The use of photography in law enforcement is as old as
photography itself. Moenssens, et
In civil as well as criminal cases, photography provides the most potent
tool in conveying facts to a jury. In criminal trials, photography plays an extremely
important role for the police and prosecutor and can play the same role for the defense.
Invented in 1839 by Daguerre, photography was used as early as 1843 to provide pictures of
arrested persons - what we now call mug shots - in Belgium. These early pictures were on
metal plates, called daguerreotypes.... The use of mug shots to identify individuals has
survived to this day.
At a very early stage, photographs were also used to record scenes of crimes and
accidents, of bodies and wounds, of suspect documents and checks and of other items of
evidence such as murder weapons. As early as 1860, courts were confronted with
photographic enlargements of questioned documents in criminal cases and by 1871 post
modern photographs, photo-micrographs and photomicrographs, and X-rays.
As photographic techniques became more sophisticated, still photographs were
made through a microscope of hairs, fibers, paint chips, tool marks, and other minute
items of trace evidence. The advent of color photography, stereo photography, and infrared
and ultraviolet picture taking, sometimes in conjunction with the use of a microscope,
also permitted the taking of photographs of small details that the human eye could not
Because of the wide uses of photography by the general public, it is probably
the one type of evidence that is best understood by all people, including police officers,
lawyers and judges. Most anybody (sic) has used a simple camera, and a good many people
can operate quite elaborate pieces of photographic equipment. All but the smallest law
enforcement agencies possess photographic laboratories as well as a wide array of
specialized and general-purpose photographic instruments. (Moenssens, Inbau, and Starrs 601)
Original photographic technology is based on a simple physics principle: certain
compounds of silver - silver halides - change their chemical structure when exposed to
light. The silver compounds are mixed with warm liquid gelatin and applied to a flat
surface, exposed to light, and processed chemically. Silver compounds were first applied
to glass plates in the 1800's and later to plastic sheets and strips. These coated strips
are called film and have been used in photo-enforcement for over forty years. Recently,
digital cameras have been developed that do not depend on film for storage of an image,
but rather the image is stored in electronic form. In the following section, we discuss
film, camera orientation and placement, lighting, Lenses, focal length, field of view, and
History of Kodak Roll Film Numbers
The following information on the history of photographic roll film is taken from
W. J.'s Photo home page http://members.aol.com/thombx19/history.html.
This excellent site is focused on IR and UV film and contains many useful links.
Roll Films Starting with 101
It first became necessary to specify which Kodak roll film was required
with the introduction of the No. 2 Kodak camera in 1889. As different models and sizes of
cameras were introduced, the film boxes were marked with the names of the cameras that the
roll would fit.
By 1908, this system had become difficult to use for ordering film. It
was now necessary to specify the image size and the camera the film was to be used in as
not all films for the same size pictures could be used interchangeably. To simplify this
system it was decided that the daylight-loading roll films on flanged spools would be
numbered in the order of introduction, starting with the first Kodak film of this type
introduced with the No. 2 Bullet camera in 1895 as number 101.
This system was gradually phased in as new film boxes and camera
instruction manuals were printed, but the numbers did not appear in Kodak price lists
until 1913. By this time, numbers 101 through 129 were used. Numbers 106 through 114 were
used for films spooled for Cartridge Roll Holders, which allowed roll film to be used with
cameras designed to use glass plates. In 1916 one more number in this series was added:
No. 130 for pictures 2-7/8 by 4-7/8 made with No. 2C Kodak cameras.
Some Kodak and Brownie folding cameras made from 1914 to the 1930's have
a little door on the back which is marked "use Autographic film A-(number)".
A-116 film, for example, was for the same size pictures as 116 film but instead of red and
black duplex paper, the film was wound with a sheet of carbon paper and thin red paper.
This film used in an Autographic Kodak camera allowed a brief message to be written on the
film in the space between the pictures. Pressure of a stylus on the backing paper
transferred the carbon to the red paper and light passing through these lines in the
carbon paper would photograph the message onto the film.
When 620 and 616 films were designed in 1931, considerable thought was
given to the numbering. These films were for the same picture sizes as 120 and 116 but the
spool diameters were smaller to allow them into thinner cameras. The "6" was to
indicate the number of pictures per roll but by the time this product had reached the
market, the decision had been made to increase the number of pictures on this size and on
sizes 120 and 116 to eight exposures so the "6" became meaningless.
In 1935, the Kodak Bantam cameras were introduced. The film for these
cameras provided for eight exposures 28 x 40 mm, and the number 828 was chosen for this
Size 220 was introduced in 1965 and is twice the length of 120 size film
although it uses the same spool. This film has only a paper leader and trailer for light
protection and no paper behind the film. It is used with professional cameras which
advance the film automatically instead of using a window on the back of the camera to
position the film.
In 1916, a very small box camera named the No. 00 Cartridge Premo camera
was introduced using a No.35 roll film. This was numbered differently as it was not the
same as the Eastman Non-Curling film supplied in the other roll film sizes but was
apparently made from unperforated 35mm motion-picture film. In 1934 when 35mm film in
cartridges were introduced with the Kodak Retina camera, number 135 was assigned to this
product. This film size could also be used in the Contax and Leica cameras.
Daylight-loading spools of film for these two cameras were also offered, and were numbered
235 and 435. In July 1952, a special length of film for 20 pairs of pictures made with
35mm stereo cameras was introduced and designated as 335.
In 1963 the Kodak Instamatic cameras were introduced. These used roll
film in cartridges from drop-in film loading. The image size is 28 x 28 mm, but slight
masking is required in printing and slide mounting so the useable image is 26.5 x 26.5 mm.
