So i got my 35,000 Lumen bulb, i got the ballast, i got the socket, i got it all wired and working!! I'm ready to begin work on my box and then. . . So i'm using a double fresnel lens i got out of my overhead projector and i realize i dont know the focal length on either of them!

I know how to do a focal length test on a single piece but not on a double, i read somewhere on here that leaving them together is better so that is why i have not attempted to separate them yet.

1) Is there any good way to be able to tell the focal length of each of them?
http://74.125.95.132/search?q=cache:edCIMM...=clnk&gl=us
Said 293mm so i'm guessing that's combined.

2) Separate or together is one even really better?

3) General recommendation for distance between a 400W MH 35,000 Lumen bulb and the fresnel lens so i dont melt it? I see people saying they just put their OHP double fresnel 190 mm from their light source, any objections? Should i go 220mm? And about far should the fresnel lens be from the focusing lens ( I'd assume ~290 mm) ? My focusing lens will be adjustable so dont worry about it needing to be exact, thanks!

4) Should i use a semi-sphere reflector or should i set up mirrors inside the box, because i have several smaller mirrors as well as a 5' x 5' mirror i can cut to fit

Thanks!

The lamp to fresnel distance should be what it was in the OHP -- or at least as close as possible to that distance.

Note that having the fresnel reversed will result in poorer light transmission, so if you can't remember which way was which anymore, leave the ability to reverse the lens.

Once in place, you'll adjust the lamp to fresnel distance to get the most light possible into the projection triplet.

Thank you as always supra guy for the advice my concern is

In the bottom of my OHP a there is a Mirror that goes like this / (slanted)


Would i just measure from the bulb to the middle of the mirror then to the Fresnel lens for the focal length?

I called 3M on their 9075 model they said it had a focal length of 11.5, im getting 13.

P.S. If i get the fresnel mounted, can i just move the bulb behind it from like 7 to 13 inches and put it up against a wall and see what puts the most light through?? I understand why the collector triplet lens has to be at a certain distance from the fresnel but i really don't understand why the bulb has to be a certain distance from the fresnel?

There are 2 sets of optics in any projector.

The primary optics are the objective lens, and it focuses the LCD on the projection screen.

The secondary optics are the fresnels, and it focuse the lamp on the objective lens.

So since the fresnels have to focus the lamp onto the projection lens, it matters where the lamp is in relation to the lens.

Yes, I'd measure the distance from the center of the fresnel to the center of the mirror, and then from there to the lamp in the OHP.

Thanks! Ok so here is where im at

I measured it got about 12.5

I checked the corner it seems to say 12.5 in the corner on the fresnel lens as well

And i'm not sure if either of these matters but maybe they can help as well

When i put the lamp 12.5 inches behind the fresnel and project it onto a wall (With no triplet lens in yet) I get a square box

When i put the lamp ~190 mm behind the fresnel lens and project it onto the wall i get a crystal clear replica of the bulb lit up

Although ~190 mm is a cool effect, i would want 12.5 inches because i dont want bright spots, i want a even spread of light like the box at 12.5 inches rather than focused areas of intensities (that replicate a picture of the bulb lit up) at ~190mm correct?

No!

In order to get even lighting, what you want is to get that clear image of the lamp -- but at the entrance to the projection lens. What this means is that you'll get as much light as possible -- from as much of the LCD surface area as possible.

Remember that these optics are separate and distinct.

So to get that clear image of the lamp, that ~190mm distance seems closer, but that only works if the projection lens is going to be about the same distance from the fresnel as the wall was. Use the focal calc to determine the apropriate distance from the LCD to the triplet (Assuming that you know the focal length of the triplet.)

1) Then lens i've salvaged out of the OHP is a singlet so i will be using it temporarily, how would i tell the focal length? Take it out in the sun and aim it concave up/concave down? And see how far it has to be to make a point of light on the ground?

2) My joint fresnel lens is slightly warped it bulges away from the side that the light comes through, is this normal or did it over heat and warp when it was in the OHP? Because the OHP was only ever on half power and had the fan was always going ???

1. The best way is to look up the specs for the OHP, but yeah, you can determine the FL of the lens using the Sun to fry ants...

2. Somewhat normal. Fresnel lenses have a tendancy to warp because of the way that they're shaped. Thermal expansion/contraction will always affect them differntly on opposing sides. I wouldn't worry about it, since it's not a factor in overall image quality.

