Is there any added bonus of splitting my fresnel? if it is, and i do, i put one behind and one infront of the lcd right?
measuring focal length
lone_wolf: you might want to actually use the whole lens and mirror assembly on top of your DIY box. People pay some money to add a front surface mirror just before the projection lens, so the box can be smaller. You already have one!
So anyway, here is my lab setup: I have a 6 cm wide halogen bulb pointing at a wall that is 1829 mm away. The bulb has a distinctive pattern on the front, so it is very easy to focus an image of it on the wall with a lens. I held a 5" diameter DCX lens between the bulb and the wall, and then moved the lens toward the wall until a sharp image of the bulb face appeared on the wall. Then I measured the distance between the lens and the wall. This distance was 420 mm. I tried doing the same with the other side of the lens, and the measurement was the same.
The distance from the bulb to the lens was (1829 - 420) = 1409 mm. Using the lens equation:
1/f = 1/1829 + 1/420
f = 323.5 mm
Then I took the lens out on my porch and focussed the image of the sun on a brick (no fire danger 😀 ). The distance between the lens and the brick was 325 mm. This is within 1% of the value I got from the lab experiment and well within the error of my measuring technique.
So what is the focal length of this lens? 420 mm or 325 mm? I could repeat the lab experiment with a different bulb to wall distance and the lens to wall distance would change. But I could still plug those new numbers into the lens equation to calculate the true focal length: 325 mm
If the distance between the lens and the wall changes when you move the bulb, than that can't be the focal length of the lens: The focal length does not change!
lone_wolf: you might want to actually use the whole lens and mirror assembly on top of your DIY box. People pay some money to add a front surface mirror just before the projection lens, so the box can be smaller. You already have one!
So anyway, here is my lab setup: I have a 6 cm wide halogen bulb pointing at a wall that is 1829 mm away. The bulb has a distinctive pattern on the front, so it is very easy to focus an image of it on the wall with a lens. I held a 5" diameter DCX lens between the bulb and the wall, and then moved the lens toward the wall until a sharp image of the bulb face appeared on the wall. Then I measured the distance between the lens and the wall. This distance was 420 mm. I tried doing the same with the other side of the lens, and the measurement was the same.
The distance from the bulb to the lens was (1829 - 420) = 1409 mm. Using the lens equation:
1/f = 1/1829 + 1/420
f = 323.5 mm
Then I took the lens out on my porch and focussed the image of the sun on a brick (no fire danger 😀 ). The distance between the lens and the brick was 325 mm. This is within 1% of the value I got from the lab experiment and well within the error of my measuring technique.
So what is the focal length of this lens? 420 mm or 325 mm? I could repeat the lab experiment with a different bulb to wall distance and the lens to wall distance would change. But I could still plug those new numbers into the lens equation to calculate the true focal length: 325 mm
If the distance between the lens and the wall changes when you move the bulb, than that can't be the focal length of the lens: The focal length does not change!
All you need is a normal light thats mounted to the ceiling turned on, thats all you need to give your light , not walls or any fancy lights lol.
Then all you have to do is focus the light bulb from the ceiling light onto a table. A 5inch DCX is kinda big and as i said in my previous post we have a limit on the size of the lens used, try a smaller one like a 100mm diameter pcx.
I just tried a 80mm PCX with a focal of 124mm, i tried it on the floor and then tried it less then a foot from the bulb, there isnt any focal difference at all, the only difference was the light bulb that was focused changed size.
Trev🙂
If that lens is 5inches round and if its not a very thick lens id doubt that its under 500mm.
Trev🙂
My test lens
>id doubt that its under 500mm
It is a 3 Diopter DCX from one of those desk magnifier/worklamps.
But we don't really NEED to guess...
Diopters = 1000 mm / focal length
3 = 1000 mm / focal length
focal length = 1000 mm / 3
focal length = 333 mm
5" diameter is only 25 % wider than 100 mm diameter. No reason that the fundamental nature of a lens should be different because it is 25% wider than the one you think is "the right size". The lens equation works just this way for any positive lens of any size.
As for your experiment, perhaps you live in a universe with different physical constants, so your lens equation works differently. I live in the one where Isaac Newton worked this out a long time ago. If you think I'm making up the lens equation, you can do a web search for "thin lens equation" and look at some of the hundreds of college physics course websites that use it.
