Sure
There are lots of ways you could make one of these, but not for less than their price! All of the design cost has to be paid (or your time invested) for a single lens or 1000000 lenses. You could skip some of that by buying one of their lenses, taking it apart, and measuring the lens curves and refractive index to figure out the glass used for each. Then you would have to buy some lens blanks of the correct materials. Then you could find someone to grind and polish the lens blanks, or you could learn how to do it yourself. Either way, a few hundred Euros to get it done. Then you would need to have some aluminum machined to hold the lenses...
There are lots of ways you could make one of these, but not for less than their price! All of the design cost has to be paid (or your time invested) for a single lens or 1000000 lenses. You could skip some of that by buying one of their lenses, taking it apart, and measuring the lens curves and refractive index to figure out the glass used for each. Then you would have to buy some lens blanks of the correct materials. Then you could find someone to grind and polish the lens blanks, or you could learn how to do it yourself. Either way, a few hundred Euros to get it done. Then you would need to have some aluminum machined to hold the lenses...
triplets
In general, a triplet is made from two positive lenses and a negative lens in the middle. The positive lenses are usually made from a crown glass and the negative lens from a flint glass. Different glasses are used because a positive crown glass lens produces chromatic aberration (like a prism rainbow) in one direction and a negative flint glass lens produces it in the other direction. Combining the two properly can cancel out the chromatic aberration, without cancelling all the refraction.
By picking the 6 different curves, two different spaces between lenses, and the two or three different types of glass, the designer finds a combination that has the desired focal length and reduces all of the distortion and aberrations to acceptable values. There are so many variables that triplet design was really an art form until computers became available. Early designers had to spend weeks doing the math to model how the lens would perform, changing each parameter a tiny bit each time through the calculations to see if it helped or hurt.
Now all of the math is done by an optical design program. Optimizations can be done in an hour or two by a designer experienced with using the program. You can download a free student version of OSLO if you want to learn how this works.
Does your brother work with human vision correction? ("optriocian" does not mean anything in english.)
If the lenses he works with are for eyeglasses, then they would not be very good for making an achromatic triplet projection lens. All eyeglass lenses are curved meniscus lenses with the eye-side curve radius fixed at 3.5 inches. You can make an uncorrected duplet that works pretty well for projection using a pair of weak positive eyeglass lenses. I use a 526 mm fl symmetric duplet lens for my 15" LCD projector, made from a pair of +1.0 Diopter acrylic lenses spaced about 100 mm apart in a piece of plastic pipe. I get a very good screen image. (I think this probably would not work as well for shorter focal length lenses. )
If you want to try this, get lenses with as large a diameter as you can. 75 or 80 mm would be good. Plastic lenses are okay, glass with anti-reflection coating would be better. The equation is:
EFL = (f1 * f2) / (f1 + f2 - distance)
"distance" is the space between the two lenses.
So if you want a 300 mm duplet, you would need two lenses with focal lengths of around 600 mm. Eyeglass lenses are measured in Diopters instead of focal length and:
Diopters = 1000 / focal length in mm
So the lenses would be sold as +1.66 Diopters. But these lenses usually come in even 0.25 Diopter units, so you would need +1.75 Diopter lenses. If you run the first equation with f1 = f2 = (1000/1.75) and distance = 55 mm, then the Effective Focal Length would be 300 mm.
In general, a triplet is made from two positive lenses and a negative lens in the middle. The positive lenses are usually made from a crown glass and the negative lens from a flint glass. Different glasses are used because a positive crown glass lens produces chromatic aberration (like a prism rainbow) in one direction and a negative flint glass lens produces it in the other direction. Combining the two properly can cancel out the chromatic aberration, without cancelling all the refraction.
By picking the 6 different curves, two different spaces between lenses, and the two or three different types of glass, the designer finds a combination that has the desired focal length and reduces all of the distortion and aberrations to acceptable values. There are so many variables that triplet design was really an art form until computers became available. Early designers had to spend weeks doing the math to model how the lens would perform, changing each parameter a tiny bit each time through the calculations to see if it helped or hurt.
Now all of the math is done by an optical design program. Optimizations can be done in an hour or two by a designer experienced with using the program. You can download a free student version of OSLO if you want to learn how this works.
