Ok, I have been through alot of the forums here and over at DIYaudio.com and I am in the design phase of making a rear projection TV. Based off of some of the components for sale here(lens,frensel etc.) I have come up with a design that may hopefully work. I have a pic (mspaint owns!).





I know it is a real rough design with no exact specifications listed, but I was hoping for some input.

What I want to do is have something with the power to do big screen computing along with movie/tv watching. I have a pretty nice liquid cooled computer (gaming machine) I was planning on putting in a compartment of the base of the TV with a tv/tuner card installed. In the front of the base of the TV I was planning on putting a couple shelves to hold Cable box, DVD player and also a keyboard and a mouse I can pull out to use the computer.

A few thoughts and concerns on this design though:

-The blue box represents the actual projector assembly. I chose a 17"LCD so I can hopefully get a higher grade resolution on screen for the computing side as well as a higher resolution for movie/tv. The 400w Halogen I am not set on and am very open for suggestion.

-Ground glass screens I have found have an excellent diffusion and clarity to them from various sources I have seen and people I have spoken to. Is a 4x3 glass screen feasable though? One of my main concerns is it taking a whack by something and breaking. I have seen alot of other screens available but cost is a concern to me since i can just make my own ground glass. Any input on this would be great !

-Of course since this picture isnt to scale, there will have to be stages of building to get the proper focal lengths and mirror placement wich I am pretty capable of doing by making a skeleton of the actual TV. My one thought about getting the distance far enough from the screen is having the blue square moveable until I find the sized projection I need.


Well, thats the main idea I am having with this project that I want to get started on. Feel free to pick it apart or offer suggestions.

Just to add, for cooling I am planning on adding a couple of 120mm computer fans that will suck are through the projector assembly from inbetween the lcd/frensel down to the light source and out the back of the unit.

don't see a collimating fresnel in your design? is that because you haven't decided on what to use, or you weren't planning on using one?

I would steer clear of halogen lights. they produce a lot of heat and have relatively low brightness.

HMM i may be wrong but i think the mirror really needs to be 45 degree angle. and make sure its an fs mirror.

correct.. without the mirror at 45* I guess you will need keystoning. didn't even think of that.

That was one thing that I was debating and can always change later. What I based this off of was the video I am sure you have all seen, the standard OHP(like in schools) with a stripped lcd placed on it. My thought behind this is that the ground glass screen its self will act as a difuser. I just couldnt find any information or elads on people that actually used a ground glass screen. Maybe my thought is flawed in this though as it will be my first project.

Its my understanding that the projector lens itself will pick up the image at a certain focal length from the LCD and project it to the mirror. I have seen other projectors with 2 frensels one 220 other like 650 but I am not sure what the top lens will do for the design I am trying to do. Any input you have would be great if you think the second FL would be necessary.





Yeah the angle I will have to mess with once I get the main components assembled in a rough fashion. The problem here I was thinking is, I dont know exactly how deep the enclosure will have to be to get the projected 4'x3' image on my screen. I may eventually have to move it to the front of the enclosure and aim the projector to the back of the enclosure to give it more "length". Am I right in thinking this?


Thanks for the input, this is some of the particulars im wondering about. BTW, what is a FS mirror?

FS = front side.. i.e. reflective material is on the front of the glass/substrate. this is compared to your standard mirror where the reflective surface is behind the glass. you want to use a front side because the light will be refracted and reflected by the glass.


OHPs use two fresnels if I am not mistaken.. they are just connected.. this is where the term "unsplit fresnels" originated. The first fresnel collects and collimates light, the second fresnel directs it towards the projection lens.

Ok , yeah I will plan on using that type of mirror. I didnt know 2 frensels were required, I will make that change. Are they supposed to be of the same type ? I noticed in your plog you used the 2 lens before your lcd panel. Still reading more in it. Im gonna get started looking into MH lamps, I saw some listed in other forum posts.

using both fresnels before the LCD gives a sharper picture with better colors. the alternate setup is one fresnel before, and the other after the triplet.

the two fresnels have different functions and are typically different focal lengths. The rear (collimating) fresnel collects the light from the bulb and makes it parallel so it passed through the LCD uniformally. The front (field) fresnel takes this parallel light and directs it towards the triplet. The most popular collimating fresnels have a FL of 220mm. this is a good distance since its far enough for the fresnel to collect enough light from our less than optimal bulb and reflector setups, but close enough so the light is still relatively intense. The choice of field fresnel depends on your triplet. you want the FL of the field fresnel to be slightly longer than that of the triplet so the light 'cone' from the fresnel fills the triplet. popular combinations are field fresnel 330mm/triplet 320mm (standard LL triplet), fresnel 500mm/triplet 457mm (18"), fresnel 600mm (pro 650mm fresnel, really 600ish mm)/triplet 500mm (pro LL triplet).

the choice of triplet depends on your desired image size and throw. just a quick guestimate for you.. with a 4x3 screen (5 ft diagonal = 60 inches), 17" LCD. you will need a throw of 4.75 feet to get a 4x3' screen with the 320mm triplet. I'd say this is possible as long as your cabinet is deep enough and tall enough it might be more practical to source a shorter FL triplet from an OHP though, but this may remove the possibility of using a 17" panel