Usual Sources of Minus Density Artefacts
This document discusses the usual sources of minus density
artifacts. Minus density artifacts cover a broad range of "defects"
seen on films; the common denominator of these types of artifacts is
that they are lighter in optical density than the area surrounding
Although not limited to appearing on single-emulsion films, minus
density film artifacts are often detected more frequently on such
films. Single-emulsion films include those for mammography, video
recording, laser printers, etc. Because there is not a second
emulsion on the opposite side of the film support or base to "hide"
shadow images, processor scratches, etc., the visibility of minus
density artifacts on single-emulsion films is inherently greater
than with double-emulsion films. For best results, it is important
to remember that many different variables must be controlled:
darkroom construction materials, processor maintenance, air
ventilation systems, housekeeping/cleaning procedures and frequency,
film box storage, intensifying screen cleanliness, and more. This
document will briefly discuss the above as they relate to minus
density artifacts on films.
Discussion in this document regarding the use of Kodak MIN-R
Screen Cleaner and Antistatic Solution pertains only to its use with
KODAK MIN-R Cassettes and screens, etc.
Mishandling of most types of films (with some exceptions, such as
Kodak T-Mat Films for medical radiography) prior to exposure can
result in unwanted minus density artifacts. The fix for such
artifacts is usually straightforward and centers upon the film
handler(s)--radiologic technologist, darkroom personnel, stockroom
personnel, loading dock and delivery personnel. In some cases
film-handling equipment (i.e., chest changers, angiographic rapid
serial film changers, etc.) can cause minus density artifacts if
they are not properly adjusted. Obviously, processor rollers and
other film processor components can cause emulsion scratches and
other emulsion "digs" and these components need to be routinely
cleaned and examined in order to eliminate these sources of minus
density artifacts. It is also possible for minus density artifacts
to be traced to certain film manufacturing and/or packaging stages.
In the majority of cases the single biggest contributor to minus
density artifacts is airborne dust contaminants, lint, and other
debris which find their way into cassettes and onto screens and
film. In some cases, debris which has adhered to an intensifying
screen will produce a minus density artifact with the same shape,
size, and location on the film. If cassettes and screens are
individually numbered for Quality Control test purposes, isolation
of the affected cassettes should not take long.
Proper cleaning of the cassette(s) and screen(s) usually brings
about a cure. However, if the air filtration is not adequate, or if
darkroom "housekeeping" is not stringent, simple cassette and screen
cleaning will offer only a very temporary respite.
Often overlooked is the air filtration/circulation system in the
darkroom. Positive air pressure in the darkroom can help restrict
the influx of airborne particles as people enter and exit this room.
The commonly used "bag filters" at the heating/AC system level may
not provide fine enough filtration for darkrooms where single
emulsion films (e.g., mammography films) and cassettes are handled.
Surgical suites and critical care areas within a hospital probably
have higher air filtration levels, but a particular film darkroom
may not. The efficiency of bag filters can be greatly reduced or
even negated by dirty post-filtration ductwork, poor or deteriorated
gasketing which allows particle "bypass," and other variables.
The use of pre-filters or "pleated" filters ahead of bag filters
is sometimes seen as a measure to prolong bag filter life. Like bag
filters, pre-filters can be had with different filtration efficiency
percentage numbers and varying micron size filtration
specifications. Such filters, depending upon their specifications,
condition, and the condition of ductwork and gaskets may not provide
the level of particle filtration deemed adequate by the film
If even "cleaner" air is desired, other types of air filtration
methods can be employed. Popular approaches include electrostatic
air cleaners and high efficiency particulate air ("HEPA") filters.
Electrostatic filters are good at capturing or "scrubbing" the
comparatively larger "smoke size" particles, but this may not be
sufficient for some medical films and exams. Regular cleaning of
such devices is recommended and this sometimes is neglected, causing
the collection plates to become loaded. In such circumstances, the
filtration efficiency may suffer.