Does your brother work with human vision correction? ("optriocian" does not mean anything in english.)
If the lenses he works with are for eyeglasses, then they would not be very good for making an achromatic triplet projection lens. All eyeglass lenses are curved meniscus lenses with the eye-side curve radius fixed at 3.5 inches. You can make an uncorrected duplet that works pretty well for projection using a pair of weak positive eyeglass lenses. I use a 526 mm fl symmetric duplet lens for my 15" LCD projector, made from a pair of +1.0 Diopter acrylic lenses spaced about 100 mm apart in a piece of plastic pipe. I get a very good screen image. (I think this probably would not work as well for shorter focal length lenses. )
If you want to try this, get lenses with as large a diameter as you can. 75 or 80 mm would be good. Plastic lenses are okay, glass with anti-reflection coating would be better. The equation is:
EFL = (f1 * f2) / (f1 + f2 - distance)
"distance" is the space between the two lenses.
So if you want a 300 mm duplet, you would need two lenses with focal lengths of around 600 mm. Eyeglass lenses are measured in Diopters instead of focal length and:
Diopters = 1000 / focal length in mm
So the lenses would be sold as +1.66 Diopters. But these lenses usually come in even 0.25 Diopter units, so you would need +1.75 Diopter lenses. If you run the first equation with f1 = f2 = (1000/1.75) and distance = 55 mm, then the Effective Focal Length would be 300 mm.
focal length
Finaly a person that knows something about focal length. Im designing a lcd projector for DVD movie watching and have not been satisfied with the range of focal lengths avalible. I have been reaserching lens at this sight http://www.optosigma.com and would like a little help trying to get teh right "distance to screen" that i require. Im looking at getting a 15`` lcd, but am only finding optics that will give me a distance to screen length of about 8 feet or so with a 5-6`` screen. I would really like to have the distance to screen of about 12-18 feet. The optics that im looking at are not triplet i dont think, so is it even worth getting the lenses that are not triplet? What about a zoom lens setup, ever thought about the lagistics of something like that? Any help would be nice. I hope you read this message.
Finaly a person that knows something about focal length. Im designing a lcd projector for DVD movie watching and have not been satisfied with the range of focal lengths avalible. I have been reaserching lens at this sight http://www.optosigma.com and would like a little help trying to get teh right "distance to screen" that i require. Im looking at getting a 15`` lcd, but am only finding optics that will give me a distance to screen length of about 8 feet or so with a 5-6`` screen. I would really like to have the distance to screen of about 12-18 feet. The optics that im looking at are not triplet i dont think, so is it even worth getting the lenses that are not triplet? What about a zoom lens setup, ever thought about the lagistics of something like that? Any help would be nice. I hope you read this message.
long throw lens
You need a projection lens with a longer focal length. Going by your parameters, I think you need something in the 621 mm to 1092 mm fl range. These are very difficult to find (and unbelievably expensive) in a projection triplet. You might be able to use an achromatic or apochromatic telescope objective, if you can find a cheap one. But it may not do very well at the edges and corners, since they are optimised for a very narrow field angle with the objects at infinite distance (ie. stars). I think you need to stick to the low end of that range, since you need a field fresnel that is a bit longer than the LCD to lens distance. Lumenlab has 790 mm fl fresnels that would work with a lens near the low end, in a split fresnel design.
Personally, I have found that a symmetric duplet lens will work fine with this long a focal length. (The field angle is very low compared to a shorter fl 15" LCD projector.) I think you should try making a symmetric duplet from two eyeglass lenses:
Find an optician who will sell you two 75 mm diameter +0.75 Diopter lenses. CR-39 plastic is okay, glass is better. Lenses with antireflection coating are the best, but more expensive. Your best bet is a small discount eyeglasses shop. Tell him or her it is for a projector lens, so you need uncut unmounted lenses. The pair of uncoated lenses should not cost anymore than their discount eyeglasses price. Where I live, that is $30.
