Film-Tech Forum ARCHIVE: How to determine distance between lamp and gate?

This is topic How to determine distance between lamp and gate? in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Gary Wachter (Member # 5833) on 03-29-2019, 05:54 PM:

There have been plenty of discussions on aligning the optical axis, but little on the proper distance from the light source to the film. How would this be determined?

Posted by Matthew Ortado (Member # 5620) on 03-29-2019, 07:56 PM:

Each lamphouse has a specific working distance. You would have to look at the manual, it is listed there.

Posted by Lindsay Morris (Member # 1430) on 03-30-2019, 06:27 PM:

Each lamphouse having a specific working distance is tied in with the mirrors used in them.
Most mirrors have a code on them something like 12 x 4 1/4 x 36 which is the mirror diam, the distance from the arc Pos tip to the mirror face and finally the working distance from the mirror face in the centre to the aperture and usually in inches. Have seen some in mm figures.
If the mirror you have is a short working distance and your lamphouse is large/long you will probably not be able to focus the light source correctly. Any old mirror does not suit any lamphouse..select carefully.