hi, this is a question i would like to ask to guy grotke about the ideal fresnell lens tread but don´t like to confuse people there, we can discus here and post there the conclusions if we get any 😀
i would like you to post your math you use to get to 550mm focal lenth. Actually, my math tell me it would be larger focal but would like to hear yours first.
If someone different to guy grotke would like to post maths or wherever they like, feel free to share it. Thanks everibody. Rox
i would like you to post your math you use to get to 550mm focal lenth. Actually, my math tell me it would be larger focal but would like to hear yours first.
If someone different to guy grotke would like to post maths or wherever they like, feel free to share it. Thanks everibody. Rox
math behind my 550 mm fl fresnel
The assumptions I used are:
1) This is for use with a 450 mm fl projector lens.
2) People using such a long-throw lens can put their projector on a table or shelf at the back of their media room, so they don't need to make the compromises of a split design to get optical keystone correction.
3) Reasonable throw distances are 10 to 14 feet.
All the math you need is:
1/projection lens fl mm = 1/LCD to lens mm + 1/lens to screen mm
For the 10' throw case, 10 feet = 3048 mm, so this yields:
1/450 = 1/LCD to lens + 1/3048 so:
LCD to lens distance = 528 mm
For the 14' throw case, 14 feet = 4267 mm so:
1/450 = 1/LCD to lens + 1/4267 and:
LCD to lens distance = 503 mm
The field fresnel in an unsplit design has to go at least 20 mm before the LCD, or you will see fresnel rings in the screen image:
528 mm + 20 mm = 548 mm (which I rounded up to 550 mm)
So you can put a 550 mm fl fresnel exactly 22 mm before the LCD to get a perfect arc image focussed at the optical center of the projection lens, for a 10 foot throw distance.
If you need a 14 foot throw distance, then you can put the same 550 mm fl fresnel (550 - 503) = 47 mm before the LCD to get a perfect arc image focussed at the optical center of the projection lens.
There are ways to make other focal length fresnels work. Just not as well.
The assumptions I used are:
1) This is for use with a 450 mm fl projector lens.
2) People using such a long-throw lens can put their projector on a table or shelf at the back of their media room, so they don't need to make the compromises of a split design to get optical keystone correction.
3) Reasonable throw distances are 10 to 14 feet.
All the math you need is:
1/projection lens fl mm = 1/LCD to lens mm + 1/lens to screen mm
For the 10' throw case, 10 feet = 3048 mm, so this yields:
1/450 = 1/LCD to lens + 1/3048 so:
LCD to lens distance = 528 mm
For the 14' throw case, 14 feet = 4267 mm so:
1/450 = 1/LCD to lens + 1/4267 and:
LCD to lens distance = 503 mm
The field fresnel in an unsplit design has to go at least 20 mm before the LCD, or you will see fresnel rings in the screen image:
528 mm + 20 mm = 548 mm (which I rounded up to 550 mm)
So you can put a 550 mm fl fresnel exactly 22 mm before the LCD to get a perfect arc image focussed at the optical center of the projection lens, for a 10 foot throw distance.
If you need a 14 foot throw distance, then you can put the same 550 mm fl fresnel (550 - 503) = 47 mm before the LCD to get a perfect arc image focussed at the optical center of the projection lens.
There are ways to make other focal length fresnels work. Just not as well.
ok, now there is a point i would like you to tell me about;
when you focus the light from the fresnell to the triplet, you are focusing exactly at the center of the lens;
thats how i thought it works, anyway, this lens we are using as example, is the thing 450mm equivalent lens, but we can do it with the 135 triplet, asuming the optical center is at midle (is 1cm from the midle actualy) so the lens we are focusing on is 7cm closer from the optical center of the triplet. i mean, if the LCD to lens distance is 528 mm, then we can say that the rear lens of the triplet is at 458mm from the lcd, so we have to focus there with the fresnell. I wouldn´t focus on the center but in full 135mm circle, so the focusing point would be farther.
i´ll try to post some pic from a very good friend of mine.
please tell me what yo you think about this GG.
when you focus the light from the fresnell to the triplet, you are focusing exactly at the center of the lens;
An externally hosted image should be here but it was not working when we last tested it.
