Okay, so, i'v not been around for quite some time. Two moves, one of which is across the country will do that.. 
But now I'm back! And with my LCD ready to strip, and money set aside to finaly do this thing (tm), I set out to rough up a design idea. I'm looking to start and hopefuly complete this project over the holiday season. (not much going on this year for me, so this is my 'keep busy' thing.
Okay, first off, this is a design rough, its just a simple 3d model of the idea. no measurements have been really made yet, no lens focal calculations or anything along those lines have really been made yet either. Those come after any problems with the inital design idea have been spotted and worked out. (thats what i'm asking you people for!
I'v carefuly thought out things like temprature concerns and cooling, keeping light leaking to a minimal on the fan holes. I also plan to put some thermal sensors on the design to keep an eye on the temprature of a few compartments, just to be paranoid.
The folded design is namely to save some space and to also keep the heat as far away from my LCD as possible. I also plan to use this projector to test out different methods of lighting once i have it actualy working, so a compartment style seemed best suited for that.
Please keep in mind, I'm a thermal paranoid type, thus the over-protection. I really cant afford to burn up an LCD, and i'm taking every precation i can to prevent it from baking itself.
The diagrams should be pretty self explanitory! Just to point out a few things, the fresnel and LCD sandwinch is 'exploded' in the diagram, the design merits the abillity to move them back, forth,together, apart,etc, before anything becomes perminantly buckled down for keeps.
No, i dont have a condensor lense infront of the light. i'm debating if i even need one or not. i'd like input on that, condensor or not?
Would the thermal window between compartments benifit from a fresnel of its own to help arrange the light some before the second mirror?
The design will be for for a 15" LCD, the A150X1 (udner the name of a 'liquid video') i bought a while back. I love this thing, it was fairly cheap, and the preformance is good enough or such a task. I'v already got it ripped apart and neatly set aside on my workbench waiting to be used. I plan on NOT testing with the LCD untill I have my lenses and tempratures and whatnot all sorted out. (I'll use a cheap printed transparancy to serve as a dummy screen for testing reasons. Did I mention I'm uber-paranoid?
The construction material will probably be your standard MDF board, later fixed up on the outside to be less of an eyesore. (the projector will be plainly visable where it will be used, so... )
And to save the need for spamming with multiple posts for attachments, here are the links.
http://hudson.ketnar.org/projector-plans/projector-crossbreed-v1-overview1.jpg
http://hudson.ketnar.org/projector-plans/projector-crossbreed-v1-overview2.jpg
http://hudson.ketnar.org/projector-plans/projector-crossbreed-v1-overview3.jpg <-- this ones labled.
It's nicknamed 'crossbreed' due to it taking some design hints from various DIY's i'v seen floating around. (and it just sounded cool..
*puts on the kevlar undies* okay boys, let me have it. *ducks*
But now I'm back! And with my LCD ready to strip, and money set aside to finaly do this thing (tm), I set out to rough up a design idea. I'm looking to start and hopefuly complete this project over the holiday season. (not much going on this year for me, so this is my 'keep busy' thing.
Okay, first off, this is a design rough, its just a simple 3d model of the idea. no measurements have been really made yet, no lens focal calculations or anything along those lines have really been made yet either. Those come after any problems with the inital design idea have been spotted and worked out. (thats what i'm asking you people for!
I'v carefuly thought out things like temprature concerns and cooling, keeping light leaking to a minimal on the fan holes. I also plan to put some thermal sensors on the design to keep an eye on the temprature of a few compartments, just to be paranoid.
The folded design is namely to save some space and to also keep the heat as far away from my LCD as possible. I also plan to use this projector to test out different methods of lighting once i have it actualy working, so a compartment style seemed best suited for that.
Please keep in mind, I'm a thermal paranoid type, thus the over-protection. I really cant afford to burn up an LCD, and i'm taking every precation i can to prevent it from baking itself.
The diagrams should be pretty self explanitory! Just to point out a few things, the fresnel and LCD sandwinch is 'exploded' in the diagram, the design merits the abillity to move them back, forth,together, apart,etc, before anything becomes perminantly buckled down for keeps.
No, i dont have a condensor lense infront of the light. i'm debating if i even need one or not. i'd like input on that, condensor or not?
