From: Sandy King (sanking@clemson.edu)
Date: 09/29/01-09:43:42 AM Z
Judy Seigel wrote:
>
>Sandy, I defer to you instantly in the matter of the *math* involved, but
>still suppose the tradeoffs for the halide bulb would be a matter of
>personal preference... I don't think an 8 minute exposure is a big deal
>(that's what my denser negs took in cyano & VDB is similar) but certainly
>an hour-long exposure would be a drag.
4-8 minute exposures are very reasonable. However, as you can see
from my tests with the Stouffer step wedge, either 4 or 8 minutes (4
for carbon and kallitype. 8 for VDB) ) would be absolutely the
shortest times possible with my BL unit for the thinnest of all
possible negatives. (In other words, it takes an exposure of 4 or 8
minutes to reach maximum black on the first step of the step wedge,
which has a density of approximately 0.07.
>
>As I recall, my bulbs are about 2-3 inches from the paper stage, that
>extra inch does I think make a difference. They're also ranked as closely
>together as the double-bulb fixtures permit, which also adds speed. The
>only other thing I can think of is that sometimes the bulbs get a coating
>of dust/grime, whatever-- perhaps mine moreso since they face up, but I'd
>suspect that even in your clean-air zone, there's stuff in the air which
>does deposit on glass from year to year.
The extra inch may make a difference, or it may not. There was some
discussion a while back on the question of spacing of the bulbs from
the paper stage and someone mentioned that the inverse square law
does not apply to diffuse light sources like BL tubes. In fact I did
some tests early on with my unit, spacing the bulbs at 2", 4" and 6"
and there was virtually no difference in speed.
The issue of a coating of dust/grime on the bulbs never crossed my
mind. Something else to worry about!!
>
>But I'm curious on one point -- which you may have mentioned, if so,
>please excuse: how big a print can you make in the cone of even light at
>20 inches?
OK, remember that one can use the lamp in one of two ways, either
with the round center filter that evens out the light, or without.
1. lamp used with no filter
a. With the lamp at 20" from the paper there is 1/2 stop of light
fall-off at the corners of a 16X20 print, and 1 1/2 stops fall-off at
the corners of my 23X29 vacuum frame. At 20" you get maximum gain in
speed, about 3 1/2 stops as compared to the BL units.s
b. If the lamp is moved to 30" from the paper stage light fall-off is
reduced to about 1/2 stop at the corners of the 23X29" frame. At 30"
the metal halide unit is about 2 1/2 stops faster than the BL unit.
c. If the lamp is moved to 40" from the paper stage light fall-off is
reduced to less than 1/4 stp at the coners of the 23X29" frame. At
40" the MH unit is 1 1/2 stops faster than the BL unit.
2. lamp used with center filter.
By using a center filter it is possible to even the light over the
entire area of the 23X29" frame, with the lamp at 20" from the paper
stage. With these conditions the MH unit is 2 stops faster than my BL
unit.
As noted, there are some operating disadvantages with the metal
halide unit compared to a BL unit: the MH unit creates a lot of heat,
draws more current, and takes about 2 minutes to reach full output,
and of course if you turn it off it can not be turned back on for
about 5 minutes. I use the MH unit with a light integrator which
minimizes most of these issues and of course gives very consistent
exposures.
Sandy King
>
>Judy
>
>
>> 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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