thats how i thought it works, anyway, this lens we are using as example, is the thing 450mm equivalent lens, but we can do it with the 135 triplet, asuming the optical center is at midle (is 1cm from the midle actualy) so the lens we are focusing on is 7cm closer from the optical center of the triplet. i mean, if the LCD to lens distance is 528 mm, then we can say that the rear lens of the triplet is at 458mm from the lcd, so we have to focus there with the fresnell. I wouldn´t focus on the center but in full 135mm circle, so the focusing point would be farther.
i´ll try to post some pic from a very good friend of mine.
please tell me what yo you think about this GG.
as said, this is the setup my friend used:
lets start;
forget about the fresnells he used by now, you can see his throw is about 350meters, so the optical center of his 135triplet is at 51,53cm from the lcd. He has 1cm between lcd and fresnells (both behind the lcd) so the distance between fresnell and triplet optical cneter is 52,53cm.
Now we want (i thought it was this way at least) the light focusing like the green cone so all the triplet rear lens is used. This is easy to find the focal lenth for the fresnell since only depends on the triplet aperture (135mm) and the lcd size. My friend needed a focal lengh of 68,7cm so full rear lens can be used.
actually he has 2*330 fresnells so he just placed the bulb at the only postion where the light is focused at 68,7 at the other side. this bulb placement he worked out teorically was 21,70cm, but this has nothing to do with us, since we´ll have 220 rear fresnell and the perfect field fresnell so lamp doesn´t need to move from the focal of the rear fresnell.
Thats why i didn´t understand your 550 mm value, the main difference is that we are focusing on the full lens aperture while you are focusing on the center, isn´t it? please tell me your thoughts. anyway, fresnells can be forced on a small range just by the lamp placement so 550-650 fresnell focal range is easily atainable.
thanks, rox
An externally hosted image should be here but it was not working when we last tested it.
lets start;
forget about the fresnells he used by now, you can see his throw is about 350meters, so the optical center of his 135triplet is at 51,53cm from the lcd. He has 1cm between lcd and fresnells (both behind the lcd) so the distance between fresnell and triplet optical cneter is 52,53cm.
Now we want (i thought it was this way at least) the light focusing like the green cone so all the triplet rear lens is used. This is easy to find the focal lenth for the fresnell since only depends on the triplet aperture (135mm) and the lcd size. My friend needed a focal lengh of 68,7cm so full rear lens can be used.
actually he has 2*330 fresnells so he just placed the bulb at the only postion where the light is focused at 68,7 at the other side. this bulb placement he worked out teorically was 21,70cm, but this has nothing to do with us, since we´ll have 220 rear fresnell and the perfect field fresnell so lamp doesn´t need to move from the focal of the rear fresnell.
Thats why i didn´t understand your 550 mm value, the main difference is that we are focusing on the full lens aperture while you are focusing on the center, isn´t it? please tell me your thoughts. anyway, fresnells can be forced on a small range just by the lamp placement so 550-650 fresnell focal range is easily atainable.
thanks, rox
drawing assumes point source lamp
Your drawing assumes that the fresnels can focus the light to a single point. But the object being focussed is nothing like a point source: It is a lamp arc that is somewhere between 15 and 25 mm long. The magnification of this image is a product of the ratio of the distances before and after the fresnels. For example, with a distance of 220 mm before the fresnels and 550 mm after the fresnels the image of the lamp arc would be:
25 mm * (550/220) = 62.5 mm
If you try to focus that anywhere but right at the "centro optico", then a lot of light will miss the lens. You don't have to take my word on this: Try an experiment! Remove your triplet and put a piece of white paper where the triplet's optical center would go. Then you can move the paper around to see how big the arc image is at different distances, and how much of the light you would lose if you don't put the arc image right at the center of the lens.
Another thing to think about: There is no advantage at all in "using the whole lens". If you could make the light from each LCD pixel strike a single point on the first surface of the lens so the ray would go through the exact optical center of the lens, you would get a fantastically sharp and bright image. (This is what would happen with a tiny point-source lamp. And also it is why commercial LCD projectors use MUCH more expensive and shorter life short-arc lamps.)
