new build question

[milihpen]

Hi all, I'm making my first projector right now. The main question I have is, with the fresnels, can I just use one piece of glass for each fresnel, or do I have to sandwich the fresnels between two pieces of glass? Thanks for all your help. Also, I'll be taking my monitor apart tonight, any tips while doing that? What about grounding? I think I'll be doing it on a wooden workbench, so can I just touch a piece of grounded metal and then go to work, or.....

Thanks for all your help, fellas

[v1d9uy]

uh actuall you dont wanna sandwich youre fresnels .. for every peice of glass you put inbetween the bulb and youre projection lens you loose aroung 10% of your light output... and trust me you need all the light you can get. just make a frame for the fresnels and lcd

[milihpen]

thanks much for the info. I've heard that 10% figure, too (I always assumed that was just for Lexan, though, but maybe not). So are those two pieces of glass that I bought useless (I figured on mounting my fresnels on them, but heck, they were just $5, so no waste )?

Regarding infrared protection, I currently have none (the Lexan XL10, which I have, is just for UV, right?). Would standard glass filter out any significant amount of ir?

So basically, what you're saying is just mount the fresnels in a frame without any glass? Will that make my fresnels warp, or no worries? Do I NEED any kind of ir protection? If I do, I think I'll just use a nice cold mirror I have (could work, right?).

Thanks much for the help, sorry about all the questions, just trying to get enlightened.

[Guy Grotke]

Acrylic fresnels will warp if they are mounted horizontally without support. I lay mine on a sheet of Lexan, which also filters out the UV. If you are mounting them vertically, then they should be fine without any support. But you do need the Lexan or other UV filter before the fresnels, or they will yellow over time from UV damage.

You need IR filtering. Cold mirror is great, but difficult to use, since you have to reflect the visible light off it. If you look at the light path in the typical projector, you would need a really large cold mirror to do that. Hot mirror is much easier, since it passes the visible light and reflects the IR. You just put it right across the light path, and it will cut most of the IR.

Lots of IR filter choices available, like DIYprojectorcompany.com's IR filter glass. Search the forum for others.

[milihpen]

Thanks for your help, Guy.

Regarding the fresnels, I think I got your gist, but when you say horizontal, you mean that the focal points would be above and below the fresnel, and when you say vertical, you mean the focal points would be in front and back of the fresnel, correct?

P.S.: Guy, I believe that you have used the cheap eBay HPS ballast in the past, correct? I just got mine (HUGE!), and am wondering if you might know, if I ever take my projector apart in the future, do I need to be wary of the capacitor, or does it discharge automatically? I imagine that thing could hurt, it's so big.

P.P.S.: Why doesn't the Lumenlab guide mention an IR filter? Their store doesn't sell any, either. Is their cooling that good?


Thanks for the help

[Guy Grotke]

1) Yes, you got it regarding the fresnel orientations. I have a first-surface mirror just before my projection lens, so my light path goes up to the mirror. Then it goes horizontally through my projection lens. Other people build a straight-shot projector because they have room for it and don't want to lose the 5-10% of the light the mirror absorbs. Of course, first-surface mirrors are not free!

2) Yes, I have that ballast. My capacitor came with a resistor across the terminals. That would discharge it pretty quickly. (ie. seconds) if yours does not have such a resistor, then be careful. You could add a 1/4 Watt 1 MegOhm resistor, if you like.

3) I have not seen the LL guide, so you would have to ask them. All I have done is to measure the equilibration temperature at the fresnels and LCD, with:

Rosco Hot Mirror
DIYBuilderGroup Hot Mirror
Rosco Thermashield
DIYProjectorCompany IR Filter Glass
No filter

Everything I tried worked much better than "no filter". I have not tried a simple piece of tempered glass, which I think is what they recommend. Maybe that does something.

[milihpen]

Thanks a lot for your help, Guy. My capacitor also has that resistor, I thought that maybe that's what it was for. Good to know it is