Wouldn't it be better if i place the bulb at 12.5" (Rather than 19") then adjust the collector lens location so bulb will be closer and thus the image will be brighter? Otherwise the sphere raduis of light given off will be another 6 inches


Edit: Okay here is where i stand

The lengths from the fresnel lens to the singlet in the OHP can be changed from 12"-16" with 14" being the average
The length from the bulb to the fresnel when it was in the projector was 12.5"
I called 3M and they know nothing they told me that 11.5 is both the focal length from the bulb to the fresnel (which i measured as 12.5 and it says 12.5 on the fresnel)
And they told me 11.5 is the distance from the fresnel to the singlet (Which isn't even possible)


(Situation 1 and 2 use the Singlet to determine the bulb placement)

(Situation 1 ) When i put fresnel at 11.5 inches away from the wall i have to put the bulb 19 inches!! from the fresnel it made a VERY clear image of the bulb, the socket, my hand moving the bulb, and the inside of the box

(Situation 2) When i put the fresnel at 14 inches away from the wall i only had to put the bulb 14.5 inches away from the fresnel, it made a pretty clear image of the bulb, however i feel the reason why i don't feel it was a VERY clear image is because it was so much brighter than the first one it was hard to look at, however the image reproduced did NOT include my hand, the socket, the inside of the box.

So would i want to go with option 2 since i really don't care if the socket/my hand are not in the image because my hand/the socket don't give off light, and it lets the bulb be closer so it will brighter, or should i go with an situation 3.

(Situation 3 uses the bulb placement to determine the Singlet placement) Place the bulb at 12.5" inches behind the fresnel lens means the image on the wall is clear at ~16" (16 is ok because while it was the max distance for the OHP i'm building a rear mount projector that could go from 14-18 for example)

P.S. Glass broke, I can get Lexan or Tempered Glass, i see Supraguy has said tempered glass is more rigid, but is either better as a heat shield? And what would the recommended thickness of each be thanks!

Second Edit: So i noticed something, considering the focal length for a triplet lens can be determines as 1/S1 + 1/S2 = 1/F I thought id plug my fresnel lens into it with the numbers provided above, they all come out to ~180mm (178,180,181). So I'm assuming that means all of the above options would work, and if so my question become how close can i/should i put the bulb to make it as bright as possible but not make it melt (ill have fans and temperedglass/lexan)? If you could let me know when you get the chance it would be appreciated as well as recommended thickness on glass/lexan, thanks! Also curious, the lexan won't block a significant amount of light will it? Cause if it doesn't couldn't i just use 2 5/16 pieces to really make a heat shield?

Any layer will block some light. Lexan is very clear, and block very little light

3mm of lexan is perfectly adequate as a heat shield and is easy to work with. It's easier to do so and work with than tempered glass. The XL10 product is my personal choice, because it also blocks UV.

19" lamp to fresnel seems to be way too distant to me. The lamp really ought to be closer to the fresnel than the triplet is. Those results you're getting seem strange to me. And no, I wouldn't expect to see the socket, your hand etc, since the lamp should be (relatively) so much brighter.

Well we shall see here soon as i complete my monstrosity V2 cause the numbers do seem goofy to me too, my last thoughts are....


1) Black outside white inside, since black absorbs light and white absorbs no light (Any objections?)

2) How close should the LCD be to the Fresnel?

3) How close should the Fresnel be to the Lexan?

4) How far should i put the mirror behind the bulb so it doesn't shatter / melt ?

5) You know how when you put the light at varying distances behind the fresnel, it will make you have different lengths in order to get it to focus? Well if i put a mirror behind the bulb, wouldn't that need a different length than the part that is just going straight through the fresnel and need to be adjusted and clear because it went however much farther and back, but then it would be the incorrect distance for the primary light that went right through?? How are you supposed to use a mirror or pro reflector considering this?

1. Black inside. Extra light coming from somewhere other than your lamp isn't going ot help the projection any, and will, in fact potentially wash out hte image. Stray light at the front half of the projector can only wash out the image, it cannot add to it. Light coming from somewhere other than the lamp becomes stray light at the front half.
   
   The advantage to the white would be to keep the temperatures down. Black also radiates heat best. However, airflow is the corect way to keep temperatures down.
   .
2. 20mm is a recommended minimum distance. There is no maximum, however, you will have to take into account what's practical.
   .
3. Anywhere from touching up. In my first projector that was about 25mm, in my pro lens one, it's about 80mm.
   .
4. I would not use a mirror in the back of the enclosure, however, glass is very heat resistant, as long as it is heated gradually, and allowed to cool gradually. Thermal stress when parts of the glass are at widely different temperatures is what cracks glass. A proper reflector made either with pyrex or stainless steel (or aluminium) will be able to take heat much better.
   .
5. Yes, it would, which is why we do not recommend a mirror. A proper spherical reflector will work much better and not change the optics requirements of the projector. A mirror may have a brighter overall picture, but because of the problem with focusing the reflection at the triplet, it will increase the projetor's vignetting, and you'll have worse "dark corners" than you would otherwise.