Lone_wolf: If you put the lens assembly at exactly the focal length from the LCD, your image would focus at an infinite distance. Use the lens equation to calculate how far away it has to be, for the size of your throw distance (projector to screen). Hint: It will be a bit more than the focal length.
>id doubt that its under 500mm
It is a 3 Diopter DCX from one of those desk magnifier/worklamps.
But we don't really NEED to guess...
Diopters = 1000 mm / focal length
3 = 1000 mm / focal length
focal length = 1000 mm / 3
focal length = 333 mm
5" diameter is only 25 % wider than 100 mm diameter. No reason that the fundamental nature of a lens should be different because it is 25% wider than the one you think is "the right size". The lens equation works just this way for any positive lens of any size.
As for your experiment, perhaps you live in a universe with different physical constants, so your lens equation works differently. I live in the one where Isaac Newton worked this out a long time ago. If you think I'm making up the lens equation, you can do a web search for "thin lens equation" and look at some of the hundreds of college physics course websites that use it.
Lone_wolf: If you put the lens assembly at exactly the focal length from the LCD, your image would focus at an infinite distance. Use the lens equation to calculate how far away it has to be, for the size of your throw distance (projector to screen). Hint: It will be a bit more than the focal length.
What? i simply stated that if its a thin lens it will have a long focal, just that! Thin ie: 10mm thick for a 5inch diameter sized lens is thin. I know very well the thin lens equation, hell its on the site i posted a few posts back!
Perhaps you need to try the way i do things for yourself first using the exact same gear to answer your own questions, a long halogen bulb wont exactly give you the best results for measuring anything, try the power saver bulbs like i use and you just may well see a difference. I dont have a problem with my lenses and out of the ones that i have measured in this room they have always measured out to their spec. I am simply telling you my findings, not trying to cause an argument.
For the thin lens equation?, i see a Edmund Halley in there somewhere..........the famous astronomer.
http://scienceworld.wolfram.com/physics/ThinLensFormula.html
Trev🙂
specious objections
Well Trev, I posted data from real experiments, and you suggested there was something "wrong" about my lens, my light bulb, and even the fact that I am sending the light horizontally against a wall instead of down from a ceiling fixture. 🙄
I tried changing from my halogen flood (which is roughly the shape of a 15 Watt incandescent bulb, not long and skinny), to a 60 watt incandescent soft white, and then to a spiral compact fluorescent 25 watt bulb replacement. All had exactly the same result: With the bulb 1829 mm from the wall, the 325 mm fl DCX lens focussed a perfect image on the wall from 420 mm. When you plug those distances into the lens equation (AKA the Gaussian Lens Formula), you calculate a focal length of 323.5 mm.
Just to prove that the lens-to-wall distance is NOT the focal length, I ran another experiment against another wall: With the bulb 2851 mm from the wall, the same 325 mm fl DCX focussed a perfect image of the bulb on the wall from 375 mm. (NOTE: Not anywhere near the previous experiment's 420 mm lens-to-wall distance.) When you plug these numbers into the lens equation, you calculate a focal length of 325.7 mm.
Sorry, but I just can't suspend my disbelief enough to try it vertical instead of horizontal. If light behaves that differently depending on the orientation to gravity, then we are being sucked into a black hole at the center of the earth! 😀
In summary, I ran several different experiments and they all showed that the distance from the lens to the image is NOT the focal length of the lens, unless the light source is at an infinite distance. Which agrees with the lens equation.
On the other hand, you said:
>I just tried a 80mm PCX with a focal of 124mm, i tried it on the floor and then tried it less then a foot from the bulb, there isnt any focal difference at all...
Using the lens equation for bulb-to-lens distances of 2311 cm and 304.8 cm (7'7" and 1 foot, respectively), I calculate that you would measure distances from lens to image of 131 mm in the first case, and 209 mm in the second case. These are so different, that I have to conclude that you must have made a huge measurement error, or it was just a thought experiment.
Since we seem to have results that do not agree, let's have another experimenter try it: Lone_wolf_84, why don't you try measuring the lens-to-image-on-the-wall distance of a simple magnifying glass, using a bulb that is 6 feet away from the wall and then again after moving the bulb 10 feet away from the wall. If this distance is the actual focal length of the lens, then it should not change. Then take the lens outside and measure the distance to another ant victim. (To hell with PETA, I do hate ants!)Post the three measurements you get.
