Film-Tech Forum ARCHIVE: Dowser - maximum closed time with lamp on

This is topic Dowser - maximum closed time with lamp on in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=16;t=001364

Posted by Gavin Lewarne (Member # 5397) on 01-27-2013, 02:42 AM:

Hi Everyone

I have actually spent the time to look back through all 50 odd pages in the digital forum, and run a few searches as well, but cant seem to find an answer to my question.

Is there a maximum "safe" time you can have the dowser closed, with the lamp on, projecting a black, or nearly black image?

I know on 35mm machines it can range from just a few seconds to a few minutes. On our Vic5, if the main lamphouse dowser is open we only ever use the shutter dowser for the leaders and tails of the film. Having left it down myself a couple times it only takes about a minute to start to smell it.

So, is there a concensus on the digital dowsers? I was thinking of it in this context-

Lets say my video monitor is broken, and i need to be Playing a bluray. So turn on the projector, cue the film and pause on black. Need to wait 15 minutes to start it, leave lamp on and dowser down or extinguish lamp and wait the 15 minutes?

Cheers!

Posted by Marco Giustini (Member # 4544) on 01-27-2013, 04:22 AM:

On a Barco the shutter is before the lens and the picture is automatically blackened by the DLP when the dowser goes down, hence there shouldn't be a time limit. (That was changed by software, previously they had trouble with bent shutters).

I don't really know about Christie, where the shutter is before the light engine.

Posted by Gavin Lewarne (Member # 5397) on 01-27-2013, 04:37 AM:

That's great info Marco, thanks (we have a barco)

Posted by Jock Blakley (Member # 6552) on 01-27-2013, 06:08 AM:

Marco, even if the picture is automatically blanked, that wouldn't have any effect on the heat from the lamp, would it?

Posted by Marco Giustini (Member # 4544) on 01-27-2013, 06:29 AM:

When the picture is blanked, all the light enters the light ngine and then is diverted to some heatsinks inside the light engine. On a Christie the heatsink is fitted on the front of the unit, it's a V shaped metal element.
Hence, almost no light reaches the lens/dowser.

The only doubt I have is whether the light engine is capable of dissipating all the light for an undefined time, but I have never heard otherwise and I have seen many machines working for several hours a day showing nothing and it did not look like a problem.

Posted by Dave Macaulay (Member # 813) on 01-27-2013, 07:33 AM:

I've wondered about the different approaches Barco and Christie have taken. The Christie dowser is able to take lamp energy indefinitely, I've never seen one damaged. The Barco dowser on the 23B and 32B had overheating issues with bright images, and an advisory went out about it: this was later handled in a software version that blanks the image with dowser closed. The light engine itself can take lamp heat pretty much forever, part of the annual maintenance involves projecting a black image for an hour then checking the system temperatures. I've never found one with unusual temperatures on the DMDs. I asked a Barco engineer about it and the idea is to have the light engine avoid focus and convergence changes as it warms up. I have looked at the convergence pattern immediately after lamp start and as it heats up and the change is small but visible. I don't know why Christie took the opposite approach.

Posted by Steve Guttag (Member # 268) on 01-27-2013, 07:58 AM:

Christie's approach is consistent with film designs. The best way to keep heat/light out of the system is to not let it in in the first place. The down side is Christie's dowser is slower and is going to have more wear problems over time due to the greater mass involved.

I'm good with both ideas.

Thus far, I have not found a time limit for leaving the douser closed. Then again, I recommend leaving the dowser open unless there is a demonstrated reason for closing it.

Posted by Marco Giustini (Member # 4544) on 01-27-2013, 08:02 AM:

quote: Dave Macaulay
part of the annual maintenance involves projecting a black image for an hour then checking the system temperatures

Hi Dave,
Where did you find the above reccomendation?
I think the DMD wouldn't heat up when showing a black picture, they're not absorbing the light. As said, the light is reflected on some heatsinks outside the light engine, hence I don't think there is any difference for the DMDs whether they're showing Black or White, but if it's black the projector needs to dissipate the extra heat from the light engine cavity - which shouldn't be a big deal.

Posted by Dave Macaulay (Member # 813) on 01-27-2013, 08:57 AM:

In the manual under service tasks...

Posted by Marco Giustini (Member # 4544) on 01-27-2013, 11:12 AM:

Found it, thanks!

That, IMHO, is not necessarily for checking temperatures but to check the entire machine's status. That said, because showing a black picture is testing the cooling capabilities of the projector, it's indeed a good test to do.

Posted by Jock Blakley (Member # 6552) on 01-27-2013, 05:35 PM:

Thank you Marco, that's good to know.