I notice my image is much better when my collector lens is placed something like 3-4 inches closer to the bulb. I thought all the optics were off until I realized the bulb envelope may be to blame. What do you think, would this extra curved layer of glass cause the path of the light to converge sooner? Or is it just because I'm collecting more light?

I've heard of people moving the light source a bit closer than the theoretical distance, and yes, it's due to the arc tube diameter. The building guide is pretty much just that, a guide. Fine tuning and adjusting is where you get your picture quality from. Wherever your components end up to give you the best picture is up to you.

Your results mimic a lot of others. I was also surprised to find that my maximum briohtness was about 1-1.5 inches closer to the back fresnal than the focal length of 220 mm.

I would suggest to anyone building a pj that adjustability in bulb placement is vital in achieving a good, bright image.

This increase in brightness is partially due to the size of the arc envelope, but also due to what happens in the front of the projector.

As you move the lamp closer to the fresnels, the light comes to a focus further forward. This usually allows more light to enter the triplet resulting in a brighter image.

If you focus a smaller image, the lamp will need to be closer. A larger image will require the lamp to be a bit further away for maximum brightness.

Edit: Corrected

My lamp is currently only 180mm away from the rear fresnel, which gives the brightest image with my test 6 foot projection.

Sorry, I made an error in my previous post...

A larger image requires the lens to be CLOSER to the LCD, and therefore the lamp needs to be FURTHER from the collimator fresnel, closer to the theoretical 220mm distance.

A smaller image requires the lens to be FURTHER from the LCD, and therefore the lamp needs to be CLOSER to the collimator fresnel. At a 72" projection, this will be signifigantly closer, so your measurement of 180mm isn't surprising.

As an aside, the lamp bing placed 180mm from the collimator also means that you are collecting almost twice as much light in total. That should be making for a very bright image.

Yes, I am considering keeping quite a small image because I really like the extra contrast and vibrance of the picture.

I'll be projecting at 72"-84". So you are saying to move the collimator lens back to about 20-25mm from the LCD instead of 15mm? I'd assume the bulb would then be at 210-215mm instead of 220mm?

I just got done building my LCD sled and lens assembly but have not built the enclosure yet. Just wanted your thoughts about this issue before I mount everything in my box when I get it done. All my stuff will be somewhat adjustable but I'd like to hopefully get things put in right the 1st time.

If I were you, I would go out of your way to make it as adjustable as possible!

At 72-84", I'd expect that the lamp will be less than 200mm from the collimator.

The distance from the collimator to LCD isn't th issue here, it can be left at whatever you designed for. The distancce from lamp to collimator is the issue.

Cooool! That's a double benefit with a 72 inch image. Not only is the image brighter due to the smaller area, but we collect more light from the bulb. That may explain why my image is bright enough to watch it with the lights on and a lot of other people complain about brightness.



It seems that at 72 -80 inches the bulb is about 20-40mm closer to the condensor fresnal. I ditto what MMc said, go out of your way to make the bulb distance adjustable. Then you simply have to adjust for maximum brightness and not have to worry if you calculated right.