Ok I need some help, I'm looking to build a new projo using the 15.4 Proview. I would like to use a pre-condenser in order to increase brightness. Most lens I've seen are between 90-120mm dia. I plan on using a tubular bulb like the s400-dd, my concern is what is the optimal FL form the condenser lens. If I understand correctly I want to get a lens that matches the condenser fresnel.
So if I were to use a 220mm FL rear fresnel, could I use a 200mm FL condenser lens, mounted 20mm from the arc of the bulb?
Could someone please explain in stupid terms 🙂
Thanks,
Squeeky
forgot one last question PCX, DCX which is best to use or do they both increase brightness?
So if I were to use a 220mm FL rear fresnel, could I use a 200mm FL condenser lens, mounted 20mm from the arc of the bulb?
Could someone please explain in stupid terms 🙂
Thanks,
Squeeky
forgot one last question PCX, DCX which is best to use or do they both increase brightness?
I just happen to have a drawing!
Pretty close, but you can't just subtract to get the answer. And you can't get anything within 20 mm of the center of a Ushio retrofit lamp's arc, because the T15 lamp (15/8") has a 23.8 mm radius! I have read posts that claim success with a 200 mm fl PCX with the flat side 1 to 2 mm from the lamp's outer envelope.
You have to do more than just move the lens toward the lamp until all of the fresnel's corner's are lit. The light has to enter the fresnel as if it was coming from the fresnel's focal length, if you want it to come out the other side parallel. (Of course, you may not want it parallel to adjust the fical point of the lamp arc image into the projection lens.)
If you do want it parallel, then you need to put the condensor lens at the position where it would perfectly intersect the cone of light going to the fresnel from a lamp at the focal distance. Then you need to move the lamp closer to the lens to compensate for the magnification. That should just light the corners of the fresnel.
Here is a graphical design you can use to make your own full-scale model. Substitute your own distances!
This example uses a 15" LCD, a 220 mm fl bottom fresnel, and a 100 mm diameter 200 mm fl condensor lens.
Start by drawing the central axis, line 1. (Make a full scale drawing.)
Draw the bottom fresnel, line 2. This is a 15 inch line, centered on the central axis.
We want the light to exit the right side of the fresnel toward the LCD perpendicular to the fresnel. (see arrow) For that to occur, draw line 3 from 7.5" up the fresnel, to a point on the axis that is 220 mm left of the fresnel. This is the path the light must follow to get refracted along the arrow at the top.
Now draw the 100 mm diameter condensor lens, line 4. Place it so it just intersects the required light path, line 3. At this point you can measure the distance between line 4 and line 2, the condensor-to-fresnel distance.
Now draw line 5, parallel to line 3, but through the center of the condensor lens. (Light that passes through the center of a lens is not refracted.)
Next draw line 6, 200 mm to the left of line 4. This is the focal plane of the condensor lens.
Since line 3 and line 5 are parallel on the right side of the lens, you can draw line 7 to the point where they focus on the left side of the lens. (This shows you how light traveling to the right along line 7 will be refracted to line 3.)
At point 8, line 7 intersects the axis. This is where you put a point-source light. You can measure the distance between point 8 and line 4, to get the light-to-condensor distance. You can also see the difference between such a projector with or without a condensor lens: Adding the condensor lets you move the light just a bit closer to capture a few more degrees of the light.
Without it, the light would be placed at the intersection of line 3 with the central axis. For a small-format LCD and a 220 mm fl fresnel, a condensor lens makes a much bigger difference.
Pretty close, but you can't just subtract to get the answer. And you can't get anything within 20 mm of the center of a Ushio retrofit lamp's arc, because the T15 lamp (15/8") has a 23.8 mm radius! I have read posts that claim success with a 200 mm fl PCX with the flat side 1 to 2 mm from the lamp's outer envelope.
You have to do more than just move the lens toward the lamp until all of the fresnel's corner's are lit. The light has to enter the fresnel as if it was coming from the fresnel's focal length, if you want it to come out the other side parallel. (Of course, you may not want it parallel to adjust the fical point of the lamp arc image into the projection lens.)
If you do want it parallel, then you need to put the condensor lens at the position where it would perfectly intersect the cone of light going to the fresnel from a lamp at the focal distance. Then you need to move the lamp closer to the lens to compensate for the magnification. That should just light the corners of the fresnel.
