Using the Parabolic Reflector, I can't get that EGG YOLK any smaller that say... 4" around, maybe slightly bigger. Arizona just pm'ed me and I don't know if this is what he meant or what, but if a person used a 550mm Collector Fresnel would the "EGG YOLK" be any smaller? Is there a difference between 220/330/550mm EGG YOLK's? Someone please?

The longer fresnell [550mm]will have a slightly smaller yoke because it has a lower magnifying power.[edit could need some help here] Each fresnel has a focal point ,that point is the smalest of the tip of two triangles. If the triplet is at either side of that point the egg will get larger but what happens when the triplet is 30mm foward of this point? the light cone gets larger and the pj image gets dimmer if all the light does not make it in the triplet.

A longer FL fresnel will project a larger image of the object. Your object in this case is the lamp arc viewed through the reflector.
You won’t get a smaller “yolk”.

DJ

Did a test using two 300mm Fresnels for the front Fresnel. The fl was cut in half to150mm and the arc image was half as long too. You are soooooo right.
Thanks for the correction

If you are using 2 lens then it is the magnification of the lens set. with a 650/220 it would be 650/220=2.95. or 2.95 X the size of your light source.

I tried a 5" lens with a -500 focal length (negative) and the virtual image is smaller, making the magnified image smaller

NOPE.

See all of you are too used to using two fresnels. The Egg and Yolk are different with a parabolic reflector because you have a huge 18" donut that hits a collector fresnel which tries to squeeze that through a 4" triplet (or 1.5" depending upon the lens used).

The Parabolic design can only use ONE fresnel.

Actually Lucky it’s not that different at all. You can use the same equations to calculate the magnification of a parabolic/fresnel hybrid that you would use for two fresnel lenses. Magnification calculated by dividing the two FL’s, 300/220=1.5, is only accurate when the to lenses have no gap between them. When you separate the lenses the magnification changes. In affect it is a thick lens and you need to apply Gullstrand’s equation. Most of the time dividing the two FL’s is adequate but in your set-up the distance between the reflector and the front fresnel is large. I set up an Exel spread sheet with all the equations so if you want I can put the values in and give you all the expected sizes and distances. I’ll probably include these equations in the new Focal Calculator when I get time.
If you want to do a quick check on your set-up, divide the front fresnel FL by the parabolic FL and multiply by the arc size, this should give you a rough size of your egg yolk.
The egg white that you are seeing is waste light directly from the arc. You will probably find that by blocking the end of the arc it will disappear.

DJ

I don't want to debate this because you guys lack the experience of working with a Parabola and I lack the experience of working with two fresnels, but I have done some. The Parabola is DIFFERENT PERIOD.

It collects and collimates the light differently, it collects it more efficiently (I believe) but it doesn't collect it as NEATLY. Basically you have a powerhouse collecting light at all kinds of different angles from the bulb and then throwing that light in rough collimation to the fresnel. The fresnel then NEATLY takes this messy light and focuses it down to the triplet. Now since that light was messy before the fresnel collected it, the neat and careful fresnel cannot focus it down to a small point.

That is the way it works. You guys and your theories of doing calculations are great but those are only ideal situations, a lot of you seem to forget that. People calculated this wouldn't be brighter, in my opinion it is (bear in mind I have not had oodles of experience with two fresnels). Now if the parabola proves to be brighter everyone is going to be rushing to calculate formulas to explain why, the same people that previously calculated that it wouldn't be brighter. This is why I will NOT debate the issue. Just another waste of time and more disinformation for people who want to learn to get mixed up on.

Waste light from the ARC? You are WRONG about that too. I have blocked the light. I am not sure (notice I try NOT to give 100% answers unless I am certain), MY OPINION is that that "WASTE LIGHT" is light refracted off the edge of the parabola, or perhaps angled light bouncing off the bulb length and then back to the parabola. Either way, it is not coming from the bulb tip because it is still there when I block the lamp.

I don't want to come across rude, but when people keep telling me things are this way, or they are that way.. and they have no experience in the matter it starts to bother me. WHY can't they preface their OPINION with I BELIEVE? It would help seperate fact from fiction.

---

Here I am coming across hard again, perhaps its my own fault from not saying something correctly in my first post. I'd have to re-read. The waste light circle could be coming form the arc, but it's not coming from the first 2-3" of radius around the bulb axis (in front of the bulb). But why am I talking about that????

I don't care about it right now, I care about the Egg YOLK! That is now goal #1. Getting all of that YOLK through the triplet. Other refinements can be tuned afterwards.

Id like to second you on your description of the differences...

I think one will find that the light that is not getting through the triplet is in fact secondary light from the bulb.... I'll explain

Effectively we have two light sources (bare with me here) , one is the REFLECTION of the bulb from the parabolic reflector dish, this reflected light is perfectly calumniated, it goes through the LCD and to the front fresnel as parallel rays of light, this is what the front fresnel is able to focus.