Would the thermal window between compartments benifit from a fresnel of its own to help arrange the light some before the second mirror?
The design will be for for a 15" LCD, the A150X1 (udner the name of a 'liquid video') i bought a while back. I love this thing, it was fairly cheap, and the preformance is good enough or such a task. I'v already got it ripped apart and neatly set aside on my workbench waiting to be used. I plan on NOT testing with the LCD untill I have my lenses and tempratures and whatnot all sorted out. (I'll use a cheap printed transparancy to serve as a dummy screen for testing reasons. Did I mention I'm uber-paranoid?
The construction material will probably be your standard MDF board, later fixed up on the outside to be less of an eyesore. (the projector will be plainly visable where it will be used, so...
)And to save the need for spamming with multiple posts for attachments, here are the links.
http://hudson.ketnar.org/projector-plans/projector-crossbreed-v1-overview1.jpg
http://hudson.ketnar.org/projector-plans/projector-crossbreed-v1-overview2.jpg
http://hudson.ketnar.org/projector-plans/projector-crossbreed-v1-overview3.jpg <-- this ones labled.
It's nicknamed 'crossbreed' due to it taking some design hints from various DIY's i'v seen floating around. (and it just sounded cool..
*puts on the kevlar undies* okay boys, let me have it. *ducks*
UV filter...
crap. Okay, so i forgot something. but a extra sheet of UV glass or lextan of the proper grade is easy enough to add in with the first pane drawer in the design. or just trade that one out instead *notes that down, makes note on parts list* thanks for the catch!
As for the electronics, i'm planning to (hopefuly) mount most of those those internaly behind the cold mirrors, given that the tempratures there don't get too out of hand. (for power supplys and whatnot), but this is all depending on what tempratures turn out to be like back there. If they end up being too high, i'll build a bracket onto the back of the projector.
As for the controll board, the LCD i'm using is basicly a computer display, so most of its guts are pretty simple, no TV tuner built in, etc.
I'm going to mainly be using it as a theater-PC display system, but getting an adapter box that lets you turn your VGA display into a 'TV' and such isnt that huge of a addition. things like that will be shown in the second draft of the design.
The way i'm planning to build the slot frame that holds the LCD will basicly allow me to mount the various electronics along with it, hopefuly i can get away with this without having to extend any ribon cables, which as everybody knows, is a pain in the backside. but the layout of the boards and stuff in the LCD is pretty easy to work with. i'm just taking it out of one frame and putting it into another one, and folding the boards at a 90 degree instead of almost all the way back against the LCD. the are roughly an inch wide and several inches long, but run along the LCD's edges at the bottom and top. Everything else is on wires and can be moved to sit with the other portions easily enough. the frame for the LCD will have insets to hold these and still let the fresnel drawers come in and set flush.
If all goes acording to plan, anyways. but seeing as this is my first projector build, i'm pretty sure i'll do alot of experimenting and trial tests before i'm 100% on some things. but hey, thats half the fun
help: i'm having a royal pain of a time calculating focal lenghts and crap. i'm not sure if i'm doing it right, anybody have a list of the process they go through to calculate all this? any help in this area would be awsome.
crap.
Okay, so i forgot something. but a extra sheet of UV glass or lextan of the proper grade is easy enough to add in with the first pane drawer in the design. or just trade that one out instead *notes that down, makes note on parts list* thanks for the catch! As for the electronics, i'm planning to (hopefuly) mount most of those those internaly behind the cold mirrors, given that the tempratures there don't get too out of hand. (for power supplys and whatnot), but this is all depending on what tempratures turn out to be like back there. If they end up being too high, i'll build a bracket onto the back of the projector.
As for the controll board, the LCD i'm using is basicly a computer display, so most of its guts are pretty simple, no TV tuner built in, etc.
I'm going to mainly be using it as a theater-PC display system, but getting an adapter box that lets you turn your VGA display into a 'TV' and such isnt that huge of a addition. things like that will be shown in the second draft of the design.
