The old adage that a picture is worth a thousand words is as true in criminal investigation as it is in any other field of endeavor. While all components of crime scene investigation are important, visual documentation stands out as the most effective tool for describing and recreating a crime scene.
There are two primary methods of preserving visual evidence: still photography and video; both have strengths and weaknesses. Videotape allows the viewer to be immersed in a real-time moving image of the scene. It is the most effective method for transporting the viewer into the crime scene. However, videotape can't match the detail of film or high-quality digital still photographs.
Consequently, still photography is the best way to capture detail at a scene. Film and digital still cameras have much higher resolution than even the best video camera, so when detail and resolution are important, make sure the scene is shot with a still camera. But still photography also has some disadvantages. Viewing the scene through a series of photographs makes it harder to understand its scope and the spatial relationships of evidence. Video is a great way to avoid this problem because the tape is continuous and hence shows all areas within the scene. This is why the best way to capture a crime scene is with a combination of video and still cameras.
Of course, video applications for law enforcement are not limited to crime scene documentation. Other police uses for video include: car video cameras to record traffic stops, covert wireless surveillance video, low-light video, videotaped interviews and depositions, training videos, and video line-ups.
Video technology and terminology can be confusing. There are so many formats and so many new products are released each year that it is difficult to know what to purchase. But here's a look at what you really need to know to make the right purchase decision for your agency.
Both digital video cameras and VHS or 8mm analog video cameras capture images using a CCD (charge coupled device), which converts light into electrical energy. The only difference between the two is that a digital camera takes the data from the CCD and converts it, via an analog to digital converter, to a digital signal. Once converted, the digital data is stored on magnetic tape. A standard video camera captures the image through the CCD then stores the data as an analog signal. The real advantage of digital video cameras is that they are designed to facilitate the export of video into computer editing stations.
There are many different digital video formats, including Mini-DV, DV, Sony's Micro-MV, Digital8, DV-CAM, and Panasonic's DVC-PRO. Additionally, cameras are being introduced that record directly to a DVD disc. DVD and compact DVD discs hold far more data than a conventional recordable CD-ROM (CD-R). A full-size DVD offers over 4 gigabytes of storage, while a CD-R is around 700 megabytes.
Mini-DV is by far the most universal format and the majority of consumer and prosumer (near professional quality) cameras are based on this format. Mini DV tapes are inexpensive-usually less than $10 for a standard 60-minute tape. Longer running times can be achieved at a reduced long-play image quality.
DV, DV-CAM, and DVC-PRO formats are more oriented toward professional markets. These formats use larger tapes, which means more footage per tape. For example, Sony offers DV-CAM tapes up to three hours in length. DV tapes are also physically more robust and can withstand the rigors of professional use more readily than Mini-DV.
Among analog formats, Betacam SP is most popular with broadcasters. Betacam SP has a horizontal resolution of approximately 600 to 800 lines. In contrast, standard VHS is 240 lines of horizontal resolution and Super VHS is 400 lines of resolution and has the capability to show a cleaner picture because the luminance and chrominance channels are separate. In standard VHS and 8mm video, luminance and chrominance channels are combined. Standard 8mm video has 260 lines of resolution and Hi8 video offers 400 lines of resolution with separate chrominance and luminance channels.
Choosing a Camera
Digital video is the best choice for most law enforcement applications, although analog technology is still viable. A Mini-DV camcorder can be purchased for as little as a few hundred dollars up to many thousands of dollars. If possible, budget at least $800 to purchase a high-quality digital camcorder. Canon, Panasonic, and Sony are industry leaders, but many companies produce high-quality, reasonably priced digital camcorders.
In higher-end digital camcorders, incoming light is split into red, green, and blue components, and three separate CCDs capture one color each. Three-CCD chip cameras have greater color fidelity, a higher signal-to-noise ratio, and can record images in lower light than less expensive single-CCD cameras. Three-CCD digital camcorders are priced around $2,000 and the higher image quality justifies the price.
Image quality is also determined by the lens and the type of zoom that it employs. Most digital camcorders offer both optical and digital zoom. Optical zoom is a measure of the available field of view of the lens element, while digital zoom is artificially created by cropping the image in the camera. Digital zoom can be useful, but it's important to remember that image quality suffers when you use digital zoom.
Another feature to look for when shopping for a video camera is image stabilization. This is especially important if you're planning on shooting a lot of high-zoom close-ups such as surveillance footage from longer distances. When zoomed in close to a subject, image stabilization dramatically reduces the amount of camera shake or vibration and makes the image clearer.
If nighttime surveillance is what you plan to do with your new camera, then you may want to consider a special feature available on some Sony cameras. Sony's Super NightShot allows images to be recorded in total darkness up to 10 feet away. The camera transmits an infrared beam of light that can't be seen by the human eye; the light bounces off the subject and is recorded by the camera. The resulting image tends to be somewhat grainy, but it's much better than no image at all.
But you don't need Super NightShot if you have sufficient light. My agency (The Boulder Police Department) used both Sony and Canon digital video cameras to record University of Colorado students rioting last year. The video was taken from a few hundred feet away under dim lighting conditions, often with a fire as the only source of illumination. Both cameras performed very well, allowing still images to be captured from the videotape. The still images were numbered and placed on the City of Boulder's Website. Thousands of people viewed the images and over a dozen people were arrested as a result of being identified from the video.