Well Trev, I posted data from real experiments, and you suggested there was something "wrong" about my lens, my light bulb, and even the fact that I am sending the light horizontally against a wall instead of down from a ceiling fixture. 🙄
I tried changing from my halogen flood (which is roughly the shape of a 15 Watt incandescent bulb, not long and skinny), to a 60 watt incandescent soft white, and then to a spiral compact fluorescent 25 watt bulb replacement. All had exactly the same result: With the bulb 1829 mm from the wall, the 325 mm fl DCX lens focussed a perfect image on the wall from 420 mm. When you plug those distances into the lens equation (AKA the Gaussian Lens Formula), you calculate a focal length of 323.5 mm.
Just to prove that the lens-to-wall distance is NOT the focal length, I ran another experiment against another wall: With the bulb 2851 mm from the wall, the same 325 mm fl DCX focussed a perfect image of the bulb on the wall from 375 mm. (NOTE: Not anywhere near the previous experiment's 420 mm lens-to-wall distance.) When you plug these numbers into the lens equation, you calculate a focal length of 325.7 mm.
Sorry, but I just can't suspend my disbelief enough to try it vertical instead of horizontal. If light behaves that differently depending on the orientation to gravity, then we are being sucked into a black hole at the center of the earth! 😀
In summary, I ran several different experiments and they all showed that the distance from the lens to the image is NOT the focal length of the lens, unless the light source is at an infinite distance. Which agrees with the lens equation.
On the other hand, you said:
>I just tried a 80mm PCX with a focal of 124mm, i tried it on the floor and then tried it less then a foot from the bulb, there isnt any focal difference at all...
Using the lens equation for bulb-to-lens distances of 2311 cm and 304.8 cm (7'7" and 1 foot, respectively), I calculate that you would measure distances from lens to image of 131 mm in the first case, and 209 mm in the second case. These are so different, that I have to conclude that you must have made a huge measurement error, or it was just a thought experiment.
Since we seem to have results that do not agree, let's have another experimenter try it: Lone_wolf_84, why don't you try measuring the lens-to-image-on-the-wall distance of a simple magnifying glass, using a bulb that is 6 feet away from the wall and then again after moving the bulb 10 feet away from the wall. If this distance is the actual focal length of the lens, then it should not change. Then take the lens outside and measure the distance to another ant victim. (To hell with PETA, I do hate ants!)Post the three measurements you get.
I never said that! lol i was simply stating that all you have to do is turn a room light on.
Try a pcx, i garentee you the difference, thats what i measured.
Well thats your calculation, mine is a real life measurement, my ruler doesnt lie and neither do i. I think you are getting somthing mixed up here, my focal didnt change, the size of the light bulb focused did.
Ill try and take you a picture but ill doubt it will turn out as my camera is dark as hell even on a sunny day.
Trev🙂
hey um, not to break up ur discussion or anything 😉 but i never did get an answer to my question regarding fresnel splitting and weather or not its better to have the
bulb-fresnel- lcd, or
bulb- one side of fresnel-lcd- other side of fresnel
and I do believe this is my last question before i start construction sometime next week and i plan on taking LOTS of pictures.
bulb-fresnel- lcd, or
bulb- one side of fresnel-lcd- other side of fresnel
and I do believe this is my last question before i start construction sometime next week and i plan on taking LOTS of pictures.
Sorry Lone_wolf_84
We have gotten rather side-tracked.
Regarding splitting fresnels: If you split them, you can make the light travel through the LCD pixels perpendicular to the panel. If you don't split them (or if you split them but put them both between the light source and the LCD), then the light will be converging as it goes through the pixels.
Fresnels are much lower quality than glass lenses, so you get the sharpest image if you don't put one between the LCD and the projection lens. On the other hand, you might have an LCD panel with very poor transmission of light at at angle (ie. viewing angle). I think this difference in viewing angle characteristics leads different experimenters to have differences of opinion about the necessity of splitting the fresnels.
If I were you, I would try an optical bench-type setup to see if you get good results without splitting. In other words, use some cardboard, wood blocks, tape, string, etc. to position your light, un-split fresnel, LCD, and projection lens so it can project an image the desired distance to a screen (use the real projection distance you want to end up with). You will probably have to make a cardboard cover to limit light leaks. Then you can move things around a bit to get the best possible image. (I totally agree with Trev on this: you need a finely- detailed high contrast image, not a brightest possible but blurry image.) If you consistently see a lot of viewing-angle artifacts around the edge of the image, then try it with a split fresnel design. If that change gets rid of the artifacts, then you have your answer.