The way i'm planning to build the slot frame that holds the LCD will basicly allow me to mount the various electronics along with it, hopefuly i can get away with this without having to extend any ribon cables, which as everybody knows, is a pain in the backside. but the layout of the boards and stuff in the LCD is pretty easy to work with. i'm just taking it out of one frame and putting it into another one, and folding the boards at a 90 degree instead of almost all the way back against the LCD. the are roughly an inch wide and several inches long, but run along the LCD's edges at the bottom and top. Everything else is on wires and can be moved to sit with the other portions easily enough. the frame for the LCD will have insets to hold these and still let the fresnel drawers come in and set flush.
If all goes acording to plan, anyways. but seeing as this is my first projector build, i'm pretty sure i'll do alot of experimenting and trial tests before i'm 100% on some things. but hey, thats half the fun
help: i'm having a royal pain of a time calculating focal lenghts and crap. i'm not sure if i'm doing it right, anybody have a list of the process they go through to calculate all this? any help in this area would be awsome.
behind cold mirrors
The way a cold mirror works is that >90% of the visible light gets reflected, but >90% of the IR goes through the mirror and out the back. So you can't put any electronics, etc. behind the cold mirrors. In fact, you might want to put the back of a black aluminum heat sink about an inch from the back of the cold mirror, and then blow some cooling air through it.
I think you should try this design with a single cold mirror. A second one might have little effect, and big cold mirrors cost a fortune! ie. hundreds of dollars
You should also make some full-scale drawings of a lamp delivering light to a 15" LCD. I think you will have to use a very long focal length fresnel in order to fit a 45 degree cold mirror in the light path. In my drawings, it takes about a 10" diameter cold mirror to fit in the path between a condensor and a 330 mm fl fresnel. I could not see any way to fit a cold mirror into the path to a 220 mm fl fresnel. But the longer the fl of the fresnel, the less light gets to it from the lamp. So you might want to think about using a 0 degree hot mirror instead.
The way a cold mirror works is that >90% of the visible light gets reflected, but >90% of the IR goes through the mirror and out the back. So you can't put any electronics, etc. behind the cold mirrors. In fact, you might want to put the back of a black aluminum heat sink about an inch from the back of the cold mirror, and then blow some cooling air through it.
I think you should try this design with a single cold mirror. A second one might have little effect, and big cold mirrors cost a fortune! ie. hundreds of dollars
You should also make some full-scale drawings of a lamp delivering light to a 15" LCD. I think you will have to use a very long focal length fresnel in order to fit a 45 degree cold mirror in the light path. In my drawings, it takes about a 10" diameter cold mirror to fit in the path between a condensor and a 330 mm fl fresnel. I could not see any way to fit a cold mirror into the path to a 220 mm fl fresnel. But the longer the fl of the fresnel, the less light gets to it from the lamp. So you might want to think about using a 0 degree hot mirror instead.
Guy:
Ah, crap. okay, i was looking at cold mirrors last night after my post and i wasnt sure if the IR really did go through or if it was just obsorbed by the mirror (IE, mirror gets really hot and needs cooling, vs the area behind the mirror.) Hmmm.
Thats significant enough to merit some heavy design changes, then. I just now noticed some of the costs, too, holy crap. (!
! how the hell did i miss that??)
Hm. Well, IR filtering glass is even more 'spensive than the mirrors, so that option is out.
Looks like its time to design some sort of better lighing system, with a smaller cold mirror and a lens of some means. Hrm. Might need to make it more of a straight shot, too. And I need to figure a better way to mount the electronics.
Questions:
Just /how hot/ is the light path of these retna-melting bulbs? I can find all sorts of specs and readouts and information about their color rating and output, but what I can't find is just how much /heat/ they spit out onto a surface (not just the bulb itself). I'v seen some straight shot style projectors with nothing more than just a sheet of glass between the bulb and the LCD/fresnels/plastics, and somehow they seem to be able to keep their LCD/plastics from going thermal meltdown. yet these are always the projectors in the 'photographs only, no real information' pages.
Meanwhile, i'm going to re-draft some ideas in the lighting department...
ancorp:
Ah, crap. okay, i was looking at cold mirrors last night after my post and i wasnt sure if the IR really did go through or if it was just obsorbed by the mirror (IE, mirror gets really hot and needs cooling, vs the area behind the mirror.) Hmmm.