Video Post Processing
Once the video images have been captured, it is often important to capture still images from the videotape, whether the video is sourced from a crime scene or a convenience store. This is an area in which digital video is superior to analog. Digital video can be directly downloaded from the camera into the computer.
But not just any computer. You need a machine with a lot of memory. Even though DV format video uses a 5:1 compression ratio, approximately 12 gigabytes of hard drive space is needed for 60 minutes of footage. A digital video computer workstation consists of a fast computer with a large hard drive and a lot of RAM memory. A Pentium 4 processor with at least a 7,200-RPM 60-gigabyte hard drive and 512 megabytes of RAM is ideal. Connectivity should include USB and preferably an IEEE1394 interface (FireWire), a newer high-speed connection designed specifically for downloading massive amounts of data. Also, an analog to digital video card is required if you are planning on digitizing analog video.
After the video is downloaded to the computer, still images can be captured using basic digital video editing software. Conventional magnetic videotape is susceptible to damage just from normal use. Every time a tape is viewed the quality is degraded, especially when using the pause button. In contrast, video on a computer hard drive can be watched without fear of damaging the evidence.
Many digital camcorders come with basic image editing software that is limited and doesn't allow for viewing or enhancement of digitized analog video. If you want to do high-quality work, you're going to need something better.
Avid Xpress is probably the premier image editing software offered today for the kinds of practical video editing necessary for police work. It is available as software only or in various turnkey systems, such as those offered by Ocean Systems. Ocean Systems markets the dTective forensic video system, which is based on Avid Xpress but includes numerous specific forensic enhancement filters developed by Ocean Systems. It can also be ordered on different hardware platforms, including a portable forensic video workstation. Avid Xpress can also be used in conjunction with image editing software such as Adobe Systems' Photoshop. The still image is captured from video in Xpress then saved as a TIFF or JPEG file and opened in Photoshop. The image can be improved using Photoshop's advanced image enhancement tools.[PAGEBREAK]
Videotaping a Crime Scene
Probably the most common law enforcement video application is recording a crime scene. The goal of videotaping a crime scene is identical to still photography: to record a reasonable and accurate representation of the crime scene. This means video should be taken before any evidence collection takes place, and usually before still photos are taken. A cover sheet helps to identify the location, date and time, type of scene, and photographer. It's also a good idea to turn on and properly adjust the camera's date and time stamp.
Before he or she begins shooting, make sure the videographer is familiar with the equipment, including advanced features of the camera. It is often beneficial to manually set the gain or white balance. Gain determines image brightness. White balance ensures that color is neutral and accurate under different light sources such as daylight, fluorescent light, and incandescent light.
Start shooting outside of the scene and work in, from a wide field of view to a narrow field of view. Many different techniques can be used to record a scene but always work slowly and methodically. It is often helpful to have a spotter walk directly behind the videographer to prevent him or her from disturbing any part of the crime scene; it's all too easy to be wrapped up in the videotaping and miss evidence right in front of your feet.
While shooting, imagine that you are a juror watching the tape and videotape the scene in such a way that it will allow a juror to understand the scene and the spatial relationships of the evidence within. Effective techniques include entering a room and slowly panning up and down, covering all of the surface areas in the room from floor to ceiling. Once the overall shots of a room are complete, zoom in to specific items of evidence. Linger on the evidence for an appropriate amount of time, slowly zoom out, then pan to the next piece of evidence.
Video and the Court
Legally, the issues surrounding video evidence in court are really no different than those for still photography, whether silver based or digital. The fundamental concern is whether the video shows an accurate reproduction of the scene.
Many law enforcement agencies worry about digital technology as it relates to forensic imaging. The primary reason for their anxiety is the ease with which digital images can be altered and manipulated.
But it is much harder to fabricate a believable altered video than most people think. Video consists of approximately 30 still images per second. In order to alter video, each frame must be altered individually-this could easily mean hundreds of frames for just a few seconds of footage. Software exists that automates special effects, but these programs are very costly and are primarily used in digital special effects for television and movies.
Still, the potential exists for both analog and digital video to be altered.
So how do law enforcement agencies ensure that video imaging is still admissible in court? The answer is simple. Demonstrative evidence such as video and still photography does not stand on its own in a court of law. The veracity of the images is not based on the technology, but rather on the person testifying to the authenticity of the video footage.
It's also important that the original videotape is kept in evidence. The tape should be copied, or digitized, and the original kept intact.
Always be straightforward with the defense if the tape has been edited. There is no problem with editing evidentiary tapes under certain circumstances such as surveillance tape that runs for three hours until the suspect can be seen. Just be sure that both the defense and prosecution are aware of the edit before the court date.
If the original tape is not used in court, John Pickering of the Boulder County (Colo.) District Attorney's Office recommends the officer, detective, or evidence technician view the copied tape before taking the stand to confirm that the copy is accurate.
Many agencies also use covert audio and video to record interviews and interrogations. While it is not absolutely necessary, it's a good idea to have signs placed at all entrances to the police department stating that persons entering the building are subject to audio and video monitoring. If a videotaped confession is used in court, the signs help alleviate protests from the defense attorney that his or her client didn't know they were being recorded.
Video is a powerful tool; if agencies follow reasonable guidelines and handle the original evidence carefully, defense attorneys will have a difficult time questioning the authenticity of video in court.
For more information
Law Enforcement/Emergency Services Video Association
David Spraggs is an investigator and firearms instructor with the Boulder (Colo.) Police Department. He teaches forensic photography and crime scene investigation.