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Old 15th February 2005, 02:36 PM   #1
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Question DIY Projection Lens Triplet

Hey Everybody

Is there any chance of making one of these yourself?
If yes then how

From Lumenlab:

PJ325v Varifocal Projection Lens Triplet

Varifocal triplet for miniature projectors.
Focal length ranges from 275mm - 325mm


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Old 15th February 2005, 11:05 PM   #2
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Default 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...
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Old 16th February 2005, 07:07 AM   #3
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Perhaps, but if anyone knew how it works in general, it would be no problem for me to figure out, my brother is an optriocian so he could help me figure out the focal points and get the lenses for cheap

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Old 16th February 2005, 10:14 PM   #4
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Default 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.
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Old 17th February 2005, 08:17 AM   #5
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Default 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.
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Old 17th February 2005, 11:38 AM   #6
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Default 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.
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Old 17th February 2005, 07:23 PM   #7
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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.
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Old 17th February 2005, 08:02 PM   #8
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What do you think about this? http://www.surplusshed.com/pages/item/l2148.html
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Old 17th February 2005, 09:14 PM   #9
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Default 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.
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Old 17th February 2005, 09:23 PM   #10
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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
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