Spot in middle of image

[Me2!]

Revived thread

Tez,

I was reviewing diffraction in my old physics book and i found a picture of your spot. Too bad i didnt remember it before.

This is called the "fresnel bright spot" and it is a diffraction from the shadow of a glass ball. I asked earlier if the was a pinch ball on your bulb. That was it. Get a magnifier and look at the tip of your bulb (with the ohp off).

There is a similar efffect from diffraction of an aperture but this would yield a spot with a few rings around it which you don't have.

Use a different type of bulb.

See:
http://badger.physics.wisc.edu/lab/manual2/node17.html
http://www.ece.utexas.edu/~becker/diffract.pdf

 

[tez]

Sounds like a thought, I had attributed it to the density of the rings in the lenses.
due to seeing it with two different bulbs in that projector and seeing the same anomoly in another OHP, which incidently uses an entirely different bulb.

I have since built a projector from scratch, and it has no problems like that... I guess I should really sell the OHP and the panel, just need to get around to listing it, but I was hoping that I had a definate answer so that I could offer a solution to the next person who gets it.

Thanks for the reply!

 

[Abner78]

I can lend a hand: I don't remember where I've read it (maybe allinbox) but the spot is due to a warp of fresnel lenses toward the glass of the OHP.
This is due to heat from the halogenous lamp.

I can confirm this theory 'cause I have the same ANNOYING spot in my configuration and ... guess what... my lenses are really warped like said above.

Make a try: take out the lenses from the OHP and lay them on a table or a plain surface. Mine haven't the whole surface in contact with the table...