Now, if there is light getting to the front fresnel that is not as parallel rays, then the front fresnel will not focus that light to a point, it will refract the light towards the focal point, but not all the way.

The front fresnel only focuses parallel rays of light, that’s what it does

Now, I said there were effectively two light sources, the second one is the unshielded light emanating forwards from the bulb, in a hemispherical arc towards the LCD, this light is not as parallel rays, it hits the LCD (and therefore the front fresnel) at an angle dictated by the line of intersection from the bulb arc centre, to the point it hits the LCD screen......

The upshot is, that the front fresnel lens will try to focus these secondary light rays... Even though they are not parallel...... It bends these light rays in toward the focal point of the front fresnel, but not enough..... (because it is only designed to refract a parallel ray in to the focal point of the lens....

If one was to do a ray trace of this one would see what i mean.


Click to view attachment


As you can see the first diagram is the light from the parabolic reflector, the second diagram is the light directly from the bulb....

In the second disagram the light rays are not parallel, hence the fresnel does not focus the light rays, they dont converge to the lens at the focal point.

I believe that this is the enlarged egg yolk, its waste light, and might not be worth worrying about.

EDIT.

I would like to point out that rather than trying to be "SMART" at LUCKY_ME's expense, Im trying ot help here, you might notice that I too am doing parabolic reflector tests.

Why did you open this thread if you don't want to know the answer?
FYI. I have experimented and used both parabolic and elliptical just not in a projector.

DAZZZLA,

I've liked everyone of your posts until this thread. Disappointing.


RobandJonk

Yes, your pics and anology are right in my opinion but.. there is one thing you are missing. It is the same thing everyone overlooks. Your ray trace of the Parabolic Reflector isn't exactly right, it is only true for a portion of the light. The real ray trace of parabolic reflection (with no direct light from the bulb) should look like a combination of BOTH your diagrams.

The Parabolic Reflector spreads light out at various angles because the of the length of the ARC, I think for the most part these angles are relatively small, but none the less angled. The light that does not hit the triplet is NOT WASTE LIGHT. This is proved everytime I widen the triplet's Aperature, the projection gets brighter.

The only waste light is the near the triplet is the Egg White, but I think that is so minor, I am not worried about it. The EGG YOLK is what needs to be pushed through that tiplet entirely for the brightest posible image. But the Egg YOLK is large and I haven't been able to find a way to make it smaller (at least not yet for anyone else who wants to step in look smart at my expense).

This is the REASON FOR THIS THREAD. Somebody commented that a different FL would change the size of the YOLK so I just thought I would ask to be sure....

Stop typing in caps, it is yelling. Keep it civil.

The collimated light is your "egg yolk" It's donut shaped because the lamp is in the way. If we could hang a little miniature sun the size of a single pixel or smaller at the FL of a complete parabola, we wouldn't have a problem.

We can't. So you do.

Because of this, you need to capture the off-axis light which is all that's directed towards the center. Because this light is practically guaranteed to be non collimated, the one place that it CANNOT be directed by the field fresnel is the center of the triplet. As such, a larger triplet is needed in order to properly gather this light. The larger the "hole" in the middle, the larger the triplet needs to be, since the light striking the center becomes less and less collimated.

Greater distance between the parabolic reflector minimises this effect, as the light becomes more collimated with distance.

Dazzala knows what hes talking about. The answer to your yolk size is in the last few sentences of his post. Tackling the large yolk with a big lens will yield a fuzzy picture. You wont be able to overcome it. Thats not because of lack of ingenuity but because there is the huge field of optics that has studied it. When I read his post i knew he saw the problem. "wrong answer" is not the right response.

You would have to find an amazingly rare lens to take the cones of light you are driving into it while having a 130mm aperature. Rare enough to be basically custom made for that system. Not a triplet either, im talking like a 6 element piece. You wont be able to focus well without it. Please prove me wrong. Id be happy to be wrong.

This is my opinion:
The way to reduce the yolk to yield a good optic system is to use a fresnel to make the light into a yolk half way between the lamp and the lcd. Use an aperature here. You then need a lens to help open it back up evenly to the next fresnel that drives it down through the LCD. A small point here will have light coming in narrow cones out of the lcd to your lens. Narrow cones means cheap available lenses can focus it! Do that and a large lens will be brighter but a small lens will also be able to focus.

Narrow cones are key. Imagine the cone of light going from each pixel to spread in the lens. Trace it back through the system. If the cone is wide - from a big yolk - most lenses become 'light flashlights'. You may get an image but it will be washed out and blurry.

What does the combination of parabola and first fresnel do? It simulates an ellipse which is the real ideal reflector. This parabola and a fresnel may be the cheapest ellipse people can come up with. For that reason the parabola can be good in a projector.

