mirror will heat up quite a lot using this whole death ray array, but will not melt.

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I'd suggest using a lens to make the rays parallel and then a prism to direct it and then destroy whatever you wayt. :)

[Easymac79]

you said use a prism to reflect it... but a prism splits white light into all colors. A laser of 1 color could bend through, but white light from the sun will split

[matroska]

You must mistake the prism principle I am talking about with dispersion. Yes, if you direct a ray of light at an oblique angle on a prism, you get a Pink Floyd cover.

However, if you direct a ray totally perpendicular, it will go through and bounce on usually 90 degree angle.

See this page for a quick visualisation:
http://en.wikipedia.org/wiki/Porro_prism

Porro prism is one of many type of prism used for redessing optical images.

Obviously, one prism I should have tought of before is the pentaprism, widely used in any single lens reflex camera.

Pentaprism:
http://en.wikipedia.org/wiki/Pentaprism

The prisms which I was referring to were really replacements for mirror, in fact good alternative to them.

[aardvarkamericaawesome]

A pentaprism uses mirror-coated surfaces to achieve its effect, so it seems it would be the same as using a mirror--two mirrors, in fact--in terms of energy loss.