Here is a graphical design you can use to make your own full-scale model. Substitute your own distances!
This example uses a 15" LCD, a 220 mm fl bottom fresnel, and a 100 mm diameter 200 mm fl condensor lens.
Start by drawing the central axis, line 1. (Make a full scale drawing.)
Draw the bottom fresnel, line 2. This is a 15 inch line, centered on the central axis.
We want the light to exit the right side of the fresnel toward the LCD perpendicular to the fresnel. (see arrow) For that to occur, draw line 3 from 7.5" up the fresnel, to a point on the axis that is 220 mm left of the fresnel. This is the path the light must follow to get refracted along the arrow at the top.
Now draw the 100 mm diameter condensor lens, line 4. Place it so it just intersects the required light path, line 3. At this point you can measure the distance between line 4 and line 2, the condensor-to-fresnel distance.
Now draw line 5, parallel to line 3, but through the center of the condensor lens. (Light that passes through the center of a lens is not refracted.)
Next draw line 6, 200 mm to the left of line 4. This is the focal plane of the condensor lens.
Since line 3 and line 5 are parallel on the right side of the lens, you can draw line 7 to the point where they focus on the left side of the lens. (This shows you how light traveling to the right along line 7 will be refracted to line 3.)
At point 8, line 7 intersects the axis. This is where you put a point-source light. You can measure the distance between point 8 and line 4, to get the light-to-condensor distance. You can also see the difference between such a projector with or without a condensor lens: Adding the condensor lets you move the light just a bit closer to capture a few more degrees of the light.
Without it, the light would be placed at the intersection of line 3 with the central axis. For a small-format LCD and a 220 mm fl fresnel, a condensor lens makes a much bigger difference.
Attachments
AWESOME
Thank you soooo much, I'll be drawing this when I get home! Also wanted to ask does it matter if it's a PCX, DCX? The lens I found is a DCX will this cause an issue? If so where can I find a similar PCX. Do you recommend I get a condenser lens close to that of rear fresnel is FL? Thanks
Squeeky
Thank you soooo much, I'll be drawing this when I get home! Also wanted to ask does it matter if it's a PCX, DCX? The lens I found is a DCX will this cause an issue? If so where can I find a similar PCX. Do you recommend I get a condenser lens close to that of rear fresnel is FL? Thanks
Squeeky
condensor lenses
The very best are Pyrex or similar heat-resistant glass with an aspherical curve on the LCD side and almost flat on the lamp side. Those are pretty expensive.
Next best are PCX: Flat on the lamp side. DCX have more spherical aberration, which translates into more light missing the projection lens.
The whole purpose of a condensor lens is to capture as much light as possible for the fresnels. You do this by getting a large diameter lens as close to the lamp as possible. You can try some drawings using different diameter and focal length lenses, to see how they fit. If you get one with too short a focal length, you can't get it close enough to the lamp arc to light the fresnel corners. If you get one with too long a focal length, it has to go farther away from the lamp and miss some light.
Lots of the DIY projection online stores have condensors. You can also look at edmundoptics.com, surplusShed.com, and rolyn.com. Find one with heat resistant glass.
The very best are Pyrex or similar heat-resistant glass with an aspherical curve on the LCD side and almost flat on the lamp side. Those are pretty expensive.
Next best are PCX: Flat on the lamp side. DCX have more spherical aberration, which translates into more light missing the projection lens.
The whole purpose of a condensor lens is to capture as much light as possible for the fresnels. You do this by getting a large diameter lens as close to the lamp as possible. You can try some drawings using different diameter and focal length lenses, to see how they fit. If you get one with too short a focal length, you can't get it close enough to the lamp arc to light the fresnel corners. If you get one with too long a focal length, it has to go farther away from the lamp and miss some light.
Lots of the DIY projection online stores have condensors. You can also look at edmundoptics.com, surplusShed.com, and rolyn.com. Find one with heat resistant glass.
OK Guy,
If I'm getting this right, this means a shorter condenser fresnel FL will give you more usable light, and add to the benifit of a spherical reflector. Based on this, a shorter pre-condeser lens FL will also provide more usable light, at least up to a point.
See my images below for my reasoning.
Am I getting this right?