The fresnels are there to send most of the light to the projection lens. You can verify that they are doing that by removing the LCD (or just driving it with a white image), and then moving a sheet of white paper along the path from the LCD or upper fresnel toward the projection lens. The light should converge toward the lens aperature. It if converges before you reach the lens and then begins to diverge, then you need a different focal length fresnel or projection lens. (The fresnel fl should be a bit longer than the projection length fresnel, if you do have the perpendicular light situation I described above. But you may not: It depends on the spacing between the lower fresnel and light, and the lower fresnel focal length.) Once you have a setup that you are happy with (hopefully ecstatic 😀 ), then measure the actual distances very precisely,and make a box for it all.
If you have trouble getting your fresnels to work right, you can always do some light condensing experiments with cheap page magnifiers from an office supply store, before ordering a more expensive higher quality fresnel. A 2 Diopter magnifier fresnel is 1000/2 = 500 mm focal length, a 3 Dipoter is 1000/3 = 333 mm, etc.
Another good trick: If your LCD is not stripped, mounted in a good protective frame with all the PCBs secured, and running, you can make a mockup frame with some clear plastic or transparency film in it. Put a test pattern on it with dry markers. Then use that for your first experiments.
We have gotten rather side-tracked.
Regarding splitting fresnels: If you split them, you can make the light travel through the LCD pixels perpendicular to the panel. If you don't split them (or if you split them but put them both between the light source and the LCD), then the light will be converging as it goes through the pixels.
Fresnels are much lower quality than glass lenses, so you get the sharpest image if you don't put one between the LCD and the projection lens. On the other hand, you might have an LCD panel with very poor transmission of light at at angle (ie. viewing angle). I think this difference in viewing angle characteristics leads different experimenters to have differences of opinion about the necessity of splitting the fresnels.
If I were you, I would try an optical bench-type setup to see if you get good results without splitting. In other words, use some cardboard, wood blocks, tape, string, etc. to position your light, un-split fresnel, LCD, and projection lens so it can project an image the desired distance to a screen (use the real projection distance you want to end up with). You will probably have to make a cardboard cover to limit light leaks. Then you can move things around a bit to get the best possible image. (I totally agree with Trev on this: you need a finely- detailed high contrast image, not a brightest possible but blurry image.) If you consistently see a lot of viewing-angle artifacts around the edge of the image, then try it with a split fresnel design. If that change gets rid of the artifacts, then you have your answer.
The fresnels are there to send most of the light to the projection lens. You can verify that they are doing that by removing the LCD (or just driving it with a white image), and then moving a sheet of white paper along the path from the LCD or upper fresnel toward the projection lens. The light should converge toward the lens aperature. It if converges before you reach the lens and then begins to diverge, then you need a different focal length fresnel or projection lens. (The fresnel fl should be a bit longer than the projection length fresnel, if you do have the perpendicular light situation I described above. But you may not: It depends on the spacing between the lower fresnel and light, and the lower fresnel focal length.) Once you have a setup that you are happy with (hopefully ecstatic 😀 ), then measure the actual distances very precisely,and make a box for it all.
If you have trouble getting your fresnels to work right, you can always do some light condensing experiments with cheap page magnifiers from an office supply store, before ordering a more expensive higher quality fresnel. A 2 Diopter magnifier fresnel is 1000/2 = 500 mm focal length, a 3 Dipoter is 1000/3 = 333 mm, etc.
Another good trick: If your LCD is not stripped, mounted in a good protective frame with all the PCBs secured, and running, you can make a mockup frame with some clear plastic or transparency film in it. Put a test pattern on it with dry markers. Then use that for your first experiments.
really laughing out loud!
Trev,
I just figured out why you saw what you did in your experiment! Sorry I doubted your findings. What we had was just a communication problem.
When you put the PCX lens a foot from the lamp, you saw a very large projected image on the floor or table top. This fits into the thin lens equation perfectly:
1/d(light-to lens) + 1/d(lens-to-image) = 1/focal length
In any converging lens projection setup like this, the direction of the light is interchangeable. So long as the bulb and the image screen are more than 2 times the focal length apart, this precedure actually produces two different points of focus for the lens. One point has the short side nearer the screen, and the other has the short side near the bulb. But for a particular lens and particular bulb-to-screen distance, both of the short sides have the same length. So you can plug the short side and long side measurements into the lens equation and get exactly the same focal length value using either point of focus.
