Martini's Build Pre-Alpha...

[dave123]

Originally posted by King_Martini
Tasty deal: www.surplusshed.com

4.75 cold mirror
FUJINON COPY LENS, 240MM FL

PLASTIC DUST COVER / FAN GUARD FOR 90mm 12VDC FANS

and shipping.

Total: $23.02

Remaining budget: $177

I thought they ran out of the FUJINON COPY LENS, 240MM FL ?

[Guy Grotke]

>so much more light with < reflector

You might get more light, but it may not be useful light. You will find out when you look at a projected image. What you need is even illumination going through all the LCD pixels in a uniform manner. It may be perpendicular to the LCD panel, or it may be converging (or even diverging) toward the objective lens. (If it is not converging, then you need an upper fresnel to make it converge.) But the key word here is "uniform": It does you no good (and probably does harm) to have any of the light not on the same set of paths.

For example, if the reflections don't come to the same focal point as the light directly from the bulb, then they won't end up at the objective lens. This is why good light engines use only a few specific designs. Test your < reflector with fresnels and a piece of paper at the proper focal distances. See if you get a reasonably uniform spot about the diameter of your objective lens, with the paper at the right distance. If you get a lot of other patches of light, then "more light" is not doing you any good.

The four useful light engine designs I have read about are:

1) Spherical reflector + condensor lens: Reflects the arc image back exactly along the direct path, so the condensor lens "sees" a twice-bright image of the arc. Needs both fresnels to converge light toward the lens.

2) Parabolic reflector: Makes a uniform perpendicular beam the size of the LCD maximum dimension, but the direct image of the bulb may result in a hot spot. Needs an upper fresnel to converge toward lens.

3) Eliptical reflector: Makes a uniform beam that converges through the LCD toward the lens. No fresnels, but again the direct light may cause a hot spot.

4) No reflector at all: Just a higher-wattage bulb + condensor lens + fresnels.

[King_Martini]

DAVE123:

Looking at the invoice, Surplusshed sent me
B&L copy lens 9-1/2" FL f/4.5 with the text "L3003 Sold out replacement" underneath, so it must be a similar replacement...


Guy Grotke:

Hmm where to start...
Well, the parabolic ( reflector collected the light very poorly. With it, I ended up with a dim center and a bright ring of light on the outside. So my light engine has undergone some changes, one being the addition of the < reflector. This change alone created an even and intense light. I have found that having the condenser lense (alone) at the right distance from the bulb perfectly displays an image of the bulb and text that is stamped into the reflector on the ceiling. Add a fresnel and it removes any and all patchy light (no image of bulb or text on reflector displayed on ceiling, just pure bright even light).

Now I have been testing putting a 6" ruler on top of the fresnel and suspending it over the light while holding the copy lens above it all. I can get a perfect sharp outline of the ruler HOWEVER some of the ruler is cut off. This means problems for me down the road I suspect.

So I move the condenser closer to the bulb so the light radius increases at where the fresnel/lcd will be. In BOTH of the condenser lens heights I am most certainly getting an even and intense light and right now I can get sharp outlines of whatever object I place on the fresnel while holding the copy lens above them.

I just need to tweak the condenser lens height with the fresnel height so I don't end up with dark corners in the projected image. This of course will correlate to how big of an LCD I decide to use. I am designing for a 6"-7" wide LCD.

I am about as novice as they come with this project, just trying to use common sense and hope for the best with the following premises:

1) Create a bright, even light thats radius can cover the LCD
2) Keep the heat down to a very low minimum
3) Keep it all adjustable for tweaking.

So far I feel the light engine is very successful so far. Using one or two fresnel will be be future tests when the time comes.

BTW thanks guy, I really appreciate your help and interest in my progress. From what you mentioned in your post (and from what I understood) so far everything is a gogogo...

Night!

[Guy Grotke]

Using a parabolic reflector is an interesting option. Theoretically, you could make one the diameter of the maximum LCD dimension, that would supply a uniform and perfectly perpendicular beam to the LCD. No condensor or lower fresnel would be needed. (In fact, trying to use a smaller reflector and lenses to "blow it up" to the size of the LCD would would be VERY difficult.)

