This question may have been covered before, and I have been looking through the forums to find it,
I am building a 17" vertical Pro Lens setup......PJ->Screen is about 13-14'
I have noticed on some of the Blogs that people are unsplitting the Frezzies....

Can some one point me in the right direction with the Pros and Cons of splitting Frezzies..

In the technical sense, split fresnels give a slightly shorter throw as there is a "magnification factor" when using the field fresnel after the LCD... but the biggest factor is that split fresnels allow for keystoning, which when using a short throw where the PJ is located in front of the sitting position, allows for floor and ceiling placement. I should note that when keystoning (split deisgn), focus becomes an issue, while the straight shooter design does not (split or unsplit design).

In the more subjective sense, I think the biggest part (IMHO) is the picture color/contrast is so much improved when using the unsplit design over the split design. I think the only way to determine this factor is to experiement and see for yourself what you prefer. You can imagine that anything located between the LCD and projection triplet can degrade the image, and IMO, that is exactly the case, so I would "vote for" unsplit when possible.

cheers,
gs

Personally, after experimenting for myself, I nearly fell over with astonishment at how much better my image was with unsplit fresnels so there was no question about which arrangement I wanted. The only issue with unsplit is keystoning.

unsplit,, by a city mile...

I have had both. Un-split is the only way to go!

Keystone distortion can be countered in an unsplit design using lens shifting. But like all things there is a trade off. I have tested the pro lens using lens shifting and the trade off is pincushion distortion.

DJ

So the general consensus is Unsplit...
Question I now have........I have built the rails and side plates for the top end of the PJ with 15mm Gaps PERMINATELY set between each of the components..ie. 15mm Gap between first Frez and LCD and then 15mm gap between LCD and Second Frez

Using these Dimentions, will having the 1st and second Frez unsplit, with a gap of 15mm be to big?
and then 15mm to the LCD?

Actually, you'll need that 15mm gap between the fresnels to avoid an unwanted "starburst" pattern from showing on your projection... (a very careful alignment can also eliminate the starburst, but it's much easier with a 15mm gap or so...)

The 15mm gap between the fresnel and LCD will also be ok (or it should) as this is mearly your cooling gap... I would have suggested something a bit larger (20-25), but you can change this if your temperatures are too high, otherwise 15mm should be sufficient to keep the fresnel rings from being seen on your projected image, unless you've removed the ag layer from your LCD, then you "might" need to increase this distance a bit more, but you can determine that for yourself once you get up and projecting.

I had also designed my first box with more or less permanent settings, but have continued to add more adjustments to the box for tweeking purposes. My suggestion now would be to perhaps add more adjustment now as it might be easier... of course you don't need to do this, but I have a feeling it might make it better for you in the long run...

cheers,
gs

A couple of things that I'd like to add.

1. There's no such thing as TOO MUCH adjustability. I tend to more or less "permanently" emplace things, but I always leave a way for me to adjust things.

2. 15mm should be adequate to prevent a "starburst" in your projection. I found 10mm to be more or less a minimum, requiring reasonably careful alignment. Note that NO alignment will remain perfect once the projector heats up, since the front and rear fresnels will tart to expand unevenly at that point, so some gap will ALWAYS be necessary, or at least highly recommnded.

3. Earlier in the thread, GadgetSmith said that split fresnels give a longer throw. This is backwards. The magnification of the field fresnel produces a larger image, which is a decreased throw. No big deal, but I thought this should be corrected.

Now, to the question: With a 15" XGA LCD and the standard lens, I did not see enough of an improvement in the image to warrant an unsplit arrangement. The split design provided the convenience of keystone correction, so that's what I went with, and I recommend in that case. With the pro lenses and a 17" LCD, I'd definitely recommend unsplit. With the longer throw, keystone correction is less a factor than with the standard lens counterpart, and the increased resolution makes a large difference. In this case, unsplit is definitely the way to go to make the most of your projector.

Supra,

I was told the split gives more brightness, can you confirm either way on that. They said it lets the light go more directly through the lcd.

Well I did several tests with split and unsplit in my projector. And split ALWAYS gave less brightness. Usually 15-20% less.

In my 15" projector I got more even brightness with a split design at the cost of slightly decreased overall brightness. Also, in a split design contrast can be greatly improved since less light is lost contacting the cell walls(which floods the cell by the way, reducing contrast) of the lcd panel(more light goes through at a perpendicular angle to the panel itself as the light is travelling versus the reconverging angle created by an unsplit design). Now, someone on this thread will probably start arguing with me(again) but try it for yourself while displaying material with low level detail.