I was talking about the point of focus nearer to the screen. When you put your 124 mm PCX very close to the bulb, then you had the short side between the lens and the bulb.
Try again with a table lamp exactly 10 feet from the wall and then move the lens starting from the wall toward the lamp until you find the short focus point. Measure the distance between the lens and the wall.
Then move the table lamp so it is exactly 6 feet from the wall and then move the lens starting from the wall toward the lamp until you find the short focus point. Measure the distance between the lens and the wall.
The two measurements will be very different. And that's why you can't use a light bulb that is a short distance away to directly measure the focal length of a lens. If you have 30 foot high ceilings, then a ceiling lamp might be far enough away that the value of 1/d(light-to lens) will be insignifigant compared to the value of 1/d(lens-to-image) for a short focal length lens. But with 8 foot ceilings or longer lenses, you have to measure both distances and use the lens equation. Or just go outside and use the sun, which has 1/d(light-to lens) ~ 0. Then the other point of focus would be with the lens 124 mm from the sun!
Trev,
I just figured out why you saw what you did in your experiment! Sorry I doubted your findings. What we had was just a communication problem.
When you put the PCX lens a foot from the lamp, you saw a very large projected image on the floor or table top. This fits into the thin lens equation perfectly:
1/d(light-to lens) + 1/d(lens-to-image) = 1/focal length
In any converging lens projection setup like this, the direction of the light is interchangeable. So long as the bulb and the image screen are more than 2 times the focal length apart, this precedure actually produces two different points of focus for the lens. One point has the short side nearer the screen, and the other has the short side near the bulb. But for a particular lens and particular bulb-to-screen distance, both of the short sides have the same length. So you can plug the short side and long side measurements into the lens equation and get exactly the same focal length value using either point of focus.
I was talking about the point of focus nearer to the screen. When you put your 124 mm PCX very close to the bulb, then you had the short side between the lens and the bulb.
Try again with a table lamp exactly 10 feet from the wall and then move the lens starting from the wall toward the lamp until you find the short focus point. Measure the distance between the lens and the wall.
Then move the table lamp so it is exactly 6 feet from the wall and then move the lens starting from the wall toward the lamp until you find the short focus point. Measure the distance between the lens and the wall.
The two measurements will be very different. And that's why you can't use a light bulb that is a short distance away to directly measure the focal length of a lens. If you have 30 foot high ceilings, then a ceiling lamp might be far enough away that the value of 1/d(light-to lens) will be insignifigant compared to the value of 1/d(lens-to-image) for a short focal length lens. But with 8 foot ceilings or longer lenses, you have to measure both distances and use the lens equation. Or just go outside and use the sun, which has 1/d(light-to lens) ~ 0. Then the other point of focus would be with the lens 124 mm from the sun!
Ok so i guess ill sit here and unpatiently wait for my lcd, then after that i will start all the testing and what not.
FYI the discussion you two are having aint bad, ive learned alot from it 😛 so on that note, Countinue on with the debate 😀
FYI the discussion you two are having aint bad, ive learned alot from it 😛 so on that note, Countinue on with the debate 😀
quite to the point
The whole thing about the lens equation stuff IS right to the point of your project:
If you can duplicate the spacing of your OHP, but replace the OHP lamp with a MH bulb, and the transparencies with an LCD panel, then you will have a working projector. But if you want it to behave differently, like having a longer throw (ie. 100" image from the back of the room), then you can use the lens equations to figure out what the proper distance will be between the new projection lens and the LCD. Then you can use it again to figure out the combined focal length you need for your pair of fresnels to get the light into the projection lens.
The whole thing about the lens equation stuff IS right to the point of your project:
If you can duplicate the spacing of your OHP, but replace the OHP lamp with a MH bulb, and the transparencies with an LCD panel, then you will have a working projector. But if you want it to behave differently, like having a longer throw (ie. 100" image from the back of the room), then you can use the lens equations to figure out what the proper distance will be between the new projection lens and the LCD. Then you can use it again to figure out the combined focal length you need for your pair of fresnels to get the light into the projection lens.