*edit for clarity
If I'm getting this right, this means a shorter condenser fresnel FL will give you more usable light, and add to the benifit of a spherical reflector. Based on this, a shorter pre-condeser lens FL will also provide more usable light, at least up to a point.
See my images below for my reasoning.
Am I getting this right?
*edit for clarity
Attachments
yes
Right. A shorter focal length pre-condensor lens can be placed closer to the lamp arc and still send all the captured light to the fresnels. By putting it closer to the lamp, it captures more of the lamp's light output. But if you get one with too short a focal length, then you will put it right up to the lamp's outer envelope and it still will converge the light into a cone too small to light the corners of the fresnel. Then you are SOL!
Using a spherical reflector is independant of using a pre-condensor lens. It will increase the amount of light coming out the front of the lamp by reflecting the light coming out the back. If you do use both together, then the spherical reflector has to match the angular width of the light being captured by the pre-condensor lens. If it is too narrow then the image edges and corners will be much dimmer.
Right. A shorter focal length pre-condensor lens can be placed closer to the lamp arc and still send all the captured light to the fresnels. By putting it closer to the lamp, it captures more of the lamp's light output. But if you get one with too short a focal length, then you will put it right up to the lamp's outer envelope and it still will converge the light into a cone too small to light the corners of the fresnel. Then you are SOL!
Using a spherical reflector is independant of using a pre-condensor lens. It will increase the amount of light coming out the front of the lamp by reflecting the light coming out the back. If you do use both together, then the spherical reflector has to match the angular width of the light being captured by the pre-condensor lens. If it is too narrow then the image edges and corners will be much dimmer.
Could you please explain what occurs when two PCX lens are placed on top of each other. I read about people doing this in diylabs, but I would like to know how that is calculated as well please.
two lenses
When you combine two thin lenses, in close contact, then the total power in Diopters is additive. Diopters is defined as:
Diopter = 1000 / focal length in mm
So Combined Diopters = D1 + D2
You can substitute one of these equations into the other and get:
1/Effective focal length = 1/f1 + 1/f2
For example, two 1000 mm fl lens together give you an EFL of 500 mm.
One interesting thing is that these equations still work for negative lenses. You just have to include the sign of the lense's focal length when you do the math.
So a +1 Diopter lens and a -0.25 Diopter lens gives you +0.75 D. Or in focal lengths:
1/EFL = 1/1000 - 1/4000 = 3/4000 = 1/1333
When you combine two thin lenses, in close contact, then the total power in Diopters is additive. Diopters is defined as:
Diopter = 1000 / focal length in mm
So Combined Diopters = D1 + D2
You can substitute one of these equations into the other and get:
1/Effective focal length = 1/f1 + 1/f2
For example, two 1000 mm fl lens together give you an EFL of 500 mm.
One interesting thing is that these equations still work for negative lenses. You just have to include the sign of the lense's focal length when you do the math.
So a +1 Diopter lens and a -0.25 Diopter lens gives you +0.75 D. Or in focal lengths:
1/EFL = 1/1000 - 1/4000 = 3/4000 = 1/1333
Equations
I came up with these equations for the different distances:
Fresnel focal length, F.
Fresnel and lcd size, D.
Condensor diameter, d.
Condensor focal length, f.
Distance between condensor and fresnel, x : D/F = d/b, x = F-b.
Distance from condensor to light arc, a: 1/a = 1/f + 1/b
Using my expected setup:
F= 210mm
D= 264mm
f=112 mm
d=76mm
I get b = 60.5mm, x = 149.5mm and a = 39.3mm.
Am I correct here?
How would I set up the corresponding equations for the projection lens?
I came up with these equations for the different distances:
Fresnel focal length, F.
Fresnel and lcd size, D.
Condensor diameter, d.
Condensor focal length, f.
Distance between condensor and fresnel, x : D/F = d/b, x = F-b.
Distance from condensor to light arc, a: 1/a = 1/f + 1/b
Using my expected setup:
F= 210mm
D= 264mm
f=112 mm
d=76mm
I get b = 60.5mm, x = 149.5mm and a = 39.3mm.
Am I correct here?
How would I set up the corresponding equations for the projection lens?
