we will cover the actual chemical processes, and the monitoring of those
processes, used to produce prints from color negative films. They include
the C-41 film process and the RA-4/EP-2 paper processes. industry.
Color Negative Film and Paper Chemical Processes
the color negative film process (C-41, C-41B & C-41RA) and the color
paper process (RA-4 & EP-2) employ several distinct chemical steps
to produce the negative or positive images contained within the emulsion
of the film or paper after exposure and processing. All conventional color
processes used to produce prints from negative films require four or five
processes combine one or more of these stages with others in the process.
For example, several of the minilab film processor models have combined
the wash and dye stabilization into a single step or bath and in the RA-4
and EP-2 processes, the bleaching and fixing stages are combined as a
single chemical bath .
and paper emulsions and the film and paper processes are very complex.
This complexity requires that the two processes be carefully monitored.
Process Monitoring is one of the most critical procedures conducted in
the operation of a photo processing lab. Product Quality is measured,
evaluated and controlled through this procedure. The successful lab will
place process monitoring as a first priority. Good process monitoring
is the only mechanism by which a lab can produce a product of good quality
with a minimum of waste and cost. Good and consistent product quality
means good business. It represents a professional attitude towards the
customer. It yields satisfied customers who receive good results from
their exposed rolls of film which in turn guarantees new and repeat business
thereby insuring the continued success of the lab.
typical lab has at its disposal several tools which are utilized to monitor
a process. They include process control strips, which are pre-exposed
strips run daily in the film and paper processes. A densitometer is used
to read, and in some cases record results from the control strips. In
addition, published information on the processes themselves along with
troubleshooting manuals and other related information are also typically
employed to monitor and maintain a process. These tools are used to match
the process to a standard and minimize any deviations 'Tom that standard.
Process deviations can result whenever process recommendations are not
followed. Although process deviations are not uncommon, the goal is to
minimize these deviations and operate the process within an acceptable
range from standard.
accomplish this, each of the individual chemical solutions (i.e. C-41
developer) must be operated and maintained within predetermined specifications.
These are typically published by the manufacturer of the film, paper,
chemicals or equipment. (The exact specifications utilized in any lab
will depend on the individual film, paper, chemical and equipment mix
for that lab.) Those specifications include among others; processing times
for each step, operating temperatures for the solutions, solution agitation
specifications, solution replenishment rates and chemical mixing recommendations.
These are commonly referred to as Processing Factors. Process monitoring
is simply a procedure by which these factors are monitored and adjusted
as necessary to keep the process running within specifications.
A. Replenishment Rates
B. Chemical Mixing
produce a top quality product, a sound understanding of processing factors
and their effects on film or paper is essential to the lab operator. However,
to fully understand how each of the processing factors affects film or
paper, it is first necessary to have a basic understanding of film and
paper composition and construction, and an understanding of the function
of each of the individual chemical steps in the processes.
Color photographic films are made up of essentially two primary
components: a transparent base and a multi-layer, gelatin-silver
halide emulsion structure. The base is typically a cellulose acetate or
polyester material which provides a support structure for the emulsion.
The emulsion consists of several different materials or compounds that are
applied to the base in individual layers during the manufacturing process
of the film.
Each of the individual layers in the emulsion has a specific purpose and
make-up. The most important of these layers are the light sensitive
dye producing layers. Within these layers are suspended light sensitive,
silver halide (silver bromide, silver chloride or silver iodide) crystals
or grains and color couplers (which are transformed into colored dyes at
development). There are at least three light sensitive layers, one
sensitive to each of the three primary colors of light; red, green and
blue. In modem films, each of these light sensitive layers is actually
made of multiple layers each sensitive to varying degrees of light
intensity. This improves the tonal reproduction, grain structure and
sharpness of the films. In these layers, through exposure and processing,
the actual dyes, cyan, magenta and yellow, are formed to make up the
actual negative image.
The green and red layers, by their nature, are also sensitive to blue
light. Immediately under the blue sensitive layer is a yellow filter layer.
This yellow filter absorbs all the blue light so it does not reach the
green and red layers. There are also several layers called intermediate
layers, which primarily prevent the other layers from intermixing or
reacting with each other.
Next to the base is an antihalation layer. This layer absorbs any light
that passes through the emulsion and prevents it from entering into the
base and then reflecting back into the emulsion which would create a halo
pattern around high intensity (density) images in the film.
the surface of the film is a top coat. It is a protective layer
that prevents damage to the emulsion that could occur during camera
loading, processing and printing.
The structure of photographic paper is very similar to photographic films.