The number 126 was used, as the original roll film for this size had been discontinued in
For the Kodak Pocket Instamatic cameras introduced in 1972, a number
lower than 126 was preferred, partly to indicate a smaller image than the 126 film. The
number 110 was chosen because it could be said as "one ten" and was easy to
Kodak's latest film number 240 for the Advanced Photo System was
introduced in 1996. Using the trade name ADVANTIX, it is the first film from Kodak to
incorporate traditional silver halide technology and a transparent magnetic recording
medium (IX - Information eXchange) allowing the following information to be recorded and
exchanged with the photofinisher:
Camera Recorded Data (if enabled)
|shutter speed and f/stop|
|date and time|
|scene brightness value|
|exposure bias setting|
|camera orientation (horizontal/vertical)|
|partially exposed roll (for mid-roll change)|
Customer Recorded Data (if camera enabled)
|C, H, or P print format which are typically printed in the following sizes
|C: Classic 4"x6"|
|H: HDTV 4"x7"|
|P: Panoramic 4"x11-1/2"|
|number of prints of a frame to be made on the initial order|
|text for back printing onto the photo|
|"Print This Frame Regardless" indication|
|"Do Not Print This Frame" indication|
Advanced Photo System also allows data recording to be read by the
|order tracking and pricing|
|order time in and time out|
|printer exposure and color data|
|reprint format change|
|customer instructions for density and color|
|lab equipment ID for quality control|
|attention flag for inspection by quality control specialist|
|remake flag (with correction data)|
|statistical information for the lab|
Modern photographic film is still made by applying a coating of light sensitive
chemicals to a plastic base. The amount of information that can be stored on film is
usually referred to as resolution.
The human eye has the equivalent of 1,893,376 pixels while a common computer
screen has a resolution of 620x480 (or about 297,600 pixels) as the table below shows.
|Eye, ( 8x10 image)
|NTSC (full screen)
|HDTV (full screen)
|Laser Printer (300 dpi)
|Offset printed magazine (8x10)
|35 mm b&w negative
|35mm film (Kodacolor Gold 100)
|KODAK Digital Camera
The resolution of film is very high. According to Norman Breslow,
"...based on the specifications for KODAK Varicolor film and a Nikon
50mm autofocus lens, show that there would be between 2.5 and 3 million pixels in the
negative, depending on the f/stop the lens was used at (Breslow, 1990)."
Others (Larish, p. 19)
suggest even more resolution from film depending on film type.
Film used in photo-enforcement includes both black and white (B/W) and color.
Color film is more commonly used today in photo-enforcement, especially if driver
recognition is required because of the greater amount of visual information contained in
color film such as the color of the vehicle. Black and white film on the other hand has a
greater dynamic range than color film and provides greater detail for difficult plate
In addition to sensitivity to color, film is manufactured with varying degrees
of sensitivity to light. The ISO standards for film rate film's sensitivity on a linear
Film for general use is marketed in the ISO range of 25 to 1000. Film used in
photo-enforcement generally has an ISO rating of 100-400. Other standard include the
European DIN and the former Soviet bloc countries GOST system.
SELECTING FILM FOR PHOTO ENFORCEMENT
35mm Print Film
There are many brands and types of film on the market. As noted above a film
with an ISO rating of 100-400 is preferred. The distinguishing feature of all film used in
photo-enforcement is graininess.
Graininess is the characteristic of film to show the individual grains of silver
halide when the negative is enlarged. When a film has a small grain pattern that is not
noticeable in a standard enlargement, it is said to be fine grained. The ability to
identify the letters and numerals in a license plate or the ability to recognize the
driver of a vehicle is determined directly by the inherent graininess of a film.
Graininess is often measured using an ISO Root Mean Square (RMS). This actually
measures the granularity of the negative. Whole values (4,5,etc.)only are reported.
However effective differences may exist in the same value.
KODAK, however, developed a new index for print film since the relative affect
of print film is noticeable more in the actual print than in the negative. The scale is
called a Print Grain Index. According to KODAK:
The Print Grain Index number refers to a method of defining graininess in
a print made with diffuse-printing illumination. It replaces RMS granularity and has a
different scale which cannot be compared to RMS granularity.
|The method uses a uniform perceptual scale, with a change of four units
equaling a just noticeable difference in graininess to 90 percent of observers. |
|A Print Grain Index rating of 25 on the scale represents the approximate
visual threshold for graininess. A higher number indicates an increase in the amount of
graininess observed. |
|The standardized inspection (print-to-viewer) distance for all print sizes
is 14 inches, the typical viewing distance for a 4 x 6-inch print. |
|In practice, larger prints will likely be viewed from distances greater
than 14 inches, which reduces apparent graininess. |
|Print Grain Index numbers may not represent graininess observed from more
specular printing illuminants, such as condenser enlargers. |
|Film (35mm, 4x6 print)
|KODAK Pro 100
|KODAK Pro 400 MC
|Fuji Super G 100
|Fuji Super G 400
|Fuji NPH 400
|Fuji NHG 400
|Fuji Reala 100
We are only able to compare differences between products
manufactured by the same company since different indices are used. Most agencies settle
for a balance of film speed and graininess by selecting a film with an ISO of 400.
Film of Choice
Since KODAK Pro 400 MC film is the recommended standard for photoenforcement,
the following technical information has been extracted from the KODAK web site. Please
consult the KODAK web site or your KODAK representative for additional and / or more
current information regarding KODAK Pro 400 and other KODAK films:
- KODAK Pro 400 MC Film / PMC is a high speed color negative film
that features moderate color saturation and contrast, and wide exposure latitude. Its
color and flesh-tone reproduction characteristics are similar to those of KODAK Varicolor
III Professional Film / VPS.
- KODAK Pro 400 Film / PPF is a high speed color negative film
that features high color saturation and wide exposure latitude. It is designed for
situations that have uncontrolled, low-contrast lighting.
NOTICE: The sensitometric curves and data in this
publication represent product tested under the conditions of exposure and processing
specified. They are representative of production coatings, and therefore do not apply
directly to a particular box or roll of photographic material. They do not represent
standards or specifications that must be met by Eastman KODAK Company. The company
reserves the right to change and improve product characteristics at any time. E-182,
35mm IR, UV, and Alternative Film
According to WJ's
photo homepage, some IR films may be useful for specific applications: Ilford SFX200/SP815/816T and Agfa APX200S
Photography Center contains information and links a variety of
photographic topics. Two examples include:
More Sensitive Film
Could Make Accurate
|By Chris Tomlinson
The Associated Press
1999 — Scientists say they have found
a way to produce photographic film that is 10 times more sensitive
to light — an advance that could make true-to-life pictures of
candlelight dinners possible without a flash or muted colors.