Attachments
looks right to me
The most useful equation for a projector lens is:
1/EFL = 1/LCD-to-lens + 1/lens-to-screen
You usually know two of these, so you can solve for the third value. Once you know both distances, you can also calculate the image magnification:
M = (lens-to-screen) / (LCD-to-lens)
Image size is:
I = (LCD size) * M
Of course, you can also work backwards from a desired I value to get the projection lens EFL that you would need.
The most useful equation for a projector lens is:
1/EFL = 1/LCD-to-lens + 1/lens-to-screen
You usually know two of these, so you can solve for the third value. Once you know both distances, you can also calculate the image magnification:
M = (lens-to-screen) / (LCD-to-lens)
Image size is:
I = (LCD size) * M
Of course, you can also work backwards from a desired I value to get the projection lens EFL that you would need.
What about the fresnel focal lenght? Don't I need to take that into account, matching it to the projection lens somehow?
picking fresnels
Yes, you get a projector that works very well if you make the fresnels focus the image of the lamp arc in the middle of the projector lens.
You can read this thread for some discussion about that:
http://www.diyaudio.com/forums/showthread.php?s=&postid=578728#post578728
Yes, you get a projector that works very well if you make the fresnels focus the image of the lamp arc in the middle of the projector lens.
You can read this thread for some discussion about that:
http://www.diyaudio.com/forums/showthread.php?s=&postid=578728#post578728
Better to use the Condenser Diameter?
Hi Guy,
Question regarding the condenser.
Reflector Diameter = 90mm parabolic reflector
I have a 120mm Diameter Aspheric Condenser.
Minimum distance from the Bulb Arc to the Front Reflector = 40mm
Fresnel focal length, F. = 210mm
Fresnel and lcd size, D. = 300mm
Condensor diameter, d.= 120
Condensor focal length, f. = 127.82
Formulas:
Distance between condensor and fresnel, x : D/F = d/b, x = F-b.
Distance from condensor to light arc, a: 1/a = 1/f + 1/b
Units in mm
lensf dia/2 a
127.82 45 42.20
127.82 46 42.82
127.82 47 43.44
127.82 48 44.04
127.82 49 44.64
127.82 50 45.23
127.82 51 45.81
127.82 52 46.38
127.82 53 46.95
127.82 54 47.50
127.82 55 48.05
127.82 56 48.59
127.82 57 49.13
127.82 58 49.66
127.82 59 50.18
127.82 60 50.69
Is it best to use the whole diameter(120mm) of the condenser but having more than 10mm
spacing between the front reflector or use only a portion of the condenser(90mm) but
close to the reflector(42.20mm)?
But if I use the whole diameter, I could even move the bulb closer to the Fresnel.
If i use the 90mm diameter then the condenser will be close to the reflector and the bulb arc.
Thanks!
Hi Guy,
Question regarding the condenser.
Reflector Diameter = 90mm parabolic reflector
I have a 120mm Diameter Aspheric Condenser.
Minimum distance from the Bulb Arc to the Front Reflector = 40mm
Fresnel focal length, F. = 210mm
Fresnel and lcd size, D. = 300mm
Condensor diameter, d.= 120
Condensor focal length, f. = 127.82
Formulas:
Distance between condensor and fresnel, x : D/F = d/b, x = F-b.
Distance from condensor to light arc, a: 1/a = 1/f + 1/b
Units in mm
lensf dia/2 a
127.82 45 42.20
127.82 46 42.82
127.82 47 43.44
127.82 48 44.04
127.82 49 44.64
127.82 50 45.23
127.82 51 45.81
127.82 52 46.38
127.82 53 46.95
127.82 54 47.50
127.82 55 48.05
127.82 56 48.59
127.82 57 49.13
127.82 58 49.66
127.82 59 50.18
127.82 60 50.69
Is it best to use the whole diameter(120mm) of the condenser but having more than 10mm
spacing between the front reflector or use only a portion of the condenser(90mm) but
close to the reflector(42.20mm)?
But if I use the whole diameter, I could even move the bulb closer to the Fresnel.
If i use the 90mm diameter then the condenser will be close to the reflector and the bulb arc.
Thanks!
Varifocal Question
What is the best Focal Length to use in a Varifocal?
For instance, 275-320mm FL Varifocal.
Is it the center, 275+22.5 = 297.5 ?
Thanks!
What is the best Focal Length to use in a Varifocal?