I have an open rated bulb so it has two "layers" the outer one has a radius of about an inch, considering that AND the fact that a pro reflector has a depth of like an inch, would i even be able to use a pro-reflector or will that wash out the image like a mirror?

P.S. Thanks for the advice on the black inside as well!!

Radius Of Spherical Reflector Calc: Helps determine focal point of soup ladle (or reflector)

I understand that but what im saying is if the lamp is 7 inches away from the fresnel lens, then by putting a reflector behind it, even if is IMMEDIATELY behind it, the light will have to travel AT LEAST 7 + Twice The distance to reflector and as i discussed above the distance from where the light come from determines where you have to place the triplet lens. So if i put a reflector in when my triplet is focused for a 7 inch light source the 7 +2x light won't focus. But i know others have used reflectors and i was wondering what they did to overcome this

Edit: Apparently my cocnern has already been discussed in depth by brainchild and jones rush, since there is a polarizer this won't happen, but that makes me wonder other than potentially breaking it or melting it, why can't i use a mirror?



Secondly my local home depot and lowes only had like 1 mm Lexan, they did have thicker Optix i think it was called but it is arcylic not a polycarbonate it nearly isn't as shatter resistant it said Lexan was 200x stronger than glass while this was only 10x stronger. However i don't plan things to be hitting against it so i was wondering if Optix would be usable as a heat shield since its an acrylic and not polycarbonate. I see it doesn't melt until 460 Celsius but i was wondering will it absorb heat? Or does the glass/lexan not actually absorb heat so much as control the air flow path? Thanks!

1. You should not use a mirror because it will result in poor light control. The pro reflector will be fine with that size lamp. That's what it was designed for. I could do up a diagram of what the problem is with a mirror, but given it's thermal issues, and the liklihood of it breaking, it's just not worthwhile. A spherical reflector is the way to go. I've seen people WANT to use mirrors, but I've never seen good results from it.

2. 1mm is awfulyl thin, are you sure? 3mm is about 1/8" and is the standard thickness at the Home Depot around here. This is what they'd sell as a replacement for a regular glass pane. It's plenty strong enough. Don't use the acryllic. It's not a matter of strength, it's a matter of temperature. Acryllic softens at 100 deg C, which it might approach with direct exposure to the lamp. It will absorb some heat, of course, but airflow will keep the polycarbonate in a safe temperature range.

I take that back its like 2.5mm but it only comes in like an 8x10 sheet and a 48"x48" sheet for like $40 so i think i'm probably going to go with tempered glass.



P.S. Would 2.5 mm work? And so this is more about controlling airflow than absorbing heat, it just has to be able to handle it?

2.5mm is fine, and they really ought to have smaller pieces. I bought a piece that was 24X36" the other day for going above the window-mount Air conditioner.

Any tips on how to cut the lexan supposing if find a better piece at the next closest lowes/home depot?
Edit: Scored and cut

And i still don't get why you would want the inside black if anyone feels like trying to explain that to me. I understand black absorbs light, white reflects ect. But since the LCD has a polarizer, and the only way out of the box is through the LCD how it would hurt the image? I understand it might be pointless and have a 0 % increase in terminal lux but how would it hurt the image and wash it out?

If'n that's not enough for you, there's plenty of more reading through a Google search: site:lumenlab.com paint inside black

Quick and im sure pretty stupid question, i got the rear of the box put together LCD/Fresnel/Lexan/Bulb/Fan now i just have to make the front half of the box, basically i had planned to make the front half of the box larger all around than the back half so i could slide it to adjust the triplet lens but thats not the point. What im getting at is i have half a box put together, i shot it on my wall, and held the triplet lens at the appropriate distance to focus the image on the wall. I was just wondering once i get the front half put on will that help the image at all as opposed to me holding the triplet lens in place?



Also a second question why does the location of the light prior to the first fresnel lens matter? I understand the second fresnel lens determines where the triplet lens goes, however what exactly does the first do? Say my first fresnel lens has a FL of 12" and i put it at 8 inches what would happen???

1) Anything that will prevent stray light from entering the lenses will be beneficial to the projection.

2) See the Light Flow section of the Guide.

If the lamp was significantly closer (i.e. 4") to the LCD than the focal point of the fresnel, you would not be able to evenly light the fresnel/LCD.

so i got my box all set up, the fresnel and lexan are 12x12 with a half inch gap on bottom for air, but my screen is 12 x 9 which means there will be an extra 3 inches of fresnel, does it matter what part of the fresnel i block off? Should i put it in the middle and block top and bottom off, should i just block off three inches at the bottom ect?

The center of your light path should remain in the center of all elements. If you were to draw a line from the center of your lamp's arc to the center of the triplet, that line should pass through the center of your fresnels and LCD. So you want to position the LCD so that it's centered on your fresnels (the center ring on the fresnels needs to remain in line with the center of the lamp's arc and triplet).