Instead, with our larger arcs we get a lot of different rays passing through each LCD pixel. They strike areas all over the lens and then get refracted to (nearly) a single point on the screen. But not to exactly the same point! That would require a perfect lens, and no such thing exists. So some of those rays end up off by a small amount, giving you lower contrast or even a blurry image. Rays passing far from the center of the lens suffer the most aberration.
If you minimize the area of the lens used by the light from each pixel, then less distortion and aberration is introduced. This is like "stopping down" a photographic lens to get a sharper image: A simple technique that every beginning photographer learns. We have a great advantage over photographers in this regard. because we don't have to close the lens aperature (and throw away a lot of light) to do this. All we have to do is setup the condensor fresnels correctly.
Defocussing the arc image past the projection lens ignores this simple fact, and there is absolutely no benefit of any kind!
Your drawing assumes that the fresnels can focus the light to a single point. But the object being focussed is nothing like a point source: It is a lamp arc that is somewhere between 15 and 25 mm long. The magnification of this image is a product of the ratio of the distances before and after the fresnels. For example, with a distance of 220 mm before the fresnels and 550 mm after the fresnels the image of the lamp arc would be:
25 mm * (550/220) = 62.5 mm
If you try to focus that anywhere but right at the "centro optico", then a lot of light will miss the lens. You don't have to take my word on this: Try an experiment! Remove your triplet and put a piece of white paper where the triplet's optical center would go. Then you can move the paper around to see how big the arc image is at different distances, and how much of the light you would lose if you don't put the arc image right at the center of the lens.
Another thing to think about: There is no advantage at all in "using the whole lens". If you could make the light from each LCD pixel strike a single point on the first surface of the lens so the ray would go through the exact optical center of the lens, you would get a fantastically sharp and bright image. (This is what would happen with a tiny point-source lamp. And also it is why commercial LCD projectors use MUCH more expensive and shorter life short-arc lamps.)
Instead, with our larger arcs we get a lot of different rays passing through each LCD pixel. They strike areas all over the lens and then get refracted to (nearly) a single point on the screen. But not to exactly the same point! That would require a perfect lens, and no such thing exists. So some of those rays end up off by a small amount, giving you lower contrast or even a blurry image. Rays passing far from the center of the lens suffer the most aberration.
If you minimize the area of the lens used by the light from each pixel, then less distortion and aberration is introduced. This is like "stopping down" a photographic lens to get a sharper image: A simple technique that every beginning photographer learns. We have a great advantage over photographers in this regard. because we don't have to close the lens aperature (and throw away a lot of light) to do this. All we have to do is setup the condensor fresnels correctly.
Defocussing the arc image past the projection lens ignores this simple fact, and there is absolutely no benefit of any kind!
ok thanks for your information.
Actually if i draw a pic with your focusing point at optical center midle point, i found that it means you are focusing on a relatively large area on the rear lens element on the triplet.
you are right when you say the bulb is not small, so is not posible focusing into a point but as i read from you at other thread when we design the projector whe asume some "trues";
1)infinitesimal light point
2)thin lens theory for all the lens used.
3)perfect light rays
anyway, fresnells will not do a perfect work, mean that it will not happen what we would like to happen but i believe in one thing (i read as well you said that starting from a optimal teorical setup is ok, and then try moving things +/- up/down);
now if we asume we have perfect elements, then we could design the perfect projector, but what would be the perfect design like? (focusing at the center or at full area?)
thanks, i like having this kind of disscussions. 😀
Actually if i draw a pic with your focusing point at optical center midle point, i found that it means you are focusing on a relatively large area on the rear lens element on the triplet.
you are right when you say the bulb is not small, so is not posible focusing into a point but as i read from you at other thread when we design the projector whe asume some "trues";
1)infinitesimal light point
2)thin lens theory for all the lens used.
3)perfect light rays
anyway, fresnells will not do a perfect work, mean that it will not happen what we would like to happen but i believe in one thing (i read as well you said that starting from a optimal teorical setup is ok, and then try moving things +/- up/down);
now if we asume we have perfect elements, then we could design the perfect projector, but what would be the perfect design like? (focusing at the center or at full area?)
thanks, i like having this kind of disscussions. 😀
ideal versus real
I think you should try to start with the ideal design: Fresnels focussing the arc image at the optical center of the projection lens. Then you can try moving things around to see if they get better. (I don't think they will get any better.)