DOH ! ... can't get anything by you Supra ! ... correction made above... Thanks ! [note to self: do not post while under the influence of Dogfish Head 120 Minute IPA...]


I can see that there would be differing results in brightness for split and unsplit fresnels. With split fresnels, the largest area of light you can capture is the viewable area of the screen as light is collimated at this point so travels perpendicular through the panel while all light outside the viewable area is blocked. For an unsplit design, you can actually have the fresnels larger than the LCD viewable area collecting more light... in this design, light is being "collected" or directed into the triplet, so maximize light gathering ability, you can place your LCD further from the collector fresnel such as the light cone is just slightly larger than the viewable area, insuring that all available light form the larger fresnels is directed to an area that is transmitted through the LCD.

Loki & Jonjandran, it would be good to post the fresnels (FL & fresnels sizes) you used when comparing split/unsplit designs, and also the approximate collector to LCD distance... It would be my guess that the 15" design using the standard LL fresnels would have better results (in terms of brightness) when split than unsplit.

cheers,
gs

Well I won't argue because I don't really care and I feel like people should try for themselves, but I found the exact opposite to be true.

With unsplit I get better contrast and 15-20 % more brightness. And this is with a Psone Lcd, a 15" Lcd, 15.4" Wuxga Lcd, Normal lens, Pro lens, 18" lens. So I've tried it both ways in a lot of different projectors.

But like I said , everyone should try it for themselves.

Well like I've mentioned before I tweak a LOT. So in all cases I adjusted everything for optimum brightness and focus. And in EVERY case I found unsplit to be brighter and more focused.

I’ve come to the conclusion that the biggest reason some people have reported better lighting using split fresnels is that it is simply a FL mismatch. The 320mmFL triplet will be at about 370mm from the LCD when projecting an image. The front fresnels are usually 330mmFL. So to get the light to enter the triplet the lamp has to be placed closer to the rear fresnel. Although this approach works it does limit the fresnels efficiency, especially around the outer edges. So when the fresnels are split the front fresnel is now 20-40mm closer to the triplet and at a much better distance. If the correct fresnel was to be used then I believe that the un-split fresnels would out preform the split every time.

DJ

I use the 'newer' 317mm and 220mm standard 15" LL fresnels. Original lens was a 300mm Carl Zeiss S-Tessar Copy Lens(removed because of FOV issue). I now use a standard LL 320mm lens. 15" LCD panel.

Hi DAZZZLA,

If I understand you correctly here, in order to have best results with a split configuration, the front fresnel (the field) should have a shorter focal distance than the triplet? So when the fresnel in placed in front of the LCD, it's focal point will be on the triplet, but the triplets focal point will be on the LCD? This would then also mean that for an unsplit configuration the field lens should have a longer focal point than the triplet. Hope that makes sense, and that I'm not completely out to lunch here .

I am currently debating between the split and unsplit configurations for my projector and after reading this thread I am leaning towards an unsplit configuration not only for it's brightness/contrast advantage, but also for the simplicity of not having to have a rotating mechanism for keystone adjustment. I am using the LumenLab pro lenses with a 17" widescreen LCD, so the focal point of the field is longer than the triplet focal point by quite a bit (the difference in focal point will be greater than the distance between fresnel and LCD).

Is there a diagram anywhere on the forums of an unsplit pro set up?

http://www.lumenlab.com/forums/index.php?showtopic=2371

The fresnel FL needs to be larger than the triplet’s FL. The FL of a triplet is only meant to represent its power. When used to project an image it would need to be further away than its FL. For example a 320mmFL triplet when projecting a 3000mm screen will be about 360mm from the LCD. The Pro triplet with a 500mmFL when used to project a 3000mm screen will be 562mm from the LCD. So when a 320mm FL triplet is used the ideal fresnel to go with it, if designing an un-split with a 20mm gap, would be 380mmFL fresnel and for the pro it would be a 580mmFL fresnel.

DJ

Aww geez this just keeps getting more confusing, or is it that I've had a long day and my brain is fried. I'm unsure.

I don't really understand why the fresnel focal length would have to be 20mm longer when it is 20mm closer to the triplet, but okay I'll go with it. I guess my question is, when designing for an unsplit configuration, would the ideal fresnel focal length be shorter or longer than the ideal focal length for a split configuration?

[edit: did some research, I understand now. For an unsplit configuration, the focal length should be even longer, ideally.]

I think I should just shut up and go with the values the Focal Calculator gives me, since I'm not going to be changing the fresnels anyway . I just want to understand how it all works.

