Understanding Pre-Condenser lens

[Megaman]

What about the fresnel focal lenght? Don't I need to take that into account, matching it to the projection lens somehow?

[Guy Grotke]

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/showt...728#post578728

[buddy123]

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!

[buddy123]

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!

[Guy Grotke]

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.

[Guy Grotke]

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.

[buddy123]

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/diyprojec...p?showtopic=41

[Guy Grotke]

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.

[joew26mpls]

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?

[Guy Grotke]

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.