Thats significant enough to merit some heavy design changes, then. I just now noticed some of the costs, too, holy crap. (!
! how the hell did i miss that??)Hm. Well, IR filtering glass is even more 'spensive than the mirrors, so that option is out.
Looks like its time to design some sort of better lighing system, with a smaller cold mirror and a lens of some means. Hrm. Might need to make it more of a straight shot, too. And I need to figure a better way to mount the electronics.
Questions:
Just /how hot/ is the light path of these retna-melting bulbs? I can find all sorts of specs and readouts and information about their color rating and output, but what I can't find is just how much /heat/ they spit out onto a surface (not just the bulb itself). I'v seen some straight shot style projectors with nothing more than just a sheet of glass between the bulb and the LCD/fresnels/plastics, and somehow they seem to be able to keep their LCD/plastics from going thermal meltdown. yet these are always the projectors in the 'photographs only, no real information' pages.
Meanwhile, i'm going to re-draft some ideas in the lighting department...
ancorp:
IR filtering glass
There are three types of IR filtering glass:
Specialty glass that absorbs some of the IR, while passing more of the visible light is one type. With this type of tempered glass filter, the surface needs to be cooled, because most of the short wave IR gets converted into raising the temperature of the glass. This will emit long wave IR that will heat other parts, if you let the temperature get too high. It also absorbs some of the visible red and does not have a very sharp cutoff point on its transmittance specturm.
Hot mirror has a dichroic thin layer deposited on one or both surfaces. This type reflects >90% of the IR striking it from the design angle, while transmitting >90% of the visible light. (The design angle may be 0 degrees or 45 degrees.) This type of filter has a much more precise cutoff point than IR absorber glass. I have seen hot mirror as cheap as $2.75 per 50 mm by 55 mm piece at surplusshed.com.
Cold mirror has a dichroic thin layer deposited on one or both surfaces. This type transmits >90% of the IR striking it from the design angle, while reflecting >90% of the visible light. (The design angle may be 0 degrees or 45 degrees.) So most of the heat passes right through it. This type of filter has a much more precise cutoff point than IR absorber glass. I have seen cold mirror as cheap as $5 per 4.75 inch diameter piece at surplusshed.com.
Neither cold mirror or hot mirror will get as hot as IR absorbing glass, but you still need to have some air blowing on them. In my experiments with the surplus shed cold and hot mirrors, the cold mirror does a great job. I was not impressed with the hot mirror. Maybe their hot mirror is very well used before they get it?
Another terrific IR absorbing filter is a centimeter or two of water! It is very selective: Hardly absorbs any visible light but the transmittance to IR goes down very steeply. It will get hot, but it is not too hard to run a little pump and a radiator. DO NOT ever heat a sealed container of water. It will expand and explode. It has to be an open system with a reservoir for expansion at the top.
There are three types of IR filtering glass:
Specialty glass that absorbs some of the IR, while passing more of the visible light is one type. With this type of tempered glass filter, the surface needs to be cooled, because most of the short wave IR gets converted into raising the temperature of the glass. This will emit long wave IR that will heat other parts, if you let the temperature get too high. It also absorbs some of the visible red and does not have a very sharp cutoff point on its transmittance specturm.
Hot mirror has a dichroic thin layer deposited on one or both surfaces. This type reflects >90% of the IR striking it from the design angle, while transmitting >90% of the visible light. (The design angle may be 0 degrees or 45 degrees.) This type of filter has a much more precise cutoff point than IR absorber glass. I have seen hot mirror as cheap as $2.75 per 50 mm by 55 mm piece at surplusshed.com.
Cold mirror has a dichroic thin layer deposited on one or both surfaces. This type transmits >90% of the IR striking it from the design angle, while reflecting >90% of the visible light. (The design angle may be 0 degrees or 45 degrees.) So most of the heat passes right through it. This type of filter has a much more precise cutoff point than IR absorber glass. I have seen cold mirror as cheap as $5 per 4.75 inch diameter piece at surplusshed.com.
Neither cold mirror or hot mirror will get as hot as IR absorbing glass, but you still need to have some air blowing on them. In my experiments with the surplus shed cold and hot mirrors, the cold mirror does a great job. I was not impressed with the hot mirror. Maybe their hot mirror is very well used before they get it?
