Definitions and Guidelines for the Use of Imaging Technologies in the Criminal Justice System
Scientific Working Group on Imaging Technologies (SWGIT)
(Version 2.2 — December 7, 2000)
Background.......Mission Statement.......SWGIT Membership
Purpose of This Document.......Part I: Definitions.......Part II: Guidelines.......
Part III: Elements of Standard Operating Procedures.......Part IV: Quality Assurance
Part V: Training
Background
Although digital imaging technologies have been used in a variety of scientific fields for decades, their application in the criminal justice system has been relatively recent. Consequently, there has been a need to gather and disseminate accurate information regarding the proper application of this and other imaging technologies (including silver-based film and video) in the criminal justice system.
Mission Statement
The mission of the Scientific Working Group on Imaging Technologies (SWGIT) is to facilitate the integration of imaging technologies and systems in the criminal justice system by providing definitions and recommendations for the capture, storage, processing, analysis, transmission, and output of images.
SWGIT Membership
The Scientific Working Group on Imaging Technologies (SWGIT) consists of more than thirty photographers, scientists, instructors, and managers from over two dozen federal, state, and local law enforcement agencies, as well as from the academic and research communities. All SWGIT documents represent the consensus opinion of this membership and should not be construed as the official policy of any of the represented agencies.
Purpose of This Document
This document is intended to serve two purposes:
- Provide definitions for use by personnel in the criminal justice system when discussing imaging and imaging technologies.
- Provide preliminary general guidelines for use by personnel in the criminal justice system as they develop specific standard operating procedures for their respective agencies.
Future documents will address other topics in the field of forensic imaging.
Part I: Definitions
(Version 2.2 — December 7, 2000)
Definitions Pertaining to Images
Image (Webster’s New World Dictionary, Third College Edition): An imitation or representation of a person or thing, drawn, painted, photographed, etc.
Imaging Technologies: Any systems and/or methods used to capture, store, process, analyze, transmit, or produce an image. Such systems include but are not limited to film, electronic sensors, cameras, video devices, scanners, printers, and computers.
Archive Image: Either the primary or original image stored on media suitable for long-term storage.
Copy Image: A reproduction of information contained in a primary or original image.
Digital Image: An image that is stored in numerical form.
Duplicate Image: An accurate and complete replica of an original image, irrespective of media.
Original Image: An accurate and complete replica of the primary image, irrespective of media. For film and analog video, the primary image is the original image.
Primary Image: Refers to the first instance in which an image is recorded onto any media that is a separate, identifiable object or objects. Examples include a digital image recorded on a flash card or a digital image downloaded from the Internet.
Processed Image: An output image (see Image Processing).
Working Image: Any image subjected to processing.
Other Definitions (alphabetically)
Archiving: Long-term storage of an image.
Artifact: Any visible feature or distortion in a recorded image or output image that is not present in the corresponding imaged object or input image. Image artifacts can be introduced inadvertently by hardware or software, or intently by an operator. The latter type includes annotation or other direct alteration of an image in order to clarify or call attention to some particular image content. Artifacts introduced by hardware and software generally degrade an image, and, if severe enough, can impair interpretation. For more information, see “Draft Recommendations and Guidelines for the Use of Digital Image Processing in the Criminal Justice System (Version 1.1 – February 2001)“; available at www.theiai.org/swgit/index.html.
Capture: The process of recording an image.
Capture Device: A device used in the recording of an image.
Compression: The process of reducing the size of a data file.
Digital Image File: A record that includes image data and related data objects.
File Format: The structure by which data is organized in a file.
Image Analysis: The extraction of information from an image beyond that which is readily apparent through visual examination.
Image Enhancement: Any process intended to improve the visual appearance of an image.
Image Output: The means by which an image is presented for examination or observation.
Image Processing: Any activity that transforms an input image into an output image.
Image Processing Log: A record of the steps used in the processing of an image.
Image Transmission: The act of moving images from one location to another.
Image Verification: A process by which an individual identifies an image as being an accurate representation.
Intermediate Storage: Any media or device on which an image is temporarily stored for transfer to permanent or archival storage.
Legacy File Management: A methodology for preserving data and images so that they are retrievable as technology changes.
Lossless Compression: Compression in which no image data is lost and the image can be retrieved in its original form.
Lossy Compression: Compression in which image data is lost and the image cannot be retrieved in its original form.
Native File Format: The file format of the primary image.
Source Code: The list of instructions written in a standard programming language used to construct a computer program. This information is not usually provided absent a court order or prior contractual agreement.
Storage: The act of preserving an image.
Storage Media: Any object on which an image is preserved.
Part II: Guidelines
(Version 2.2 — December 7, 2000)
Documented Procedures.......Preserving Original: Storage.......