For instance, 275-320mm FL Varifocal.
Is it the center, 275+22.5 = 297.5 ?
Thanks!
aspheric condensor
buddy123:
I am a bit suspicious about an aspheric condensor lens for use in this application. They are compensated so that you get less spherical aberration when you put the lamp arc right at the focal length distance. But that would give you a parallel beam out the other side of the lens. Not what we need!
Instead we put the lamp arc closer than the focal length distance, so we get a diverging cone of light out the other side. I don't think the aspheric compensations work if you do that. But maybe they will... Sounds like you need to run some experiments.
As for the distances, it is best if you can get the pre-condensor lens as close as possible to the lamp arc and then get or make a reflector that matches the maximum light angle to the edge of the lens. If the focal length of your pre-condensor lens dictates that it has to be far from the lamp arc to work, then you have the wrong lens.
buddy123:
I am a bit suspicious about an aspheric condensor lens for use in this application. They are compensated so that you get less spherical aberration when you put the lamp arc right at the focal length distance. But that would give you a parallel beam out the other side of the lens. Not what we need!
Instead we put the lamp arc closer than the focal length distance, so we get a diverging cone of light out the other side. I don't think the aspheric compensations work if you do that. But maybe they will... Sounds like you need to run some experiments.
As for the distances, it is best if you can get the pre-condensor lens as close as possible to the lamp arc and then get or make a reflector that matches the maximum light angle to the edge of the lens. If the focal length of your pre-condensor lens dictates that it has to be far from the lamp arc to work, then you have the wrong lens.
varifocal
You could ask the supplier, or just try it at different settings. None of them will be as good as a fixed focal length lens, but it probably will not matter. An LCD projector lens does not have to be anywhere near as good as a photographic lens, because the pixel size on the screen is so large.
Since the fresnels will focus all the light to a certain point, if you can put the projection lens at that point and use the varifocal lever to focus it there, then you will get a good image. If you put the lens too far from that point, you may get a sharp image but it will be very dim. The important thing is to get the condensor system and the LCD projection lens system adjusted so they intersect at the right point.
You could ask the supplier, or just try it at different settings. None of them will be as good as a fixed focal length lens, but it probably will not matter. An LCD projector lens does not have to be anywhere near as good as a photographic lens, because the pixel size on the screen is so large.
Since the fresnels will focus all the light to a certain point, if you can put the projection lens at that point and use the varifocal lever to focus it there, then you will get a good image. If you put the lens too far from that point, you may get a sharp image but it will be very dim. The important thing is to get the condensor system and the LCD projection lens system adjusted so they intersect at the right point.
We can't use a condenser Reflector in a parabolic Reflector?
Guy,
So you think it's hard to use a condensor lens on my 90mm
diameter parabolic reflector.
I just saw an image related here...
http://s6.invisionfree.com/diyprojectorphils/index.php?showtopic=41
Guy,
So you think it's hard to use a condensor lens on my 90mm
diameter parabolic reflector.
I just saw an image related here...
http://s6.invisionfree.com/diyprojectorphils/index.php?showtopic=41
parabolic reflector
The problem is that a lamp sends out light in all directions. A condensor fresnel can be used to refract some of that diverging cone of light into a wide parallel beam for the LCD. A parabolic reflector sends out light already in a parallel beam, so you don't need a condensor fresnel. If you try to use both direct light and a parabolic reflector, then you either use a condensor fresnel or you don't. Either way, you lose about half of the light!
The only way you can use a parabolic reflector in an LCD projector, is to make the reflector as large as the LCD, add a tiny spherical reflector between the lamp and the LCD (so no light goes directly to the LCD), and then use only a field fresnel.
Most builders have found that a spherical reflector (with the lamp arc at the center of curvature) works a lot better. Then all of the direct and reflected light goes from the lamp arc to the condensor fresnel.
The problem is that a lamp sends out light in all directions. A condensor fresnel can be used to refract some of that diverging cone of light into a wide parallel beam for the LCD. A parabolic reflector sends out light already in a parallel beam, so you don't need a condensor fresnel. If you try to use both direct light and a parabolic reflector, then you either use a condensor fresnel or you don't. Either way, you lose about half of the light!
