Home About Us Laboratory Services Forensic Science Communications Back Issues April 2002 General Guidelines for Capturing Latent Impressions Using...

General Guidelines for Capturing Latent Impressions Using a Digital Camera

fsc_logo_top.jpg
fsc_logo_left.jpg

April 2002 - Volume 4 - Number 2

Standards and Guidelines

General Guidelines for Capturing Latent Impressions Using a Digital Camera

Scientific Working Group on Imaging Technologies (SWGIT)
Version 1.2, December 6, 2001

Message from the Chair

Introduction | Equipment | General requirements for portable computers
Procedures | Procedure 1 | Procedure 2 | Procedure 3

Introduction

The purpose of this document is to describe the proper documentation of latent print evidence in the field by qualified personnel when using a digital camera.

The recommendations 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.

Equipment

It is recommended that a professional digital camera kit consist of, at a minimum, the following items:

  • Professional 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
  • Dedicated electronic flash designed for digital camera, capable of off-camera operation
  • Dedicated electronic flash cable to permit off-camera operation
  • Remote shutter release
  • Tripod capable of various angles and positions
  • Artificial light sources (e.g., flood lights, flashlights)
  • Digital storage media including additional media, as needed
  • Portable computer with appropriate software for downloading and viewing images at the scene
  • Appropriate cable connections (e.g., USB, Firewire, parallel) and/or PCMCIA capability
  • Scaling devices graduated in millimeters
  • Photographic log

The 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.

General requirements for portable computer dedicated for field use:

  • Processor speed 400 MHZ or faster
  • Color display
  • System memory of 128 MB or higher
  • Minimum hard drive of 10GB
  • Connectivity capability with camera
  • Appropriate camera software for image file acquisition
  • Appropriate software for file management
  • Network capability
  • AC power adapter/12V DC power adapter
  • CD-R/W (for CD-R media)


Procedures: Agency-specific step-by-step instructions for the capture of latent images should be followed.

Images from separate incidents should be clearly delineated by a change of storage media or by proper documentation.

Procedure 1: Lens calibration to achieve a minimum of 1000 ppi

The following 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.

  1. 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,

  2. 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).

  3. Make a template to the exact dimension of the area of coverage, (37 mm X 25 mm).

  4. Place template on a flat surface and fill image area in viewfinder with template and scale.

    After achieving the camera-to-subject distance,

  5. Mark lens with pencil, photograph template with scale, and acquire image into computer.

  6. Calibrate image with calibration software to actual size (1:1).

  7. Print image and measure the scale to verify size. If this is correct, the lens is calibrated.

    If this is larger than actual size,


  8. Move the camera away from the template, refocus, and mark the lens.

  9. 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,

  10. 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 ppi).

  11. Physically 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).

Procedure 2: Camera setup for latent impression photography

A typical standard operating procedure includes the following techniques used for field and laboratory settings.

  1. Locate visible impressions to be captured.

  2. Capture overall view of impression area without a scale with appropriate lighting.

  3. Capture overall view of impression area with scale and appropriate lighting.

  4. 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).

  5. Light visible impression appropriately.

  6. Place scale/identification tag adjacent to impression without obscuring detail. A tag should contain the following minimum information:

  7. Fill image area in viewfinder with impression and scale using lens calibrated with camera in use.

  8. Take light meter readings and adjust camera settings to capture image.

  9. 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.

  10. Visually verify images.

  11. If images are unacceptable, re-photograph.

  12. Repeat steps 7 through 11 for each lighting position used for that impression.

  13. After the evidence is processed for impressions, follow steps 5 through 11 as appropriate for all latent prints developed.

  14. Prepare photographic log or worksheet per agency policy.

  15. When using more than one storage media, uniquely identify each device.

Procedure 3: Use of a portable computer in field operations to acquire file/images

  1. Connect camera or removable media to computer according to manufacturer's specifications.

  2. 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.

  3. Prepare subdirectories for receiving downloaded images from camera or media. Create and name a subdirectory, using unique naming convention, in sequential order.

  4. Copy all original files to appropriate subdirectory.

  5. Verify that all images have been copied into the correct subdirectory.

  6. Set file permissions to preclude accidental deletion of files.

  7. If appropriate, erase removable media for reuse. Agency-specific standard operating procedures should dictate whether reuse of storage media is acceptable.

 

Top of the page