Preserving Original: Post-Capture Processing.......Documentation of Image Processing
Verification of Original and Processed Images.......Preserving Original: Chain of Custody
Guidelines for Software.......Guidelines for Image Compression
Guidelines for Image Capture.......Guidelines for Image Output
Guidelines for Image Transmission
Documented 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.
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, reliability, and ease with which copies may be generated:
- Silver-based film negatives in 35-mm or larger format
- 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:
- Photographic prints including prints produced through instant photography (Users are cautioned that individual photographic prints do not usually contain the full resolution and dynamic range available in original film negatives).
- 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:
- Inkjet prints
- Solid ink prints
- Thermal wax paper prints
- Dye-sublimation prints
- Dry-silver prints
- Laser prints
- Electro-static prints
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.
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 these 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. (See “Draft Recommendations and Guidelines for the Use of Digital Image Processing in the Criminal Justice System (Version 1.1 – February 2001)“; available at www.theiai.org/swgit/index.html).
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 use of such steps should be documented in the case notes in sufficient detail that comparably trained personnel can repeat the steps and produce similar output.
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 individuals 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 individuals who had custody and control of the archived image.
Guidelines for 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.
Guidelines for 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. Note that if lossy compression is used, critical image information could be lost and unwanted artifacts introduced as a result of the compression process. Repeated saving of a file using lossy compression may exacerbate the loss of image information.
Guidelines for 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 appropriate 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)
When documenting major crime scenes, it is strongly recommended that a camera capable of manual override, with interchangeable lenses, off-camera flash, and a tripod mount be used as the primary capture device. Conventional silver-based film in 35-mm format or larger is recommended for use as the primary media in this case. This documentation may be supplemented by video and/or digital still imaging.
It should be recognized that some agencies may wish to utilize equipment other than that recommended above. In such circumstances, the agencies should demonstrate and document that the selected equipment is adequate to meet the agency’s anticipated needs.
Further information and additional SWGIT recommendations relating to different law enforcement field applications may be found in the SWGIT document Guidelines for Field Applications of Imaging Technologies (Forensic Science Communications, January 2000).
Further information and additional SWGIT recommendations regarding image capture devices for forensic laboratory applications are not yet available.
Guidelines for 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
Guidelines for 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
Part III: Elements of Standard Operating Procedures
(Version 2.2 — December 7, 2000)
The following should be considered when formulating standard operating procedures:
Title: The title should be a descriptive name for the procedure.
Purpose: Why, when, and by whom the procedure is used.
Equipment/Materials/Standards/Controls: Identifies what items are required to perform the procedure. This may include protective equipment, hardware, software, and configurations.
Procedures: A step-by-step description of how the procedure is conducted. If appropriate, instructions should include precautions to be taken to minimize degradation.
Calibration: Describes any steps required to ensure the accuracy and reliability of the procedure. Where applicable, instrumentation setup and calibration procedures should be documented.
Calculation: Describes any mathematical operations that are applicable to the procedure.
Limitations: Describes any actions, interpretations, or equipment that are not appropriate for the procedure.
Safety: Identifies and addresses potential hazards in the use of the procedure.
References: Identifies documents both internal and external to the user agency regarding the procedure, related procedures, and principles behind them.
Part IV: Quality Assurance
(Version 2.2 — December 7, 2000)
Equipment.......Software.......Personnel
Personnel utilizing images and imaging technologies in the criminal justice system should implement quality assurance programs to ensure that results achieved are repeatable and valid. As part of the programs, performance checks and corrective actions should be documented.
Equipment
Where applicable, equipment utilized in imaging should be checked regularly for proper performance and calibration, and findings documented. Where applicable, an end-to-end system check for consistency within specified system parameters should be performed on a regular basis and whenever modifications are made to the system. All equipment should be maintained according to the manufacturers’ specifications and recommendations as contained in the operating manuals.
When a piece of equipment or system falls outside the specifications and recommendations, the equipment or system should be taken out of service until it has been corrected. Evaluation of equipment and system checks should be documented inclusive of corrective actions.
Software
If software errors that significantly affect the results of a processing step are detected, corrective actions should be taken. If the manufacturer identifies software errors and provides corrective remedies for them, the remedies should be implemented before the software is used again. Once corrective actions have been taken, an end-to-end system check should be performed prior to putting the system back into operation.
Personnel
All personnel utilizing imaging technologies shall be trained and tested for competency and proficiency in the agency’s standard operating procedures and the operation of the relevant imaging technologies. A formal training program should be documented and maintained, with the results of competency and proficiency tests documented. Proficiency testing should be repeated on a regular basis or when significant changes in hardware or software are made.
Part V: Training
(Version 2.2 — December 7, 2000)
Issues relating to training in imaging technology are addressed in the SWGIT document Draft Guidelines and Recommendations for Training in Imaging Technologies in the Criminal Justice System (available at www.theiai.org/swgit/index.html).
This document is identified as Version 2.2. Information referenced to this document can be identified as Version 2.2, 12/7/00.
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