Agfa, the European film manufacturer that
sponsored the study and holds the patent, would not comment on
when the film might become available commercially. And researchers
acknowledged more work is needed to determine how well it can
reproduce certain colors.
But if the approach works, it could
revolutionize photography, improving on the basic design that has
been around since the 1840s.
In a study published in Thursday’s issue of the journal Nature,
researchers at the University of Paris-Sud said they have managed
to capture every bit of available light on film by adding a simple
“A real breakthrough,” said Richard
Hailstone, a scientist at the Rochester Institute of Technology.
A camera focuses light from an
object onto film, which is made of plastic with a chemical layer.
The film uses two kinds of light-sensitive crystals — halide
crystals and silver crystals — to produce an image.
When a bit of light, called a photon,
strikes one of the halide crystals, it breaks an electron loose.
Ideally, that electron combines with a nearby silver crystal.
Up Electrons Recorded
Later, when the film is placed in a developer, the silver crystals
that picked up electrons darken and stick to the plastic while the
rest are washed away. The result is a negative.
One photon of light cuts loose one
electron, but most of the time the electron quickly returns to the
halide instead of combining with the silver. As a result, most
film is not very efficient. In dim light, long exposure times are
needed to capture enough photons to create an image.
The French researchers added a chemical
called formate to the crystals. That kept the loose electrons from
recombining with the halide crystals. So every electron knocked
loose by a photon was captured by a silver crystal.
Options Ruin Film
Other chemicals can keep electrons from recombining with the
halide crystals, but they ruin the film’s ability to produce an
The chief researcher, Jacqueline Belloni,
said her technique could be used to make images with greater
clarity or to take pictures in very low light without a flash.
The new film could also widen the gap in
picture quality between conventional photography and no-film
digital photography, which has been growing in popularity.
One remaining question is how the
technique will work in with dyes that allow film to record red
light, Hailstone said. Belloni said further research will have to
look at the question.
Researchers at Eastman Kodak Co. said
they have been experimenting with chemicals that do the same thing
that formate does. They said it may be hard to make commercial
film with formate.
Copyright 1999 The Associated Press. All rights
THE AFFECT OF TEMPERATURE ON FILM
The film most often used in photo-enforcement is an ISO400 color print film
consisting of a base layer of acetate and three dye layers made of gelatin with a
protective overcoat. (See the KODAK Pro 400 images above.)
According to I. J. Marvoka, a technical representative of Fuji, Inc. (Marvoka), film is frequent subjected
to extreme temperatures by consumers who leave cameras and film in their vehicles in the
summer. Since temperature in a closed vehicle may reach 200° F. considerable empirical
evidence exists which demonstrates the effect of temperature on the Fuji product. This is
in addition to the standard tests performed by manufacturers.
Michael Davignon (Davignon),
a technical representative of KODAK, reports that film is damaged by prolong exposure to
extreme heat. Typically heat causes the base density of the film to increase which results
in what is frequently called "heat fog." The problem of heat fog is temperature
dependent and additive. That is, the problem worsens as the temperature increases and the
longer the film is exposed to extreme temperatures. Heat fog primarily affects the color
tones of the film. This is of more concern to consumers than to those in photo-enforcement
since film is used for identification purposes only.
However, even when heat fog bias occurs, some of the bias is correctable in
processing. According to Mr. Marvoka, "Film used in 120°F environments for one or
two days is only marginally affected. (Marvoka)"
Since heat damage is temperature dependent and cumulative, jurisdictions should
take the following measures in situations where heat may be a problem, i.e. where the
ambient temperature may rise above 100 F°:
|Film should be shipped and stored under refrigeration|
|Film should be chanced every day|
|Processing time and temperature may need to be adjusted |
|Film should be installed in cabinets designed for extreme heat conditions|
Based on the comments of the Fuji and KODAK representatives and actual field
practice by Gatsometer, the manufacture of the camera who has installed and operated
cameras in Australia, Kuwait, Jordan, Egypt, Morocco, and the United Arab Emirates;
ambient temperatures of up to 120° should not impact the use, readability, and
admissibility of the film used.
FILM HANDLING AND STORAGE
As noted above, film is sensitive to light and heat. It is also sensitive to
X-rays and other environmental factors. Fuji, a leading manufacturer of film recommends
the following for film handling and storage:
|Be sure to expose an process before the expiration date indicated on the film
package and process as promptly as possible after exposure|
|Films stored under cold storage conditions should be allowed to stand at room
temperature for 1 hour or longer before use and should come to equilibrium with room
temperature before opening. Sheet films should be handled in total darkness, exercising
care not to touch emulsion surfaces. Roll films should be handled in subdued light,
whether indoors or outdoors.|
|Film loaded into cameras or film holders should be exposed and processed as
promptly as possible.|
|Under certain conditions the X-ray equipment used to inspect carry-on baggage at
airport terminals will adversely affect photographic film (causing fogging). The adverse
effects of this are increased with the intensity of the X-rays, the speed of the film, and
the cumulative number of inspection exposures. |
|Therefore it is recommended that at each inspection, films be removed from
baggage and that airport security personnel be asked to inspect film manually. This is
absolutely necessary for ultra-high speed films with ISO ratings of 1000 or above, said
films being particular sensitive to X-rays and other additional sources..|
|Film fogging may occur in hospitals, factories, laboratories, and other radiation
sources. Therefore utmost care must be exercised in these environments.|
Unprocessed Film Storage
Whether exposed or not, at higher temperatures and humidities, films display
greater photographic property variations (e.g., speed, color balance, and minimum
density), and are more susceptible to adverse physical effects . Formalin vapors and other
harmful gases can also adversely affect photographic properties. It is recommend,
therefore, that care be exercised as suggested below.
