Film-Tech Forum ARCHIVE: 350mm Diameter mirror vs. 380mm diameter mirror with XBO 2500OFR

This is topic 350mm Diameter mirror vs. 380mm diameter mirror with XBO 2500OFR in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Marin Zorica (Member # 1716) on 03-19-2005, 05:34 PM:

In now setup i have two lamphouses with XBO 2500 OFR in vertical positing with 350mm mirror, i also got 380mm mirrors and i can put them (but i must move xenon forward to mirror, but that isn't's problem to do), i only want to know is does will be more light on screen with bigger mirrors if i put them?

Thanks!!!

Posted by Gordon McLeod (Member # 33) on 03-19-2005, 09:19 PM:

not always
the larger mirror will only gather more light if the electrode spacing matchs the shadow they produce on the reflector at the correct cathode focus

Posted by Marin Zorica (Member # 1716) on 03-20-2005, 09:34 AM:

quote: Gordon McLeod
the larger mirror will only gather more light if the electrode spacing matchs the shadow they produce on the reflector at the correct cathode focus

Gordon, can you explain this little bit?????

also what is with little mirror which is in front of bulb, some people told me that isn't matter if is installed????

Also what is proper way to focus vertical lamp together with small and large mirror??

Posted by Gordon McLeod (Member # 33) on 03-20-2005, 09:46 AM:

the front mirror collects the light that is lost normally towards the front bulkhead and reflects it back through the arc to the main mirror
It produces some more light and definitly improves the eveness of light
But if improperly focused the inverted arc image can burn the electrodes off the lamp in seconds

Posted by Marin Zorica (Member # 1716) on 03-20-2005, 03:39 PM:

Can anyone explain me this???:

quote: Gordon McLeod
the larger mirror will only gather more light if the electrode spacing matchs the shadow they produce on the reflector at the correct cathode focus

Any how is right procedure to focus vertical xenon lamp together with big and small mirror??

Thanks!

Posted by Gordon McLeod (Member # 33) on 03-20-2005, 07:14 PM:

1st get a small flat mirror like a lady's compact mirror and hold it between the large main mirror and the lamp on a 45 degree angle so you are seeing thtough the lamp on the optical axis
Adjust the front mirror till an inverted image of the electrodes is located directly in the centre of the lamp and the inverted image is almost but not touching the electrodes

Then close up the lamphouse and light the lamp and normally there is a viewing hole in the back to look through the mirror and one should see the main arc flame with an inverted one superimpossed and inverted inside it
Then focus the main mirror for best screen light

Posted by Dave Macaulay (Member # 813) on 03-21-2005, 08:49 PM:

Marin - I think Gord's first answer assumed you had horizontal lamps. In that case the anode lump casts a large shadow - direct light from the arc is blocked towards the front of the lamphouse. Putting in a larger deeper collector that extends into this shadow won't improve your light efficiency as there isn't any light there to collect. The "pointy" cathode doesn't throw a very large shadow and the centre hole in the mirror is usually designed to be close to matching the shadow area. Asking about the front spherical mirror tells us you have a vertical lamp, where the shadow answer doesn't make much sense.

Posted by Marin Zorica (Member # 1716) on 03-22-2005, 05:03 AM:

Dave,

yes i also assumed that Gordon is talking about horizontal lamp.

But I'm still interested will i get more light in a screen with vertical lamp if i put bigger mirror, now is 350mm diameter and i planning to put 380mm diameter mirror, will i get more light with this bigger mirror?

Thanks!

Posted by Gordon McLeod (Member # 33) on 03-22-2005, 11:14 AM:

Not always the larger mirror will have different focal points so the bulb may have to be mounted in a different location and the working distance to the apperture may be different
You have too many variables to give a awnser

Posted by Marin Zorica (Member # 1716) on 03-22-2005, 03:24 PM:

Yes it's true that with changing from 350mm to 380mm mirrors i must move xenon bulb to focal length of 380mm and that isn't problem because here on xenon lamp i have two point on cathode and anode holder, one is for 350mm mirror and second is for 380mm mirror, and that isn't problem because in other cinema i got same xenon and mirror holders in same projector except there is 380mm mirrors, so i can put mirrors but i want to know are there be more light on screen?

I don't want panic but i have 2500W xenon lamp and distance from projector to screen is 41 meter, screen is not plastic is only white wall because it's open air cinema!

And i wonder will there be enough light? So i want put bigger mirrors.

Posted by Gordon McLeod (Member # 33) on 03-22-2005, 04:11 PM:

If you have the mirror it will not cost anything but your time to try it
It s impossible to advise on this with out being there

Posted by Marin Zorica (Member # 1716) on 03-22-2005, 05:38 PM:

Yes gordon you said best answer!!!

Because i have two projectors i can put 380mm mirror on one and leave 350mm mirror, then i will can see difference, if there will be!!