If you have a very large diameter and high quality projector lens, then it will not matter much if the arc is not focussed at the optical center of the lens. I am using an excellant lens that is only 65 mm in diameter, so I can not afford to have the fresnels off at all. I would lose too much light that way.
When you focus at the optical center, quite a bit of the area on the first surface of the triplet will be used. (Even if the lamp was a tiny point source.) But since the lamp is not a point source, that area will be larger than you would think from a drawing that assumes a point source lamp. You can make a drawing that shows the light coming from both ends of the arc, going through a single pixel on the LCD, and then getting focussed to the optical center of the projection lens. That will use a lot more area of the lens's first surface.
It is easy to verify this idea by putting a white paper ring around the triplet to see if light is missing the lens. If you focus the arc image at the optical center of the projection lens, I think you will minimize the wasted light.
I think you should try to start with the ideal design: Fresnels focussing the arc image at the optical center of the projection lens. Then you can try moving things around to see if they get better. (I don't think they will get any better.)
If you have a very large diameter and high quality projector lens, then it will not matter much if the arc is not focussed at the optical center of the lens. I am using an excellant lens that is only 65 mm in diameter, so I can not afford to have the fresnels off at all. I would lose too much light that way.
When you focus at the optical center, quite a bit of the area on the first surface of the triplet will be used. (Even if the lamp was a tiny point source.) But since the lamp is not a point source, that area will be larger than you would think from a drawing that assumes a point source lamp. You can make a drawing that shows the light coming from both ends of the arc, going through a single pixel on the LCD, and then getting focussed to the optical center of the projection lens. That will use a lot more area of the lens's first surface.
It is easy to verify this idea by putting a white paper ring around the triplet to see if light is missing the lens. If you focus the arc image at the optical center of the projection lens, I think you will minimize the wasted light.
i am with you when you say it is a good starting point focusing at the center of the optical center.
But please, think we are in a perfect world just by now. So you now, infinitesimal light, thing flat lens... everithing just works perfectly.
now think we have just one lens 450 focal, a perfect singlet. (as we are in the perfect world, the image would be perfect even with this single lens).
Now if you focus at the center exactly as shown at left on my first pic, the light hitting the center of the lens is not refracted, so the efect is like if there was no lens, is like if there is a pinhole that will project a image on the wall.
please, always thinking on perfect world, i think it is not the propper way for the design to do this. I think focusing beyond the lens is fine so most of the lens area is used like the image i posted at the right.
It what i think, the perfect world design should be a good starting point, then i´m shure the focusing point should be somewhere between mine and yours because of the imperfections of the real world.
Don´t you think in perfect world it is my way the best?
But please, think we are in a perfect world just by now. So you now, infinitesimal light, thing flat lens... everithing just works perfectly.
now think we have just one lens 450 focal, a perfect singlet. (as we are in the perfect world, the image would be perfect even with this single lens).
Now if you focus at the center exactly as shown at left on my first pic, the light hitting the center of the lens is not refracted, so the efect is like if there was no lens, is like if there is a pinhole that will project a image on the wall.
please, always thinking on perfect world, i think it is not the propper way for the design to do this. I think focusing beyond the lens is fine so most of the lens area is used like the image i posted at the right.
It what i think, the perfect world design should be a good starting point, then i´m shure the focusing point should be somewhere between mine and yours because of the imperfections of the real world.
Don´t you think in perfect world it is my way the best?
Rox if we are talking about a perfect world then the objective lens would not be needed at all. If we then come back to reality again even a little bit, aberrations will start to effect things again. The best way to alleviate them is to use as little refraction as possible and that is the optical centre of the lens.
DJ
DJ
faith versus reason?
I've explained all the reasons it is better to at least try to focus the arc image at the optical center of the projection lens. If you want to convince anybody that it is better to focus it past the lens, then I think you will need to list an actual reason or two, show a drawing explaining why, or tells us about some experiment results that support that idea.
I don't know of any...