|FUJICHROME film packs should be kept at temperature below 15°C (59°F) and
FUJICOLOR film packages below 10°C (50°F) if stored for extended periods. Unopened
film packages should be stored in polyethylene or vinyl bags.|
|Loaded cameras and film holders should be kept in cool, dry places free from
harmful gases. Formalin vapors may escape from adhesive agents used in new building
materials, notably plywood, and from modern furniture. Film storage near such Formalin
vapor sources should be avoided.|
Processed Film Storage
Processed films are subject to color fading and discoloration from light
(especially ultraviolet), high temperature and humidity. To avoid the adverse affects of
light, heat, and moisture, it is recommended that processed film should be kept in
sleeves, envelopes or mounts and stored in dry, cool and dark locations where there is
|Recommended Storage Conditions|
Temperature: below 25°C 77°(F)
Humidity: 30% to 60% RH
|Extended Duration Conditions|
Temperature: below 10°C (50°F)
Humidity: 30% to 50% RH
(Fuji p. 62-63)
Light is usually defined as, " radiation that is capable of affecting the
retina of the human eye (Shortley and
Williams, 469). It is also the radiation that causes a chemical change in silver
halide compounds (photographic film). Adequate lighting is essential in any photograph. It
especially important for photographs taken for law enforcement use. Primary lighting is
usually provided by a single high power -- 100 to 300 ws-- strobe (pulsed) flash. If
conditions warrant, a secondary flash - often called a slave flash - is sometimes used. It
is of particular use in red-light and rail crossing enforcement where the intersection is
unusually wide. Lighting a red-light violation involves balancing the amount of light
reflected to the camera from a vehicle, a driver (if driver recognition is required), and
a vehicle's license plate.
The placement of the primary light source is very important especially if night
and/or driver recognition is required. It has been known for some time that "The
intensity of the light varies inversely as the square of the distance from the luminous (Norton, P. 243). Simply stated,
doubling the distance results in one fourth the light intensity, not one half. This power
function of light means that the observed and recorded effect of light on an object
changes dramatically as the distance between an object and the source of light
illuminating it increases. Table 2 shows the effect of locating the light source at
different distances from a vehicle photo point.
Known as the Inverse Square Law, The intensity of light diminishes rapidly as
the distance from it increases.
If a flash is measured at 10 feet from its source using a flash meter and the
meter indicates that an f-stop of 16 should be used, the effect of moving another 10 feet
is shown by a value of f8 - twice the aperture opening. By 60 feet an aperture of f2.7
must be used to record the same image. In this example, at any distance greater than 60
feet, the flash has virtually no effect. It would be much the same as the spectator at a
sporting event using a standard flash to try capture his or her favorite athlete on the
stadium floor many feet below - one can snap the shutter, the flash unit can fire, but the
photos come back unviewable - unless enough ambient light is available.
There are three basic ways to handle this effect. The first is rather simple:
increase the flash output. The second is equally simple: move the camera closer to the
vehicle (or wait until the vehicle is closer to the camera). The third way is more
difficult but is very effective: add a slave flash.
If both flashes are in sync with the shutter- and they should be- then there
effects will be additive. That is two, 200 ws strobes will have the same intensity
(assuming that they are the same distance from the subject) as one 400 ws flash.
Of course there are many possible arrangements of flash units. And flash units
are usually duel power. That is, they may have different output levels on the first and
second flashes. Since flash intensity is directly related to the amount of energy stored
in the units' capacitors at time of discharge, and capacitors take time to charge, it is
usually best to have the brightest shot first.
It should also be noted that in red-light and rail crossing enforcement the use
of enhanced ambient light from overhead luminaires should be considered. Overhead lighting
may allow the use of lower wattage primary strobes which may have an added benefit in that
they may reduce the effects of glare from some plates.
A flash unit is composed of:
As discussed, the flash tube and capacitor play an important role in flash
intensity. Another factor also involved is the reflector. The reflector is usually a
silvered bowl surrounding the flash tube. The bowl is designed to focus the output of the
flash unit. The resulting spread of light is known as the light cone. The light cone is
the cone of light emanating from the strobe. It is common to have a light cone with a
angle of 30°- 40°.
The angle should match the camera angle if recognition of the driver is required
to insure that the interior of the vehicle is sufficiently illuminated. Flash cones can be
controlled by changing the shape of the reflector. Like lenses, flash units can be
obtained with wide, medium, or narrow focused beams.
When driver recognition is not required, most emphasis regarding the flash
is centered of reducing the glare from license plates. Polarizing filters can be of
significant help in reducing glare, but a standard windshield blocks a considerable amount
of light from reaching the drivers face. Empirical testing has shown that the light
reaching a driver's face may be as little as 1/5 as much as hits the front of a vehicle.
There are several techniques which can improve overall lighting of a vehicle.
Since the reflectivity difference between a vehicle and a licensee plate may be 100%,
bouncing the flash from the pavement to the plate by directing the flash in front of the
vehicle and by increasing the distance between the camera and the flash unit may also
improve the readability of plates.
According to Rossi (p. 83),
"A piece of glass or other transparent material bounded by two spherical surfaces (or
one plane and one spherical surface) forms an optical system called a simple lens."
Professional high quality lenses are used by most makers of photo-enforcement equipment.
These usually range from 45-150 mm depending on the distance from the camera to the
vehicle to be photographed. Lens quality is very important in photo-enforcement since it
is the lens which determines to a great degree the clarity of a photograph.
The selection of lenses include consideration of its focal length. There are
three main categories of lenses: wide angle (<45mm), normal (45-75mm), and telephoto
(>75mm). Generally, normal and telephoto lenses are used in photo-enforcement.
The field-of-view is how much the camera "sees" or records on film.
For example, a camera with a 45mm lens at a distance of 20 feet will produce a
field-of-view of 15'x11'. That is, the camera "sees" an area twenty feet wide by
eleven feet high. At the same distance, on the other hand, a 150mm lens "sees"
only 4' x 3'. Field of view must be taken into account when selecting a lens. One may use
different lens depending on the distance the photopoint is from the camera and how wide
the frame must be.
Photographic filters are glass or plastic media placed between the lens of a
camera and an object to be photographed that change the light entering a camera. The most
common filter is a skylight filter that reduces the amount of UV light reaching the film
or CCD. Filters are usually placed in front of the lens but can also be placed on the
light source to change the color of the flash. This is often used to reduce the effect on
the drive's vision. Some filters commonly in use in photo-enforcement are shown below:
||Absorbs the UV rays which often make photographs hazy. Often used
as a permanent lens protector
||Reduces blue providing more natural skin tones
||Reduces reflections from non-metallic surfaces. Enables clearer
view of driver.
||Enhances contrast. Used over flash to reduce flash effect on driver
||Enhances contrast between reds and yellows
||Enhances contrast between sky and foreground
CCD technology, like that of film, is also based on a principle of physics.
