Being inspired by HF's design i got thinking, instead of using aluminium to form a cone linking directly into the back of the lcd foregoing the need for a rear frensel...in theory. what would happen if you used mirrors, and the only way i could think this would be feasible is if you used a pyramid shape... a quick GS drawing, with the light poking thorugh the bottom a la HF.


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although all the light would bounce around inside and eventually be directed towards teh rear frensel would this cause superior brightness to using a simple rear refelctor or would it simply produce many virtual arc's leading to hot spotting?

just an idea. do not have a frim enough grasp of optics to figure this out myself, anyone?

bump?

For one, the light wouldn't be distributed well by those planar surfaces. It's far from an optimal setup.

I’ve done ray traces using this type of reflector. It will produce a very even distribution and direct allot of light towards the LCD. The problem is that the arc needs to be small and the reflector apex needs to be very close to the arc. I’ve contemplated using a small 150W lamp that has had the outer glass envelope removed. But I’m slake and haven’t done anything about it yet.
Another possible lamp could be one of the car HID Xenon lamps.

DJ

@DAZZ: I thought we'd pretty much discarded the idea due to just those constraints? You really think it's feasible to find something small enough and bright enough that can be squeezed down into the "point" of the pyramid to make it work? Not to mention the severe thermal stress gradient I'd think would exist down there but I suppose that's not even close to an insurmountable problem given some of the other things peeps have done around here. Just what kind of size constraints have your ray traces suggested?

how is this any different then a spot light design wise. I mean the both are enclosed, both use bright bulbs - I have a 1,500,000 candle power spotlight that uses what looks to be a super small halogen bulb - its the size of a side blinker replacment bulb but holy crap this thing will blind you. The reflector looks like a stainless salad bowl imo.

Perhaps this is uneducated - however I cant help but think - at the bottom of the "cone" why not place in a spherical reflector and integrate it in to the cone this wouldn't this help direct the light a bit more?

DAZZ- how would this raytrace out?

I think this type of design is great and if you don't ask you wont know
bear with me

Yeah i was just wondering that if the light was at the very bottom and the sides were mirrored then surely all light would eventually be reflected upwards. then although it would be following a very disorganised set of paths the rear frensel if flush with the top should direct 100% of the light towards the lcd.

How would this design function if you made a complete pyramid from mirrors then using a dremel or something similar made two small holes in opposite mirror sides to insert a small quartz metal halide. all the light that would be lost would be that escaping through the arc chamber and through the holes in teh mirror.

it just seemed Hitesfiero is using the principle but having to make an fantastically complicated and well made reflector out of alluminium that innevitably will absorb some light whereas a mirror being less malleable will reflect more. compromises i guess.

Thanks Dazz for the info!

just read my post, my text is less than helpful to understand what i mean... here some pics

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i just think the difficult bit would be first cutting the mirror then cooling it. sealing the top would mean the whole thing would get very hot, and melt the frensel. a tempered glass top prob would be needed with the frensel on top to prevent warping.

if it was sealed and then you just cooled the lcd would you need to cool the bulb as being sealed it would be exposed to a lot higher temps than normal, and how would a mirror cope...tarnish? you wouldn't need a fs mirror as ghositng would not be an issue so pos the glass would be safe, or just crack.

10” LCD 330-550 rear fresnel, 330 front fresnel, 10-15mm arc vessel. If the lamp is too large and too far from the apex it will cause the virtual arc images created by the mirrors to be too large. After magnification by the fresnels the arc images at the triplet will simply miss.
To get a better idea of the sizes involved, make yourself a pyramid from aluminium foil. Place a small object in the apex and look into the pyramid. Notice how placement and size affect the virtual images created. What your eye sees is what the fresnels will see as the source of light. For example if you use a 10mm object in the pyramid and there were 8 virtual images created that were 50mm apart, then using a pair of 330 fresnels will create a 50mm arc image at the triplet (magnification= 330/330= 1). If a 220-330 fresnel pair were used the magnification would be 1.5 so the arc image at the triplet would be 75mm.

