Hello, I hope I'm in the right spot and that this hasn't been answered somewhere but I looked around it seems to have not come up. Anywho, my problem is this:
If I set up a double convex lens and shine a light through it, but before it reaches it's focal point I put a plano concave, I've been told that it will straigten out into a normal beam again. If this fact is wrong already, let me know. But if that beam of light is a projection, projected straight for whatever reasons, when it goes through those lenses and becomes smaller, will the image scale down with it and maintain quality, just smaller?
Please let me know, or tell me if I make absolutely no sense.
If I set up a double convex lens and shine a light through it, but before it reaches it's focal point I put a plano concave, I've been told that it will straigten out into a normal beam again. If this fact is wrong already, let me know. But if that beam of light is a projection, projected straight for whatever reasons, when it goes through those lenses and becomes smaller, will the image scale down with it and maintain quality, just smaller?
Please let me know, or tell me if I make absolutely no sense.
If you start with a beam made from parallel rays, then a positive lens (EG. a convex lens) will converge those rays to a point at the focal distance of the lens. If you put a negative lens before that point, you could diverge the rays back to a parallel beam. (The negative lens would have to have the correct focal length to do that.) The end result would be a parallel beam of smaller diameter and more light per square centimeter of cross section.
But you can't form an image from a parallel beam! It would "focus" at an infinite distance. Image projection occurs when all the rays coming from a point on the object are focussed by a lens or lens system back to a single point in the image. The rays coming from another point on the object are focussed to the corresponding point in the image, etc. You have to have a lens or lens system with a net positive focal length to do that.
The basic parameter you need to deal with is the focal length. That tells you everything about the geometry of throw distance, image size, object size, and object-to-lens distance. The image size is a product of the object size times the ratio of the throw distance divided by the object-to-lens distance. The equations are simple. Given an object of a certain size and a throw distance, you can calculate the focal length you will need to get a particular desired image size. Or you can solve for whatever is unknown. There are some freeware and online calculators for this.
A lens system can be adjusted with a very weak positive or negative lens to alter the focal length. It may introduce some distortion, but is worth an experiment if that is what you need to do. But it is not easy to find such weak lenses! (like a 1000 mm fl lens)
The next level of issues are all about cancelling the distortions and abberations introduced by spherical-section lenses. That is the difference between a $2 magnifier and a $1000 camera lens.
But you can't form an image from a parallel beam! It would "focus" at an infinite distance. Image projection occurs when all the rays coming from a point on the object are focussed by a lens or lens system back to a single point in the image. The rays coming from another point on the object are focussed to the corresponding point in the image, etc. You have to have a lens or lens system with a net positive focal length to do that.
The basic parameter you need to deal with is the focal length. That tells you everything about the geometry of throw distance, image size, object size, and object-to-lens distance. The image size is a product of the object size times the ratio of the throw distance divided by the object-to-lens distance. The equations are simple. Given an object of a certain size and a throw distance, you can calculate the focal length you will need to get a particular desired image size. Or you can solve for whatever is unknown. There are some freeware and online calculators for this.
A lens system can be adjusted with a very weak positive or negative lens to alter the focal length. It may introduce some distortion, but is worth an experiment if that is what you need to do. But it is not easy to find such weak lenses! (like a 1000 mm fl lens)
The next level of issues are all about cancelling the distortions and abberations introduced by spherical-section lenses. That is the difference between a $2 magnifier and a $1000 camera lens.
You can find an assortment of focal lengths in positive meniscus lenses. (They are the type of lenses found in eyeglasses)
Including 1000mm fl here.....Rolyn Optics .
Or you could make a symmetrical duplet projection lens. Duplet projection lenses are the kind used in mid-range overhead projectors and in slide projectors.
Here's the quick and easy way to figure out what you need.
Decide on the final focal length of your projection lens, the focal length you need to get the size projection you want. As an example, lets say you need a 300mm EFL projection lens. To find out what focal length the two individual lens elements have to be, multiply the EFL by 1.71. So, your two lenses have to be 513mm fl lenses. (get reasonably close anyway). To find out how far apart to mount the two lenses in the mounting tube, divide the EFL by 2. So they need to be mounted 150mm apart. Now you have a 300mm EFL symmetrical duplet projection lens.