Thanks for your help with this.

Personaly I love the unslpit but then again I have a vertical pj no need for keystoning. I did test out the unspilt on it just for the hell of it and I still prefer the unsplit over the spit just need to make sure you give yourself some distance between the lcd and fresnel or you'll get rings.

if u take ur time with the build im sure both have their benefits but just to report my experience...

my build is pretty sloppy and not much is selaed etc. i originally went split and the thing was working fine. I took the beast apart to do AG removal and decided to give unsplit a shot. Well, for me the results were not good. I dont know if unsplit is less forgiving or what but I had all kinds of weird lighting irregularities and leaks etc. Needless to say its back to split which works fine (except the front fres has a scratch). In my next build i will experiement with both as it will be much more precise and finished.....

Brian

Microlens array for split design

Most people prefer the unsplit design. A fresnel is a terrible quality lens. Edmunds claims 5% FL variance on their fresnels. Their fresnels are probably better than most. This translates to a muddy, poorly focusable image.

What about a microlens film for the field fresnel? If each microlens is the diameter of the pixels and designed to focus that pixel's light at a set FL it might make a very nice split design.

http://www.ntu.edu.sg/home/yqfu/PDF...31104-JVSTB.pdf

http://rogers.mse.uiuc.edu/files%5C...APL_82_1152.pdf

Thoughts?

Heh. I should check this thread more often.

GS: No problem, I just thought I'd point that out.

medio: You can get more brightness from an unsplit design. That was the whole point to my pro lens projector design, to "push the envelope" as it were. But it turned out that for best viewing I found it best to give up some of that brightness.

For the record, the diagonal size of the "light cone" that I was capturing would have been about 21", funnelled back through a 17" LCD. Currently it would be about 18.5" but I haven't actually worked it out, because the virtual size is only one factor of many. (This size estimate is also making many assumptions that I know from a mathematical standpoint are absolutely indefensible, which is why I didn't really bring the numbers into play, but it does show the relative difference between my 440 lumen setup and the current 250 lumen setup.

In terms of brightness, unsplit rules. Period.

Evenness is prone to tweaking. I don't think that one design or the other is clearly superior, that's something that's entirely up to the buulder.

Me2: the problem with the microlens idea is that the whole array must work as one lens. In effect, this is what a fresnel lens already is, with the whole area directing parallel light towards a common focal point. Just focusing each individual pixel (or sub-pixel) is worthless. Also with the design variances between LCD manufacturers, the end product would be more expensive than just a large singlet projection lens the size of the entire LCD. (Actually, you could probably build a 450mm diameter triplet for the cost of such a precision lens. There! No field fresnel required!) Suffice it to say that it's just not practical.

Oh, and DarkMeat: Welcome to the Moderator team (The Mod Squad?) in case I didn't say it earlier.

reet: The FL of the fresnel should be LONGER than the FL of the triplet, actually. This is because in order to acheive a finite focus, the triplet must be a distance GREATER THAN it's focal length from the object (LCD) -- I did a write up on this that is in the wiki guide, so I won't go into a lot of detail here. Since the field fresnel is part of a working pair, it's focal length should be paired to the distance between it and the intended position of the triplet. In 99% of the cases with a usable throw, this will still be further than the focal length of the triplet. (Though this can be adjusted based on the distance of the collimator lens to the light source, but that's also beyond the scope of this discussion.)

Yes, I did some reasearch and I now understand how the optics work, and have decided to go with an unsplit configuration for my projector. Thanks though.

I think you missed my point. The whole array would work as one lens. The problem is with the fresnel. The rings cut across pixels at different points. This means the pixels will send light through the sharp points on the lens. This light will be scattered. With a microlens film, which is not much harder to make than the other films already put in LCD panels (everyone who has taken the panel apart has thrown away a few sheets of films), the boundaries of the lenses match the boundaries of the pixels. The light from the pixel is sent where it is wanted.

You are right this would need precision manufacturing. There are a few standard pixel sizes. Yes there are many panel sizes. If these sheets are made off a large die you can cut smaller ones to fit. Im not saying someone is going to get a company to make tools for this so they can have one lens. I'm not sure what would bring you to that conclusion. What I am saying is rear projection TVs currently use a high quality fresnel but that is going to change. They are OK with the fresnel because of the large viewing angle. The customer is not impressed with the image. They will have to move to large microlens sheets or be unable to compete with other designs. This means companies will have large microlens arrays already made to send DLP projection to the back of the screen. They will also have the tools to make large arrays.

