Cold Mirror

[Dazzzla]

There has been a lot of talk about using cold mirrors to remove heat from the light path, but nobody has actually described how to implement it using small cold mirrors.
The only way I can think of designing it is to use two condensers. I’ve attached a drawing that might help explain it.
The design uses two condensers with very short focal lengths (20-30mm). The first condenser bends the rays to be parallel allowing all of the light to strike the cold mirror at 45 degrees for maximum IR removal the second condenser converges the rays to a new point source for the Fresnel to focus on. Although it may be possible do with out the second condenser on a smaller LCD, I can’t see a way to do it on larger LCD, as the cost of the cold mirror may become expensive.
I would be interested if anyone else has any ideas or any improvements

DJ

[Dazzzla]

oops

[faithblinded]

I believe you can put together a number of smaller cold mirrors to achieve a larger one without paying the exhorbitant price of haveing a large one made. Just tile them to whatever size you need and secure them to something flat so they act as one big cold mirror.

[Dazzzla]

Hello faithblinded

Thanks for your input. Yes you can tile the mirrors but I can’t see how to make it work without the lamp and condenser getting in the way of the light path. You possibly could split the Fresnel and put the mirror in between them but that makes the mirror and the pj allot bigger.
Or maybe I’m missing something?


DJ

[Faites777]

I believe that this idea has been implemented before. According to this picture, we only need to use a small cold mirror. Place the cold mirror near the focal point of the condenser and all the IR light will be filter out completely. The Heatsinker will absorb the IR light and the fan will help to cool the Heatsinker faster.

In the picture, I did not have access to a CAD progrom so I was unable to calculate all the lightrays. But placment of the Coldmirror at a focal point is possible.

[WMT]

Why not use a hot mirror?
It is the same think but it reflects the ir back.
Also if you do not want to warm up the lamp a lot you can use a 45 degrees hot mirror.The light goes through with 45 degrees angle and the ir reflects to the side.I use an 0 degrees hot mirror and my lamp does not seem to get more hot than without the hot mirror.i use this setup more than a year now with no problem.Beleive me even with the 400w hqi-ts the lcd is very cool with this mirror.It also has a uv filter.it is 90mmx90mm square and i bought it from a 3m supplier for 146 euros last year.it can work upto 500 celcious temperature!!!!

Try edmundoptics.com

http://www.edmundoptics.com/onlineca...productID=1492

[unit24]

The main problem is it looks like the cost of the hot mirror would be more expensive than the cold mirror of the same dimensions:

• MIRROR HOT 45 DEG 4" X 5" $184.80

• MIRROR COLD 45 DEG 4" X 5" $121.20

[ancorp]

I think for a hot mirror setup, it would be better to tile up the small ones as well - look at the price difference!!!

55x50mm HotMirror at surplusshed

$2.75!!!

[Guy Grotke]

I have some of the cheap hot mirror tiles from surplusshed.com, and I am not impressed. I think they may be worn out or defective. I don't notice the heat being decreased when I test them with a halogen lamp.

But I also have a 4.75" diameter cold mirror I got from them for $5 and it works very well! I can really feel a lot of heat coming through it, and the reflection of visible light is very bright.

I noticed the same problem with putting a cold mirror into the light path between a condensor lens and a 15" LCD. Very difficult with a 220 mm fl fresnel. A bit easier if you change to a 330 mm fl. But you would still need a huge mirror. I agree that you need to make a stronger condensor so you can reflect a focal point. Or another option is to get the light down to that small focal point using an elliptical reflector.

I am still looking for a better hot mirror, since that would be very easy to use.

[Rox]

I tried those cheap hot mirrors from surplushed and aparently didn´t noticed any good result. But then took one of them and did an experiment; Do you know how night vision works? it is IR light lamp on a camera and then the camera receives the IR reflexion from the objets. So I placed the IR mirror on the cameras IR lamp and found it works nice. Much of the IR waves were reflected and lost (I could notice there was image on the camera visor but trust me, most of it was lost. I took it in front of my bathroom mirror and tried some esperiments. It was intersting...