Using the same calculation, if you need a 6 inch EFL projection lens,
The two lenses inside need to be 10.26 inch focal length
They need to be mounted 3 inches apart.
The field of view on projection lenses made this way is about 30 degrees, so get the biggest diameter lenses you can. Positive meniscus lenses from Rolyn are from about 65mm diameter for the glass ones and about 75mm for the acrylic ones.
Hope this helps you out.
Tgreenwood
Including 1000mm fl here.....Rolyn Optics .
Or you could make a symmetrical duplet projection lens. Duplet projection lenses are the kind used in mid-range overhead projectors and in slide projectors.
Here's the quick and easy way to figure out what you need.
Decide on the final focal length of your projection lens, the focal length you need to get the size projection you want. As an example, lets say you need a 300mm EFL projection lens. To find out what focal length the two individual lens elements have to be, multiply the EFL by 1.71. So, your two lenses have to be 513mm fl lenses. (get reasonably close anyway). To find out how far apart to mount the two lenses in the mounting tube, divide the EFL by 2. So they need to be mounted 150mm apart. Now you have a 300mm EFL symmetrical duplet projection lens.
Using the same calculation, if you need a 6 inch EFL projection lens,
The two lenses inside need to be 10.26 inch focal length
They need to be mounted 3 inches apart.
The field of view on projection lenses made this way is about 30 degrees, so get the biggest diameter lenses you can. Positive meniscus lenses from Rolyn are from about 65mm diameter for the glass ones and about 75mm for the acrylic ones.
Hope this helps you out.
Tgreenwood
Okay, the comment on not producing an image. This is ultimately for a DIY projector, but when you shine the lightsource through the LCD panel, are you saying that through that, it is no longer a parralel beam? And if it's off by a little to not be parralel, is that a major issue?
My main question is that LCD screens seem to be larger than the emission matrix (or tube thing at the end, I don't know what the formal name is) so when I put my light through it, in order to make it fit through the emission matrix, could I focus the image. I know it would take a lot of calculation to find the proper kinds of lenses to focus it out, but could I condense the image, (or colored rays of light, or some other fancy term I should be using) and then put it through the emission matrix to produce my final projected image? I was just under the impression that if I were to do that, all the different little rays of color would be condensed into a smaller section of such rays but in the same realitive posistion after all the other rays were condensed. If I am completely wrong on how that works, do not hold back explanations.
My main question is that LCD screens seem to be larger than the emission matrix (or tube thing at the end, I don't know what the formal name is) so when I put my light through it, in order to make it fit through the emission matrix, could I focus the image. I know it would take a lot of calculation to find the proper kinds of lenses to focus it out, but could I condense the image, (or colored rays of light, or some other fancy term I should be using) and then put it through the emission matrix to produce my final projected image? I was just under the impression that if I were to do that, all the different little rays of color would be condensed into a smaller section of such rays but in the same realitive posistion after all the other rays were condensed. If I am completely wrong on how that works, do not hold back explanations.
There's a little animation thingy at Geometric Optics webpage that really helped me understand how projection works.
Take a look at the attachment, I did this one to kind of show you how basic projection works. The small arrow on the left is the "object" and represents the LCD. The lines from the point of the arrow through the lens shows how light goes throught the lens and gets projected. The bigger Arrow on the right show how much the projected "object" is magnified and how far away it focuses an image from the lens.
When you say emission matrix, do you mean the projection lens?
Take a look at the attachment, I did this one to kind of show you how basic projection works. The small arrow on the left is the "object" and represents the LCD. The lines from the point of the arrow through the lens shows how light goes throught the lens and gets projected. The bigger Arrow on the right show how much the projected "object" is magnified and how far away it focuses an image from the lens.
When you say emission matrix, do you mean the projection lens?
Attachments
Ya, I couldn't think of the right word but when I say emission matrix, I mean the final projection lens. And thanks for that optics simulator link.
