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| Forensic Science Communications | April 2002 — Volume 4 — Number 2 |
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 Standards
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Guidelines for Field Applications of Imaging Technologies in the Criminal Justice System Scientific
Working Group on Imaging Technologies (SWGIT) Introduction.......Advantages and Disadvantages of Major Image Capture Technologies in Field Applications.......Silver-Based Film Cameras.......Discussion.......Instant Print Cameras.......Digital Cameras.......Video Cameras.......Hybrid Imaging Systems.......Guidelines for Standard Operating Procedures (SOPs) The purpose of this document is to:
This document addresses the photographic documentation of events or subjects that are not in a controlled environment (e.g., forensic laboratory or studio). The field applications addressed in this document include the following imaging:
Advantages and Disadvantages of Major Image Capture Technologies in Field Applications The selection of an acquisition device (camera) is driven by the purpose and requirements of the end product. Therefore, the final use of the image should determine the choice of the camera. The recommendations for primary and secondary image capture devices are dependent on current technology and may change subject to changes in technology. These recommendations are made based on the practical experience of the SWGIT membership in the acquisition and analysis of the images discussed herein. Agencies should decide what technologies are best suited to meet the requirements of their mission-specific tasks. As long as an agency can demonstrate and document that its choice of technologies is adequate to meet its anticipated needs, the agency should not feel required to adhere to the SWGIT recommendations. 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 along with conventional silver-based film in 35 mm format or larger as the primary capture media for evidentiary photography/imaging. Advantages of silver-based film cameras in field applications:
Disadvantages of silver-based film cameras in field applications:
Resolution: The best measure of resolution is the evaluation of output imagery, which is the product of a series of steps. However, since this document is devoted to field applications, the following discussion is restricted to camera evaluation only. Users should verify resolution by visually examining images of test targets in their specific environments. SWGIT uses the following definitions of resolution: "Measure of capability to delineate picture detail;" and "The ability of a photographic system to record fine detail." These definitions are a subset of those found in ANSI/A Technical Report TR26-1993: Resolution as it Relates to Photographic and Electronic Imaging.
Traditionally, film manufacturers measure resolution in terms of line pairs per millimeter (LP/mm) or lines per millimeter. Figure 1 illustrates how a line pair consists of a black line and an adjacent white line. Lines per millimeter refers to each line (black or white), thus there are always twice as many lines as there are line pairs over a given distance. Manufacturers of digital cameras frequently describe image size in terms of number of pixels. As stated in ANSI/A TR26-1993, "There is a common practice of referring to digital resolution as simply the total number of pixels within a frame or field, or alternately as the number of pixels in the horizontal and vertical directions. Alone, this information is useful to indicate the amount of data that can be handled in a single field at a given moment, but tells nothing about the unit's ability to resolve spatial information." (Note that the words frame and field in the quote refer to a photographic field of view. These should not be confused with the terms used in video technology.) The ability to resolve spatial information can only be determined by testing a specific imaging system. In order to compare technologies, the total information that can be represented in the sensor is discussed. This constitutes an upper limit to the actual achievable resolution.
It is possible to directly compare the maximum amount of information that can be represented by any two sensors by comparing the total number of pixels per frame. To compare film to the sensors in digital cameras (CCD/CMOS), it is necessary to relate line pairs per frame to pixels per frame. Figure 2 illustrates how, under ideal conditions, two pixel columns (or rows) represent one line pair. Common film types encountered in law enforcement field applications have optimal resolutions in the range of 40 - 160 line pairs per millimeter. Black and white films typically used at crime scenes have resolutions at the upper end of this range. Color films used at crime scenes have resolutions at the lower end of this range. A single frame of 35 mm ISO 200 color film is 36 mm wide by 24 mm high. With a resolution of 50 line pairs per millimeter, such a frame can resolve the following:
The equivalent number of pixels in a single frame is then calculated:
This represents a total of 3600 x 2400 = 8,640,000 pixels. A digital camera with a detector that is 3040 x 2008 pixels in size contains more than six million (6,104,320) pixels. This is referred to as a 6-megapixel camera. There is a tradeoff between the field of view and the resolution that can be represented by a sensor. If the field of view is held constant, then the resolution will vary with the number of pixels. If resolution is held constant, then the field of view will vary with the number of pixels.
If the field of view is held identical for both sensors above (35 mm ISO 200 color film and a 6-megapixel CCD), the film, with 40 percent more pixels than the CCD (8.64 million versus 6.1 million), will provide an improvement in resolution of approximately 20 percent. Alternatively, if the resolution is held constant for both sensors, the film will cover a field of view that is 40 percent larger than the CCD, as illustrated by Figure 3.