Mount each lens at the end of a 50 mm long piece of 3" ABS pipe. (That's black plastic sewer pipe sold in short lengths at Home Depot.) The lenses should be mounted with their convex curves facing out. You can make retaining rings, etc. from pieces cut from mailing tubes. (Be careful! I had to have several stiches in my thumb from cutting a mailing tube with a razor knife!)
This will give you a 679 mm focal length duplet that can handle a 40 degree field angle. (I have one made from two +1.0 Diopter lenses that gives me a 526 mm fl: Focusses sharply from corner to corner.) Your lens should give you a 66" image from 12 feet.
If you do try this, let us know how it works for you.
You need a projection lens with a longer focal length. Going by your parameters, I think you need something in the 621 mm to 1092 mm fl range. These are very difficult to find (and unbelievably expensive) in a projection triplet. You might be able to use an achromatic or apochromatic telescope objective, if you can find a cheap one. But it may not do very well at the edges and corners, since they are optimised for a very narrow field angle with the objects at infinite distance (ie. stars). I think you need to stick to the low end of that range, since you need a field fresnel that is a bit longer than the LCD to lens distance. Lumenlab has 790 mm fl fresnels that would work with a lens near the low end, in a split fresnel design.
Personally, I have found that a symmetric duplet lens will work fine with this long a focal length. (The field angle is very low compared to a shorter fl 15" LCD projector.) I think you should try making a symmetric duplet from two eyeglass lenses:
Find an optician who will sell you two 75 mm diameter +0.75 Diopter lenses. CR-39 plastic is okay, glass is better. Lenses with antireflection coating are the best, but more expensive. Your best bet is a small discount eyeglasses shop. Tell him or her it is for a projector lens, so you need uncut unmounted lenses. The pair of uncoated lenses should not cost anymore than their discount eyeglasses price. Where I live, that is $30.
Mount each lens at the end of a 50 mm long piece of 3" ABS pipe. (That's black plastic sewer pipe sold in short lengths at Home Depot.) The lenses should be mounted with their convex curves facing out. You can make retaining rings, etc. from pieces cut from mailing tubes. (Be careful! I had to have several stiches in my thumb from cutting a mailing tube with a razor knife!)
This will give you a 679 mm focal length duplet that can handle a 40 degree field angle. (I have one made from two +1.0 Diopter lenses that gives me a 526 mm fl: Focusses sharply from corner to corner.) Your lens should give you a 66" image from 12 feet.
If you do try this, let us know how it works for you.
Thanks for the info Guy Grotke. I will be going to my local optics shop in the next few days. Do you have any good advice on optics to use with some sort of zoom lens? I have the ability to machine very accurate parts for the design. Is there any difference between "zoom" lens and standered? Or is it just getting the correct focal lengths for each lens? Keep checking back i will post my results on the design that you specified. Im in the middle of midterms so it may be a week or more tell i post again. Thanks for your help.
surplus shed lens
That is about what you would end up with by building the lens I described, except the focal length is a bit long for any fresnels I know about. You might be able to make it work by adjusting the lamp-to-condensor-fresnel distance to make the rays going to the field fresnel into a slightly diverging cone. This increases the distance from the field fresnel to the focussed image of the lamp arc.
I have a 700 mm fl telescope objective that I have tried with my projector. Not too bad, but I could not get the center and the corners both perfectly focussed at the same setting. Like I said earlier, telescope objectives are optimized for very narrow field angles.
For $19 I would try it! If it doesn't work for your projector, you could always make a telescope.
That is about what you would end up with by building the lens I described, except the focal length is a bit long for any fresnels I know about. You might be able to make it work by adjusting the lamp-to-condensor-fresnel distance to make the rays going to the field fresnel into a slightly diverging cone. This increases the distance from the field fresnel to the focussed image of the lamp arc.
I have a 700 mm fl telescope objective that I have tried with my projector. Not too bad, but I could not get the center and the corners both perfectly focussed at the same setting. Like I said earlier, telescope objectives are optimized for very narrow field angles.
For $19 I would try it! If it doesn't work for your projector, you could always make a telescope.
