Guidelines for Capturing Latent Impressions Using a Digital Camera
Working Group on Imaging Technologies (SWGIT)
Version 1.2, December 6, 2001
from the Chair
requirements for portable computers
of this document is to describe the proper documentation of latent
print evidence in the field by qualified personnel when using a
offered in this document are made in accordance with current National
Institute of Standards and Technology (NIST) guidelines specifying
1000 pixels per inch (ppi) at 1:1 as the minimum resolution.
It is recommended
that a professional digital camera kit consist of, at a minimum,
the following items:
digital camera capable of interchangeable lenses, manual override
for exposure and focus, off-camera flash, remote shutter release,
and tripod mount
- Macro lenses
capable of 1:1
electronic flash designed for digital camera, capable of off-camera
electronic flash cable to permit off-camera operation
- Remote shutter
- Tripod capable
of various angles and positions
light sources (e.g., flood lights, flashlights)
- Digital storage
media including additional media, as needed
computer with appropriate software for downloading and viewing
images at the scene
cable connections (e.g., USB, Firewire, parallel) and/or PCMCIA
- Scaling devices
graduated in millimeters
camera/capture devices listed in Chart
1 will meet the required 1000 ppi standard. However, this is
not an endorsement or a recommendation for the brand names or models
of equipment listed. This is not an all-inclusive list; there are
other cameras that will achieve this resolution.
requirements for portable computer dedicated for field use:
speed 400 MHZ or faster
- Color display
- System memory
of 128 MB or higher
hard drive of 10GB
capability with camera
camera software for image file acquisition
software for file management
- Network capability
- AC power
adapter/12V DC power adapter
- CD-R/W (for
step-by-step instructions for the capture of latent images should
separate incidents should be clearly delineated by a change of storage
media or by proper documentation.
1: Lens calibration to achieve a minimum of 1000 ppi
procedures should be completed prior to field use.
The pixel dimensions
on the sensor determine the camera-to-subject distance, and therefore,
the area of maximum coverage for 1000 ppi.
- Find the
pixel resolution on the chart to determine what the image size
will be. If the camera is not listed on the chart, see the manufacturer's
specification sheet for the pixel resolution size.
To determine the area in millimeters,
- Divide the
pixel resolution by 1000, then multiply by 25. (Twenty-five millimeters
equals approximately one inch.) For example, 1524 X 1012 equals
37 mm X 25 mm (rounded to the lowest millimeter).
- Make a template
to the exact dimension of the area of coverage, (37 mm X 25 mm).
- Place template
on a flat surface and fill image area in viewfinder with template
After achieving the camera-to-subject distance,
with pencil, photograph template with scale, and acquire image
image with calibration software to actual size (1:1).
- Print image
and measure the scale to verify size. If this is correct, the
lens is calibrated.
If this is
larger than actual size,
camera away from the template, refocus, and mark the lens.
- Repeat steps
5, 6, 7, and 8. Once verified, this becomes the camera-to-subject
maximum distance to provide 1000 ppi.
When the above procedure has been completed,
- Scribe a
line on the side of the lens over the pencil mark (recommended).
This is the known setting to achieve the correct resolution (1000
move the camera in and out to focus the image. This will ensure
that the resolution will not change (which would happen if you
adjust the lens focus).
2: Camera setup for latent impression photography
A typical standard
operating procedure includes the following techniques used for field
and laboratory settings.
- Locate visible
impressions to be captured.
- Capture overall
view of impression area without a scale with appropriate lighting.
- Capture overall
view of impression area with scale and appropriate lighting.
- Mount camera
on tripod with camera at a 90-degree angle to the impression.
Do not use a magnetic level. (Avoid contact or proximity of magnetic
fields with storage media and camera because these fields may
erase stored images and data and interfere with image capture).
- Light visible
- Place scale/identification
tag adjacent to impression without obscuring detail. A tag should
contain the following minimum information:
- Fill image
area in viewfinder with impression and scale using lens calibrated
with camera in use.
- Take light
meter readings and adjust camera settings to capture image.
- Capture impression
using correct exposure as indicated by the light meter. If necessary,
capture additional images of the impression by bracketing exposures
up one f-stop and down one f-stop.
- If images
are unacceptable, re-photograph.
- Repeat steps
7 through 11 for each lighting position used for that impression.
- After the
evidence is processed for impressions, follow steps 5 through
11 as appropriate for all latent prints developed.
- Prepare photographic
log or worksheet per agency policy.
- When using
more than one storage media, uniquely identify each device.
3: Use of a portable computer in field operations to acquire file/images
- Connect camera
or removable media to computer according to manufacturer's specifications.
- Create and
name a file folder on the computer's hard drive to receive original
image files. The camera or software may require unique file folder
names for each download. Care should be taken not to overwrite
existing image files from previously downloaded media. Some cameras
reset their file counters whenever media is changed.
- Prepare subdirectories
for receiving downloaded images from camera or media. Create and
name a subdirectory, using unique naming convention, in sequential
- Copy all
original files to appropriate subdirectory.
- Verify that
all images have been copied into the correct subdirectory.
- Set file
permissions to preclude accidental deletion of files.
- If appropriate,
erase removable media for reuse. Agency-specific standard operating
procedures should dictate whether reuse of storage media is acceptable.
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