Instead of silver, however, charged coupled devices (CCDs), as they are called, use
silicone. Photons of light pass through a lens and strike a cell of the CCD. The amount of
light (intensity) is electrically converted to digital information and stored. Merrill and
Lichty describe this as:
A brief review of how a CCD camera imagine is recorded will help in
understanding imagining system optimization. [For more details, see Laser Focus World,
Dec. 1994, p. 53 -- Ed.] The LCD imaging array itself has no inherent gain. Every incoming
photon generates a single free electron in the pixel it impacts, with a probability
ranging from 0% to 70%. This probability is known as the quantum efficiency and is
wave-length-dependent, loosely following the absorption curve of silicon. The number of
electrons is determined by the light level in the image and the exposure (integration)
time. There will also be a few "rogue" background electrons bound in each pixel,
known as dark electrons. The number of' these dark electrons is strongly dependent on the
CCD operating temperature. The correspondence between object "pixels" and CCD
pixels is determined by the magnification of the optical system being used.
The array of' active pixels is referred to as the parallel register. Prior to
readout, the inherent signal-to-noise ratio (S/N) of the image stored in these pixels is
determined by the shot noise -- the natural statistical variation in the number of photons
arriving at each pixel. Shot noise is the statistical variation in the light-absorption
Each exposure results in an array of captured charges in the form of a
two-dimensional matrix. This n x m matrix is averaged, row by row, into the serial
register. The individual pixels in the serial register are then transferred in serial
fashion into an output node. Camera electronics measure the charge in this node, digitize
the chordate, and clear the node electrons, leaving it ready for the. next serial
transfer. This readout process introduces statistical variations known as read noise. The
readout speed is usually limited by the speed of the camera analog-to-digital (A/D)
converter, with a maxim of several megahertz (millions of pixels/second).
A process known as binning allows the charge from several pixels to be combined
before readout. This reduces the number of individual readout events and thus decreases
the total readout time, allowing the user to set the camera to a faster frame rate.
Binning is possible in both the x and y directions either by combining multiple rows into
the serial register before undergoing serial transfer or by reading multiple serial pixels
into the output mode between individual readouts. Because it increases the S/N, binning is
also used for low light applications. The trade off for higher frame rate and increased
S/N is reduced spatial resolution. (Merrill
and Lichty 121)
As discussed, the resolution of CCDs is measured in pixels. Although
similar to CCDs used in video cameras, CCDs used in law enforcement are typically
high-resolution using an array of 1,000 x 1,000 pixels (1,000,000) or more. Although not
as high as film, digital camera resolution has progressed rapidly in the last five years
and is now sufficient for most enforcement purposes.
An excellent explanation appeared in Scientific American in June, 1998.
by Michael D. McCreary
Director of Operations, Microelectronics Division
|At the heart of any digital camera is a light-sensing semiconductor
array, the camera's endlessly reusable "film." The most commonly used light
sensors are charge-coupled devices (CCDs), which were developed in the early 1970s and are
also incorporated in such products as video cameras, facsimile machines and desktop
scanners. CCD-based cameras make it possible to capture images that can be instantly
transmitted, for example, from a photojournalist in the field or from a reconnaissance
satellite in space.
A CCD is an array of light-sensitive picture elements, or pixels, each measuring
five to 25 microns across. The camera's lens focuses the scene onto this pixel array. Just
as the resolution of conventional photographic film is related to the size of the grain,
the resolution of a CCD chip is measured by the number of pixels in the array. A digital
still camera intended mainly for nonprofessional use has an array of, typically, 640 by
480 pixels; a top-of-the-line professional camera would have an array of millions of
CCD chips are fabricated in a process that requires hundreds of steps and
several weeks. Although CCDs are the dominant light-sensitive semiconductor, companies
such as Eastman Kodak, Motorola, Intel, Rockwell and Toshiba have invested heavily in a
competing technology, the CMOS image sensor, which is expected to be used in products such
as digital cameras, especially lower-end models intended for nonprofessionals.
Illustrations by: Jared Schneidman Design
WHY IS THE USE OF CCD CAMERAS GROWING?
Regardless of the method used to capture a violation, it is still necessary to
convert the image of a violation to a form that a computer can use: digital. With
film-based systems, film must be exposed, developed, scanned, digitized, and stored on a
computer in digital form.
CCD cameras eliminate most of these steps. In addition, a digitized image can be
sent electronically from the point of capture to a processing center by way of telephone,
cellular phone, FM signal, or satellite eliminating the need (and danger) of manually
retrieving film and reducing the time required to issue a citation.
But the growth of digital cameras is also guided by economic reasons as Hedgecoe writes:
"A replacement for silver-based image recording is inevitable.
Silver is an increasingly rare and thus increasingly more expensive resource, and the film
industry uses millions of ounces a year. Video is an attractive option, especially since
more of the technology is already in place because of the popularity of video cameras and
recorders. Another feature of digitized images is that they can be transmitted down normal
telephone lines, even computer-to-computer with the help of a modem, making them of vital
interest to the news gathering industry. On the minus side, at present, no matter how
sensitive the CCD within the camera, it cannot compare with the literally millions of
'image grabbing' surfaces of the silver-halide grains that are present in traditional
camera film." (Hedgecoe 24)
And 1998 may be the year. In a news story in EE Times,
03/03/98, By Margaret Ryan writes:
Digital Cameras Expect Big Exposure In '98
With more than 100 different digital-camera models offering improved
image quality, ease of use, and cheaper prices than older designs, the market for digital
cameras is poised for explosive growth in 1998.
Forecasts for worldwide shipments of digital cameras hover between 3.8
million to 5.3 million units for 1998. One long-term prediction from researcher Frost
& Sullivan, in Mountain View, Calif., expects the United States market to reach
2,357,800 units and $478.1 million in revenue by 2000.
Jonathan Cassell, senior industry analyst of the
semiconductor-applications program at Dataquest, in San Jose, Calif., said he believes
1998 will be the biggest year -- "a barn-burner" -- in terms of unit production
of digital cameras. Cassell said he projects 5.377 million digital-camera units will be
produced worldwide in 1998, more than double the 2.089 million units produced in 1997.
