Appropriate Techniques for Law Enforcement
Division of Criminal Investigation Criminalistics Laboratory
State of Iowa Dept. of Public Safety
This article is intended to update all Law Enforcement personnel on new and/or suggested practices for the use of digital photographic and computer equipment. As you all are aware, the digital photographic world is here to stay, and the Law Enforcement community is no exception.
There are a number of suggested practices for the correct use of digital forensic photography. First and foremost, it must be understood that precise photographic techniques are a must for quality results. Over the last year, the DCI Criminalistics Laboratory has been required to analyze submitted digital photographic evidence. Most point-and-shoot digital cameras today still fall below the minimum resolution requirements, as established by SWIGIT (Scientific Working Group on Imaging Technologies). Their guidelines state that it is necessary to capture a single fingerprint at a minimum of 1000 X 1000 pixels per inch. Most of the digital point-and-shoot cameras on the market default at a setting of 640 X 480 pixels per inch, far below the required minimum for analytical quality photographs. Most agencies have used this resolution as the norm because of the large number of images that can be stored at that level. A misconception about the lower resolution is that it “looks just fine” on the computer monitor. Remember, a computer monitor normally views at a low 72 dots per inch! Even personnel who try to abide by the 1000 X 1000 pixels per inch, can make a grave mistake. Shooting a large overall photograph at that resolution does not allow for a high quality image of a single fingerprint within that area. By the time a 1:1 enlargement is made of the fingerprint, that resolution will again fall below minimum requirements. It is a requirement for all submitted fingerprint evidence to be sized 1:1 before being entered into the AFIS system. So, you can see the problems that can be created.
Remember to “fill the frame” of the camera with any fingerprint evidence. It is also a good idea to mark the areas involved, as it can be very difficult to see on the computer monitor. Ruler placement is of utmost importance with digital photography. (There are a few companies currently producing software that will allow for accurate re-sizing to 1:1 images). To make this procedure easier, make sure to place your rulers along the horizontal or vertical edge of the camera viewfinder area, if possible. This will allow for accurate image orientation and allow for easy cropping of the image. It is unnecessary to print out the entire image area during analysis, so a cropped image will allow for a much faster printing time from the computer to the printer. When a large number of fingerprints must be analyzed, it will shorten the production time by a large margin. Also remember that the ruler must be placed at the same plane as a footwear or tire track impression. Critical focus is imperative. A slightly blurry digital photograph can be impossible to analyze on the computer monitor. Once enlarged to 1:1, the problems are expanded!
Currently, digital cameras with a 5 mega-pixel resolution capability or higher, are working at an acceptable level. It requires a digital camera with approximately 10 mega-pixel capabilities to be equivalent to 35mm film. Because of the current high cost of these cameras, it still isn’t economically feasible for most law enforcement agencies. The other important parts of the “Digital Pipeline” include a compatible computer system with the appropriate amounts of memory, storage media and output devices, such as CD-R writers and printers. It is a good idea to establish an agency-wide standard operating procedure, in writing, that sets the required guidelines. Be consistent with the production of your digital imaging files.
Another very important point that must be covered is the use of JPEG files and TIFF files. I know that most officers would rather not have to deal with computer jargon such as JPEG and TIFF, but for imaging and archiving purposes, you must get to know these two terms. The JPEG file format is probably the most widely used photographic format today. In fact, this is what allows the less expensive cameras on the market to shoot so many images on a storage card or disc. The JPEG file format is a compression format. Which means, as soon as you take a photograph, image information is lost to allow for higher storage capability. Every time an image is saved as a JPEG, you will continue to lose information. On the other hand, the TIFF file format is a lossless type. TIFF files are generally of a much larger size and higher resolution, so it is difficult for most agencies to shoot TIFF images at the scene. For those who still use cameras that use the JPEG file format, I would suggest that once the images have been taken, down load your images to your computer and save them in a file folder, under the current case number, as TIFF files. This will guarantee that there will be no more image information loss during copying, moving or saving.
As mentioned before, the computer and the output devices are just as important as the camera. A highly acceptable scenario for a total digital system, would include a lap-top computer with at least 128-256 megabytes of RAM memory, 40-Gigabyte hard drive and an on-board CD-R writer. That way, you could down load your images directly to a computer hard drive and “burn” them to an archival CD-R disc. If your camera memory cards are of a limited storage capability, then you put the current images into the computer, re-format your card and begin shooting again. Keep accurate photo logs, as once you re-format your storage cards, you may lose the sequential numbering of your images. There are also external CD-R writers available that will plug right into the computer. The future of conventional prints could be questionable. Only those images needed for a court proceeding would need to be printed on conventional photo paper. It is not cost or time effective to try to print out large numbers of images on an ink-jet type printer. Photo quality papers are still higher in price and the printers themselves cannot keep up with the high print output pace of a sophisticated chemistry based photographic printer. There have been huge strides made in commercial digital printing over the last few years. There is a system currently in use that will allow for a quality chemical-based print to be produced up to 50 inches wide, from a relatively small digital file! Only a few of these systems exist in the world right now and we have one of them sitting in the middle of Iowa. So, high quality courtroom exhibit photos will be possible from our digital cameras.
Something that is not mentioned very often, but must be considered, is the problem of “reverse technology”. In other words, how will you retrieve old case digital images, when they have been placed on an obsolete storage media. Technology continues to change day by day, so think of equipment that must be maintained and used for recovery in the future.
A number of leading professional camera manufacturers are predicting the presence of film for some time yet. If you have doubts in your current digital system, always back up your photographs that you know will require analysis, with a film type camera. The day will come when CCD chip technology and price will bring the sophisticated single-lens reflex type digital cameras within reach of the law enforcement community. Then the choice will be time/work flow driven.
Hopefully, you have been able to draw some helpful hints from this brief. There are no set answers for our unique forensic problems, but we should be able to take a substantial “bite” out of them as technology and economics allow.