Another terrific IR absorbing filter is a centimeter or two of water! It is very selective: Hardly absorbs any visible light but the transmittance to IR goes down very steeply. It will get hot, but it is not too hard to run a little pump and a radiator. DO NOT ever heat a sealed container of water. It will expand and explode. It has to be an open system with a reservoir for expansion at the top.
Thats right but what ever is behind it ( cold mirror) does as the object behind it absorbs it.
Same. That cold mirror from surplus shed isnt too bad, its far better then any hot mirror ive used. The best way to go about it is deffinatley with cold mirror, hot mirrors can also loose us more light and change our colour temp to undesired results. That cold mirror from surplus shed is about 80% visible reflection btw and its used.
I was looking at some new film from 3M the otherday, its dichoric filter film so it may be an alternative cheap fix in filtering out ir or making our own large cold mirror if you'd like on the cheap, if the films cheap enough.
Those hot mirrors from surplus shed are used, most of their stuff is, and those hot mirrors are only out of a slide projector so their specs are probally under rated for the power of bulb that we use, hence why they dont do too much or explode.
Comments on this design? it wont fit, unless your rear frensel is gonna be over 400mm focal (rough guess), you wont fit 2 cold mirrors in with the lamp being at its focal. Having 2 cold mirrors will make a difference but you will loose more light if anything, also dont think that you will be taking 2x the ir away as it dont work like that.
The first will filter 90% of IR, the second will filter 90% of whats left over from the first, (10%) so the second will only be filtering 9%. Not enough to worry about.
Also why so much glass? each peice of glass will loose you lots of light, its not needed and will more then likley blur your image.
Somone asked about how hot the mh lamps are, the 250w throw out 300deg c, that heat is less the further you travel away from the bulb hence why our lcd's dont melt.
Other then that keep up the great work.
Trev
Guy:
Thanks for the input, the information you provide is much help. thanks for letting me pick your brain.
I'm workin on the final touches on a different lighting setup, its probably about a billion times more effective than the last pile of crap. Relys on only first surface mirrors and perhaps a hot mirror or a smaller cold mirror if i need it
And your 'water filter' idea, seen that one before! I'v been tempted to use an oil filled window, see how well that works. with thr tight type of oils, you can get some very interesting light filtering propertys.
Thankfuly, the design still merits alot of room for experimentation, which is half the reason i'm building this thing, i need something to tinker with and experiment on that doesnt get me in trouble.. *ahem*
Ace
Er, a *what* focal lenght?! O_0
I'm thinking you didn't read something right, those fresnels will be much closer together later on, the diagram is exploded. when its all finished, the fres-lcd-fres setup will be a tight sandwinch. collimator-LCD-collector with nary a gap to spare unless needed. Either that, or i missed something WAY huge in my reading. 0_o
And I already said several posts ago that a heavy redesign was underway due to the cold mirror issue,cost, useless need, etc. so, no more double extra large break-the-bank cold mirrors.
Not to sound snappy or anything, i want as much input as i can get, and I thank you for yours, you have some really awsome projectors..
Oh, and the new design has /way/ less glass than the first draft. I went a little glass crazy, i know. thats what first drafts are for, you should see my other first drafts. Often leading to questions like 'Uhm, hudson? just how are you going to get the FAA to pass on that?' and ' won't NASA have something to say about this?' Etc.
I'll have some new images in the next day or so, me and a couple buddys are brainstorming on this now, working out some engeneering aspects for the light system and whatnot.
Thanks for the input, the information you provide is much help. thanks for letting me pick your brain.
I'm workin on the final touches on a different lighting setup, its probably about a billion times more effective than the last pile of crap. Relys on only first surface mirrors and perhaps a hot mirror or a smaller cold mirror if i need it
And your 'water filter' idea, seen that one before! I'v been tempted to use an oil filled window, see how well that works. with thr tight type of oils, you can get some very interesting light filtering propertys.