Surge To Come From Businesses, Not Consumers
The growth will mostly come from business users, rather than consumers,
according to market researchers. That's primarily because business users have a compelling
reason to buy digital cameras: They need digital images that can be manipulated for
inclusion into computer databases, presentations, and Web pages. There's nothing to compel
consumers yet. Nevertheless, consumer acceptance of the devices is increasing, analysts
What may entice consumers to purchase digital cameras is the introduction
of higher-resolution mega-pixel cameras this year. In 1998, digital cameras will move
beyond Video Graphics Array (VGA) resolution (which is not adequate for applications,
especially photo prints) to devices that produce images closer to photo-print quality.
At the same time, there will be a glut of VGA 640 x 480 basic
point-and-shoot cameras on the market, and prices of those cameras are expected to drop to
less than $200 by midyear. Prices of these VGA digital cameras are likely to drop once
Complementary Metal Oxide Semiconductor (CMOS) sensors replace charge-coupled devices
(CCD), which captures the image, in digital cameras this year. CMOS sensors offer the
potential for lower power consumption and greater chip integration for digital cameras
The main manufacturers of digital cameras for the consumer market include
Casio, Dycam, Eastman Kodak, Fuji Photo Film USA, Olympus, and Canon.
There are also a host of consumer-electronics company participants
including Sony and Samsung. Other competitors making and selling digital cameras include
Agfa Division Bayer, Apple, Canon Computer Systems, Chinon America, Epson America,
Logitech, Nikon, Olympus America, Ricoh, Ritz Camera Centers (Ritz sells a camera made by
Chinon), and Sanyo Fisher.
There are also a host of companies that design and manufacture chips for
the cameras, such as Analog Devices, 8x8, LSI Logic, and companies that design and sell
photo-editing software. Adding to the long list of players are companies that produce
digital desktop cameras including Connectix, Pixera, StarDot Technologies, and
In addition to all those players, Ron Tussy of International Data said a
number of Taiwanese companies jumped into the market this year.
Something's got to give this year with more than 45 vendors and more than
100 digital-camera models, price erosion, and product life spans shrinking from one year
to six months in 1998. Companies will find it difficult to make a profit (as they have for
the past two years) on digital cameras, and some players are likely to exit this year.
A San Jose, Calif.-based hardware-software company called Flashpoint ...
believes the intelligence is in the camera in the form of Flashpoint's Digita software.
The cameras are complex with LCDs and infrared radiation capability for connections
between camera and printer, camera and camera, and camera and computer. Motorola allied
with Flashpoint, while camera makers Kodak, Sharp, and Minolta committed to building
Digita software into their cameras.
In addition to buying trends, technology trends will drive the market for
digital cameras. The introduction of CMOS sensors to replace CCDs will enable lower-cost,
lower-power consumption and greater integration of the circuitry of digital cameras.
The use of CMOS sensors will allow more chip makers to expand into the
digital cameras and that will drive prices of cameras down even further.
While current technology limits the use of digital cameras to enforcement of one
to two lanes of traffic, rapid development in CCD technology is likely to continue and
digital cameras will vie with film based systems for use in law enforcement. Many agencies
are already using digital cameras for many forensic activities. For a description of some
of the uses, see Digitizing the
There are two types of CCD cameras in use today in automated law enforcement:
analog and digital. Most of us are familiar with analog video cameras. These are the
NTSC/PAL analog cameras which have been in wide use for many years by consumers and are
often referred to as camcorders or videocams. These video cameras record analog signals
(usually NTSC or PAL) on moving magnetic tape.
In August 24, 1981 the first still frame camera was introduced by Sony. Called
the MAVICA, an acronym for magnetic video camera (Larish 3), it was an analog NTSC/PAL type system which recorded its
captured images on 2-inch video floppy disk.
It wasn't until the fall of 1988 that the first digital still camera was
released by Fuji. It used an 8-bit/color, 400,000 pixel CCD sensor combined with a fixed
focus 16mm lens with a 1/60 to 1/2000 electronic shutter and storing captured images on an
S-RAM card. (Larish 44).
In 1998,IMAGEK introduced an adaptor for
standard 35mm cameras that converts them to digital. The company claims that:
|The system shoots and stores 30 pictures at a time and is reusable up to
100,000 times. |
|Connects to your PC for instant viewing, electronic storage, immediate
e-mail transmission and artistic manipulation. |
|Enables custom and standard photos with your printer. |
|One EFS-1 Electronic Film system saves you the expenses of 100,000 rolls
of film and processing. |
|Compatible with all standard digital printing methods. |
|Provides creative freedom with your existing lenses and attachments. |
|Allows flexibility to choose between standard 35mm film or EFS format at
Imagek CCD Adapter
for 35mm Cameras
CCD CAMERA REQUIREMENTS
While CCD cameras are analogous to film cameras and both produce images, digital
cameras present some issues which must be addressed. KODAK Motion Analysis Systems
Division located in San Diego, CA suggests the following as camera/system requirements:
|Data management: facilitate the ability to easily capture, transmit, process,
store and recover captured data for both image and text formats.|
|Resolution: sufficient to meet all the intended uses for the image-reading of the
licensee, clear detail of the vehicle and if required, allowing identification of the
|Anti-blooming: prevention of spreading of overexposed portions of the image
(i.e., vehicle headlights or sunlight from highly reflective surfaces.)|
|Contrast latitude: adequate differentiation of light-to-dark areas within an
image to add detail recognition.|
|Stopping power: Blur-free images of moving vehicles.|
|Sensitivity: ability to detect at low-light levels as well as into near-IR
|Image enhancement circuitry: camera electronics to eliminate major sensor defects
such as bright or dark columns, which detract from the visible presentation of the image.|
|Frame rate: continuous read out of images to support monitoring along with single
frame capture capability for recognizing several successive vehicles committing a
|Installation flexibility: ability to mount into permanent or mobile settings.|
|Environmentally friendly: minimize the impact of any by-products of system. (Erickson 2.)|
The Redflex System
Redflex Traffic Systems Pty Ltd expects its new digital SMARTCAMred
red-light enforcement system will supercede the 'twin-shot' operation of conventional
red-light cameras. SMARTCAMred combines advanced high resolution digital imaging with
computer vision and communication technologies and has been developed specifically for
The system has just concluded trial under the Technology Review
being conducted by the ICBC in British Columbia, Canada. A separate three month trial
starts this month in Howard County, Maryland, USA, where the system's multiple imaging
will suit the County's requirement that a vehicle be imaged prior to reaching the
intersection stop-bar in addition to conventional shots.