I'm not sure what you mean though, by that image. I understand that a projection image is a collection of beams of light with secific color to them. Correct me if I'm wrong there. And that the initial image is small, so then after it exits the projection lens, it gets larger. Thus making a projector possible. I'm asking if it works the opposite way. Take your image and make it a little smaller, but before it hits the focal length, put a concave lens there to straighten the beams out again. I'll try to get an image soon.
I'm not sure what you mean though, by that image. I understand that a projection image is a collection of beams of light with secific color to them. Correct me if I'm wrong there. And that the initial image is small, so then after it exits the projection lens, it gets larger. Thus making a projector possible. I'm asking if it works the opposite way. Take your image and make it a little smaller, but before it hits the focal length, put a concave lens there to straighten the beams out again. I'll try to get an image soon.
main question
The answer to your "main question" is yes! You have re-invented the light condensor system used in almost every projector.
If you build a projector with a small LCD (or CRT) and a very large lens that can be very close to the LCD or CRT, then you don't need a condensor. Most of the rays coming from each pixel will hit the lens and get focussed to a point in the screen image.
If you build a projector with a large LCD and a small diameter projection lens, then you need a condensor lens to direct the light from all the pixels into the projection lens. Without the condensor, you would see a very dim image because most of the rays from each pixel would not even hit the lens.
Most DIY LCD projectors use a pair of plastic fresnel lenses (like page magnifiers) to implement the condensor system. These can be as cheap as $5-10 by using actual page magnifiers, or you can buy better fresnels for $20-35 each. Since almost all such projectors use a Metal Halide lamp as the light source, they place the first fresnel (the condensor fresnel) so the lamp arc is at the fresnel's focal point. (Usually 200-220 mm from the fresnel.) The result is a fairly uniform beam of pure white light coming out the other side of the fresnel. The LCD is placed at least 1 centimeter from the condensor fresnel (so the focussed screen image will not include the fresnel's ring pattern).
The field fresnel is placed at least 1 cm after the LCD, to bend the parallel rays into the projection lens. The field fresnel does slightly magnify the screen image, but it is placed so close to the LCD that this effect is minimal.
So the standard DIY LCD projector really consists of two interacting optical systems. The "projection system" uses the projection lens to focus LCD pixel rays into a screen image. The "condensor system" uses the fresnels to focus an image of the MH lamp arc into the projection lens. That gets most of the usable light on the screen for a bright image.
The answer to your "main question" is yes! You have re-invented the light condensor system used in almost every projector.
If you build a projector with a small LCD (or CRT) and a very large lens that can be very close to the LCD or CRT, then you don't need a condensor. Most of the rays coming from each pixel will hit the lens and get focussed to a point in the screen image.
If you build a projector with a large LCD and a small diameter projection lens, then you need a condensor lens to direct the light from all the pixels into the projection lens. Without the condensor, you would see a very dim image because most of the rays from each pixel would not even hit the lens.
Most DIY LCD projectors use a pair of plastic fresnel lenses (like page magnifiers) to implement the condensor system. These can be as cheap as $5-10 by using actual page magnifiers, or you can buy better fresnels for $20-35 each. Since almost all such projectors use a Metal Halide lamp as the light source, they place the first fresnel (the condensor fresnel) so the lamp arc is at the fresnel's focal point. (Usually 200-220 mm from the fresnel.) The result is a fairly uniform beam of pure white light coming out the other side of the fresnel. The LCD is placed at least 1 centimeter from the condensor fresnel (so the focussed screen image will not include the fresnel's ring pattern).
The field fresnel is placed at least 1 cm after the LCD, to bend the parallel rays into the projection lens. The field fresnel does slightly magnify the screen image, but it is placed so close to the LCD that this effect is minimal.
So the standard DIY LCD projector really consists of two interacting optical systems. The "projection system" uses the projection lens to focus LCD pixel rays into a screen image. The "condensor system" uses the fresnels to focus an image of the MH lamp arc into the projection lens. That gets most of the usable light on the screen for a bright image.
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