I've explained all the reasons it is better to at least try to focus the arc image at the optical center of the projection lens. If you want to convince anybody that it is better to focus it past the lens, then I think you will need to list an actual reason or two, show a drawing explaining why, or tells us about some experiment results that support that idea.
I don't know of any...
yes, pinhole projection is posible as well in perfect world. But also is posible with a lens projection (no focusing at center only but full area of the lens aperture).
i am just discussing design concepts, how should perfect design be, nothing else.
For example, on the pre-condensor optimal placement process (first adjust the diameter of the lens where the cone light diameter fits and then place the bulb at the best place so the light is like if it came from the focal of the rear fresnell (virtual light source, cos really it will be something closer than without the pre-condenser lens) I agree 100% with this theorical optimal placement as a stating point. The assumtions taken here are the same assumtions i am taking on my perfect world.
i think that the concept of focusing at the center of the lens is not the perfect design, since you can remove it and nothing will change.
i am just discussing design concepts, how should perfect design be, nothing else.
For example, on the pre-condensor optimal placement process (first adjust the diameter of the lens where the cone light diameter fits and then place the bulb at the best place so the light is like if it came from the focal of the rear fresnell (virtual light source, cos really it will be something closer than without the pre-condenser lens) I agree 100% with this theorical optimal placement as a stating point. The assumtions taken here are the same assumtions i am taking on my perfect world.
i think that the concept of focusing at the center of the lens is not the perfect design, since you can remove it and nothing will change.
Re: faith versus reason?
Well, i repeat that i am in perfect world all the time, so ther eis no arc lengh.
anyway, if i return to real world, i am ok with your statemet of the 25mm arc becoming 65mm arc on the projection lens area. But even now we can get farther focusing cos the aperture is 130cm and not 65mm, mean there is a midle point between yours and mine.
but, please lets return to perfect world for a while; all the posts about reflectors shapes (parabolic, eliptical, spherical...) are perfect as well, infinitesimal light source... i agree working on the perfect world as a starting point. Don you agree with me?
i seems to me that the concept of focusing on full area, is erroneus for you, is it right?
Guy Grotke said:
Well, i repeat that i am in perfect world all the time, so ther eis no arc lengh.
anyway, if i return to real world, i am ok with your statemet of the 25mm arc becoming 65mm arc on the projection lens area. But even now we can get farther focusing cos the aperture is 130cm and not 65mm, mean there is a midle point between yours and mine.
but, please lets return to perfect world for a while; all the posts about reflectors shapes (parabolic, eliptical, spherical...) are perfect as well, infinitesimal light source... i agree working on the perfect world as a starting point. Don you agree with me?
i seems to me that the concept of focusing on full area, is erroneus for you, is it right?
real results too!
All of this is not just theory for me. I have a projector that uses a very good projection lens that is only 65 mm in diameter. If I focus the arc image at the optical center (as I suggest), then I get most of the light through the lens and onto the screen.
If I focus the arc image past the projection lens, then I get a much dimmer image because a lot of the light does not get through the lens.
Since I am using one of LL's 790 mm fl fresnels, I have added quite a bit of space between the field fresnel and the LCD. This space puts the arc image right at the optical center of the projection lens. The result is a very bright and even image, compared to other arc image locations.
I used to think that focussing the arc image past the lens was best, because I read that on a DIY forum. Then I actually experimented with my projector. Now I know better. But anybody with doubts should try some experiments.
All of this is not just theory for me. I have a projector that uses a very good projection lens that is only 65 mm in diameter. If I focus the arc image at the optical center (as I suggest), then I get most of the light through the lens and onto the screen.
If I focus the arc image past the projection lens, then I get a much dimmer image because a lot of the light does not get through the lens.
Since I am using one of LL's 790 mm fl fresnels, I have added quite a bit of space between the field fresnel and the LCD. This space puts the arc image right at the optical center of the projection lens. The result is a very bright and even image, compared to other arc image locations.
I used to think that focussing the arc image past the lens was best, because I read that on a DIY forum. Then I actually experimented with my projector. Now I know better. But anybody with doubts should try some experiments.
ok so you found that midle point between perfect world and and focusing at center (thats what i would do as well).
but do you agree with me on the statement in perfect world focusing at center of the singlet lens is not correct?
but do you agree with me on the statement in perfect world focusing at center of the singlet lens is not correct?