This is not negative, it is trying to present the reasoning as i see it. If you dont believe me get lens design software and try to make a lens to the parameters we are discussing. I recommend oslo.

Here is the link for anyone interested:
http://www.lambdares.com/techsupport/osloupdate.phtml

It was the right response. Perhaps I was a little harsh because I felt people were taking the point of this thread off topic and twisting it to... I don't know, something that didn't sit right with me. You should know all about that.

In my opinion you are wrong about the focus. My Ednalites(3 piece lens) focus perfectly, the vari-focal(4 piece lens) catches every piece of lint corner to corner, the Docter-Wetzlar (3 piece) perfect focus again. The old BUHL(3 piece) lenses don't focus perfectly. This to me is lens quality, not an issue of aperature size. Can you see the relevance? If a wider aperature decreases focus, why is everyone waiting for brain's Pro-Lens? Why? More light, yes... but it is possible to have a larger aperature and still retain focus (to an extent that is financially viable anyways).



Well you do go deeper into depth than I am willing to at the moment, however this is (all of it) off topic. Perhaps I should have titled the thread differently, my fault again. The thread was simply a means to be sure that there is no difference in egg "YOLK" size when using different FL Fresnels after a particuliar person stated something to the effect.

My apologies, but I am not willing to get sidetracked in these "deeper" topics because I am busy with other things. If there is a problem with the Pro-Lens I can come back to this. The 135mm's... well I have three interested sellers, but others have convinced me that it's not the quality of lens I am looking for either.

Nice one.

Very "Community Spirited"

Arc image = “yolk”
For a fresnel / fresnel set-up the arc image is calculated using the front fresnel FL, the back fresnel FL, the distance between them and the distance that the arc is from the rear fresnel FL.
For a parabolic / fresnel set-up the arc image is calculated using the front fresnel FL, the parabolic FL, the distance between them and the distance that the arc is from the parabolic FL.
The arc image will be larger if you increase the front fresnel FL.
The arc image will be larger if you decrease the rear fresnel FL or parabolic FL.

I still don’t know what I said wrong to make you flip out so I’ll try not to post in this thread as I seem to have upset you some how.

DJ

DAZZZLA,

I don't know either.


RobAndJonK,

I probably deserve that, but I don't fully see it.


Me2!,

I get you're point about the lenses, thre rest... I'm not absorbing at the moment.

Dazz,

Could you please post the equations for the calculations listed above.
Perhaps a new thread would be best.

thx

Here are diagrams of the yolk from a parabolic in oslo:

http://www.lambdares.com/data/Ttutorials/T...rLightpipes.pdf

Look at the diagrams on page 22 and 34. 34 in particular. See they are trying to funnel light into a pipe with a lens( a real one not a fresnel which would do a much better job). They are trying to make a smallest yolk. See they are modeling this with a .5mm source. A 20mm arc will yield a huge yolk in comparison. The idea is the same though.

The edits are my lousy typing...

Please explain perfect focus. On my screen i can see the individual colour subpixels. Blue looks like a dark line. Im using a smaller 150w to achieve this. 60" wide screen. Your pictures didnt seem focused at all. I have a cr@p pinhole camera so I know how bad the camera can make the picture look. Try a thin line cross. Does this project as a cross or as a ghosted smear?

Me2!,

Please explain perfect focus... well that depends on the lens. The best lens I used could capture and display the finest scratches, fingerprints and lint on the transparencies from corner to corner. As I have said in the other threads, the aperature is too small, so moving up to the big BUHL lenses they let more light pass but they do a much poorer job of focusing. It may be true that larger lenses with perfect focus are hard to find, hard to make and hard to buy but I beleive there is something out there that suits what is needed.

Back on topic, interesting link I am surpized that a light pipe is so messy, but then I guess it could make sense. last night I as thinking I am going to do something I haven't done yet. I am going to place a fresnel 330mm from the wall and then I am going to move the lamp/reflector assembly back and forth and I am going to see what distances produce the smallest Egg YOLK. It is the least I can do.

Here are those equations you wanted to look at.
http://www.lumenlab.com/forums/index.php?showtopic=8281
DJ

Thats a good idea. As you increase the distance you remove more light that is off angle. A possible way of reducing the size of the projector is to seperate out the light box. You could have the parabola blast light from the corner or the attic to a mirror with the LCD and objective lens. The box you would have to keep in the living room would be much smaller.

Also, this would greatly reduce the amount of heat anywhere near you lcd! Woot!
That could only be done with a parabola.

I think this is the type of lens your looking for. Imagine a cash register sound when you look at it. cha-ching!

Brain will find a cheaper compromise lens for us.

I have one, it is the best lens of the six that I have. However, the aperature is too small.

P.S. The owner got it FREE.