From: John Campbell (tojohn@texas.net)
Date: 03/20/02-09:30:23 PM Z
Judy--
Now THAT"S good testing! Your array was wired in parallel, I would assume
(given that the whole thing worked, even with "out" bulbs), and ballasts
matched? Resistance differentials among the tubes would explain the four
tube uneven results-confirmed by moving the tubes. I would also be curious
as to the grounding of the total array.
Anyway, it's a good test. I'm adding your method to the QC of every Gecko
that goes out of the shop from now on.
Thanks and best wishes,
John
http://www.photogecko.com/
Home of The Gecko UV Light Box
"Get The Gecko!"
-----Original Message-----
From: Judy Seigel [mailto:jseigel@panix.com]
Sent: Tuesday, March 19, 2002 9:30 PM
To: alt-photo-process-l@skyway.usask.ca
Subject: RE: Light source for big prints
On Tue, 19 Mar 2002, John Campbell wrote:
> How about:
>
> ||================= ||
> ||================= || add *2* fixtures *on each side*
> ||================= || perpendicular
> ||================= || to the others
> ||================= ||
> ||================= || *or-1 48" fixture per side. . . ?*
> ||================= ||
> ||================= ||
> ||================= ||
> ||================= ||
> ||================= ||
> ||================= ||
> ||================= ||
>
> I dunno why. . . just a twisted aesthetic of balance, I guess.
Judging by my own tests (shown Post-Factory #7), any of these
configurations should be fine. Probably. But I found an easy "low tech"
way to test & advise something of the sort for every system because there
may be pitfalls.
Dilute cyanotype emulsion 1 to 2 with water, coat on some cheapo paper the
size of your light bank and expose for the minimum exposure to get tone
(pale shows uneven better than dark). In my case that was 30 seconds.
Don't try to do it with 21-steps, in which the readings are too close &
with too many variables, and the difference between wedges can throw
things off. You can't do every inch anyway. Coat the whole paper & read by
overall glance.
Even with every other bulb covered, it still came out even, even at only
1-1/2 inches from the bulbs.
*BUT*, surprise! Top to bottom of the light table there was a difference,
that is, about the bottom 4 bulbs gave noticeably less density than the
top. I tried coating from different directions, I put a white reflector at
the open end.... Could the electricity be wearing out on the way down?
Finally I switched the top 4 bulbs with the bottom 4 bulbs & all was even.
No I can't explain it.... but I show it ... scanned the whole sheet &
copied a wedge from the bottom in Photoshop & pasted into the top.
Difference is very distinct. I'd suggest a test of this type for EVERY
light table, but ESPECIALLY if you didn't get all bulbs at same time from
same lot. Interestingly (baffingly), even when several bulbs in a row were
out (by accident), the difference was less marked.
Judy
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