The hard part is the light coming from the lamp: Even with a perfect parabola (and the lamp at the exact focus), the lamp itself would either block axial light to make a central cold spot or else supply it directly and make a central hot spot (since the direct light is brighter than the reflected light).

It may also be very difficult to make a "perfect" parabola. The light reflected near the center would strike the reflector at a pretty steep angle, while the light reflected at the far edges would strike the reflector at a much lower angle. So the reflectance may be different.

One problem is that you can't judge how well you are doing by looking at a piece of white paper at the LCD position: It may look even, but if a region is lit only by rays that are not perpendicular (or converging the same way as all the other rays in a design without a fresnel above the LCD), then that light will not end up at the objective lens.

I think a spherical reflector, consensor, and lower fresnel may be a lot easier to get right, compared to a parabolic reflector. This combination does put about 65% of the available light through the LCD in a useful manner. A parabola would have to be quite deep (and very reflective!) to collect that much light, and don't forget that the rays coming directly from the lamp are only useful near the center of the image, the rest don't hit the objective lens.

[King_Martini]

--> snip <--
(In fact, trying to use a smaller reflector and lenses to "blow it up" to the size of the LCD would would be VERY difficult.)
--> snip <--

Guy, must you always be right!!!! Well maybe all is not lost, but I think I stumbled across the problem you mentioned in your prior post.

What is odd is that:

lamp+reflector= even light
lamp+reflector+condenser = brighter light with cold spot in middle

However when I play with the projection lens and a small piece of fresnel (3"x4.5") and put a film negative on top, the fresnel edges wont fit into the lens.

My only thought is that since the light circle my setup makes can cover 6-7" in diameter, using a second fresnel might get everything into the copy lens, guy am I wrong on this?

Otherwise it's back to scratch for me

At least I got to the point I could reflect a film negative.

If you have the patience I can use your advice in a few days...

[Guy Grotke]

I think I understand why point sources work better than big "blobs" of light. It is true that any light going through a particular LCD pixel (in any direction) that then strikes the objective lens, should then be focused to the same point on the screen by the objective lens. That means that every part of the projected image would be lit by rays coming through many different parts of the objective lens. The problem with that is that the lens would have to be perfect, and there is no such thing! So some of a pixel's rays will end up a bit off in the image.

If you can make sure that the light traveling through a pixel strikes only a very small area of the objective lens, then objective lens aberrations will have less effect on the image. Then you get a much sharper image.

I will try to attach a drawing. The top part shows a point-source design. Each example point (A, B, & C) in the image are lit with rays that follow a unique path for each pixel, so each ray uses only a tiny part of the objective lens.

The bottom part shows a design with a big blob of light source. Some rays like #1 do not hit the objective lens at all. The central ray strikes the center of the LCD and the center of the lens. So far, so good. But the outlying rays (2 & 3) travel through the same central pixel, but fall on outer edges of the objective lens. This focuses them near the central point of the image, but not exactly on it. Thus a blurry image.

[Guy Grotke]

>Guy, must you always be right!!!!

No, I'm wrong about a lot of stuff. Just ask my wife!

I just have read thousands of these posts on several projector forums, read many websites on optics, tried to remember my college physics, and run some lens experiments. Guess how many sucessful projectors I have built: 0 so far. I have a stripped LCD panel, but no FFC extender to run it. (HINT HINT, JCB!!! Where's that darn FFC extender I ordered? )

I have a DIY achromatic doublet I built with surplus shed lenses. I have the ballast, but my MH bulb hasn't arrived yet. On the other hand, the big fresnels I ordered from LL have arrived. So I have been experimenting just like you with a little halogen lamp, trying to fill my transparency film "LCD" with uniform light and getting a very non-uniform and truncated projected image.

Keep trying. Getting there is 90% of the fun. I think you can succeed with a parabola, but it will have to make a parallel beam as wide as your LCD diagonal measurement. And you will have to fiddle a bit to get the central spot the same brightness as the rest.