The resolution that can be achieved in a digital camera will be lower than the upper limit (ideal conditions) discussed above. Some research has found that it takes approximately four pixels to capture and reproduce a line pair under test conditions, instead of the representation by two pixels. Using this standard, the pixel resolution of film is effectively doubled. The practical result of using four pixels per line pair leads to the comparisons in Table 1 and Figure 4. NOTE: Discussing the resolution of video systems is beyond the scope of this document. SWGIT will address issues relating to video resolution in future documents. To compare the size of images generated from each of these sensors, a common output resolution must be defined. A conservative divisor of 200 pixels per inch (ppi) is generally used to determine the output size. Table 2 lists the size of the images generated using the sensor resolutions in Table 1. In the above discussion, only film and digital sensors were considered. In reality, an image is captured and later displayed using an entire system in which several factors may affect the resolution of the particular image. The quality of the optics (lens or lenses) used to focus the image on the sensor plays an extremely important role and may greatly limit the overall resolution of the system. Also, differences in display methods may limit or enhance the perceived resolution. CCD sensors often achieve color acquisition by using one sensor array with pixel cells of alternating color sensitivity. Thus the overall resolution of the image will be less than the specified number of pixels on the sensor. One way to ameliorate this situation is to use a camera that contains three CCDs instead of one. This allows one to have a separate CCD for each primary color, red, green, and blue. Therefore, for the same specified number of pixels at the sensor, a three-CCD camera will provide higher resolution than a single-CCD camera. Two other factors that can effect resolution include the contrast in the scene and noise. In this context, contrast refers to the apparent difference between the brightest and darkest parts of the scene. As contrast increases, resolution increases. Noise refers to random variations that limit the fidelity of detection and reproduction systems, such as the granularity of photographic images. As noise increases, resolution decreases. Identifying the specific effects of these factors on each of the sensors discussed above is beyond the scope of this document. Dynamic Range: The difference between the brightest highlight and darkest value that a sensor (film or CCD) can detect and record in a single image. Negative film provides two to four f-stops more than most digital cameras. This increased dynamic range allows capture of both shadow and highlight details in a single frame of film. These same details might require several different images (at different capture settings) when recorded with a digital camera. Color Range: The range of colors that can be detected by a sensor compared to normal human vision. Negative film has a color range that is superior to CCDs. Flexibility: The selection of speeds and types of film available. Silver-based photography provides a wider selection of film speeds and types than digital cameras. Film can be selected for specific applications in the field, and the film speed or type can be changed on-site to meet specific needs. Advantages of instant print cameras in field applications:
Disadvantages of instant print cameras in field applications:
Advantages of digital cameras in field applications:
Disadvantages of digital cameras in field applications:
NOTE: Fixed camera video surveillance systems are not addressed in this document. They will be addressed in future SWGIT documents. Advantages of video cameras in field applications:
Disadvantages of video cameras in field applications:
Notes on video formats:
A hybrid system is the combination of silver-based photography and digital imaging technology that typically involves the conversion of silver-based film or print images to digital images through the use of scanners. Hybrid imaging systems incorporate some of the benefits of both film and digital image technologies and are recommended for those agencies and organizations seeking to add digital imaging technologies to their photographic resources. Advantages of hybrid imaging systems in field applications:
Disadvantages of hybrid imaging systems in field applications:
Guidelines for Standard Operating Procedures (SOPs) General Guidelines for a Crime Scene Photography SOP TITLE: Crime Scene Photography SOP PURPOSE: To permanently document, by qualified personnel, evidence and other details at a crime scene for future reference NOTE: Crime scene photography generally requires the ability to:
Crime scene photography is usually a time-limited activity when there is only one opportunity to correctly complete the task. Depending on the nature of the crime or incident, conditions at a crime scene may dictate the selection and use of differing equipment and techniques. EQUIPMENT: (Image Capture Devices)
PROCEDURES: Agencies should follow agency-specific step-by-step instructions for documenting crime scene evidence. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: See Advantages and Disadvantages of Major Image Capture Technologies in Field Applications SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation and manufacturers' manuals. General Guidelines for a First Responder (A) SOP The only images taken are those taken by the first responding officer(s). TITLE: First Responder Photography SOP PURPOSE: To document conditions found at an incident by the first law enforcement official(s) on the scene where a crime scene photography unit or specialist will not be requested NOTE: Examples may include: domestic violence incidents, traffic accidents, minor property crimes, and other incidents as defined by agency-specific policies. Photography generally is not the first responder's primary responsibility, and the first responder may have only a minimal amount of photography training. EQUIPMENT: (Image Capture Devices)
PROCEDURES: Agencies should follow agency-specific step-by-step instructions for documenting crime scenes. NOTE: Images from separate incidents should be clearly delineated through a change of storage media or through proper documentation. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: Successful capture of close-up images will require adhering to manufacturers' specifications. Images captured digitally typically have a limited enlargement capability that is less than those captured using 35 mm film (See Advantages and Disadvantages of Major Image Capture Technologies in Field Applications). Successful capture of images also requires utilization of fresh media (e.g., film that is not past expiration and has been stored properly) and well-maintained equipment. SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation and manufacturers' manuals. General Guidelines for a First Responder (B) SOP Images taken by the first responding officer(s) prior to arrival of a crime scene photographer. TITLE: First Responder Photography SOP PURPOSE: To document conditions found at an incident by the first law enforcement official(s) on the scene prior to the arrival of a crime scene photographer NOTE: This guideline is directed toward documenting transient conditions that might be lost prior to the arrival of crime scene photographers. Examples include situations in which evidence must or might be moved, lost, or altered. Photography generally is not the first responder's primary responsibility, and the first responder may have only a minimal amount of photography training. EQUIPMENT: (Image Capture Devices)
