David W. Brooks. ~. Richard M. Hedges, and Dan 1. Philen Texas A & M University ~
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College Station 77843
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Some New Light on Blatk and White Slider
During any given academic year we are called upon to produce from 300-2000 2-in. slides. Our needs for slides vary from the simple graduate student seminar presentation to the glossy multiprojector synchronized slide-tape talk. Regardless of the intended use, there is always a need for the simple black-on-white slide to present simple statements, formulas, graphs, etc. When convenient, such slides may he prepared using direct positive color film. While this is a relatively inexpensive route ($0.144/slide), it requires long delays in commercial processing or tedious do-it-yourself color film development. Also, whites are not pure white hut an off shade of blue or green. Negative slides (white-on-hlack) are easy to prepare using Kodak high contrast copy film on Kodalith films. While these have their place in audiovisual work, they can be overdone. Black-on-white slides can be achieved by using negative slides as subjects for a second exposure with B&W film, or using a reversal process film. Simple direct process black-on-white slide films will find extensive application. Recently, an article by Robert Speck1 called our attention to a little known and somewhat difficult to obtain product, Kodak high speed duplicating film 2575. We have experimented with this film, and we commend it to our readers as an excellent all around black-on-white slide medium for educational work. Kodak requires a minimum order for the film which is tw large an amount for most educational organizations to handle. Therefore, a supplier must be found that will provide this material in usable amounts. State Photo Supply, 226 N. Allen Street, Alhany, New York has recently supplied us with 35-mm X 150-ft rolls for $16 per roll (minimum order: two rolls). The film is easily handled in a dark room using a type OA safe light, and convenient lengths may be cut and spooled into reusable cartridges. Exposure of the film to achieve high contrast is a t f8-9.5 for 5 sec under illumination of four 300-W photoflood lamps placed 15 in. from the illuminated subject. (Yes, that is a lot of photons!) Ordinary pica typewriting on heavy white paper provides excellent copy for alphameric information when photographed using a macro lens. Once exposed, the film is removed from the cassette under safety light and placed on a reel. I t is developed for 60 sec in 1:2 Dektol a t 70°F. although these conditions do not seem to he extremely critical. Development is then stopped with dilute acetic acid, and the film is fixed using ordinary fixer for 3 min. The film is then washed, dried, cut, and mounted. Actual darkroom time, excluding drying, is less than 20 min. Continuous tone copies of blaek-&-white materials of lower contrast may he obtained by "flashing" the film as described by Speck. The film cost is less than $0.03/slide as compared to
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/ Journal of Chemical Education
$0.30/slide or more for a Polaroid slide. The cost of developing chemicals is trivial, being less than $0.01 per slide. The mounting costs are less than $0.04, assuming some plastic slide mount is used. A total cost of $0.08 per slide is really very reasonable. [Our university photo aids lab charges $1 per black-on-white slide produced by a double Kodalith film technique.] The critical cost factor is usually a time factor. Assuming you have got to have slides "right away-seminar hegins in 2 hr," how does this medium compare with Polaroid slides? We figure that we break even on time when ten or more slides are to he made. Of course, if money is your sole object, you can run 30 graduate student seminars with 30 slides each using less than $16 worth of film. This is a feat with which no-alternate process can easily compete. The photochemistry of this film is rather unique and is worthy of special comment. As purchased, the film is already fully exposed. If developed without further exposure, the film is extremely opaque. An exposure plot made for most black and white films (log density plotted versus exposure time) usually shows two inflection regions. The first shows an almost linear increase in density with exposure time which eventually levels out. At very long exposure times, a decrease in developed film density versus time is observed. Since Kodak 2575 film is already fully exposed for maximum density, further exposure is described by the second portion of the curve. Exposure of the film causes electrons to he transferred from halide ion to silver ion forming silver atoms and hromine atoms. Normally, silver atoms thusly formed which happened to he a t the surface of a silver halide grain would subsequently act as catalytic centers for the developer (a reducing agent). However, in cases of extremely heavy exposure such as with this film, the chemistry of the bromine hecomes critical. Bromine atoms caneffectively move to the grain surface by "hole" migration. At the surface, bromine atoms can react with surface granules of silver metal coating them with a surface layer of silver bromide. Once coated, the catalytic effect of the silver is eliminated. This process, called solarization, accounts for the inability of the developer to react with the most heavily exposed areas of film.2 This mechanism is consistent with the unusually long 5-sec exposure a t high light levels. We recommend this film to you as one that you will have lots of fun with while getting good results. 'Speck, R., Photo Methods forlndustry, 15.43 (1912). zThe fog observed in clear regions of the exposed developed film is due to the silver metal produced by the rather extensive primary photochemical reaction. This fag, however, does not noticeably interfere with slide projection. ~
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