PHOTOGRAPHIC CHEMICALS
=General Considerations.=—Developing, fixing and other chemicals for
aerial work differ in no essential respect from those used in ordinary
photography. Full discussions of these are to be found in numerous texts
and articles. The aerial photographic problem is to select those most
suited for the under-exposed flat negatives characteristic of
photographs from the air. At the same time selection from among the
chemicals of appropriate quality must be governed by considerations of
the conditions surrounding work in aerial photographic laboratories.
These laboratories, especially in war-time, are apt to be most primitive
in their facilities.
=Characteristics of Developers for Plates and Films.=—From the
standpoint of practicability, aerial negative developers should have
good keeping power, be slow to exhaust, and work well over a
considerable range of temperatures. From the standpoint of the
photographic quality desired in the negative, the developer should bring
up the maximum amount of under-exposed detail. This means that it should
impart the highest possible speed to the plate, with good contrast, and
low fog or general reduction of unexposed silver bromide.
There are many characteristics to study in a developer: its effect on
inertia or speed, gamma infinity, fog, time of appearance, “Watkins
factor,” speed of development, temperature coefficient, dilution
coefficient, keeping power, exhaustion, length of rinsing, stain, color
coefficient and resolving power. These are defined and described as
follows:
_Effect on inertia._ The meaning of inertia has already been given under
the discussion of plate speed. While this is a constant, independent of
time of development, for any one developer, it is altered appreciably by
change of the latter.
_Time-gamma relation._ Contrast, symbolized by γ, has likewise been
discussed under plate sensitometry. Viewed from the standpoint of the
developer, the point of interest is the rate at which γ varies with
development, and the maximum contrast which can be reached or γ
infinity. Speed of development is commonly defined by the _velocity
constant_, symbolized by κ, which is arrived at mathematically from a
consideration of the time of development to produce two different
contrast values. High γ infinity is desired for aerial negatives, and
for rapid work κ must also be high.
_Fog._ The opacity due to chemical fog is to be kept at a minimum in
aerial negatives, as it is chiefly prejudicial to under exposures.
_Time of appearance and Watkins factor._ The time of appearance is
measured in seconds. The Watkins factor is a practical measure of the
speed of development, and is determined by the ratio of the time of
development required for a definite contrast, to the time of appearance.
It is useful also as a guide to development time.
_Temperature coefficient._ This is the factor by which the time of
development at normal temperature (20 Cent.) must be increased or
decreased in order to obtain the same quality negative, for a change of
seven degrees either side of normal.
_Temperature limits_ are the temperatures between which development can
be carried out with any degree of control or without serious damage to
the negative. These factors are of great importance where climatic or
seasonal changes have to be endured.
_Dilution coefficient._ This is the factor by which the development time
is increased in order to maintain a given quality negative in different
dilutions of the developer. It is useful in tank development.
_Keeping power._ The keeping power of a developer, mixed ready for use,
is determined by its ability to resist aerial oxidation. A developer of
poor keeping power, which must be made up immediately before use, causes
delay and waste of time whenever emergency work has to be done, whereas
a developer of good keeping power may be left in its tank ready for
instant use.
_Exhaustion_ of a developer is the rate at which it becomes useless for
developing, due both to aerial oxidation and to the using up of its
reducing power by the work done in developing plates. It is conveniently
measured by the area of plate surface developable before the solution
must be renewed.
_Length of rinsing._ The time required for rinsing between development
and fixing bath plays a not unimportant part in total development time.
Dichroic fog is caused with some developers if, due to insufficient
rinsing, any of the caustic alkali is carried over to the fixing bath.
Stains develop also if the fixing bath is old, or if light falls on the
unfixed plate while any developer remains in the film.
_Color coefficient._ The function of the sulphite, which forms a
constituent of all developing solutions, is two-fold. It acts partly as
a preservative, and partly to prevent the occurrence of a yellow color
in the deposit. The yellow color, if present, increases the photographic
contrast. This phenomenon has been purposely utilized, particularly in
the British service, to give “stain” to negatives which otherwise would
show insufficient printing density. The color index or coefficient of a
negative (with a given printing medium) is the ratio of photographic to
visual density. If we take a pyro developer containing five parts of
pyro per thousand and ten parts of sodium carbonate, and then vary the
amount of sulphite from none to fifty parts per thousand, the color
index varies as follows:
Sulphite Color Index
Parts per Thousand
50 1.16
25 1.24
15 1.30
10 1.45
5 1.80
0 2.75
The color index is somewhat different with various kinds of printing
media.