So... if you're going to block off part of your fresnels(s), you'd block off the same amount from the top and bottom. Keep in mind that you'll want a slightly larger area exposed on your fresnels then the active area of your LCD.

I hope I explained that well enough.

True.

It woud also be acceptable to cut 1.5" off of the top and bottom of the fresnels. (Well, you probably want it larger than the LCD, but not much.)

The center point on the fresnels is important, and should be aligned to the projector's principal axis. The center of all the rings of the fresnel is the important part.

Is there any reason why I should cut the fresnel lens supposing i block the unused area off from the front?

Only to save space. Otherwise, there's no reason that this should need to be done.

Really? I have serious issues getting the entire screen to be focused. I end up finding a happy middle and deal with it, but my fresnels are warped as well, and moving the LCD panel to compensate for the blurring seems to point to the fresnels... if the front fresnel is warped, wouldn't it have some impact on the ability to focus at the triplet end?

or is it just a bad triplet?

Well, for a SPLIT build the quality of the collector fresnel will have an effect on the image, but keeping the gap to a minimum will reduce that. (Hold the fresnel over a sheet of paper, and read text on the paper. Try it at 10mm, 20mm and 50mm. The further from the paper the fresnel gets the greater the magnification, and the more distortion will appear.)

For an unsplit build, the only thing that it will affect is light distribution.

If you cannot get even focus, it's more likely your triplet. (Suggestion: Try reversing the triplet direction. It makes a difference.)

If you cannot get even focus, it's more likely your triplet. (Suggestion: Try reversing the triplet direction. It makes a difference.)
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thanks SG!
Do you just flip it around in the enclosure, or do you dismantle the lens assembly and reverse the internal lenses?
Sorry if this is one of those "Jack Handy" questions, but I looked at my triplet and there is only this ring with 2 notches that seem to hold it in place, and they don't want to budge!

No dismantling required just flip the entire triplet housing backwards towards the fresnels.

Thanks guys for all of your help!

So tonight I flipped the triplet... it sounds like a cheesy dance move from prom... and while it's far from perfect, it made my toms hardware special look tons better! thanks again!

Now back to Left 4 Dead @ 720P!

Is it possible to use the optics from the OHP, or is that really where the quality breaks down?

I have built one using the OHP and other than only getting a 10" x 10" coverage of the LCD (which is enough to run XBMC in window mode) the quality it quite good, but not good enough for me.

Is going through the effort to build one with a good triplet len worth it?

Also, can I use the light from an OHP?

Thanks guys for an awesome thread, very useful!

It is entirely possible to use everything from the OHP and, provided that the OHP is reasonbly well made, get an excellent image.

OHPs are generally a bit limited though. They don't generally have to be as bright, since they're intended for use with highly transparent materials, which LCDs are not, but they do work.

OHP lamps are cheap, though in terms of cost per hour, they come out as being more expensive than MH... But you pay it out over time.

The other limitation of OHPs is that the bed size is generally small. A 15" LCD is probably about your upper limit.

Can I improve the design of the OHP for an LCD? Can the image be improved with straight through alignment without all the mirrors?

I am getting an image that is good but a little "fuzzy" and I was wondering if it is worth redoing the OHP and using a triplet lens.

I can focus it but not to the point where I don't think I need glasses.

hey there! I'm also using an "OHP" and get great results! I do have a metal halide OHP, but I was getting by with a standard halogen for the longest time... I'm guessing that yours doesn't have a triplet lens, which would be why you aren't able to get a clear image. I'd definitely change that and procure one!

Great! Thanks for the input.

I am using the 3M 910 (http://www.kpsurplus.com/products/view/21719) for pic. ($10 on craigslist )

So, did you just rotate your whole box on it's side and add a triplet lens kit at the fresnel focal point?

I have it hidden behind a wall so I'm not really worried about size or looks.

My OHP has a triplet in it.. I created a panel to house my LCD, mounted the controller on the back end of the OHP, and that's that.... I have an enclosure drawn up, but want to wait till I move to build it for simplicity sake....

Yes, that is what I am doing too.

My image it really good but it is just a little to soft for my liking and I can't adjust it any further.

Do you think I can order a the LL 80mm triplet lens and place it at the focal point of the OHP fresnels and get a clearer picture?

I am wondering if the focal point will be too large for the triplet.

well, right now you can't order any LL parts, but there are other sites out there that have plenty of parts in stock... I can PM you one I've been looking at, also you could try eBay as well....

Well, according to what I've found, your fresnel has a FL 182.9...
To view the manual for your OHP, use the following link:

http://www.mbelectronics.com/quickreferenceguide/900_955_ipb.pdf