Film-Tech Forum ARCHIVE: Carbon arc and uneven color temperature

This is topic Carbon arc and uneven color temperature in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Fredrik Sandstrom (Member # 8171) on 11-03-2014, 02:55 PM:

I'm having problem with one of the carbon arc lamps. It gives a slight bluish color on the right hand side of the picture. Center and left is white and nice. It seems more pronounced if I make the arc very short, but no matter where I place the tips of the carbons I can't seem to make the problem go away completely.

The other lamp gives better and more even illumination and does not have this issue at all. Interestingly, to achieve a given amperage, the good lamp can be used at a lower setting on the rotary knob that controls current. And furthermore, the good lamp consumes noticeably less carbon. I don't know if this is related to the color temp issue.

These are Cinemeccanica Zenith 350 lamphouses.

What can be amiss, any ideas?

Posted by Paul H. Rayton (Member # 1817) on 11-03-2014, 03:17 PM:

Have you opened up the lamphouse and looked at the reflector(s)? Possibly one is clouded by dust, or even cracked. Considering your description, that would be the first place I'd look.

Posted by Fredrik Sandstrom (Member # 8171) on 11-03-2014, 03:36 PM:

Yes, both reflectors are in pretty bad shape. Lots of small pits and copper droplets on them. I was advised to use a razor blade to remove the copper droplets, and while this indeed works, it leaves behind a new small pit in the surface. No cracks however.

We have four other reflectors that are also used but in much better shape, so we should put two of them in instead. I don't know why that hasn't happened yet.

However, I feel both reflectors are very similar in this regard, and if so this would not explain the problem. But I'll take a closer look.

Posted by John Eickhof (Member # 323) on 11-03-2014, 07:56 PM:

check the working distance from the center of the reflector to the film gate one lamp may be too far back, also did you adjust the mirror horizontally? You are getting fringe light from the spot or the carbon support is slightly off center. If you have a lamp alignment kit (a set of rods and lens holder adapter you can make sure all the optics are ligned up. It should be a simple fix.