Very high levels of air filtration can be achieved through the
use of a terminal-ducted, HEPA filter. These filters usually post
specifications of 99.9% efficiency in removing particles down to
around 0.3 microns (the diameter of a single human hair is about
75-100 microns). A step below this level in both efficiency and
particulate size filtration would be a minimum rated HEPA filter
without a fan. Filtration efficiency is often quoted in the mid-90%
range and these filters are advertised to be near 100% effective in
removing particles above one micron in size. This may be an
effective measure to employ, as it offers both lower cost and less
pressure drop vs. a HEPA filter + fan, so that other rooms, which
may feed off the same ductwork, are not "starved" for ventilation.
A professional, not-for-profit organization, The Institute of
Environmental Sciences (Illinois; 847-255-1561; FAX # 847-255-1699),
is a good source for "recommended practices and standards" regarding
air filtration methods and testing. This organization also offers
various reports and videotapes on a variety of subjects for a fee.
Subjects include cleanroom design and housekeeping, clothing
recommendations, movement and behavior recommendations in cleanroom
environments, laminar flow clean air devices, microorganism control,
and related issues. Although a number of the above documents involve
"cleanroom" environments which exceed normal photographic and/or
radiographic darkroom environments, there is good information to be
found for air filtration information.
As for darkroom construction, it is recommended that the darkroom
walls and ceiling have solid, smooth surfaces if possible. If
painted, a semi-gloss finish is preferred over a flat finish. This
allows for easier cleaning via wiping and/or vacuuming. If a
suspended ceiling is in place, there are a number of drop-in panels
available which do not "shed" fibers or drop debris. Improper
selection of ceiling tile material and/or improper installation (for
darkroom conditions) can be a significant source of airborne debris
continually introduced into the darkroom, which can reduce the
effectiveness or even defeat other housekeeping efforts such as air
filtration, screen cleaning, etc.
If a conventional door is fitted to the darkroom, the vibrations
and possibly air pressure changes produced by opening and closing
the door many times a day can result in unnoticed movement of the
ceiling tiles. Such movement can cause ceiling tile particles to
dislodge and fall. To help relieve changes in air pressure caused by
opening and closing the door, installation of a lightproof vent in
the door or in a wall of the light lock may be considered. Revolving
darkroom doors or "barrel doors" alleviate the air pressure transfer
somewhat, but these can transmit vibrations into the walls and
ceiling structure too. If special ceiling tiles cannot be used, some
effort to seal the tile edges and surfaces (via painting with a
semi-gloss paint, polyurethane, or other means) should be
considered. The tiles should also be checked for movement within the
suspended grid and this movement should be minimized as much as
possible, taking into consideration ceiling access needs and
especially local building and fire codes, any of which may prohibit
certain measures to "lock" or otherwise adhere ceiling panels into
Continuing with darkroom design considerations, the floor beneath
benches, sinks, and equipment should be accessible for cleaning as
much as possible. Storage facilities below the countertops should be
closed cabinets. Shelving should not be open. Dust-holding ledges
and overhead surfaces that are too high for easy cleaning should be
minimized. Any storage above the countertops should be closed
cabinets that preferably reach all the way to the ceiling.
Non-essential items such as newspapers, magazines, facial tissues,
notebooks, writing pads, etc., should not be kept in the darkroom
loading area where they could attract or create dust and lint.
Darkroom countertops and the processor feed tray should be wiped
with a damp, lint-free damp sponge or cloth a number of times daily
to remove settled dust and lint. A chamois cloth, available at most
auto parts stores and discount stores, often works well.
Be sure to use an appropriate solution that will
not leave a flaky residue after drying to moisten cleaning sponges,
mops, or cloths. Obviously, such solutions should not cause any
adverse effect to films, cassettes, countertops, feed tray, walls,
etc. Floors and open shelves should be damp wiped or mopped at least
daily. Often it is advisable to damp mop at the end of the day so
that any dust settles overnight, before cassettes are reloaded the
next morning. This timing can sometimes have a positive effect on
the incidence of dust/lint artifacts.
