Not all in focus. Brighter in centre.

Hi Guys. I've been lurking around reading for a few days. I bought a 400W OHP from eBay and I'm using a 15" AOC TFT. It works, but I can't seem to get the whole screen in focus. If I get the centre in focus, the edges are out of focus and vica versa.

Not only that, I've noticed how the centre of the screen is brighter than the rest. I'm guessing this is just an issue with the projector style :(

I am writing an article on this, which needs to be in by Friday morning - so I'd like to get the best results I can :S

In the longterm though, I'd like to have a decent projector built for my living room - so I'll be sticking around to learn more. It seems to me that building a custom chassis might be a step forward.


2005-03-14 12:47 am
You probably have an OHP that uses a singlet lens, or a not-very-good duplet.

A singlet will have what looks like one lens and one mirror. Duplets often have one lens facing down to the stage and one lens facing the screen, after the mirror. A triplet lens will look like a camera lens, usually a couple inched long and all in one housing, then the mirror after that.

Ability to focus center and edges at the same time is worst for singlets, better for duplets, and best for triplets. You won't run into quads.
dimmer at the edges

Light spreads out at the square of the distance. (AKA The Inverse Square Law) If you measure the distance from the lamp filament to the center of the fresnel, and the distance from the lamp filament to the corner of the fresnel, then you can calculate how much of a difference in brightness you should expect.

I have a MH lamp arc about 220 mm from a fresnel that sends the light to my 15" LCD. In my PJ, that expected ratio is around 1.8:1!

When you try to use your projector for a graphics application like windows, this difference is very apparent. But video is funny: Lots of video content has darker edges already, and not much happens at the edges anyway. As long as the transition is smooth, you hardly even notice it.

The one big advantage of going to a custom PJ is that you can get fresnels big enough to fill a whole 15" LCD, instead of having some cut off.
Light spreads out at the square of the distance. (AKA The Inverse Square Law) If you measure the distance from the lamp filament to the center of the fresnel, and the distance from the lamp filament to the corner of the fresnel, then you can calculate how much of a difference in brightness you should expect.

So would you say that an advantage of using a longer FL on the back fresnel (between the bulb and the LCD) is more uniform lighting? Moving the blub back as far as you can would reduce the difference between the center distance and the edge distance.


2004-07-25 10:06 pm
yes, that is true superdaveumo, but there is some big BUT.

if you use 330 rear fresnell instead of 220 then it will be more even light (the inverse square law is less significative) but the light took from the bulb is less (narrower angle on the 330 case).

this is the main disadvantage, the light loose. So the light evenes is in expense of brightness. Now there is a posible fix to this problem; you can add a precondenser lens.

The precondenser lenes will capture a wider angle form the bulb and will condense it to a narrower angle and direct it to the rear fresnell. This way you will have even light and "same" amount of light.

the precondensor lens choice is something hard work and needs to be done well. You can have a wrong lens and you can have less light than the lensless setup.


2005-03-14 12:47 am
It's my opinion that most of the "darker in corners" problems are caused less by the inverse square, but by the viewing angle of the LCD. An LCD works best if light passes through at 90 degrees to the plane of the LCD. In an unsplit setup, or in an OHP, the light is already being condensed to the projection lens before it hits the LCD. So out near the edges, the light is coming in at a significant angle.

In a split setup, the light goes through the LCD in a more or less parallel path. Then the second fresnel focuses the light towards the projection lens.

I like the sharpness of an unsplit setup, but recently I am using split fresnels because of keystone correction and also a very noticeable improvement in brightness across the whole LCD. It's difficult to notice any dimming at all in the corners.
viewing angle

Thanks for the info cbm5,

I was thinking about trying my new 550 mm fresnel in split mode, just to see if I would get more even lighting. It has a very fine pitch, so it should do pretty well in a split design.

Each type of LCD has a different maximum viewing angle, and it is usually not the same value from all directions. So you might only see a viewing angle problem along one edge! This is one of the reasons that builders using unverified designs need to experiment a bit, just to see what works best with their particular set of components.

I will definitely try it.
Hi Guys,

For some strange reason, I didn't get an email notifying me of a response - more than likely, I didn't check the box when I posted :p

Some interesting responses there - there is obviously a lot more to this than I thought. I think I might have chosen a rather inferior OHP. I'm tempted to box this thing back up, flog it on ebay again and then choose another one (preferably 4000 lumens at least, as the brightness on this isn't all that great). Or failing that, build my own.

Does anyone remember the CTX EzPro 550? It was a TFT based projector. I have one of those that is annoyingly broken, and I think it's something electronic rather than electric that has broken on it - otherwise I'd carry on trying to fix it. But it's got a decent lens on it. Maybe I could use this lens?

There it is.

I'm even slightly tempted, to just buy a TFT that's a similar size to the original in the EZ Pro, build a switch mode power supply to power the bulb and just use the original chassis!!


2005-03-14 12:47 am
The Hami actually has 800x600 resolution and VGA input, compared to other LCDs which might have much less resolution and be intended to watch only NTSC video.

The halogen light might have worked "fine" but on the page you posted it said 300 lumens, you might like to kick that up a bit, plus the page also says 70 hours which starts to get really expensive. A nice metal-halide system will pay for itself in a few months.
Ouch, 300 lumens isn't much at all. Yet this projector here is some 2000 lumens I think, but the image is inferior to that CTX (IIRC). It uses a 36V 400W bulb, surely that would give out more than 300? It might be a mistake and it mean 3000.

So I just need an LCD that has VGA input - not that specific brand. I'll have to which the TFT out and measure it first. If I'm REALLY lucky, I might be able to find some driving electronics for it somewhere (I mean REALLY).