Thankfuly, the design still merits alot of room for experimentation, which is half the reason i'm building this thing, i need something to tinker with and experiment on that doesnt get me in trouble.. *ahem*
Ace
Er, a *what* focal lenght?! O_0
I'm thinking you didn't read something right, those fresnels will be much closer together later on, the diagram is exploded. when its all finished, the fres-lcd-fres setup will be a tight sandwinch. collimator-LCD-collector with nary a gap to spare unless needed. Either that, or i missed something WAY huge in my reading. 0_o
And I already said several posts ago that a heavy redesign was underway due to the cold mirror issue,cost, useless need, etc. so, no more double extra large break-the-bank cold mirrors.
Not to sound snappy or anything, i want as much input as i can get, and I thank you for yours, you have some really awsome projectors..
Oh, and the new design has /way/ less glass than the first draft. I went a little glass crazy, i know. thats what first drafts are for, you should see my other first drafts. Often leading to questions like 'Uhm, hudson? just how are you going to get the FAA to pass on that?' and ' won't NASA have something to say about this?' Etc.
I'll have some new images in the next day or so, me and a couple buddys are brainstorming on this now, working out some engeneering aspects for the light system and whatnot.
Oh no! I thought a cold mirror transmitted IR and UV...all this time I've had my projector set up with only the cold mirror as my defense against heat and UV...what kind of glass should I use (or what could be found at homedepot ) that can get rid of the UV? And should I just put it right before the lower fresnel?
) that can get rid of the UV? And should I just put it right before the lower fresnel?
LOL np buddy.
The focal i was talking about was the rear frensels focal length, unless its over 400mm long you cant get 2 mirrors folding the light behind the lcd, and this is only with a 7inch lcd im talking about, the bigger the lcd the bigger the mirror will have to be on that design.
The best thing to do is to draw up a full size plan. If you know what optics you are using (focal lengths) its all go to the plan then build with only minimal final ajustment. Another way to go about things is see what size you want as an end product first, then see if those optics are available to maintain that size, if its not possible either change the optics or the design to suit.
Trev
Froz
Need a UV filter? Go hit home depot and look for UV filtering lextan, it will have a green film on one side, pretty cheap. i think the green film side is the side you want your light coming in from, not sure. *pause for dramatics..* search the forum.. the stuff is cheap cheap tho, you can buy quite a bit of it for just a few bucks.
As for how much of a dent it makes in your optics, i have no idea. yet. but thats what i plan to use. There is UV glass, too. since UV is easy to filter vs stuff like IR, UV filtering materials are /way/ cheapr than hot and cold mirrors. everything and its dog filters UV, car windows, sunglasses, inlaws, etc, the material is so common, its almost scary. I wish the same were true for IR related stuff, it would make life alot easier about now.
Ace
Thats roughly what i'm doing. The 3d images you'v seen are, at the moment, just winged. Once we nail down at least a design theory thats sound and workable, we plan to go back, make it to 1:1 scale with real measurements (in 3d anyways, but measurements that are taken from real life measurements) and then we start crunching the focal math to see what needs to change.
Thats the cool thing about this - i can, in theory, build thousands of projector variations, and not really spend a dime. Once i find the 'happy spot', i can take all the numbers and measurements, know exactly what i need to buy and how much, and then do the nike. (just do it)
Well, thats the plan, anyways. We start crunching focal math later today or tomarrow, so..lets hope it doesnt throw /too/ much to peices.. i'm still iffy on just how to calculate all this focal stuff. math, sadly, isnt my best feild. but i'v been finding some handy online lens calculators that make things a little easier.
Need a UV filter? Go hit home depot and look for UV filtering lextan, it will have a green film on one side, pretty cheap. i think the green film side is the side you want your light coming in from, not sure. *pause for dramatics..* search the forum..
the stuff is cheap cheap tho, you can buy quite a bit of it for just a few bucks.As for how much of a dent it makes in your optics, i have no idea. yet. but thats what i plan to use. There is UV glass, too. since UV is easy to filter vs stuff like IR, UV filtering materials are /way/ cheapr than hot and cold mirrors. everything and its dog filters UV, car windows, sunglasses, inlaws, etc, the material is so common, its almost scary. I wish the same were true for IR related stuff, it would make life alot easier about now.