EDIT:
And using a 550-330 fresnel pair (550 at the rear) a negative magnification of 0.6 would be archived. So the arc image at the triplet would be 30mm.

Nice explanation!


So pretty intriguing. Still seems to me that engineering that apex is gonna be a tough nut to crack. Especially fabricating something with high reflectivity. Or maybe not? And having everything so tightly packed along with the "closed" nature of the design is gonna make dealing with the heat a problem. But then again - if using a relatively low wattage lamp there's less heat to deal with ITFP.

so if we are talking about small arcs something like this looks like it could suffice...

http://www.lampspecs.co.uk/Light-Bulbs-Tub...light-White-BLV

either then to have a small hole at the bottom of the pyramid and simply allign this bulbs arc with it. or to actaully inset this bulb through a hole at the bottom, having the arc at the theoretical peak of the pyramid.

I haven't been able to find any specs on this bulb with regards to it size but it looks quite small.

Edit heres a similar one

http://www.lampspecs.co.uk/Light-Bulbs-Tub...-Cool-GE-CDM-T-

arc lenght 7.6mm, not too bad at all!

Here’s a rough conceptual diagram:

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Notice that it doesn’t have to be a complete pyramid. The size is dependent on the angles of the sides.

An inaccurate trace
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I see what you mean about the added virtual images increasing the effective arc size. am not sure i understand the first diagram as surel if there is a reflective surface on either side of the arc, there is going to be reflections.

it seems that having the arc within the pyramid is leading to many artifact arcs, which could lead to problems passing all the light through the triplet. however as this system brings teh advantage of a lot more light being used a diffuser over the top of the tempered glass could bring uniformity at the cost of a small amount of light loss but with the postential of still being brighter than a conventional diesgn...it makes you think. if only i had a bit of money ant time at the moment this could be an interesting design concept!

What's the reflectivity index of mirror-polished Al and does it require any protective coating? I know Al kinda "protects itself" but would it stay highly polished if it got as hot as one could reasonably expect in such an application? Or would the efficiency of such a configuration offset any diminished reflectivity when compared to a material with a higher reflective index? Probaby doesn't matter it gets hot but I just get this image (virtual ) in my head of the Al discoloring near the lamp for some odd reason.

I had the same thoughts as although al is fairly resistant to corrosion, being close to the very hot bulb could cause it to begin to oxidise over time (produces a white salt i think), so it would not remain highly polished. with an actual second surface mirror, they usually use aluminium these days but being behind the glass does not allow any o2 in so hopefully preventing oxidation.

I did a fair amount of testing with reflector systems and came up with a few rules of the behavior.

The larger the reflector the larger the arc image.

You can make the arc image smaller by using a longer rear FL but this necessitates the need of a reflector with a tighter light pattern which will make the reflector larger or flatter, A flatter reflector will wast more light so it becomes less efficient.

You can use a vary large reflector system but the triplet will have to be large also. I did like the idea of the guy with the 5" doublet but I think the FL was too short.

For any of the larger LCD's a condenser system will be as bright or brighter than a reflector system because of the inability to make a perfect reflector compared to a lens. If you have a larger triplet (lower F value) then both a condenser or reflector system would be brighter so all remains the same.


There are some pre made reflectors that might just work but no one has tried them yet.

Where would one find one of these pre-made reflectors?

I too have been messing around with reflectors. Heat is tough to dissipate with these designs, even with only a 150w bulb. I am currently using a mini cone (4" tall cone, bulb is ~8" away from heat shield) and it distributes light very nicely across the entire LCD. I had a full sized cone but nearly melted everything because of the lack of cooling.

I think these designs have some great potential, but there are also different and difficult problems to overcome compared to the traditional DIY projector design.

You flatter me sir!
I would be bright, but I think the 90 deg seams would show in the projection.