Mines are 12ft lol, as i explained before it wont make a difference so long as you are out of the lenses focal length you have to be past its focal length measurment for it to work, if you are within its focal length then i think its obvious you wont focus the bulb on a table. The only difference i saw was the size of the bulb when focused, the focal point didnt change.
As for most of the calculations ill be honest with you, you dont need to use them, most of those are for designing lenses from scratch rather then using them on lenses that you've bought that already have the specs. All of my lenses have the specs for them and only at certain times i may do a calculation to make sure all is correct in a multi lens light engine array for abberations. The room light test is just a quick an simple way to measure the EFL, and yess its just a very estimated EFL, (the EFL on a bought lens is also very estimated i might add). Building a projector isnt also what you think it is, having everything at its perfect focal points doesnt always work out, most of the time they need to be just off, this applies especially to condensers. So realy at the end of the day the thing that realy matters most to a projector builder is his/others hands on experience rather then his maths. Ive got 2years of full time experience behind me, both practical and theroy, i can draw you up a projector in 5mins and it will be garenteed to work perfectly, i wouldnt garentee you it would work if i didnt have any background practical experience. We have our base points in theroy aswell as in practical.
I did read in another thread that you have tried to build a projector and havnt succeeded, there is nothing wrong with that and for first time builders that is very common, but ill give you some friendly advice. Use this forum and others to help you, forget the calculations for now, use the parts that others have worked with and make a working model of your own first, then calculate your theroy differences, and at times you will soon see that the theory sometimes conflicts with a working design in reality. Why you may ask? ask yourself the very same question of where the calculations came from to begin with, they came from mostly cameras and telescopes from over 100 years ago. The theorys are old and resonably acurate, but in a modern real world environment somtimes they arent always acurate enough, and because of this new ones emerge every year.
There was just an upgrade to a calculation widley used by world leading scientists that was just proven wrong, i cant remeber the name of it (was somthing in quantum mechanics), but anyone who has a PHD in such and such using that old calculation is now polictically wrong.
I hope that made sence i just woke up🙄
Trev🙂
theory versus rule of thumb
Trev,
I think you may draw some incorrect conclusions from things you read in these posts: I have never written that I tried to build a projector and failed. I wriote something like "the number of sucessful projectors I have built so far = 0". But that's just because I am still waiting for jcb to send me an FFC extension I ordered. My projector is coming along just fine, thank you!
I have very high confidence that it will work, because I do understand the math, I have run several experiments with projection lenses, fresnels, a transparency mock-up of the LCD panel, and a light source (effectively lots of prototypes), and the results of the experiments agree perfectly with the math.
That has been my experience with all sorts of engineering endeavors. I have worked in medical research labs, written embedded software for air pollution monitors and the baseband chip used in most of the Japanese PHS cell phones in the world, designed and built a multi-camera automated body volume measuring system on a NASA grant, designed and built a three-D mouse and drawing system for the PC using a pair of CCD cameras and LCD shutter goggles, etc. Now I write functional verification software for a company making 100 Mbit/second DSL chips, so we can have high confidence in the designs before spending $1000000 for the first silicon. (Writing software for chip companies pays a LOT better than any of the optical stuff.) In all of these, working designs are based on the known properties and relationships of materials and forces. And that means designs based on physics and math.
And the the lens equation I have been suggesting IS a thin lens simplification of the general lens equation. It is also exactly the same form equation as the one used for calculating the combined focal length of two lenses, or the effective resistance of two parallel resistors! Funny how the same math describes how so many different thing work.
Reading all of the posts in this and several other forums has been educational, in the sense of learning what works together well, from the items available now at reasonable prices. But I have been looking at LCDs with the idea of building a projector for about 20 years. Now their price/performance lines have intersected at the point that you can build a great projector for less than you would pay for a new TV, and I have the time available to put into it. The LCDs available in 1984 were pretty worthless for this purpose and ungodly expensive. At that time, my sister-in-law's boyfriend started writing a flat panel display industry report and newsletter. I read some of his early reports and got interested. (And no, I didn't pay the $250,000 report fee!
)
On the other hand, experience with all of the useful items that now fit within the budget of a DIY builder, is a great asset that I am short on! So, based on all your experience with projectors, have you ever seen a real 14" or larger LCD projector that used a 45 degree angle cold mirror? These have wonderful IR removal properties and there are several of these available at surplus prices, but those are all under 5 inches in diameter. That means the light beam has to fit in about a 3 inch diameter circle when it strikes the mirror. That seems way too small for any part of the light path before hitting the LCD, unless the condensor gets a lot more complex. So for all the suggestions to use a cold mirror, I don't think anybody is actually doing it in state-of-the-art DIY projectors. Maybe that is why these cold mirrors are so cheap! 😀
Have you seen one used?
