Understanding Pre-Condenser lens

[joew26mpls]

I got that much. I should have been more specific. I am asking about an aspheric pre-condenser. Is the job of that to correct and even the light from the source whether it be bulb with hemispherical, no reflector, or elliptical? Do I have to make sure my elliptical reflector is sending the light out at the proper angle? This thing (elliptical reflector) is concentrating A LOT of the light from this bulb into an area about an inch to inch and a half in diameter. It is so powerfull at that point that it burns cardboard almost like a fresnel and sun. I REALLY want to figure out how to harness that or I will be forced into a 1000w... which I may end up upon anyway. The "traditional" 400w with hemisphere setup works pretty good, but it is obvious it isn't pushing nearly enough light through the LCD judging by the amount of brightness, contrast, and gamma tweaking needed. I would also LOVE to REALLY even out that light spread.

Thanks so much for your informed help! School or just well studied on the subject?

[Guy Grotke]

This is a bit trickier than measuring a parabola, since an ellipse has two focal points. You could successively trim a piece of cardboard until it fits into your reflector perfectly, and then use that template to find a graphical solution to the ellipse equation. That would consist of tracing the ellipse cross section curve onto a piece of paper, and finding a length of string and two focal point pins on the central axis that give you the same curve. Pretty tedious!

Or you could precisely measure the diameter at two different distances from the center of the reflector, and plug those values into the general ellipse equation to find the parameters of the equation for your ellipse. That would give you the location of the two focal points.

I would just use the experimental approach: Move a small point source lamp (ie. a flashlight bulb) along the central axis. At each position of the lamp, use a small piece of white paper to see if you can find a point on the axis where an image of the lamp filament is formed. When you do find it, the lamp position and the image position are your two focal points.

To use an elliptical reflector in a projector, you still have to make the fresnel "see" it as coming from the fresnel's focal point. So the fresnel's focal point has to coincide with the reflector's second focal point. The reflector has to be wide enough to send light to the fresnel's corners.

[Guy Grotke]

You can get even more efficient if you add a large spherical reflector (red in the drawing below) with a hole in the center. This reflector sends most of the rays that missed the elliptical reflector back through the lamp arc and into the elliptical reflector.

The amount of added light depends on the shape and reflectivity of the reflectors, but without the spherical reflector most of the light getting to the fresnel directly from the lamp would be wasted, since it would not get to the projection lens.

[Guy Grotke]

These are used to collect a lot of light from a very close lamp arc, and then to send that in a less divergent cone to a condensor fresnel. It works just by intersecting more of the light sphere coming from the lamp.

If your elliptical reflector light cone (after the second focal point)does not match the focal length and diameter of the condensor fresnel, then you could use a pre-condensor lens to adjust it. (But you would lose some light in the lens. Better if you can use a reflector and fresnel that match.) Determining the right pre-condensor lens to use would be done the same way you would for a non-reflector design: The second focal point of the reflector would act as the light source.

[Gunawan W]

Hi,
I'm curious about condenser lens, how important is this lens for our DIY 15" LCD PJ?
Because from the drawing at firt page of this thread, I saw not much light can be saved, I think it's only a fraction of light and don't forget to count overall light loss due to condenserlens thickness.

[Guy Grotke]

For a 15" LCD with a 220 mm or shorter fl condensor fresnel, a pre-condensor lens does not capture much more light. Look at the angles: The condensor fresnel is so close to the lamp that it already gets a lot of light.

If you have a longer focal length condensor fresnel or a smaller LCD, then a pre-condensor lens would help a lot.

[juice_e]

What kind of reflector would be best for a 15" LCD but using a larger MH bulb? Because the one i have is a 250 watt MH, but its about 6 " long. And where would i get a reflector that wouldnt cost an arm and a leg?

[Guy Grotke]

If your bulb is about 2" in diameter, then you can probably use one of these:

http://www.culinarycookware.com/cata...0&prodid=16100

http://www.dr.ca used to have some various sizes of hemisperical stainless steel quiche molds, but I don't see them there anymore. You might call them to ask.

If you have one of those huge 3-4" diameter bulbs, then you may have to use a big spherical mixing bowl.

Either way, you will have to cut out a hole on one or both sides to get your lamp arc right at the center of the sphere. Make sure you have plenty of air flow around the bulb if you use one of these: They send the light right back through the arc, so the bulb can overheat if you don't cool it well.

[juice_e]

your bulb is about 2" in diameter, then you can probably use one of these:

http://www.culinarycookware.com/cat...00&prodid=16100

What size bowl is best? will these only refleft about 65% of the light?

[joew26mpls]

You can buy an IKEA napkin holder reflector from here for $4:
http://www.ireplica.com/?page=shop/b...3a64b4cf292bd&
It should fit most bulbs. No cutting needed