MH bulb with Enclosed Luminaires - What do you think?

[Rolm]

Yes, I found the same by-the-case-only problem as well.

I've been doing some thinking about this, and I think I've come to the conclusion that the MH bulbs with a U-turn element will not be able to create a clean, focused beam if they are end-mounted in a "conical" reflector. This has to do with the uneven distortion caused by the glass of the bulb a result of the point-source being mounted off-center in the bulb. And, to a lesser exent, the presence of the returning element on the other side of the bulb.

Consider that all glass distorts light to some extent. Assuming an even thickness on all points of the glass, this distortion can be calculated as a product of the angle of the beam of light relative to the surface of the glass. Therefore, an "ideal" bulb is a perfect sphere with a 0-diameter point-source in the exact center. That way, the angle of every beam emanating from the point-source is 90deg, therefore no distortion.

Move the point-source away from the center, and now almost _every_ beam emanating from it strikes the bulb glass at an angle other than 90deg, so almost every beam suffers from varying degrees of distortion. Another terribly inaccurate illustration:
<img src="http://members.cox.net/rolm/images/bulb_distortion.jpg">

If you imagine this offset bulb mounted end-on in a conical reflector (even if you manage to offset the bulb to put the point-source in the exact center of the reflector), you can see how the resulting beam will not be even. Since the source of the light is distorted, there is no way the focused beam can be even. This is part of what I ran into iwith the MH175RSP, because it uses a U-shaped element; even spreading the 60K-lumen hotspot resulted in an uneven beam.

If you look at MH floodlights using standard (U-shaped element) bulbs, they ALL mount the bulb sideways in a "rectangular" reflector. This rotates the distortion so that the reflector is reflecting complementary "distorted" beams rather than uneven distorted beams.

Ok, that description was a little hard to follow. But if you imagine this:

<img src="http://members.cox.net/rolm/images/bulb_distortion1.jpg">

It appears that the sideways-mounted bulb could give you a much cleaner beam than the end-mounted bulb.

The popularity of the U-shaped element probably means that it is the cheapest to mass-produce, and the most stable design. This would explain why the bi-pin and HQI bulbs are so expensive and limited to >=250W, while there are 1000W bulbs with the U-shaped element.

What do you think?

[rajkumar]

I finally order the following bulb and compatible ballast:

http://store.yahoo.com/elights/methal40unbu1.html

It seems like this bulb has the element centrally located compared to the other I researched. Also, the size is more managable than 10 inch plus lamps.

I have not worked with any MH bulb so far so I am not sure how the placement of element affects the distribution of light - but I will find out as soon as I get the bulb and ballast delivered.

Since the dimension of LCD is more rectuangular than round I am thinking of building rectuangular reflector based on the elliptical reflector theory ( I had posted a link earlier) - basically two circles one for the length of the panel and the other for the width and then fuse them together.

I am not going with the spider style reflectors - used for the acquarium lighting.

I am also looking into the Iwasaki 6500K 250W bulb (double ended - small unit) - that's sold with IceCap electronic ballast etc. for around $250. I still believe that If I have a small bulb, I may be able to get a ceramic reflector made. The bulb I have ordered has 5 inch light center length from the base and is Universal so I can mount is at any angle. I am thinking on mounting it horizontal with the arc facing the reflector side to reduce possible hotspot effects.

Since I will have to use the large bulb, I dont thik I can use the existing OHP set up so am thinking of working on the box design while waiting for the bulb/ballast.

[rajkumar]

While waiting for the lamp and ballast.. I am thinking of working on the box and reflector.

Thoughts on ellipsoidal reflector.

Since I am going to use the fresnel, I think I dont need to build an ellipsoidal reflector. Let the fresnel collect and focus the light. All I think I need to do is feed straight light to fresnel.

The first diagram shows how to bounce point source light straight and the second shows my design using the measurements of the bulb I am going to use. THe only thing I can think is that I have three inch room between the bulb element and the reflector surface (see my design) - that might be too narrow.

If this design is true, than I am going to draw a 6 inch radius circle and cut a half round of the circle in cardboard. Once I have that I can take that with me and look around for ceramic/glass/stainless steel/pyrex/etc bowls that can resemble similar curve. If not, I can have a friend make a ceramic half round with same radius that I can use for the reflector.