Trev,
I think you may draw some incorrect conclusions from things you read in these posts: I have never written that I tried to build a projector and failed. I wriote something like "the number of sucessful projectors I have built so far = 0". But that's just because I am still waiting for jcb to send me an FFC extension I ordered. My projector is coming along just fine, thank you!
I have very high confidence that it will work, because I do understand the math, I have run several experiments with projection lenses, fresnels, a transparency mock-up of the LCD panel, and a light source (effectively lots of prototypes), and the results of the experiments agree perfectly with the math.
That has been my experience with all sorts of engineering endeavors. I have worked in medical research labs, written embedded software for air pollution monitors and the baseband chip used in most of the Japanese PHS cell phones in the world, designed and built a multi-camera automated body volume measuring system on a NASA grant, designed and built a three-D mouse and drawing system for the PC using a pair of CCD cameras and LCD shutter goggles, etc. Now I write functional verification software for a company making 100 Mbit/second DSL chips, so we can have high confidence in the designs before spending $1000000 for the first silicon. (Writing software for chip companies pays a LOT better than any of the optical stuff.) In all of these, working designs are based on the known properties and relationships of materials and forces. And that means designs based on physics and math.
And the the lens equation I have been suggesting IS a thin lens simplification of the general lens equation. It is also exactly the same form equation as the one used for calculating the combined focal length of two lenses, or the effective resistance of two parallel resistors! Funny how the same math describes how so many different thing work.
Reading all of the posts in this and several other forums has been educational, in the sense of learning what works together well, from the items available now at reasonable prices. But I have been looking at LCDs with the idea of building a projector for about 20 years. Now their price/performance lines have intersected at the point that you can build a great projector for less than you would pay for a new TV, and I have the time available to put into it. The LCDs available in 1984 were pretty worthless for this purpose and ungodly expensive. At that time, my sister-in-law's boyfriend started writing a flat panel display industry report and newsletter. I read some of his early reports and got interested. (And no, I didn't pay the $250,000 report fee!
)On the other hand, experience with all of the useful items that now fit within the budget of a DIY builder, is a great asset that I am short on! So, based on all your experience with projectors, have you ever seen a real 14" or larger LCD projector that used a 45 degree angle cold mirror? These have wonderful IR removal properties and there are several of these available at surplus prices, but those are all under 5 inches in diameter. That means the light beam has to fit in about a 3 inch diameter circle when it strikes the mirror. That seems way too small for any part of the light path before hitting the LCD, unless the condensor gets a lot more complex. So for all the suggestions to use a cold mirror, I don't think anybody is actually doing it in state-of-the-art DIY projectors. Maybe that is why these cold mirrors are so cheap! 😀
Have you seen one used?
I never said you did 😉.
Ive seen a 15inch projector in person, a guy down the road from me has one that i helped him build but not one with a ir cold mirror in it.. Its very doable but as you say 3inches diameter is kinda on the limit, 4 inches should work though and surplus shed sell these quite cheap, about $5 from memory. Im all for the cold mirror and dichoric range of reflectors, in the new design im doing now i use a dichoric reflector and the image is very white without the red tinge often given from the low band ir. I personally wouldnt go back. The colour shift of the reflector i use has a shift of 300k in colour, most good quality cold mirrors shift the colour about 300k up from the bulbs normal colour, it just makes those colours just stand out! and gives us an even better contrast level. You would think with the light being whiter the contrast would be less, but for some reason atleast to my eyes its a whole lot better. Using the dichoric reflector will also save the life of your bulb if its spherical because the light going back to its source is cooler then what it would be if the ir was being reflected back. I say go for your design, it sounds interesting and its totally doable, i recomend using 2 condensers in that setup and all should be sweet.
Trev🙂
WhoA WHOA WHOA
How come noone informed me of this whole cold mirror, dichoric thing? seems like good info to know, so ace, mind giving me some words of wisdom about them?
How come noone informed me of this whole cold mirror, dichoric thing? seems like good info to know, so ace, mind giving me some words of wisdom about them?
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