MH bulb with Enclosed Luminaires - What do you think?

[rajkumar]

HOw did you make your elliptical reflector? Is it just the one curve (length or Width) based on gunwan's design? just curious.

I found a source (theaterical supply) in SF where I can possibly find some ellipsoidal reflectors made for stage lights. I will go there Monday and hunt for some thing pre made.

Part-express reflector arrived - Its a typical parabolic shape and I dont this it will help with my project but I will try anyway when my lamp arrives.

I was reading about various screens and found out that the glass bead screens are not suitable for LCD projection - less contrast. White matte is the best and if you apply a thin coat of gray color (5%) mixed with gesso (clear) with another coating of some transluscent gesso, the screen gives more contrast (due to gray) and 3D effect (clear coat). I will have to try that.

[Rolm]

Yes, my reflector was just the one curve, based pretty much on Gunwan's method. I experimented with adding side reflectors as well as using a flexible mirrored plastic, but nothing really improved the brightness too much. This lead me to my conclusion that there was something fundamentally wrong with my setup.

I have constructed another much deeper elliptical reflector, this time with a 26" FD. I've made the reflector, but haven't mounted the lamp yet (I hope to do that tonight). I'm trying for design1 on this one. The challenge is to make the reflector large enough to reduce the angle that the light enters the LCD without making the entire projector 6 feet tall. It would be easy to do: due to the diameter of the bulb (4-1/2"), I am forced to make very wide reflectors (this one is 18" wide). Even so, my beam angle is close to 55deg. In this case, F2 is only 5" above the theoretical placement of the 7"-high LCD. This may be too shallow.

The trouble is that trying to increase this height is difficult: I can only do this by either increasing the FD or decreasing the ellipse height. The difference in F2 height above the LCD when I went from a 20" FD to a 26" FD was only 3/4". And the 4-1/2" diameter of the bulb forces me to make the ellipse taller as I increase the FD. If I went with the smaller ($30) 2-1/2"-diam bulb, I could reduce the width of the reflector to around 12" and increase the angle of light passing thru the LCD dramatically, and probably raise F2 another 5". But that means shelling out more $$$, which I'm not willing to do yet.

Hmm, a theatrical store... I'd be interested to know what they have to offer in the realm of elliptical reflectors.

Screens are an area that I have read a fair bit about but haven't experimented that much with: once I get the projector working well, then I may have time to mess with that.

[rajkumar]

Finally, I got the reflector from the theater supply store. Its indeed a piece of art (see pics) has more than 700 flat mirror like surfaces and according to the literature can reflect up to 90% of the light. Its about 6 inch diameter and around the same height and comes with the cover with approx. 3 inch opening where the beam focuses.

They also had a glass reflector (same dimensions) which reflects all the light but transmit heat so that the bounced light is not hot -if this aluminum reflector works I might upgrade it to the glass one later. Glass reflector can transmit almost all the light and according to the theater people has a very sharp focused beam since it does not have small flat mirror like surfaces of the aluminum one.

Finding F1 for the elliptical reflector was fun. See the next post.

[rajkumar]

To see how the reflector works and where the F1 is, I used a salvaged ballpoint pen with LED light and tryied to mimic the dimension of the MH lamp I plan to use (still have not received - more than a week overdue). The reflector is amazing - I can get a very focused beam (about 2 inch diameter) at the F2 and if I look inside the reflector from F2 I can pretty much see the LED on most of the mirror surfaces. The pictures are not sharp enough since I took them with available light holding the reflector with one hand and my heavy digital SLR on the other so kind of blurry but you can get the idea.

[rajkumar]

Here is my version of the MH lamp using LED light from a ball point pen (freebie from western union!) to find the F2 and observe the beam behavior.

[rajkumar]

Now I have the reflector and the lamp is on its way, I am thinking of looking into how to expand the focused beam from the reflector. I know other people on the forum have been using a condensor lens to expand the focused light on to the frasnel. In my 3M overhead, it has a halogen lamp with elliptical reflector and the focused light goes through a narrow iris on to the mirror and bounced on the frasnel.

My question is is it necessory to use condenser lens or let the beam to expand itself (like the 3M ohp setup). I dont have anything close to the condensor lens that I can try so any other suggestions?

[Rolm]

WOW! That's a pretty kick-A reflector! Nice job figuring out the F1/F2, also.

I think that people use and "atomoshperic" or "condenser" lens when they want to change the angle at which the beam expands in order to bring F2 closer or push it farther away. My OHP also omittted the condenser lens, so I think that's a pretty common way of doing it.

On the other hand, I don't know whether or not a condenser lens performs other magic on the beam, like evening it out and reducing hotspots. I don't think so, but perhaps.

Unfortunately, the bulb that I'm working with is the el-cheapo 4-1/2"-diameter one, so a 6" reflector is too small. I ended up building 4 different reflectors, each more complex then the last. None of them managed to give me a clean, clear beam: most weren't much better than the bulb with nothing more than cardboard boxing it in.

I think I'm ready to abandon trying to build a "rectangular reflector, because I think they waste too much of the available light. I'm taking a step back and eying the metal bowls like you talked about very early in this thread. I found a source for mixer bowls, which elliptically shaped, and usually about 8-10" in diameter and 10-12" deep. The ones I found are stainless steel, so they should be resonably shiny to start with, and should polish up nicely.

They're cheap (around $20), so I'll order one this week to play around with. I may even pick up a cheap aluminum spherical mixing bowl this weekend to build the alpha test kit.

Again, good job with the reflector. If I decide to go down a bulb size, I think I'll definitely have to find one of those.

[zardoz]

I havent worked out the focals ..just some "eyeball" tests. I'm using the giant 4 1/2 inch HID, I located some dirt cheap bowls. The reflector is 13 3/4 wide by about 7 inchs deep $7.00, it seems to give really good direction to the light, the second is 12 1/4 by 6 inchs deep $5.00. The second failed as a reflector, but seeing the stage reflector I'm wondering if there is benefit in using the failed bowl to close the "clam shell" on the out put side of the lamp housing. Also does anyone know how an HID likes to have it's light rays reflected back to it's arc tube? Not to mention cooling...

any thoughts?

zardoz

[Rolm]

<b>>clamshell bowls</b>

As far as I know, the clamshell arrangement is only beneficial for an elliptical reflector, where the rearward reflector (where the bulb is mounted) is an elliptical reflector (as opposed to a parabolic or spherical), and the apeture in the forward bowl corresponds to the F2 of the rearward bowl. The second "bowl" serves more to block errant rays that are not in line with the focused beam passing thru F2.

I can't think of of how a clamshell arrangement would benefit a parabolic reflector (where the reflector only serves to spread the beam) or a spherical reflector (where the refl. sends the beam straight out).

Sooo, where'd ya find the cheap bowls? I found a pretty-much elliptical reflector on ebay, in the form of a refubished Kitchenaide mixer bowl.

[zardoz]

I got these at the local "bargain department store" Saan's in nova scotia. They are stainless stell and wicked shiny! The larger bowl is somewhat eliptical, I've spent most of my time working out how to feed my laptop panel so havent done a lot of work on the reflector yet. Still learing

zardoz