Ace
Thats roughly what i'm doing. The 3d images you'v seen are, at the moment, just winged. Once we nail down at least a design theory thats sound and workable, we plan to go back, make it to 1:1 scale with real measurements (in 3d anyways, but measurements that are taken from real life measurements) and then we start crunching the focal math to see what needs to change.
Thats the cool thing about this - i can, in theory, build thousands of projector variations, and not really spend a dime. Once i find the 'happy spot', i can take all the numbers and measurements, know exactly what i need to buy and how much, and then do the nike. (just do it)
Well, thats the plan, anyways. We start crunching focal math later today or tomarrow, so..lets hope it doesnt throw /too/ much to peices.. i'm still iffy on just how to calculate all this focal stuff. math, sadly, isnt my best feild.
but i'v been finding some handy online lens calculators that make things a little easier.
cold mirror in light path
The real problem Trev mentioned is that your lamp has to be at the focal point of your lower fresnel. With a 15" LCD that means the path is pretty wide, unless you use an extra-long fl fresnel. Look at the drawing I have attached. This is the light path between a 24 mm arc and a 15" 220 mm fl fresnel. I tried to see where I could fit a cold mirror (blue line), to reflect the light (red lines). That is a very large cold mirror!
If you use a longer focal length fresnel, then the cone is not so fat. So it is easier to use a smaller cold mirror.
The real problem Trev mentioned is that your lamp has to be at the focal point of your lower fresnel. With a 15" LCD that means the path is pretty wide, unless you use an extra-long fl fresnel. Look at the drawing I have attached. This is the light path between a 24 mm arc and a 15" 220 mm fl fresnel. I tried to see where I could fit a cold mirror (blue line), to reflect the light (red lines). That is a very large cold mirror!
If you use a longer focal length fresnel, then the cone is not so fat. So it is easier to use a smaller cold mirror.
Attachments
no UV filter?
frozenbum_87:
You actually DO have a UV filter, because an acrylic fresnel will filter out quite a bit of the UV. Maybe you have both fresnels below the LCD, so that would filter out even more UV. Ordinary window glass is not very good at removing UV. That is why they sell window film you can apply to prevent your carpet and furniture from fading. You can pay quite a bit for special glass that does filter more UV. Or just go to Home Depot and get a piece of Lexan XL for a few bucks. That is a great UV filter, and you can use it to support your lower fresnel so it doesn't sag and screw up the focal point. (It has much better heat resistance than acrylic.)
As for your cold mirror's transmission/reflection spectra, you might have a visible light bandpass mirror that does let some UV through along with most of the IR. Probably not. Lowpass or highpass filters are simpler to make. Your cold mirror probably does reflect most of the IR.
You can test a dicroic filter to see if it reflects or passes IR using a remote control and a TV camera. A TV camera will "see" a standard IR remote control as a brilliant point source. If you interpose a good IR filter, then it should be much dimmer.
You can test a UV filter using a blacklight and something fluorescent. (Maybe you have an old poster, some paint, a T shirt?) If it glows brightly under the blacklight, see what happens when you interpose the UV filter.
frozenbum_87:
You actually DO have a UV filter, because an acrylic fresnel will filter out quite a bit of the UV. Maybe you have both fresnels below the LCD, so that would filter out even more UV. Ordinary window glass is not very good at removing UV. That is why they sell window film you can apply to prevent your carpet and furniture from fading. You can pay quite a bit for special glass that does filter more UV. Or just go to Home Depot and get a piece of Lexan XL for a few bucks. That is a great UV filter, and you can use it to support your lower fresnel so it doesn't sag and screw up the focal point. (It has much better heat resistance than acrylic.)
As for your cold mirror's transmission/reflection spectra, you might have a visible light bandpass mirror that does let some UV through along with most of the IR. Probably not. Lowpass or highpass filters are simpler to make. Your cold mirror probably does reflect most of the IR.
You can test a dicroic filter to see if it reflects or passes IR using a remote control and a TV camera. A TV camera will "see" a standard IR remote control as a brilliant point source. If you interpose a good IR filter, then it should be much dimmer.
You can test a UV filter using a blacklight and something fluorescent. (Maybe you have an old poster, some paint, a T shirt?) If it glows brightly under the blacklight, see what happens when you interpose the UV filter.
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