This staining effect is a variable one, depending upon length of
development, dilution of the developer, length of rinsing, temperature,
the fixing bath used (plain hypo being necessary for a maximum effect),
the length of washing after fixation and the properties of the water
used. Standardization of these conditions in the field is difficult;
hence any developer which will give the same effective contrast without
resorting to stain is to be preferred.
_Resolving power._ Some developing processes and conditions will
introduce bad grain into the negative. Hence the resolving power which a
developer brings up must be investigated among its other
characteristics.
=Practical Developers for Aerial Negatives.=—In the English service a
pyro metol developer was generally used, producing stained negatives.
The French, American and Italian practice was to use metol-hydrochinon,
without staining. A special chlor-hydrochinon developer, worked out by
the Eastman Research Laboratory for the United States Air Service, has
probably the greatest merit of any yet tried. A comparison, given below,
between it and a pyro metol formula used on a representative plate,
illustrates the use of the various bases of study given above.
PYRO FORMULA
Solution A Solution B
Pyro, 3.75 grams Sodium carbonate, 53 g
Potassium metabisulphite, 3.75 g
Metol, 3.05 g
Potassium bromide, 1.5 g
Water, 500 c.c. Water, 500 c.c.
Use 1 part of A to 1 of B
CHLORHYDROCHINON FORMULA
Solution A Solution B
Chlorhydrochinon, 25 g Sodium carbonate, 30 g
Metol, 6 g Sodium hydrate, 10 g
Sodium bisulphite, 2.5 g Potassium bromide, 3 g
Sodium sulphite, 25 g
Water to 670 c.c. Water to 330 c.c.
Use 2 parts of A to 1 of B
Pyro Chlorhydrochinon
H & D speed 150 180
Gamma infinity 1.45 2.12
Fog (at maximum gamma) .32 .60
Time of appearance 5 seconds 5 seconds
Watkins factor 25 10
Velocity factor “κ” .320 .400
Temperature coefficient 1.40 2.0
Temperature limits 4° to 32° C 4° to 32° C
Keeping power 45 minutes 8 days
Exhaustion (100 c.c.) 30 sq. in. 300 sq. inches
Dilution coefficient 2 2
Color coefficient 1.50 1.00
Resolving power 47 53
Owing to the difficulty of securing pure chlor-hydrochinon a metol
hydrochinon of very similar properties has been worked out. Its
composition is
Metol 16 grams
Hydrochinon 16 grams
Sodium sulphite 60 grams
Sodium hydroxide 10 grams
Potassium bromide 10 grams
Water to 1 litre
To keep the ingredients in solution in cold weather, 50 c.c. of alcohol
should be included in every litre of solution. All things considered
this is probably the most practical and satisfactory developer for
aerial negatives.
=Developers for Papers.=—The following formula has been found very
satisfactory for papers:
Metol .9 gram
Hydrochinon 3.6 gram
Sodium carbonate 20.0 gram
Sodium sulphite 14.0 gram
Potassium bromide .5 to 1.0 gram
Water to 1 litre
=Fixing Baths.=—For plates the following fixing and hardening bath is
recommended:
Sodium thiosulphate (hypo) 350 grams
Potassium chrome alum 6 grams
Sodium bisulphite 10 grams
Water to 1000 c.c.
During hot weather, the above quantities of chrome alum and bisulphite
are doubled.
For papers the following:
Hypo, 35 per cent. 100 volumes
Acid hardener 5 volumes
The acid hardener is constituted as follows:
Alum 50 grams
Acid acetic 28° 400 c.c.
Sodium sulphite 100 grams
Water to 1 litre
=Intensification and Reduction.=—These processes have been little
employed in air work. Reduction is rarely necessary, for obvious
reasons. Intensification would often be of value, but the common
practice, which saves some time, is to use printing paper of strong
contrast for those negatives which are deficient in density and
contrast. When intensification is desirable or permissible, either the
ordinary mercury or uranium intensifier may be used.
=Water.=—In the field it is found necessary in many cases to purify the
water that is to be used in mixing up chemicals. Water may contain
suspended matter or dirt, dissolved salts, and slime. It is important to
remove the suspended matter, as it may cause spots on the plates and
papers, while any slime would coagulate, forming a sludge in the
developer which would also tend to settle on the plates and cause marks
during development. The dissolved salts may or may not cause trouble.
Two methods of purification are possible:
(_a_) Filter the water through a cloth into a barrel, add about one gram
of alum for every four litres of water, and allow to settle over night.
Draw off the clear liquid from a plug in the side as required.
(_b_) Boil the water and allow it to cool over night. If the water
contains dissolved lime, boiling will often cause this to come out of
solution.
V
METHODS OF HANDLING PLATES, FILMS AND PAPERS