The corrugated shipping boxes with boxes of film inside should
not be opened in the darkroom--this can introduce cardboard fibers
into the darkroom. The stiffener boards commonly found in film boxes
should be removed from the boxes and from the darkroom. The same
should be done with the tops of the film boxes. Do not remove the
film from the envelope or store the film loose in the film bin.
Film(s) kept in a film bin should be sufficiently immobilized to
prevent shifting back and forth as the bin drawer is opened and
closed. This can help minimize or eliminate "chucking" abrasions on
films, which are multidirectional scratches, usually grouped
together in approximately 1-2 mm diameter clusters. Cases and boxes
of film should be stored vertically to help prevent (with most
films) minus density marks due to pressure.
The use of a darkroom patient I.D. stamper or printer requires
the handling of many paper I.D. cards in the darkroom, which can
create another source of fibers and debris in very close proximity
usually to films and open cassettes. A daylight-type (i.e.,
non-darkroom) I.D. camera is preferred for not only this reason, but
such a device can reduce the possibility of mis-labeled films, be
faster in operation, and produce a more clear I.D. imprint in most
Selection of garments worn by those working in the darkroom is
important too. To the best extent possible, lint-free clothing is
desirable. If such clothing is not provided or worn, uniform smocks
and other lint-free gear worn over clothing are usually very
helpful. Wool clothing should be avoided, and articles of clothing
should not be hung in the darkroom.
An ultraviolet radiation or "light" device (commonly sold to
check intensifying screens), used with the proper precautions, can
be an excellent aid to help illuminate dust and lint on counter
surfaces, feed trays, walls, etc. Not all dust particles fluoresce,
however. If an ultraviolet light is utilized, it is advisable to
check all the areas of the darkroom before cleaning and again after
cleaning to see the effect of the cleaning efforts. ALWAYS
follow the recommendations of the manufacturer for eye protection
and handling when using such UV devices.
Relative humidity and temperature play a part in the generation
of film artifacts too. A range of 30-50% relative humidity is
prescribed for opened boxes of most types of Kodak medical x-ray
films. Increased humidity reduces the potential for static discharge
and electrostatic attraction of dust and lint to film and screens.
However at higher humidity drying becomes less effective and films
may be tacky on exit from the processor or exhibit more pronounced
Very low humidity can cause respiratory discomfort in people,
static buildup on film and equipment, and curl and brittleness in
films. Static charges on film and cassettes attract dust; they can
also cause static discharge marks (artifacts) on processed images.
Longer-term storage in very low humidity conditions can cause
another type of unwanted film artifact: emulsion cracking.
Temperature recommendations for opened packages of film are 50-75
Cleaning of cassettes and screens can be accomplished using both
wet and dry means.
The use of intensifying screen cleaner and antistatic solution as
recommended by the manufacturer of the screens is highly
recommended. Depending upon individual circumstances, the frequency
of cleaning can vary widely--from weekly to daily to more than once
each day, as necessary. Excessive rubbing of plastic or
polycarbonate cassettes (seen commonly in mammography cassettes)
when cleaning should be avoided as this can possibly increase the
static attraction of dirt, dust, lint, etc.
In addition to the use of screen cleaner, a solution of 70%
isopropyl may be used occasionally when thorough cleaning of KODAK
MIN-R Intensifying Screens is necessary. If this practice is
employed, it is highly recommended that the user follow the alcohol
cleaning with an application of
Carestream Screen Cleaner.
Such antistatic brushes can be used to clean the internal
surfaces and channels of the cassette, and it may be additionally
effective (or recommended by the manufacturer) to hold the opened
cassette either vertically or upside down while using the antistatic
brush to allow gravity to assist in debris removal. Be sure to
follow the brush manufacturer.s directions for handling, use, and
replacement (if applicable) of brush components. Other products such
as canned air and a rubberized roller device with a special
adhesive, which have been used in the graphic industry and in the
photographic trade, have been marketed for screen cleaning too.
This article was
on the Kodak website but I cant find it now! 2006