135mm long-throw lense kit from diylabs

[Guy Grotke]

Since we don't know the focal lengths of the individual lenses that are in the 450 mm fl triplet, we can't really say what happens to the light inside it. If the lenses were +450 mm fl, -450 mm fl, +450 mm fl, then I think the first lens would refract rays at the edge by about 8 degrees. But then the middle lens would refract them back by some amount.

Instead of guessing about what happens inside the triplet, my estimate is based on the size and directions of the light paths coming from opposite corner LCD pixels for this particular application. In the drawing below, the blue lines represent the path of all of the light going through an LCD corner pixel. (I know that this is the path, because if I remove the triplet and put a piece of white paper where the center lens would go, I can see the focussed 60 mm long arc image.) The width of each light path will actually be a little bit smaller than 60 mm inside the middle lens, because of refraction by the first lens.

The green lines represent the path of all of the light going through the opposite corner pixel of the LCD. If we send light along these paths, at least we know it will get into the lens and most of the light from each path will get converged to a single spot on the screen 10 feet away. If we want to know exactly what happens inside the triplet, then we need to know the lens materials, the 6 surface curves, and the spacing. Then we can put all of that in a ray tracing program to make some better predictions.

But maybe all of this is not necessary: If anybody reading this has one of these triplets, they could use a laser pointer to find the maximum field angle through the lens in just a minute or two, and then post the result here!

[ancorp]

Originally posted by Guy Grotke

But maybe all of this is not necessary: If anybody reading this has one of these triplets, they could use a laser pointer to find the maximum field angle through the lens in just a minute or two, and then post the result here!

I have a laser pointer, and an identical looking lens... how can I measure the fov with a laser pointer?

[Hezz]

ROX,

You are thinking but you are in error. The refraction caused by the lenses system is taken into account as is part of the chromatic correction and these issures are taken into account when the FOV is stated.

All light is refracted by all of the lenses in differing amounts. The light being refracted does not lessen the stated FOV because the designed FOV assumes that such things are occuring and the lens correction is what takes this into account. The FOV of a lens system is not based on simple geometry but we have used simple geometry to show how the light cone of the fresnel will fit in the aperature of the lens. As long as the field fresnel light cone is within the FOV of the lens there will be little wasted light output. But this is a seperate issue from the objective lens system's FOV.

The FOV of the objective lens system is not based on such simple geometry. It is determined by the corrected refracted indexes and dispersion of the combined lens system. Each lens element is a different glass type and shape and spacing from the others lenses. This is what is used to correct for the different amounts of refraction at different light frequencies.

Repeat, the stated FOV is an engineered level of performance and assumes that there is refraction therefore the stated FOV is what it is unless the company was lying to you. But what they have stated is inline with what Guy has measured and what common lens design sense and experience indicate it shold be.

Hezz

[Inkog]

UPS should be delivering this lens to me tomorrow! (It only took 45 days or so for DIYPC to ship it to me )

I have been following this discussion for quite some time and will be happy to do some experimentation in attempts to help resolve these issues once and for all.

I have a laser pointer around somewhere also...

[Guy Grotke]

You just lay the lens on its side on a table with the screen 10 feet away, then point a narrow laser beam through it. You should see a laser spot on the screen. (Use some masking tape or a couple of small paperback books to hold the lens in place.)

Create a line on the table (on paper maybe?) that is perpendicular to the lens central axis, and 530 mm back from the center of the lens tube. Then you can move the laser pointer (with its tip on that line) as you keep it pointing at the center of the lens. (Not the center of the first lens surface, but the center of the entire lens tube.)

When the screen spot dissappears, you may have gone past the maximum field angle. You can try and move the laser direction a bit on the first lens surface to see if you can get the screen spot to reappear. If not, then you have gone past the maximum angle. Move the laser pointer back along the line until the spot just reappears on the screen. Measure the angle of the laser beam relative to the central axis. If you don't have a protractor, you can just measure the distance of the laser pointer tip from the central axis along the 530 mm perpendicular line. If that is more than 8.5 inches, this triplet will work fine for a 17" LCD.

Tip: Make sure the laser pointer is at the same height above the table as the middle of the triplet, (ie. 67.5 mm?)

[Rox]

Hezz said; "The FOV of the objective lens system is not based on such simple geometry. It is determined by the corrected refracted indexes and dispersion of the combined lens system. Each lens element is a different glass type and shape and spacing from the others lenses. This is what is used to correct for the different amounts of refraction at different light frequencies"

I agree with you Hezz, SO the test Guy Grotke described (my friend did it also with his 135 tirplet and my laser) i would say it is not the Field angle, this test would show the max critical light angle, mean the last light angle (light stop angle), but i believe the fov is smaller because it is not geometrical defined but the aperture where the focus, light intensity, aberrations... are the minimal expected by design as hezz said.

I still have not received the reply from manufacturer about the field angle 24 (12+12) or 48 (24+24) but i think the spec they will give will be more acurate than any test we could do.

[Hezz]

Rox,

I think that they may not respond to you because it is common knowledge that the stated FOV is measured from the lens axis to the furthest corrected light angle. So if they told you that it is 24 degrees then the total usable imaging angle is twice that number.

I thinks Guys test was still reasonably accurate in describing the approximate FOV. It would'nt make much sense from a design point of view to make a lens system this large have only a 24 inch total usable imaging angle because it would have little utility except in the case where a huge amount of light ( several kilowatts) had to be put through the lens at a narrow angle.

Hezz

[Rox]

well, if they have told me that the angle is X, then they have not problem telling me where is the angle defined .

I still have no reply, but will tell you soon. Just wonder, have you ever seen corner to corner focused image with 15"?


this is focused ok at center but i would not say it is "corner to corner focus" check it.

http://img.photobucket.com/albums/v6.../DSC01328b.jpg

[Guy Grotke]

The laser beam used in the test I described will be a very narrow beam, so it will let you determine the maximum angle at which 100% of the light in a particular ray will get through the lens. Once you know that angle, then you can use it to see how wider beams (ie. all the light from a corner LCD pixel) will pass through the lens.

You can refer to the last drawing I included in a post. The laser will show you if the upper edge of the corner-pixel beam can get through the lens. Then you can rotate the laser slightly to see if the lower edge of that beam will also get through the lens. If both edges of the beam get through the lens, then it will work fine for a 17" LCD projector.

Also, I think that most lens designs are made so the edges and the center have "good enough" performance. Some distance part way between the edges and the center, the performance will be the best. But that particular distance does not define the FOV. The full field of view uses much more of the lens.

[Guy Grotke]

Isn't that image from your friend's projector that uses a 330 mm fl field fresnel? That probably has more to do with the poor corner appearance than the use of the 450 mm fl triplet.

gg