Brightnes irregularity

[AndrejK]

Hi,
I have already buld my DIY projector 3 months ago (some pictures are here:
http://www.fs.uni-lj.si/lavek/Studenti/Razno/S1.jpg
http://www.fs.uni-lj.si/lavek/Studenti/Razno/S2.jpg
http://www.fs.uni-lj.si/lavek/Studenti/Razno/S3.jpg).
The overhead is VEGA Junior GLS 400W (see http://www.vega.si/prospekti/Junior.pdf) and the LCD monitor is
Philips 105S.
But I have some troubles with brightnes irregularity - the center of the projected image (around 10cm at 130cm diagonal) is very bright. So bright that the picture is indistinct.
Is there some way to reduce the brightnes in the center of the image?

[dj_holmes]

o.k bruv. the only thing i can think of is........

if your monitor is completely flat on top of the ohp then......

open your ohp and take out your frenel lens its more than likely warped. by that i mean that, due to the heat the centre of the fresnel has been dented outwards towards the glass. i don't hink i'm wrong on this!! open your ohp and find out for sure.

[AndrejK]

if your monitor is completely flat on top of the ohp then......

The monitor is completely flat on top of the ohp, because it is built in CNC machnined Kerroc plate.

open your ohp and take out your frenel lens its more than likely warped. by that i mean that, due to the heat the centre of the fresnel has been dented outwards towards the glass. i don't hink i'm wrong on this!! open your ohp and find out for sure. [/B]

I have already taken out the fresnel lens and it is truly dented outwards towards the glass (about 5mm) . But isn't that the way it should be?
Where I could buy the new fresnel lens?

[dj_holmes]

nooooooooooooooo my friend its not suppose to be bent like that. this will conentrate all the light in the middle if u actually press flat on the bump you will see more light in the corners (if that made any sense??)

exclusiv-online
izzotek

if u europe...i'm in the same position as you i need frenel also!!

its bent because it go too hot and MELTED the fresnel like that. hottest in the middle

[AndrejK]

I see, I will try to bend the frasnel lens to see if there is any impact on picture brightnes!
I will probably order the frasnel lens online from http://www.3dlens.com/enter.html - check it, because some guy from UK already ordered it and it is quite satisfied!
It cost 24$ for 280x280mm, or 24$ for 310x310mm + shipping

AndrejK

[18wheeler]

some OHPs use CURVED fresnels. and it is actually designed so to compansate the light intensity a corners. It is documented in my 3M 9550 and other OHP's manual.

[spamh8r]

Some of those lenses look to be pretty nice. VERY reasonably priced too!