Sorry, new to all this and just learning optics. But is this a triplet, and would it work? http://www.surplusshed.com/pages/item/l2196.html
doublet
No, a triplet is made with three lenses, a doublet has two lenses. In this case they have a little bit of air space between them. Others are glued together and are called "cemented doublet". It might work for a small-format projector, but not as well as this:
http://www.diyprojectorcompany.com/catalog/product_info.php?cPath=23&products_id=38
Or this:
https://secure.lumenlab.com/shop/group.php?id=2
One of the challenges of projector design is to find a combination of projection lens, LCD, and fresnels that will all work together. If you are just starting out, think about buying a set of lens and fresnels designed to match a particular LCD size.
No, a triplet is made with three lenses, a doublet has two lenses. In this case they have a little bit of air space between them. Others are glued together and are called "cemented doublet". It might work for a small-format projector, but not as well as this:
http://www.diyprojectorcompany.com/catalog/product_info.php?cPath=23&products_id=38
Or this:
https://secure.lumenlab.com/shop/group.php?id=2
One of the challenges of projector design is to find a combination of projection lens, LCD, and fresnels that will all work together. If you are just starting out, think about buying a set of lens and fresnels designed to match a particular LCD size.
So what your saying is lens with focal lengths of that distance always have a narrow feild of focus? Is there a general rule about matching fresnals? You say that you dont know of a fresnal that will match that lens. How do you match the fresnel to the lens? Does the fresnel focal length need to be similiar to the focal length of the lens? Would a smaller LCD help with my distance to screen problem? I really appreciate your time and information. Its very helpfull. Thanks....
>lens with focal lengths of that distance always have a narrow feild of focus?
No, not at all. There is a lot more to a multi-element lens than just the focal length. When a lens designer starts a design, he has a set of goals. Telescope objective designers don't care how their lens performs with light entering at a 20 degree angle, because it will be mounted in a long tube with a tiny eyepiece at the other end. Rays coming in at more than 1 degree will never make it to the eyepiece. A particular telescope objective might work okay with a 20 degree off-axis ray, but it might not.
>Is there a general rule about matching fresnals?
The lamp and the two fresnels form a second projection system that follows all the rules of such systems. The fresnels form a projected image of the lamp arc. You get the best performance if that arc image is focussed at the middle of the projection lens. That gets all of the light on the screen. The default setup for the fresnels is to put the lamp arc at the focal length of the condensor fresnel (ie. 220 mm), and the projector lens at the focal length of the field fresnel (ie. 790 mm). But the distance between the LCD and the projector lens is determined by the lens focal length and the throw distance. You can figure out what that distance will be:
1/fl = 1/LCD-to-lens + 1/lens-to-screen
It is always longer than the projection lens focal length. Since you are stuck with that distance, you have to work backwards to see how you can get the arc image focussed right at the projection lens. You can move the field fresnel to before or after the LCD. You can also play with the lamp-to-condensor-fresnel distance a bit to move the focal point of the field fresnel.
And yes, a smaller LCD would give you a smaller image with more conventional projection lenses. But it isn't easy to find a smaller format LCD that is as good as even a cheap 15" monitor LCD.
No, not at all. There is a lot more to a multi-element lens than just the focal length. When a lens designer starts a design, he has a set of goals. Telescope objective designers don't care how their lens performs with light entering at a 20 degree angle, because it will be mounted in a long tube with a tiny eyepiece at the other end. Rays coming in at more than 1 degree will never make it to the eyepiece. A particular telescope objective might work okay with a 20 degree off-axis ray, but it might not.
>Is there a general rule about matching fresnals?
The lamp and the two fresnels form a second projection system that follows all the rules of such systems. The fresnels form a projected image of the lamp arc. You get the best performance if that arc image is focussed at the middle of the projection lens. That gets all of the light on the screen. The default setup for the fresnels is to put the lamp arc at the focal length of the condensor fresnel (ie. 220 mm), and the projector lens at the focal length of the field fresnel (ie. 790 mm). But the distance between the LCD and the projector lens is determined by the lens focal length and the throw distance. You can figure out what that distance will be:
1/fl = 1/LCD-to-lens + 1/lens-to-screen
It is always longer than the projection lens focal length. Since you are stuck with that distance, you have to work backwards to see how you can get the arc image focussed right at the projection lens. You can move the field fresnel to before or after the LCD. You can also play with the lamp-to-condensor-fresnel distance a bit to move the focal point of the field fresnel.