The only way you can use a parabolic reflector in an LCD projector, is to make the reflector as large as the LCD, add a tiny spherical reflector between the lamp and the LCD (so no light goes directly to the LCD), and then use only a field fresnel.
Most builders have found that a spherical reflector (with the lamp arc at the center of curvature) works a lot better. Then all of the direct and reflected light goes from the lamp arc to the condensor fresnel.
Thanks so much for the detailed explenation Guy.
I remember someone saying Edmund Optics tech guys are all optical engineeers in another thread. Couldn't be further from the truth. If you call with the overconfidence the person that made that statement has you will EASILY sway the average Edmund Optics phone tech person into agreeing with you. It is a customer service area that has a mixed group of people with mixed backgrounds. Some have a lot of experience and some not, but to make the company and themselves look good they will say they are optical engineers if asked. You can hear the hesitation in the response. I called looking for condenser help and explained my typical DIY 15" setup and elliptical reflector to a couple employees. I was recommended by the first person a tiny diode laser condenser lens and the second person I talked to couldn't help me any better to find what I need. None of the required questions were asked by either person. The second was at least honest upon further questioning and frankly informed me that they don't all have very technical knowledge of optics, but can find out if they need to by asking around.
This condenser mystery has been bugging the s^%$ out of me for a while now. Your explenation made a world of difference Guy. I am now much more on the right track, but have one last curiosity. Is the condesor going to take most light fed into it and send it out at a consistant angle and path on the other side? I ask because I have an elliptical reflector for a MH bulb and I am not sure of its exact optical measurements. It is easy to find the approx focal length by shining the light on a surface and observing the focused light diameter, but I don't know the FL or angle etc. Is there anyone that can figure that using precise measurements I can take of this reflector?
I remember someone saying Edmund Optics tech guys are all optical engineeers in another thread. Couldn't be further from the truth. If you call with the overconfidence the person that made that statement has you will EASILY sway the average Edmund Optics phone tech person into agreeing with you. It is a customer service area that has a mixed group of people with mixed backgrounds. Some have a lot of experience and some not, but to make the company and themselves look good they will say they are optical engineers if asked. You can hear the hesitation in the response. I called looking for condenser help and explained my typical DIY 15" setup and elliptical reflector to a couple employees. I was recommended by the first person a tiny diode laser condenser lens and the second person I talked to couldn't help me any better to find what I need. None of the required questions were asked by either person. The second was at least honest upon further questioning and frankly informed me that they don't all have very technical knowledge of optics, but can find out if they need to by asking around.
This condenser mystery has been bugging the s^%$ out of me for a while now. Your explenation made a world of difference Guy. I am now much more on the right track, but have one last curiosity. Is the condesor going to take most light fed into it and send it out at a consistant angle and path on the other side? I ask because I have an elliptical reflector for a MH bulb and I am not sure of its exact optical measurements. It is easy to find the approx focal length by shining the light on a surface and observing the focused light diameter, but I don't know the FL or angle etc. Is there anyone that can figure that using precise measurements I can take of this reflector?
condensor fresnel
What a condensor fresnel does with the light, depends on where the light is coming from. In the drawing below, the light drawn in red is coming from the focal point of the fresnel so the light on the other side is parallel.
The light drawn in blue is coming from less than the focal length, so it will make a diverging cone on the other side of the fresnel.
The light drawn in green is coming from more than the focal length, so it will make a converging cone on the other side of the fresnel.
If you put your lamp arc right at the focal length from the condensor fresnel, then you will get parallel rays out the other side. If you are using something more complex, like a pre-condensor lens or an elliptical reflector, then the light has to strike the fresnel as if it was coming from the fresnel's focal point if you want parallel rays out the other side.
What a condensor fresnel does with the light, depends on where the light is coming from. In the drawing below, the light drawn in red is coming from the focal point of the fresnel so the light on the other side is parallel.
The light drawn in blue is coming from less than the focal length, so it will make a diverging cone on the other side of the fresnel.
The light drawn in green is coming from more than the focal length, so it will make a converging cone on the other side of the fresnel.
If you put your lamp arc right at the focal length from the condensor fresnel, then you will get parallel rays out the other side. If you are using something more complex, like a pre-condensor lens or an elliptical reflector, then the light has to strike the fresnel as if it was coming from the fresnel's focal point if you want parallel rays out the other side.
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