Redflex says SMARTCAMred operates more efficiently than
conventional red-light cameras and generates superior sets of image information (that
record the whole violation episode). And it allows violation data to be communicated
direct to the processing back-office using communications channels (POTS, ISDN, cable
modem or fiber optic) for rapid citation issue. Or data can be transferred on a removable
disc. The digital image sets and violation data are encrypted to provide absolute
Film-handling and printing are, of course, non-issues.
Digital SMARTCAMred cameras image vehicles continuously
as they move towards and over the 'stop-bar', cross and exit the intersection. The systems
hold up to ten (10) separate images of the entire violation event. These 'overlap' with
the actual violation detection that is recorded on a special, high resolution violation
image of the license plate (shot at a position to optimize later plate recognition and
The violation image is imprinted with a 256-character datablock.
Because this dataline is fully software configurable, it is completely flexible to client
requirements. This allows full description and records of the intersection details, type
of violation, violation data, time of violation, time elapsed into 'red' cycle', frame
number, direction and speed of the vehicle, date etc. as required.
The systems 200 MMX industrial Pentium Computer Control
Unit allows it to be flexibly configured to manage variable enforcement requirements
including known variations in traffic flow or altered policy objectives. With the
communications systems, the Unit also allows remote status monitoring and direct modem
communication of deployment data, status reports, or alarm warnings to enforcement
SMARTCAMred systems monitor up to four lanes of traffic
simultaneously, and are capable of sustainable imaging at two violations per second or
better, giving effective enforcement of multiple offences in busy intersections. System
housing is environmentally controlled to allow operation in extreme weather conditions on
a 24 hour basis.
Alternatively, Redflex' film-based SMARTCAMred camera
system may be used if specific jurisdiction rules require film images. The two systems are
interchangeable. Redflex high performance technologies take both well beyond
By far one of the most confusing aspects of photo-enforcement is the meaning and
comparison of resolution. We speak of the resolution of cameras -both film and digital-,
of monitors, and of printers. And we use different terms or the same term in a different
way: lpi, dpi, pixels, etc.
In general, the resolution of an instrument (including the human eye) is the
ability to differentiate or resolve objects individually. Devices are said to have high
resolution if they are able to provide many unique pieces of information and have low
resolution if they present few.
Erickson, identifies five classes of resolution associated with CCD photography
which he calls the NTSC-type camera which have resolutions up to 760 x 480 pixels:
|Very High Resolution|
He further identifies a new class: Mega-resolution with 1,000 x 1,000 Pixels and
above. (Erickson 3)
||Bytes per pixel
|RGB true color
|CMYK true color
The Photographic and Imaging Manufacturers
Association (PIMA), which is accredited by the American
National Standards Institute (ANSI). , has sponsored a standard for measuring
resolution of digital cameras. It is known as The Electronic Still Picture Imaging
DRAFT INTERNATIONAL STANDARD ISO 12233.
In part, the draft states:
The ISO (the International Organization for Standardization) is a
worldwide federation of national standards bodies (ISO member bodies). The work of
preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has
been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in
the work. ISO collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
Draft International Standards adopted by the technical committees are
circulated to the member bodies for voting. Publication as an International Standard
requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 12233 was prepared by Technical Committee
ISO/TC 42, Photography.
The spatial resolution capability is an important attribute of an
electronic still picture camera. Resolution measurement standards allow users to compare
and verify spatial resolution measurements. This standard defines terminology, test
charts, and test methods for performing resolution measurements for analog and digital
electronic still picture cameras.
One of the most important characteristics of an electronic still
picture camera is the ability of the camera to capture fine detail found in the original
scene. This ability to resolve detail is determined by a number of factors, including the
performance of the camera lens, the number of addressable photo-elements in the optical
imaging device, and the electrical circuits in the camera, which may include image
compression and gamma correction functions. Different measurement methods can provide
different metrics to quantify the resolution of an imaging system, or a component of an
imaging system, such as a lens. Resolution measurement metrics include resolving power,
limiting resolution (at some specified contrast), spatial frequency response, MTF, and
A method for measuring resolution is to capture an image of a suitable
test chart with the camera under test. The test chart must include patterns with
sufficiently fine detail, such as edges, lines, square waves, or sine wave patterns. The
test chart defined in this standard has been designed specifically to evaluate electronic
still picture cameras. It has not been designed to evaluate other electronic imaging
equipment such as input scanners, CRT displays, hard copy printers, or electrophotographic
copiers, nor individual components of an electronic still picture camera, such as the
The resolution measurements described in this standard are performed in
the digital domain, using digital analysis techniques. For electronic still picture
cameras that include only analog outputs, the analog signal must be digitized, so that the
digital measurement can be performed. The digitizing equipment is characterized, so that
the effects of the digitization process can be removed from the measurement results. When
this is not possible, the type of digitizing equipment used shall be reported along with
the measurement results.
The spatial frequency response (SFR) measurement method described in this
standard uses a computer algorithm to analyze digital image data from the electronic still
picture camera. Digitized image values near slanted vertical and horizontal black to white
edges are digitized and used to compute the SFR values. The use of a slanted edge allows
the edge gradient to be measured at many phases relative to the image sensor
photo-elements, in order to eliminate the effects of aliasing. This technique is
mathematically equivalent to performing a moving knife edge measurement.
Storage of Digital Images
Whether a CCD captured image is in analog or digital format, it still must be
stored. Analogue images are usually stored on magnetic tape while digital images may be
stored in random access memory (RAM), magnetic media such as diskettes, hard disk drives,
or on magneto-optical (MO). These devices may be local or the a signal (analog or digital)
may be transmitted by wire or radio. Generally, if the data are not to be stored locally,
analog signals are much quicker to transmit.
