Film-Tech Forum ARCHIVE: Dark Picture

This is topic Dark Picture in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Paul Rich (Member # 243) on 05-03-2001, 11:49 PM:

We have three houses with dark pictures. I thought it might be the bulbs, so they were changed, but the picture is still too dark. What could be the problem and solution?

Posted by Aaron Sisemore (Member # 145) on 05-04-2001, 12:03 AM:

If the bulbs are new, a dark image could be caused by one of the following:

-Bulb of too low of a wattage for the screen size and throw.

-Dirty, damaged and/or misaligned reflector(s) or other lamphouse internal optics

-Projector misaligned to lamphouse

-Misaligned bulb focus

-Dirty optical path (lens/portglass/screen)

-Lamphouse power problems (bad diode(s), etc)

-Fire shutters or electric dowsers sticking partially closed

Aaron

Posted by Brad Miller (Member # 2) on 05-04-2001, 05:01 AM:

Start with the simplest checks and work your way up until you find the problem. Before you go any further, tell us what make/model lamphouses are these? Also, what wattage are they?

First step, check the bulb focus.

Second step, what is the amperage and voltage on the meter (which could be off) on the side of your lamphouse?

Let's take it from there.

Posted by John Pytlak (Member # 331) on 05-04-2001, 06:43 AM:

Standard SMPTE 196M specifies an aim of 16 footlamberts, with an allowed range of 12 to 22 footlamberts. If your lamphouses were originally bright enough, I suspect either misalignment/misfocusing of the optics, or deterioration of the optical components (especially the reflector).

As a "rule of thumb", a gain=1 matte white flat screen requires approximately 5 watts per square foot to light properly with a properly aligned lamphouse and normal (2 blade) shutter and optics. With a gain=1.5 curved screen, figure about 4 watts per square foot.

Here are some articles on the subject:
http://www.kodak.com/US/en/motion/newsletters/reel/spring98/pointers.shtml
http://www.kodak.com/US/en/motion/newsletters/notes/june2000/pytlak.shtml
http://www.kodak.com/US/en/motion/newsletters/notes/march2000/pytlak.shtml
http://www.kodak.com/US/en/motion/newsletters/reel/december99/pytlak.shtml
http://www.kodak.com/US/en/motion/newsletters/reel/december99/harknessHall.shtml

------------------
John P. Pytlak, Senior Technical Specialist
Worldwide Technical Services, Entertainment Imaging
Eastman Kodak Company
Research Labs, Building 69, Room 7419
Rochester, New York, 14650-1922 USA
Tel: 716-477-5325 Cell: 716-781-4036 Fax: 716-722-7243
E-Mail: john.pytlak@kodak.com
Web site: http://www.kodak.com/go/motion