And yes, a smaller LCD would give you a smaller image with more conventional projection lenses. But it isn't easy to find a smaller format LCD that is as good as even a cheap 15" monitor LCD.
put your optician brother to work
There won't really be much work to do, unless he is grinding extra-large glass lenses for you. Stock lens blanks should just be sitting on somebody's shelf already made up and polished.
I think the plastic lens blanks are available up to 75 mm diameter, but the grinding machines used to make custom lenses may be able to handle 100 mm diameter. If he can make you extra-large lenses and get them anti-reflection coated, then you will have a very good duplet.
I know (from experience) that a 1000 mm + 1000 mm duplet works very well with a 15" LCD. The field angle is about 32 degrees. With a shorter focal length and a 15" LCD, the field angle increases so it may not work as well. If you use a smaller LCD, then the field angle may decrease. The angle is the critical limit. You could try making one with off-the-shelf CR-39 plastic lenses to see if it works for your projector. Then he could make you a fancy one with coated glass lenses.
There won't really be much work to do, unless he is grinding extra-large glass lenses for you. Stock lens blanks should just be sitting on somebody's shelf already made up and polished.
I think the plastic lens blanks are available up to 75 mm diameter, but the grinding machines used to make custom lenses may be able to handle 100 mm diameter. If he can make you extra-large lenses and get them anti-reflection coated, then you will have a very good duplet.
I know (from experience) that a 1000 mm + 1000 mm duplet works very well with a 15" LCD. The field angle is about 32 degrees. With a shorter focal length and a 15" LCD, the field angle increases so it may not work as well. If you use a smaller LCD, then the field angle may decrease. The angle is the critical limit. You could try making one with off-the-shelf CR-39 plastic lenses to see if it works for your projector. Then he could make you a fancy one with coated glass lenses.
Guy Question
Hi there.
First off...thank you so much for sharing your knowledge with all of us here.
Okay...here's my question.
Due to the constraints of the room I'm building for, my projector needs to be around 16 feet away from the screen. I'm using a 15" LCD and have a 220mm fl and a 317mm fl fresnel lenses. The screen I'm projecting to is huge...approx 122". The focusing duplet I have is way off base...image is 2-3 times bigger than the screen when projected from 16 feet. If I were going to build a duplet from eyeglass lenses, can you offer this noob a bit of help on the diopter needed and spacing.
Is it even possible to build a focusing lens to be used with the fresnels lenses I have given the room requirements or do I need to replace the fresnels with others?
Any help you can offer is greatly appreciated.
Dee
Hi there.
First off...thank you so much for sharing your knowledge with all of us here.
Okay...here's my question.
Due to the constraints of the room I'm building for, my projector needs to be around 16 feet away from the screen. I'm using a 15" LCD and have a 220mm fl and a 317mm fl fresnel lenses. The screen I'm projecting to is huge...approx 122". The focusing duplet I have is way off base...image is 2-3 times bigger than the screen when projected from 16 feet. If I were going to build a duplet from eyeglass lenses, can you offer this noob a bit of help on the diopter needed and spacing.
Is it even possible to build a focusing lens to be used with the fresnels lenses I have given the room requirements or do I need to replace the fresnels with others?
Any help you can offer is greatly appreciated.
Dee
DIY Projection Duplet
To find the focal length that you need, go to
Projected Image Size
and use the calculator there.
Once you have calculated the focal length, click on "Lens Designer" at the top of the page and it will take you to another calculator that will tell you what focal length lenses you need and how far apart they have to be to give you the projection duplet you need. (Lenses designed with this have a 30 degree field of view.) I usually just use the chart at the bottom of the page.
For those without an optician brother, they can get eyeglass lenses from Rolyn Optics at Rolyn's Positive Meniscus Lenses .The page has both focal length and diopter values for each lens, thank goodness, since I'm never really sure how to convert from one to the other.
The glass ones are expensive - $16.56US each, but the acrylic ones starting on page four of the positive meniscus (eyeglass) lens section are made of acrylic and cost only $8.79US each. And they come in 72 to 74 mm diameter.
Hope this is helpful.
