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October 1999 Volume 1 Number
3
Definitions and Guidelines for
the Use of Imaging Technologies in the Criminal Justice System
Part
II: Guidelines
(Version 2.1 —
June 8, 1999)
Documentation of Policies and Procedures
Personnel engaged in the
capture, storage, processing, analysis, transmission, or output
of imagery in the criminal justice system should ensure that
their use of images and imaging technologies are governed by
documented policies and procedures.
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Preserving Original: Storage
The original image should
be stored and maintained in an unaltered state. This includes
maintaining original digital images in their native file formats.
Duplicates or copies should be used for working images when applicable
(see Post-Capture Processing).
The following media are recommended
for the preservation of original images because of their quality,
durability, permanence, and reliability:
- Silver-based film with the
exception of instant film
- Write-Once Compact Disk
Recordable (CDR)
- Digital Versatile Disk Recordable
(DVD-R)
The following are acceptable
for the preservation of original images but care must be taken
to avoid loss of data:
- Instant film separately
fixed
- Photographic prints
- Diskettes
- Magnetic tape
- Fixed hard drives
- Removable magnetic media
- Compact flash cards
- PC cards
- Smart media
- Removable magneto-optical
drives
- Write-once magneto-optical
drives
The following are not considered
to be acceptable for the preservation of original images:
- Instant film packs
- Inkjet prints
- Solid ink prints
- Thermal wax paper prints
- Dye-sublimation prints
- Dry-silver prints
- Laser prints
- Electro-static prints
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Preserving Original: Post-Capture
Processing
Film: Can process the original if the
processing is non-destructive.
Analog Video: Recommend minimal processing of
original to avoid degradation of signal. If the original is used,
a copy should be made prior to processing and analysis.
Digital: Make a duplicate image and use the
duplicate as the working image.
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Documentation of Image Processing
Techniques common to traditional
darkrooms and digital imaging stations, such as cropping, dodging,
burning, color balancing, and contrast adjustment that are used
to achieve an accurate recording of an event or object, are standard
processing steps. When the results of the steps are visually
verifiable, documentation of such steps is not considered mandatory
except when the image is subjected to image analysis.
Techniques such as unsharp
masking, multi-image averaging or integration, and Fourier analysis,
that are used to increase the visibility of specific details
in an image at the expense of other image details are standard
processing steps. However, the use of such steps should be documented
in the case notes in sufficient detail that comparably trained
personnel can repeat the steps and produce the same output when
the image is subjected to image analysis.
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Verification of Original and Processed
Images
Personnel who captured the
original image or were present at the time the original image
was captured can verify that the image is a true and accurate
representation.
Any processed image subjected
to image analysis should be documented with an image processing
log. An image not subjected to image analysis does not need a
log.
It is recommended that the
image processing log document steps such as dodging, burning,
color balancing, contrast adjustment, unsharp masking, multi-image
averaging or integration, and Fourier analysis. The steps should
be documented in the case notes in sufficient detail that comparably
trained personnel can repeat the steps and produce similar output.
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Preserving Original: Chain of Custody
A chain of custody must be
maintained for the film or video tape upon which original images
are recorded.
For digital images, the chain
of custody should document the identity of the personnel who
had custody and control of the digital image file from the point
of capture to archiving. Once the file has been archived, the
chain of custody should document the identity of the personnel
who had custody and control of the archived image.
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Software
Software used in the processing
and analysis of digital images should produce consistent results,
permitting comparably trained personnel to achieve similar results.
Legal Note: Manufacturers of software used for
image processing may be required to make the software source
code available to litigants, subject to an appropriate protective
order designed to protect the manufacturer's proprietary interests.
Failure on the part of the manufacturer to provide this information
to litigants could result in the exclusion of imaging evidence
in court proceedings. This should be considered when selecting
software.
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Image
Compression
Original images and images
expected to undergo image analysis should not be subjected to
lossy compression. If compression is necessary, lossless compression
is strongly recommended. If lossy compression must be used, then
the highest quality option is recommended because critical image
information could be lost and artifacts introduced as a result
of the compression process. Repeated saving of a file using lossy
compression may exacerbate the loss of image information.
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Image
Capture
Image capture devices should
be capable of rendering an accurate representation of the item
or items of interest. Different applications will dictate different
standards of accuracy. At a minimum, the following should be
considered when selecting devices:
- Characteristics (size, movement,
location, etc.) of the scene, item, or items of interest
- Lighting of the items of
interest
- Dynamic range of the scene
- Time constraints
- Required end product(s)
It is strongly recommended
that conventional silver-based film be the primary media for
documenting crime scenes. This documentation may be supplemented
by analog video and/or digital imaging.
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Image
Output
An output device should be
capable of producing an accurate representation of the input
image. The following should be considered in the selection of
output devices:
- Final use of image
- Time constraints
- Longevity/permanence of
output image
- Spatial resolution required
- Range of colors and brightness
to be produced
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Image
Transmission
Received images should accurately
reflect the transmitted images. The following should be considered
in the selection of transmission methods and devices:
- Final use of image
- Time constraints
- File size
- Security of transmission
- Integrity of transmission
Hardware and software compatibility
of transmitters and receivers
File format compatibility
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Return
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Part III: Standard Operating Procedures
Part IV: Quality Assurance
Part V: Training, Qualifications,
and Proficiency
Information referenced
to this document can be identified as Version 2.1 ¾
June 8, 1999.
FORENSIC SCIENCE COMMUNICATIONS OCTOBER 1999 VOLUME
1 NUMBER 3 |