However there are a few key differences. Each of the red, green and blue
light sensitive layers are generally composed of a single layer rather
than multiple layers as in film emulsions. The position of the red and
blue light sensitive layers (cyan and yellow dye layers) are reversed and
paper does not contain a yellow filter layer.
base is, in most cases, a paper material that has a resin coating.
The resin coating prevents the paper from absorbing any of the chemical
solutions during processing. The base typically will have an anti-static
layer coated on the back side. It prevents static electricity from
building up and discharging on the surface of the paper as it travels
through the printer.
Due to the complex, multi-layered emulsions of film and paper, and
the sensitivity of the emulsions to chemical solution variances, each of
the processing factors plays a critical role in proper development and
further processing of these two light sensitive medians. Proper control of
each of the processing factors for each of the chemical solutions is the
key to good quality processing. Let us now look at each of the individual
chemical solutions and the role they play in film and paper processing.
A. Color Negative and Paper Developers
The color negative film and color paper processes have as their first
step a developer. The developer is the chemical step that is responsible
for the formation of the actual image in the film or paper. The developer
contains many different chemical compounds, each with its own function
However, the primary compound in the developer is the color developing
agent (also referred to as a reducing agent).
When the silver halide grains in a light sensitive material are exposed
to light, a potential for an image is formed. This potential image is
called a latent image. During processing, the color developing agent
converts the exposed silver halide grains to metallic silver through a
chemical reaction called reduction. During this reaction 'the developing
agent becomes oxidized. The oxidized developing agent then reacts with the
color couplers in the emulsion to form a dye image. A cyan colored dye is
formed in the red sensitive layer, a magenta dye in the green sensitive
layer and a yellow dye in the blue sensitive layer.
These steps can be represented by the following:
Silver Halide + Color Developing Agent
Silver + Halide + Oxidized Developing Agent
Color Developing Agent + Color Coupler - Dye Image
B. Color Negative Bleach
Bleach is the second chemical step in the color negative film process.
The bleach solution first stops the development process through a change
in pH and the dilution of the remaining developer solution contained in
the emulsion. The pH of the developer is approximately 10 (alkaline) and
the pH of the bleach is approximately 5 (acidic). Developing agents must
be alkaline to be active.
The bleach has as its primary function the task of converting the
developed metallic silver in the film to a nonmetallic silver compound
which can then be removed from the film by the fixer leaving only the dye
image. (In some films, the yellow filter and antihalation layers are also
silver compounds that the bleach also converts to nonmetallic silver
compounds.) The compound that performs this function is referred to as the
bleaching agent or oxidizing agent. During this process the metallic
silver takes on a halide ion and becomes a nonmetallic compound, and the
bleaching agent is reduced from an active form to an inactive form. To
maintain a proper balance of active bleaching agents, air is usually
bubbled through the bleach during the process. The air converts the
inactive bleach to an active form.
steps can be represented by the following:
Silver + Halide Ion + Bleaching Agent
Halide + Reduced Bleaching Agent
Reduced Bleaching Agent + Air Active
C. Color Negative Fixer
The fixer converts the unexposed silver halides and the bleach converted
silver halides in the film to complex soluble silver salts which then
dissolve out of the film into the fixing solution. This silver is later
removed from the fixing solutions through various silver recovery
D. Color Paper Bleach-Fix
The color paper bleach-fix is essentially a combination of the film
bleach and fixer into a single solution. This combined solution performs
the same functions in the paper process as the separate solutions in the
E. Color Film and Paper Wash and Washless Stabilizers
In the emulsions of both film and paper that have been processed, there
still remains residual quantities of bleach-fixes, fixers, soluble
silver complexes and other processing chemicals. These must be removed
from the emulsion prior to drying to prevent image staining and fading.
This is done through diffusion of water into the emulsion which removes
these compounds. In the past, this was simply done with water. However,
due to increased water costs and conservation efforts, new "washless"
stabilizers were developed to enable photographic films and papers to be
washed with reduced quantities of water. Most small photofinishing
operations employ these washless stabilizers in place of water washes.
F. Color Film Stabilizer
In most of the current processing systems in smaller labs, this step is
combined with the washing step in the process. However in larger labs the
film passes through a final stabilizer solution following the wash prior
to the film dryer. In both systems, the stabilizer compounds perform
several functions The stabilizer contains an agent that helps stabilize
the dyes in the film. This prevents the dyes from fading or changing color
over time. The stabilizer contains a compound which hardens the emulsion
protecting it from scratches during drying and further handling. It also
contains a wetting agent that allows for uniform, spot free drying. In
those systems in which the wash and stabilizer are combined, the
stabilizer contains an agent that prevents algae growth in the processing
tank. Algae will cause a stain in the film emulsion and can attack the