PROCEDURES: Agencies should follow agency-specific step-by-step instructions for documenting crime scenes. NOTE: Images from separate incidents should be clearly delineated through a change of storage media or proper documentation. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: The successful capture of close-up images will require adhering to manufacturers' specifications. Images captured digitally or with video typically have a limited enlargement capability that is less than those captured using 35 mm film (See Advantages and Disadvantages of Major Image Capture Technologies in Field Applications). Successful capture of images also requires using fresh media (e.g., film that is not past expiration and has been stored properly or new, unused videotapes) and well-maintained equipment. SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation and manufacturers' manuals. General Guidelines for a Surveillance Photography SOP TITLE: Surveillance Photography SOP PURPOSE: To document acts and individuals engaged in those acts as they occur NOTE: Surveillance activities may involve highly specialized techniques and equipment that require technical training and knowledge and are best accomplished by trained specialists. EQUIPMENT: (Image Capture Devices)
NOTE: State laws may limit the legality of audio taping. Check local statutes for legality of all surveillance activities. SPECIALIZED EQUIPMENT: Night vision or thermal imaging equipment PROCEDURES: Agencies should follow agency-specific step-by-step instructions for documenting crime scenes. If audio is used, simultaneous recording of audio with video on the same media and testing of the system is strongly recommended. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: The successful capture of images sufficient for identification of depicted individuals and/or objects (e.g., license plates) will require close attention to the selection and appropriate use of equipment. Images captured digitally or with video typically have a limited enlargement capability that is less than those captured using 35 mm film (See Advantages and Disadvantages of Major Image Capture Technologies in Field Applications). The successful capture of images also requires utilization of fresh media (e.g., film that is not past expiration and has been stored properly or new, unused videotapes) and well-maintained equipment. SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation and manufacturers' manuals. General Guidelines for a Tactical Survey SOP TITLE: Tactical Survey SOP PURPOSE: To document conditions at a location so that plans can be made for future law enforcement activities NOTE: This type of photography is directed at obtaining general information regarding the physical layout and major contents of a location in preparation for law enforcement activities. EQUIPMENT: (Image Capture Devices)
PROCEDURES: Agencies should follow agency-specific step-by-step instructions for documenting crime scene locations. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: Location-specific limitations may apply. SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation and manufacturers' manuals. General Guidelines for a HAZMAT Scene Photography SOP TITLE: HAZMAT Scene Photography SOP PURPOSE: To permanently document, by qualified personnel, evidence and other details at a crime scene that may contain biological, chemical, and/or radiological hazards NOTE: A careful assessment of each HAZMAT scene is necessary prior to committing any personnel or equipment. The means for inserting, removing, and sanitizing equipment and personnel from a scene should be planned prior to initial entry. HAZMAT scene photography generally requires:
HAZMAT crime scene photography is usually a time-limited activity when there is only one opportunity to correctly complete the task. This type of photography usually involves the use of protective clothing and gear that will make the operation of photographic equipment more difficult. EQUIPMENT: (Image Capture Devices)
SPECIALIZED EQUIPMENT: Equipment for protecting personnel and camera systems PROCEDURES: Agencies should follow agency-specific step-by-step instructions for documenting crime scene evidence. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: Protective equipment will hamper the use of some equipment, and conditions may not permit the use of other equipment (e.g., flash, film). (See Advantages and Disadvantages of Major Image Capture Technologies in Field Applications) SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation and manufacturers' manuals. General Guidelines for an Aerial Photography SOP TITLE: Aerial Photography SOP PURPOSE: To document locations, activities, and geographical relationships using aircraft as a platform NOTE: Aerial photography may involve highly specialized techniques and equipment which require technical training and knowledge and is best accomplished by trained specialists. EQUIPMENT: (Image Capture Devices)
SPECIALIZED EQUIPMENT: Night vision or thermal imaging equipment PROCEDURES: Agencies should follow agency-specific step-by-step instructions for aerial photography activities. CALIBRATION: If necessary, agencies should develop procedures specific to their needs. CALCULATIONS: If necessary, agencies should develop procedures specific to their needs. LIMITATIONS: Check the legality of all surveillance activities. Images captured digitally or with video typically have a limited enlargement capability that is less than those captured using 35 mm film (See Advantages and Disadvantages of Major Image Capture Technologies in Field Applications). Successful capture of images also requires using fresh media (e.g., film that is not past expiration and has been stored properly, and new, unused videotapes) and well-maintained equipment. SAFETY: Agencies should develop procedures specific to their needs. REFERENCES: Agencies should refer to agency-specific documentation, manufacturers' manuals, and local statutes. General Guidelines for an Accident Scene Photography SOP
General Guidelines Covered Under General Guidelines for a First Responder SOP
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| Forensic Science Communications | April 2002— Volume 4— Number 2 |