T.
To find the focal length that you need, go to
Projected Image Size
and use the calculator there.
Once you have calculated the focal length, click on "Lens Designer" at the top of the page and it will take you to another calculator that will tell you what focal length lenses you need and how far apart they have to be to give you the projection duplet you need. (Lenses designed with this have a 30 degree field of view.) I usually just use the chart at the bottom of the page.
For those without an optician brother, they can get eyeglass lenses from Rolyn Optics at Rolyn's Positive Meniscus Lenses .The page has both focal length and diopter values for each lens, thank goodness, since I'm never really sure how to convert from one to the other.
The glass ones are expensive - $16.56US each, but the acrylic ones starting on page four of the positive meniscus (eyeglass) lens section are made of acrylic and cost only $8.79US each. And they come in 72 to 74 mm diameter.
Hope this is helpful.
T.
long throw projector
Dee:
First of all, I would not recommend a 122" image. I think that will be too large to view comfortably. (Your neck gets tired after a while from moving your head back and forth ) But I will show you the math, so you can work it out if you really do want 122".
Here is the optical geometry for a 100" image, assuming a 16' throw distance and a 15" LCD:
M is magnification
A is the LCD to projection lens distance
B is the projection lens to screen distance
FL is the focal length of the projection lens
M = 100" / 15" = 6.67
FL = B / (M + 1) = 16' / (6.67 + 1) = 2.086' = 636 mm
A = B / M = 16' / 6.67 = 2.40' = 731 mm
So you would need a 636 mm fl projection lens. You can try making that from a pair of positive meniscus lenses using this:
EFL is the effective focal length of a multi-element projection lens
FL1 is the focal length of the first simple lens
FL2 is the focal length of the second simple lens
D is the distance between the lenses
FL = (FL1 * FL2) / (FL1 + FL2 - D)
If you space them too far apart, then you limit the field angle too much. Eyeglass lens blanks come in 1/4 Diopter increments.
Diopters = 1000 / focal length in mm
So you can get +1.0 Diopter (1000 mm fl) and +0.75 Diopter (1333 mm fl) lenses. The large steps make it difficult to make specific long-throw projection lenses from eyeglass blanks. So let's see what happens with a 1000 mm and a 1333 mm lens with 30 mm of space:
EFL = (1000 * 1333) / (1000 + 1333 - 30) = 578.81 mm
Feeding this back into your room and LCD size constants:
M = (B - FL) / FL = 7.43
Image size = 7.43 * 15" = 111"
Now we can recalculate A for this lens:
A = B / M = 16' / 7.43 = 2.15' = 656 mm
So that tells us you need about 656 mm from the LCD to the projection lens. If you use a split design, then you would put a (656 - 20) = 636 mm fl fresnel 20 mm after the LCD. If you use a non-split design, then you would put a (656 + 20) = 676 mm fl fresnel 20 mm before the LCD. These long throw fresnels may be very difficult to find! Lumenlab.com used to sell a "790 mm" fl fresnel (that I measured to actually be about 770 mm fl). You could ask them if they have any left. They were changing to a shorter focal length (650 mm?, 600 mm?), for use with a 450 mm fl triplet, so they may have a "790" they could sell you. Even if they don't, you could still use a 650 or 600 mm fl fresnel by adjusting the distance between the lamp arc and your condensor fresnel.
So the short answer is: Keep the 220 mm fl fresnel. Find a 770 mm field fresnel. Buy +1.0 and +0.75 Diopter eyeglass lenses and put them in a tube. The Rolyn lenses that tgreenwood mentioned only go down to +1.0 D in the acrylic. You will have to buy a glass +0.75 D. This will give you an image that is good enough for watching video, but a bit blurry around the edges for running windows. If you want a sharp image from corner to corver, you will need to find a triplet or tessar lens designed for somewhere in the 550 to 650 mm fl range.