Any input?

[rajkumar]

drawing

[Rolm]

So, you're not going to try for the "conical" (ie bowl-shaped) reflector? By "half-round", do you mean that it will be a "rectangular" reflector (like a tube cut in half)?

If so, you might save a little time and effort and visit your local Home Depot (or the local equivalent). There are very sturdy cardboard tubes with a 12" diameter that are used to pour concrete footings. Cut one in half (you'll need a hacksaw, that's how sturdy they are) and layer the inside with aluminum flashing (also at Home Depot, comes in 12"x10' rolls, and it's usually almost as reflective as a mirror), and I think that'll be a good beta version of your design. If I understand your "half-round" design correctly, that is.

As far as the design itself, I'm not immediately aware of a reason why it _wouldn't_ work. It seems like a sound idea to me. If the reflector is large enough, and your trig is right, then you should end up with a large enough "parallel" beam. The only problem is that there's a pretty big margin of error in lining up the bulb and the reflector just so. I would make sure that I have some "wiggle room" to make some pretty large adjustments.

My light should be here tomorrow (according to UPS tracking), so I may have more results to post soon.

[rajkumar]

Rolm,

I think I didnt explain my design correctly. I am not looking into the tubular half round because it wont work for the light escaping on sides - it will only bounce light in the center of the tube. The reflector I am thinking is like a half of the hollow ball. Imaging a 12 to 15 inch ball thats used in bowling and cut it in half - something like that - the only thing is its hollow so I can put some sort of reflective film/aluminum sheet inside.

The placement of lamp inside should not be a huge deal since it has to align with the center of the lcd and at half the radius of the overall radius of the ball.

[Rolm]

Ok, so a conical (or, more accurately, hemispherical) reflector.

Hm, if you could go smaller than the 12" diameter you mentioned, you might try this:

<img src="http://www.homedepot.com/cmc_upload/HDUS/EN_US/asset/images/pii/9/8/2/2/A22289_2.JPG">
6' Cord Clamp Lamp with 8-1/2" Shade
Model: #010-HD277-894
SKU: #277894

The aluminum reflector is 8-1/2" diameter, and it sells for about $8. With a standard incandescent bulb, it throws a pretty even expanding beam. This was going to be my next test if the reflector that comes with my MH light isn't good enough.

The shade is rough-spun aluminum, so there are ridges inside the reflector that would have to be sanded down before you could polish it. It's easy to do, just get some wet sandpaper in 200, 400, and 600 grits (comes in a combo pack for about $5), and then spend an evening sanding down the shade, starting with the roughest grit to get rid of the ridges, and then progressing to the finer grits in stages. After it's as smooth as you can get it with the 600, use some Mothers Mag Polish (aluminum polish from an automotive store) to polish it to a mirror surface.

Er, come to think of it, it's an awful lot of effort just to see if it works. And then it might still be too small for you to use...

In any event, this is probably what I'm going to have to do with the reflector I'm getting.

[rajkumar]

Seems like a decent set up for a cheap price. I have seen similar but smaller reflectors in home depot/IKEA etc.. I just ordered a PAR56 reflector with bipin base from partsexpress. Also, I found a retailer for that GE bulb I was refering before. Its around $62 each with 1000 hr. life. 10K lumens. Its very small (<1.8 inch length and <1 inch diameter) and can burn in universal position. The arc is fairly centrally located so it may work with the reflector from part express or even with my EIKI OHP built in reflector. So, I am going to order that bulb and another ballast for 150W set up while waiting for my 400W metal halide bulb/ballast. Hope in between this two set up one will work.

10K lumen seems lower but a proper reflector may help. If its three times brighter than my junky 3M projector (3K lumens), I will be happy since I dont have to build a box for the bigger bulb setup for now. I will continue working on 400W MH set up in a box with custome made reflector and a long throw projector lense.

[Rolm]

Grrr.

I received the MH setup today, and UPS chipped the bulb. So I can't test it until I replace that.

Attached is an image. The metal ruler at the bottom of the image is 24" long, so it gives you an idea of the size of this thing.

[Rolm]

<img src="http://members.cox.net/rolm/images/Img.jpg">