I can only see the idea of a micro lens array working if there was a point source used. Or the micro array was place directly on top of the LCD. The problem is that the micro lens array will only be affective for the rays that originated from the centre of the arc. Rays from the ends of the arc will end up entering the adjacent micro lens so they would still be affected by lens edges the same way as a fresnel. Just my thoughts.

DJ

Sounds interesting, but neither of those links is formatted properly.

Thats a good point DJ. I think thats the reason a microlens could only work on the field side of a split design. It would have to capture light that has come through the LCD pixel. It would have to be right against the LCD. It would not improve anything if it is before the LCD.

Here are the proper links:

http://www.ntu.edu.sg/home/yqfu/PDF%20paper/031104-JVSTB.pdf

http://rogers.mse.uiuc.edu/files%5C2003%5CAPL_82_1152.pdf

I copied my post from DIYaudio. I guess the links got truncated. It seems noone posts over there any more.

This very informative thread makes me want to go with an unsplit design (and just use a motorized pulley driven arm to lower my projector when in use, 70's era James Bond super criminal style) ... but there's one big issue I'd like help answering first:

I have a 317mm fresnel from the LL store which, given a 20mm fresnel-LCD gap + 360mm LCD-triplet gap, will be trying to focus my hard-earned light about 63mm short of the triplet. I'm wondering a couple of things -

- How negative is the effect of this 63mm fresnel FL mismatch? Reduced brightness? *Much* reduced brightness? Crappy edges?

- How are other folks dealing with this? I looked for 380mm fresnels for a long time on the web and found nothing, so if anyone knows of a source, I would much appreciate it.

Thanks!

Welp, I'm sold. Unsplit it is for me, now...

Short fresnels bring light from a wide area together faster. This means the angles of light entering your field lens is big. If you have a small LCD it should be ok. Otherwise you need a big lens or a special wide lens. A metrogon is an example of a lens designed for very wide angles. You can make a smal box with all 220mm fresnels with a metrogon. There is no free lunch. a metrogon is a slower(dimmer) lens than the standard cook triplet.

noob question:

why is there a need for the rear fresnel in a non split design if the front frenel will simply and immeadiatly redirect the light narrowly?

I believe that the front fresnal makes the expanding light rays parallel, while the rear takes those parallel rays and focuses them back down as they head toward the triplet.

I have a question one of you may be able to help me with.

Recently, I've decided to go with an unsplit setup, but my 317mm fresnel has been slightly modified at the bottom to accommodate a LCD cable, which ran underneath it. I'm a little worried that with an unsplit design being 10-15mm behind the LCD to avoid starbursts that all of the available surface area of the fresnels will need to be used to insure all of the LCD is illuminated. I plan to make a frame for the fresnels tomorrow and see how it turns out, but I want to be prepared if it dosn't go well. After searching I have learned that lumenlab acquires its fresnels in 12in squares. These squares consist of two sides that are heat welded at the sides to insure proper alignment. To prepare the fresnels for us, luminlab removes sections from the top and bottom of the set, THEN carefully remove the heat welds to separate the forward and rear lens. For those of us wanting to go unsplit wouldn't a properly aligned / heat welded one piece fresnel be the way to go? If so, does anyone know if luminlab would consider selling them this way? It seems like it would really simplify the process of realigning the lenses + it would save space, as most people space the lenses 10 - 15mm apart to avoid starbursts caused by poor alignment.

Any thoughts on this would be greatly appreciated.
Lane

I don’t believe LL receives it’s fresnels welded. They come from the manufacturer in their raw form.

DJ

I agree with Dazz.
I have 11 different sets of fresnels in my hands ,those I purchased from different website stor. some of them are heat welded and some not when they came to me. As far as I know , the fresnels are heat welded together that for OHP. If you go with split- setup, you have to serparate them by yourself,and it's very easy to do that.
Best regards.
Kiat

This was posed by brainchild 01/27/04 in http://www.lumenlab.com/forums/index.php?s...hl=heat++welded thread.



Now, 2 years is a long time and lumenlab could certainly be sourcing their fresnels from other sources now. I suppose I could always PM brainchild and see if they still get their fresnels pre-welded or if they are buying from other sources, but I would like to know if ya'll think getting a properly aligned / heat welded one piece fresnel would be benificial enough to bother with?

Thank you for your replies

Lane

It doesn't seem like it would be very hard to weld the fresnels yourself. I mean, each one will have a center bump or circle, line up the 2 circles and glue 'em together. I bet even welded fresnels get messed up when heat starts warping them, but I don't know.