Dee:
First of all, I would not recommend a 122" image. I think that will be too large to view comfortably. (Your neck gets tired after a while from moving your head back and forth
) But I will show you the math, so you can work it out if you really do want 122".Here is the optical geometry for a 100" image, assuming a 16' throw distance and a 15" LCD:
M is magnification
A is the LCD to projection lens distance
B is the projection lens to screen distance
FL is the focal length of the projection lens
M = 100" / 15" = 6.67
FL = B / (M + 1) = 16' / (6.67 + 1) = 2.086' = 636 mm
A = B / M = 16' / 6.67 = 2.40' = 731 mm
So you would need a 636 mm fl projection lens. You can try making that from a pair of positive meniscus lenses using this:
EFL is the effective focal length of a multi-element projection lens
FL1 is the focal length of the first simple lens
FL2 is the focal length of the second simple lens
D is the distance between the lenses
FL = (FL1 * FL2) / (FL1 + FL2 - D)
If you space them too far apart, then you limit the field angle too much. Eyeglass lens blanks come in 1/4 Diopter increments.
Diopters = 1000 / focal length in mm
So you can get +1.0 Diopter (1000 mm fl) and +0.75 Diopter (1333 mm fl) lenses. The large steps make it difficult to make specific long-throw projection lenses from eyeglass blanks. So let's see what happens with a 1000 mm and a 1333 mm lens with 30 mm of space:
EFL = (1000 * 1333) / (1000 + 1333 - 30) = 578.81 mm
Feeding this back into your room and LCD size constants:
M = (B - FL) / FL = 7.43
Image size = 7.43 * 15" = 111"
Now we can recalculate A for this lens:
A = B / M = 16' / 7.43 = 2.15' = 656 mm
So that tells us you need about 656 mm from the LCD to the projection lens. If you use a split design, then you would put a (656 - 20) = 636 mm fl fresnel 20 mm after the LCD. If you use a non-split design, then you would put a (656 + 20) = 676 mm fl fresnel 20 mm before the LCD. These long throw fresnels may be very difficult to find! Lumenlab.com used to sell a "790 mm" fl fresnel (that I measured to actually be about 770 mm fl). You could ask them if they have any left. They were changing to a shorter focal length (650 mm?, 600 mm?), for use with a 450 mm fl triplet, so they may have a "790" they could sell you. Even if they don't, you could still use a 650 or 600 mm fl fresnel by adjusting the distance between the lamp arc and your condensor fresnel.
So the short answer is: Keep the 220 mm fl fresnel. Find a 770 mm field fresnel. Buy +1.0 and +0.75 Diopter eyeglass lenses and put them in a tube. The Rolyn lenses that tgreenwood mentioned only go down to +1.0 D in the acrylic. You will have to buy a glass +0.75 D. This will give you an image that is good enough for watching video, but a bit blurry around the edges for running windows. If you want a sharp image from corner to corver, you will need to find a triplet or tessar lens designed for somewhere in the 550 to 650 mm fl range.
Thanks tgreenwood.
If I've done this correctly...it is a 15" LCD to be projected....approx 10 foot projected image...and 16 foot distance to the screen...which suggests a 21.3" focal length.
If I go to Rolyn's...the lens closest is the 21.5" lens EFL which gives the following...
Lens EFL 546.1mm
Element FL 935.8mm
Distance 273.1mm
Aperture 168.7mm
Stop 91.8mm
So from what I see this is a +1 diopter lens which is not hard to find, but I think there is going to be a problem finding a lens with a diameter even close to 168.7mm! Can I use a 100mm diameter lens blank from an eyeglass shop? Will I just lose light or will I introduce abberations of some sort? Have I misinterpreted this stuff?
Any help is greatly appreciated.
Dee
If I've done this correctly...it is a 15" LCD to be projected....approx 10 foot projected image...and 16 foot distance to the screen...which suggests a 21.3" focal length.
If I go to Rolyn's...the lens closest is the 21.5" lens EFL which gives the following...
Lens EFL 546.1mm
Element FL 935.8mm
Distance 273.1mm
Aperture 168.7mm
Stop 91.8mm
So from what I see this is a +1 diopter lens which is not hard to find, but I think there is going to be a problem finding a lens with a diameter even close to 168.7mm! Can I use a 100mm diameter lens blank from an eyeglass shop? Will I just lose light or will I introduce abberations of some sort? Have I misinterpreted this stuff?
Any help is greatly appreciated.
Dee
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