IN PERSUIT OF A LOW CONTRAST HIGH-SPEED DEVELOPER
FOR REWIND PROCESSING OF AERIAL FILM
by
R.W. Graham - Polytechnic of Central London (PCL)
M. Hassan - Polytechnic of Central London (PCL) .- UNITED KINGDOM
R.W. Lorenz - International Institute for Aerial Survey and Earth Sciences (ITC)
THE NETHERLANDS
Introduction
here t —
There can be no doubt that Rewind-Spool-Tank-Processing (RWST) is not an ideal method for processing
rolls of aerial film and particularly films exposed for survey. Indead it is quite remarkable that the
RWST processing method works at all! For most of the time that the film is in the developer any image
can only expect to "see" fresh developer solution for something like only one percent of the total
development time. For approximately 99% of the time, each image point has to sit in a tightly encased
environment soaked with a rapidly overworked developer - a worse condition could hardly be imagined in
terms of chemical processing, bat a condition nevertheless enforced by economic and physical factors.
what little work has been done on RWST has been thorough, analytical and consistent (1)(2)(3) and should
be referred to by all who have an active interestin the process. The first reference is to a paper by
Lloyd Walker and it is often to this paper that the critics of RWST refer since it tends to highlight the
known weaknesses of the method, nevertheless, in Worton's paper (2) it is made clear that a number of
different approaches to RWST technique can be made and he refers to an earlier paper with Smith (4) and
their suggestions for optimising the process. Thomson's paper (3) is a good reference to the problems
associated with RWST imagery and the authors of this work would seek no argument with his findings,
indeed it is interesting to note that with respect to the well known problem concerning loss of contrast
at the end of the RWST processed film, Walker quotes such losses at values of between 22% and 27%,
Thomson suggests 22% from middle of film to each end, and Graham (from two 76 meter length trials at
ITC, using Kodak Double X with DK50, and D76) a contrast of 1.3 starting from a step-tablet at 2 meter
from end to 10 meter into the length of the roll. The equivalent contrast was then seen to remain stable
for the entire length up to the last ten meters at the other end of the film roll. Since a contrast
increase to centre of factor 1.3 is the same as saying a contrast loss of 23% towards the ends it is
obvious that there is full agreement on this RWST characteristic as measured by three independent
sources. Similarly, Thomson's report and measurement of "Bromide-streamers" is equally substantiated
by numerous examples taken from ITC training missions, prticularly from flights recording river and canal
traffic at scales of about 1:10 000 where ships and barges create strong highlights against a low-density
ground of water, resulting in streamer density decrements of the order of 0.3 over distances as large as
two centimeters. However, although sufficient work has been done to establish the problems of RWST
processing there has been little, if any, work related to chemical formulations which could counter the
problems of inconsistent contrast, Bromide-streamers, and above all - "bar-marks". The work explained in
this paper is the result of pilot trials aimed at establishing a developer idealised for the RWST system
and it is a joint exercise between the School of Scientific Photography, Polytechnic of Central London
(PCL) and the ITC Enschede.
The Required Developer
The ideal developer for RWST processing should be capable of replacing any and all of those developers
now in use but with the added advantage that the ideal rewind developer (IRD) would provide the following:
(1) Maximum possible film speed.
(2) Low-contrast processing as required.
(3) High-contrast processing as required.
(4) Freedom from well known RWST artifacts. (Bar-marks, streamers, etc.).
(5) Low fog-level.
(6) The best possible -image quality for a given speed and contrast.
(7) Have good working qualities. (i.e., be useful with a number of different types of film and to retain
activity over its operational life time).
(8) A formula that is easy to prepare with chemicals that are easily obtained.
Item (4) was considered to be the obvious start for an IRD and in order to come to terms with the physical/
chemical aspects of the problem is was thought that only a two-bath developer would be the answer. As a
consequence chemical formulation was established as follows: (a) a first bath in a low pH, low energy,
developer that would minimise the early establishment of physico-chemical defects, followed by (b) a
second bath in a reasonably energetic developer operating at a moderate-to-high pH level.
After two different types of formulation for (a) and (b) where experiments were carried out with 70mm
roll film in rewind-simulated conditions (Hassan),-changes in the relative concentrations of Metol
and Hydroquinone, and absolute strength of the remaining chemical components were established only
after numerous trials with 70mm stock and subsequent sensitometric analysis. The third basic formulation
was then established IRD-3, and a full test with 76 meters of 24 centimeter wide aerial film made with
the Zeiss FE 120 Rewind-Spool-Tank-Unit. This test consisted of a full 76 meters of calibrated step-
tablets down the entire length of the roll (Agfa-Gevaert Aviphot Pan 33) accompanied by three-bar
resolution targets (USAF type) of both high (100:1) and low contrast (1.6:1) groups. Both, step-tablets
and resolution targets were exposed onto the film via the EG&G flash sensitometer Mk.VI, and care was
taken to ensure that the exposure levels of the targets were not too high and that their contact with
the film was as perfect as possible.
203