" Does anyone know of a way to prevent this burning?"

Put the lens out at night.

It would help to know why you are deploying the lens. What does it do?

Yes i agree with lyn what are you doing with the lens, So we can work out what would be the best approch.

Night time use would solve the problem

or dont put anything underneath it

what is it that you are putting under it.

In the absence of knowing what you are doing an infra red filter will stop the heat rays getting through, But it may be the infra red that you need for whatever it is your doing.

Its not a nuclear bomb busrt indicator is it, Like the ones used by the Royal Observer Corp in the uk in the 50s ? yes i was a member of that as well. Sad i know

http://www.youtube.com/watch?v=LWcsOz2_XpY

http://freespace.virgin.net/richard.wordsmith/roc/rochist.htm

http://www.ringbell.co.uk/ukwmo/Page222.htm

this is the high tech device that worked out where in the sky the burst took place.

there was another that measured the presure wave, equally high tech (not)

and a for want of a better name a glass ball with paper under it to trace out the suns postion during the day

Campbell-Stokes recorder

From Wikipedia, the free encyclopedia Jump to: navigation, search A Campbell-Stokes recorder adapted for use in polar regions (The right sphere is facing south) Close up of a summer sunshine card for the Campbell-Stokes recorder. Campbell-Stokes recorder used in a tropical region.

The Campbell-Stokes recorder (sometimes

Ground Zero Indicator

From Wikipedia, the free encyclopedia Jump to: navigation, search Ground Zero Indicator, showing the interior graticules (and the insert at bottom left shows the instrument mounted with its cover in place)

The Ground Zero Indicator, known by the acronym GZI was a specially designed shadowgraph instrument used by the British Royal Observer Corps during the Cold War to locate the Ground Zero of any nuclear explosion.

Peter... wow. ga.

Thanks

You may know about this stuff too?

I forgot to add that polarizers won't work. Infrared light goes right through them; optical polarizers only work on visible light. You can buy polarizers that work on IR, but they are expensive.

One other thought. If the lens you are using is large, you might need a large piece of heat-absorbing glass. Contact a local dealer of Pilkington window glass and see if you can get a piece of "Evergreen" glass. Here is a link to some info on it and other heat-absorbing glasses from Pilkington.

http://www.pilkington.com/resources/336224pg.pdf

Change the orientation of optic axis of lens to horizontal along N-S direction.

Yea Dear,

I have been working with focussing/ magnifying devices including Fresnel's lens for some 25 years. Only thing I can say is – as long as the input radiation contain heat in it, any focussing device is going to burn the thing that you will put under it as long as the device is not covered. Yes, The only exception is the radiation emitted by LEDs, as this radiation does not contain any heat component, definitely you can work safely without burning any thing that you may put under focussing/ magnifying device.

There is always a ray of hope and silver lining around dark clouds.

I definitely would love to know the application of your device if it is not a closely guarded secret and you wish to share.

I am Dr N P Singh and available at npsdhillon@gmail.com

Best of luck,

Dr N P singh

Don't put anything that will burn near the "plane" where the lens focuses.

You can put heat absorbing glass on the sun side of your lens. The filter will get warm and may need to be cooled with a fan or other means. The radiation that passes through the filer will still be focused by the lens, but the power density will be much lower as the bulk will be absorbed by the filter.

Having purposely burned things with a magnifying glass as a kid, it won't burn that much if the object under it isn't at it's focal length. Of course, if you are wanting it to magnify something visually at the same time then, yes, you may burn this object depending on how flammable it is.

What exactly are you doing? Others will be able to help you more if you explain.

If the purpose is only to put the lens under Sun, then put on other lens below this lens which can spread the heat (concave lens) instead of focusing at focus.

SurplusShed.com, sells surplus Jaegers heat glass filters, 57mm diameter by 2.5mm thick. The heat absorbing filters transmits visible light while absorbing infrared energy, which where most of your heat comes from.