I’m sure Guy and myself are of the same thinking on this topic. That for the optimal focus of light from the lamp should be at the centre of the objective lens, not beyond it or somewhere in between. As I said in my last post, in a perfect world no objective lens would be required, you could include one but it would be pointless so we have to consider a near perfect world to base our design concepts on. In a near perfect world anything that refracts light will still introduce distortion so we need to keep the amount of refraction to a minimum. To do this we need to use as little of the objective lens as possible. The outer edges of the lens are where the greatest angle of refraction occurs and also where the most distortion is introduced. So keeping our light focused at the centre, even a near perfect singlet, will keep distortion to the lowest. I hope this makes it clearer.
DJ
DJ
don´t you see the lens is not working at all that way?
your design is ok as well, but is based on pinhole projection way.
i would like you to introduce a perfect singlet lens on your design so pinhole is not needed. Make the lens work somehow, you would need to hit the lens on some part of it (not center) so it starts interacting with the opical sistem, so the lens is needed and if you remove it you would see something changed on the image.
remenber that if you remove your lens, nothing will be noticed on the proyected image.
i am always talking in perfect world even the absurd it could be.
your design is ok as well, but is based on pinhole projection way.
i would like you to introduce a perfect singlet lens on your design so pinhole is not needed. Make the lens work somehow, you would need to hit the lens on some part of it (not center) so it starts interacting with the opical sistem, so the lens is needed and if you remove it you would see something changed on the image.
remenber that if you remove your lens, nothing will be noticed on the proyected image.
i am always talking in perfect world even the absurd it could be.
That is why the objective is not needed.
Although this is simular to a pinhole lens it is not the same. A pinhole lens blocks out unwanted rays. In this situation there won’t be any unwanted rays to block.
If the perfect singlet was placed at the focused light position a very small change to the image would occur and this would be due to the physical thickness and FL of the lens. Moving the focused light beyond the singlet would make the projected image larger. Moving the focused light so that it is before the singlet will make the projected image smaller. This occurs because the rays that are converging onto the perfect singlet are not hitting the entire lens surface.
This is becoming very difficult to explain, trying to mix perfect world with real. The terms don’t translate that well when used in the perfect hypothetical.
It is also very late hear so forgive me is this doesn’t make sense.
DJ
Dazzzla said:
let me ask a stupid cuestion; do you notice any difference on the projection when you remove the projection lens form your setup?
sorry if my words are offensive but i have two problems;
first my english is very limited (i write with a english dictionary on the desk always).
second, i think in perfect world but you push me to real world with your arguments.
about the answer on the stupid question i asked, i now your answer 😀. Then the conclusion i get is that your setup does not represent the reality at all.
please, i just would like to discuss by arguments and without using unwanted words, i believe thats the forum idea. thanks for your replys. Rox
Every optics theory book I've read says that for projection systems the idea of the condenser (the fresnel in this case) is to create an image of the light source at the aperture of the objective lense. This is to get the best light throughput, and even field illumination of the slide (lcd in our case).
Question Guy,
If you have an unsplit frenel with back focal length of
330mm and front focal of 550 mm with an Objective lens of 450
mm fl.
you said,
528 mm + 20 mm = 548 mm (which I rounded up to 550 mm)
you can put a 550 mm fl fresnel exactly 22 mm before the LCD to get a perfect arc image focussed at the optical center of the projection lens, for a 10 foot throw distance.
Should the lamp be placed with the combined focal length of Fresnel, 330mm and 550mm ?
Other says it should be place at 330 no matter what.
What is correct in real world based on experiments?
If you have an unsplit frenel with back focal length of
330mm and front focal of 550 mm with an Objective lens of 450
mm fl.
you said,
528 mm + 20 mm = 548 mm (which I rounded up to 550 mm)
you can put a 550 mm fl fresnel exactly 22 mm before the LCD to get a perfect arc image focussed at the optical center of the projection lens, for a 10 foot throw distance.
Should the lamp be placed with the combined focal length of Fresnel, 330mm and 550mm ?
Other says it should be place at 330 no matter what.
What is correct in real world based on experiments?
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