Since images require much storage space, the decision on the method of storage
is often made by the size of the file storing the image. To calculate the raw storage
required use the following formula:
When the number of images is great or the size of each image file is
large, some thought must be give to storing the images in compressed form.
- Where: FS=file size
- H=height of the image in pixels
- W=width of the image in pixels
- R=resolution in pixels
- B=bytes per pixel
One cannot discuss digital technology without discussing compression. Since
digital images are usually quite large and storage costs relatively high, some method must
be used to reduce the size of images for storage. Two types of compression are generally
available: lossy and lossless. As the names imply, the difference is in whether or not
information is lost during the compression process. While there are many different
compression algorithms used for compressing images, the most popular is probably JPEG.
The Joint Photographic Expert Group (JPEG) created a compression scheme that
includes both lossy and lossless algorithms. The actual name of the standard is
"Digital Compression and Coding of Continuous-Tone Still Images." It is also
known as ISO standard 10918. "JPEG is not one standard but a suite of standards - 29
distinct coding processes in all (Brown
and Shepherd 382)."
Brown and Shepherd's book on graphic file formats (Brown and Shepherd) define in detail the
JPEG's baseline sequential scheme is a combination of the Discrete Cosine
Transform(DCT), reduced precision of the DCT coefficients (quantization), run-length
encoding, and Huffman or arithmetic encoding.
To compress an image, the image data is divided into blocks, where each
block is an 8 x 8 array of values. Each block of data is put through three processes: a
Forward Discrete Cosine Transform (FDCT), a quantizer, and a coder. To decompress an
image, these processes are reversed: an un-coder, a de-quantizer, and an Inverse Discrete
Cosine Transform (IDCT) (Brown and
Common Compression Encoding Methods
Once an image of a violation has been captured either on photographic film or in
digital format it must be processed so that information about the violator can be
obtained. This process depends on the media on which the image was stored.
The basic processes
involved with the use of photographic film involves film development, scanning, and
storage. Exposed film must be developed to expose the latent image. It then must be
transferred into a computer for viewing. The device at left is a film scanner manufactured
by ROBOT FOTO UND ELECTRONIC GmbH and used to scan 33mm film. The scanned images must also
be stored if later retrieval is required. If desired by the jurisdiction digitized images
of the violation may be included as part of the citation mailed to the violator. This
image should provide a good picture of the vehicle, surroundings and data block. If
frontal photography is used, the driver's face may be included, but faces of other
occupants should be blocked. This increases the response-by-mail rate without infringing
on the violator's privacy. According to KODAK:
- You can easily scan Pro Film negatives with a variety of linear-array-CCD,
area-array-CCD, and PMT film scanners. You can scan negatives on desk-top scanners as well
as high-end drum scanners.
- Because no standards exist to define the colored filter sets that film
scanners use to capture the red, green, and blue information of the film image, each
manufacturer's scanner has its own characteristic output. The output depends on the
scanner's sensitivity to the dyes in the film. This sensitivity is determined by the
spectral distribution of the colored filter sets and/or the spectral sensitivity of the
charge-coupled-device (CCD). In addition to these spectral specifications, scanner output
depends on the look-up tables or matrices that the scanner uses to output information for
CRT monitors, transmission, etc. These tables or matrices are part of either
"plug-in" programs used with specific software packages designed for image
manipulation, updateable ROMs included with the equipment, or fixed algorithms for
calibrating and balancing, similar to those used in photographic color printing equipment.
- The generic "color negative film" channel designation available
with scanner software is only a starting point. You can adjust the final color balance and
the scene-dependent contrast and brightness of an image by using the scanner's controls
during pre-scan, or by using an image-manipulation software program or workstation after
- Some scanners allow you to use "plug-in" programs to make
calibrations based on D-min film stock. Because different types of color negative films
have different colored-coupler masks, the optimum D-min balance is different for each type
of film. Therefore, for optimum results, set up a specific channel for each type of film
you are scanning.
- Note: For more information, visit the KODAK World Wide Web site
Most film today is developed using automated film developers frequently seen in
fast film processing labs throughout the world. Many professional labs also offer custom
development which allows for correction of over or under exposures. Custom labs can also
vary the contrast of film during development. However, this level of development is rarely
necessary if appropriate film is used and the camera is adjusted properly.
Because images contain so much information, they usually require a significant
amount of space for storage. There are several ways to store the information contained in
images. If film is the media used to record the image, the film itself may be used as an
information storage system. Images may also be stored electronically. If digital
recordings are used to capture the violation or if the images from film are captured in
digital form, some form of magnetic or optical media must be used to store the
As a storage media, photographic film is cheap and dense. If the original
violation was captured on film about 6 million bits of information are recorded. With the
exception of Hollywood type equipment, the information on film can not be completely
captured in digital form.
Magnetic media include the ubiquitous hard drive, zip drives, and magnetic tape.
Hard drive have an advantage when rapid access is desired or copies of the image need to
be transmitted over great distances in a short amount of time. Access times are quick and
the cost per unit of storage is falling. Frequently violations recorded on film are
captured and stored on hard drive for processing then discarded. The original film serves
as the long-term storage media.
Optical and magneto- optical storage
systems provide a large amount of storage capacity and may be made available on-line if
placed in juke boxes. The unit cost is lower that magnetic media but access time is
longer. Optical storage (CDs) (Shown Below) can be permanent or re-writable. The KODAK PIW
6600 Film Scanner (Shown here) can produce high
quality scanned images on CD. Permanent CDs offer the added protection that the images
remain unchangeable once recorded. This may satisfy some courts as a long-term storage
method. On the other hand, re-writeable CDs are more cost effective since they can be
reused after the images are no longer needed.
The storage media is often dictated by the type of citation issued by a
jurisdiction. If images are to be printed on the citation, they must be stored at least
temporarily on magnetic or optical media.
License Plate Recognition
For a full disdussion of automatic license plate recognition technologies,
please see click here.
The traditional techniques of forensic photography may also be of interest. Many
related topics and links can be found Steven Staggs book on Forensic photography titled Crime
Scene and Evidence Photographer's Guide. A description of the book and other
interesting links can be found at: http://www.pe.net/~staggs/index.html
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