Item No: M2927 Price: $2.00 each or 10 for $15.00

http://www.surplusshed.com

Laughing Jaguar

You need to apply a partially reflective metal coating to the lens. The coating will act as a neutral density filter across ALL wavelengths and will greatly reduce the amount of energy that passes through the lens. If the coating is made of the right density you still be able to see through it to read what is behind the lens but it won't transmit enough energy to cause thing to burn.

DG

mars, Please tell us more about what you're trying to do and why?

We are happy to help you out. Which way did you come in?? Actually a beam splitter will redirect a controlled fraction of the incoming light without affecting its spectral make-up. Another approach is to use photo-chromic glass between the light source and the lens. It kind of depends on your application. You have not disclosed that information, so you may not receive an optimum answer.

Sorry about not informing you of what the application is earlier. It's very simple. The lens is going to be used to help someone read. It will be elevated enough that a person could read a sheet of paper, book, whatever, but also have enough room to write. In other words the lens will be elevated far enough from the page that they can get their hand in there and write on it.

It may seem silly that this thing is going to be outside all day but it will be attached to something that is usually kept outside (that's the part I wont tell you). But really the application is just to magnify text on a sheet of paper. That's what I was worried about burning since someone may just leave the paper under the lens by accident.

I believe I have though of a way to solve this and would like to hear your opinions. Regular magnifying lenses are circular and focus the light to a single point. On the other hand, bar magnifiers are long and rectangular and focus light into a line. They only "stretch" text in one direction, vertically, but I think this will be enough magnification. Many people use bar magnifiers to help them read. I am going to make one that has a much longer focal distance than traditional bar magnifiers. This way it can be suspended above the page and leave room for someone to write. Any thoughts?

By the way, the heat absorbing glass was a little too expensive but very good idea anyway. Thanks!

If the lens is at say an angle of 45 degrees and orintated west to east with the open side facing south that should do it with no further additions

Good idea by the way, Ive guessed what you want it for.

I suggest you try longer focal length lenses located close enough to the eye that sunlight won't get through them. The print would be illuminated exclusively be ambient (unfocused) light. The lens formula could be cylindrical, spherical, or asymmetric. The lenses could be temporarily attached in front of the viewer's eyes using a removable clip type device. This would minimize the chances that the lens assembly would be left out in the sunlight

Yes. Cylindrical lens is better choice if magnification in vertical direction of letters is enough to read, because it focuses input energy in to a line rather than to a point, and hence the intensity at focused region is less. Also as you thought (and Welderman suggested), the 'object-to-focal point distance' (f-u) will be more for longer focal length lens relative to short focal length lenses and hence the chances of heating/burning would reduce. The distance from lens-to-object (u) is D*f/(D+f) for simple microscope where D=25cm. So for a f=25cm lens, 'u' is 12.2 cm or f/2, whereas if you choose a f=100cm lens, 'u' would be 20cm or f/5. So, for the f=100cm case the paper is 80 cm away from focal point and hence buring may not be an issue. Because 'u' is also the distance available for the reader to write, longer focal length lens will take care of this aspect too. As readers suggested earlier a reflective broadband IR coating on input surface will further optimize the system but increase cost. Two websites which may be useful to you are

http://www.goalfinder.com/product.asp?productid=26

and

http://www.tutorvista.com/content/physics/physics-ii/optical-instruments/simple-microscope.php

Forgot to add that the longer the focal length the smaller the magnification for single lens magnifier (eg., which is just 2 for f=D). So you have to optimise the focal length of the lens for 3 aspects namely., a)required magnification,b) how far away the object is from the focal plane ('focus-to-object distance'), to reduce chances of burning paper c) object-to-lens distance to facilitate writing by reader/user, which is just difference of focal length of lens and the 'focus-to-object distance'. I hope when you optimize the focal length the lens, the user/reader's eyes may strain less than those of watch maker/repairers.

Why don't think to a TVcamera with a monitor for reading?