From: Sandy King (sanking@clemson.edu)
Date: 09/28/01-10:13:54 AM Z
Judy,
From similar data we have reached some very different conclusions. In
my estimation the two-to-three and one-half stop increase in speed
offered by the metal halide unit over the bank of BL tubes is a huge
improvement in performance, even though some persons might consider
the operational trade-offs (extra wattage, heat) as significant.
However, since your opinion as stated is based primarily on the speed
comparison I think we should explore a bit more that aspect of the
problem.
First, my comments about relative speed of the two units were based
on testing Stouffer TP 4X5 step wedges, with the speed point
determined, by convention, as the step wedge that gives the first
maximum black. In those circumstances, with the BL tubes set at 4"
above the printing frame and the metal halide unit at 20" and
printing with Vandyke, the first maximum black was obtained with the
BL unit at 8 minutes, the first maximum black with the metal halide
unit at 45 seconds. Tests with carbon and traditional kallitype show
a similar gain in speed for the metal halide unit, though these
processes are about twice as fast as Vandyke.
Now, unless something is wrong with my BL bulbs , which I doubt, the
absolute minimum exposure time to reach the first maximum black of
the process is 8 minutes for Vandyke and 4 minutes for carbon and
traditional kallitype, with the BL exposing unit. This observation
is not based on a comparison of *my* negatives to *your* negatives,
but on printing with a step wedge the qualities of which are known
within fairly narrow parameters.
The assumption that your typical printing times are consistent with
those of others (45 seconds for digital negatives, 1.5 minutes for
silver gelatin negatives) may or may not be generally true. Maybe we
will hear from others on what are typical exposure times.
In my case it is definitely at odds with reality as my typical
exposure times (with carbon printing) range from a minimum of about
15 minutes (for negatives with a Dmin of about .20) to an hour or
more for negatives with a Dmin of .45-60. The two stop increase in
speed provided by the metal halide unit effectively allows me to
complete exposures in the 4-15 minute range as opposed to the 15-60
minutes with the BL unit.
Sandy King
>On Thu, 27 Sep 2001, Sandy King wrote:
>> 1. With the end of the bulb fixed at 20" from the exposing plane the
>> metal halide unit is 3.5 stops faster than my BL unit, with the
>> processes I tested: carbon, Vandyke and traditional Kallitype.
>
>Since my fluorescent blacklight bulbs expose gum from a digital negative
>in 40 to 45 seconds and from a silver gelatin negative in 1-1/2 minutes,
>and I assume others have similar times, I'd say it sounds like the metal
>halide bulb simply introduces more complications -- the on-off wait, the
>light falloff, AND greater heat & eye exposure. Seems to me Sandy you've
>done a public service with this info (especially about the contrast being
>the same -- scotch one more rumor) but, well --- what are your usual
>exposure times?????? Where's the fire ????
>
>PS. If you gave the wattage of the metal halide, I missed it -- but total
>watts of 8 fluorescents (24") is 160... The halide is probably 1000???
>Even with the shorter time, still takes more electricity.
>
>PS. Is it politically OK to say "scotch" a rumor?
>
>Judy
>
>
>>
>> 2. Light fall off at the corners of a 16X20 print is about 1/2 of a
>> stop with the bulb at 20" from the exposing plane. Light fall off at
>> the extreme corner of the 23X29 frame is about 1.5 stops. Keeping the
>> lamp fixed at 20" from the exposing plane it is possible to even out
>> the illumination completely over the 23X29" area of the vacuum frame
>> by placing a round, black center filter of about 6" diameter directly
>> under the center of the lamp at about 12 inches from it. This
>> reduces illumination at the center but the unit is still 2 full
>> stops faster than the BL unit. Some would find it more convenient to
> > simply increase the distance from the lamp to the exposing plane to
>> three or four feet, with about 1-2 stops loss in light compared to
>> the 20' distance.
>>
>> 3. The metal halide lamp takes about 2 minutes to reach full output,
>> and if you switch it if off it is necessary to wait about 5 minutes
>> before turning it back on.
>>
>> 4. The metal halide unit produces a lot of heat, and I mean *mucho*.
>> Use of a cooling fan is necessary.
>>
>> 4. Image contrast is approximately the same with BL tubes and the
>> metal halide unit, with the processes I tested.
>>
>> I think this kind of exposing unit deserves some consideration by
>> anyone looking for a good UV exposing unit. It is quite a bit faster
>> than a bank of BL tubes, costs about the same (or perhaps less) than
>> to assemble a bank of BL lights, and comes basically ready to go in
>> that all you need to do to use the unit is connect the wires to an
>> extension cord and either hang it or place it on some solid support.
>>
>> Sandy King
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> >I forgot to say yesterday that P-F #6 has an article on "Light Carpentry"
>> >(& I admit to some pride in the title): four UV light systems with
>> >instructions to build, most requiring only light carpentry skills (such as
>> >mine own).
>> >
>> >Jarek Mirkowicz gets the romance prize: "My first experience with the
>> >beauty parlor lamp was preparing gelatin matrixes for underground
>> >bulletins against the communist regime in my country" (Poland). His
>> >current design folds up compactly for storage.
>> >
>> >Nick Makris made his from off-the-shelf parts, except for the bulbs,
>> >which he jiggered for stronger light for platinum.
>> >
>> >I show my first REALLY basic system as a thumbnail & the new improved
>> >"reversible" version in photographs with glass in "open" and "closed"
>> >position.
>> >
>> >Bob Schramm's brainstorm is a set of mercury vapor arc lights hung high
>> >enough on the garage ceiling to cover an area 6 feet square (tho exposure
>> >is hours). Bob, nuclear physicist, explains difference bet. sodium &
>> >mercury vapor bulbs -- both are yard lights, but one works for our
>> >purposes, the other doesn't.
>> >
>> >And, speaking of CONTACT FRAMES: last year I sold my supersized $224
>> >custom built contact frame -- on this list as a matter of fact. It was a
>> >thing of beauty (Great Basin) but NOT as practical & easy to use (at least
>> >for a weak woman) as a simple plate glass sandwich, which lets really
>> >large paper stick out the ends, and doesn't require the MUSCLE those
>> >heavy-duty springs do. Contact, especially with a weight on top, is at
>> >least as good, maybe better.
>> >
>> >Don Bryant's customized version of the Edwards construction plans
>> >(he improved design of ventilation slots, etc.) is in Issue #5. He also
>> >covered it with formica, if you could believe.
>> >
>> >cheers,
>> >
>> >Judy
>> >
>> >.................................................................
>> >| Judy Seigel, Editor >
>> >| World Journal of Post-Factory Photography > "HOW-TO and WHY"
>> >| info@post-factory.org >
>> >| <http://rmp.opusis.com/postfactory/postfactory.html>
>> >.................................................................
>>
>>
>> --
>>
--
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