Guidelines for Testing a Camera or Scanner for the Capture of Friction Ridge Detail

16-P-002-2.0

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1. Introduction

The purpose of this document is to describe a procedure to ensure that a digital camera or scanner can capture an image of friction ridge detail at an achievable resolution that enables the recording of level three detail. Friction ridge detail includes latent, patent, and plastic print evidence created by fingerprints, palms and the soles of feet.

2. Limitations

Resolution references are only for the image being viewed on a monitor or uploaded into an automated fingerprint identification system and are not for output to printed media.

3. 1000 Pixels per Inch (ppi) Requirement

There are several references to a 1000 ppi resolution requirement for images of friction ridge detail [1]Scientific Working Group on Friction Ridge Analysis, Study and Technology, “Standard for Friction Ridge Impression Digital Imaging”. [Online] https://www.nist.gov/system/files/documents/2016/10/26/swgfast_standard_imaging_final_2.0_130427.pdf, [2]Mangold, K. (2016), Data Format for the Interchange of Fingerprint, Facial & Other Biometric Information ANSI/NIST-ITL 1-2011 NIST Special Publication 500-290 Edition 3, Special Publication (NIST SP), National Institute of Standards and Technology, Gaithersburg, MD, [online] https://nist.gov/publications/data-format-interchange-fingerprint-facial-other-biometric-information-ansinist-itl-1-1., [3]K. Jain, Y. Chen, and M. Demirkus, “Pores and Ridges: High-Resolution Fingerprint Matching Using Level 3 Features,” IEEE Transactions on Pattern Analysis & Machine Intelligence, vol. 29, pp. 15-27, January 2007.. While the 1000 ppi resolution requirement permits the capture of level three detail. It does not mean that an image recorded at a lower resolution would contain insufficient detail and therefore be of no value for comparison. The important requirement for determining the needed resolution of images should be a determination made by an examiner and should be based on the resolution necessary to visualize those characteristics for analysis.

Using the Nyquist theorem [4]Olshausen. “Aliasing” (Handout), PSC 129 – Sensory Processes. Bruno Olshausen Homepage, Professor, Redwood Center for Theoretical Neuroscience (University of California, Berkeley). October 10, 2000. [Online]. http://redwood.berkeley.edu/bruno/npb261/aliasing.pdf, a 1000 ppi nominal resolution can theoretically achieve a maximum resolution of 500 line pairs per inch. Practically, three to four samples are required, resulting in a resolution between 250-330 line pairs per inch, or 9.8-13 cycles per millimeter (mm) [5]Zhang, F. Liu, Q. Zhao, G. Lu, and N. Luo, “Selecting a Reference High Resolution for Fingerprint Recognition Using,” IEEE Transactions On Instrumentation And Measurement, vol. 60, no. 3, pp. 863-871, March 2011..

4. Resolution Test Charts

Use a Resolution Test Target Capable of Measuring Between 9.8 to 13 Cycles per Millimeter (e.g., T-90 “Ultra High Resolution Target”). To determine if a digital camera is capable of capturing an image at a given resolution, it is necessary to use a test target. The test target used in this procedure is the T-90 “Ultra High Resolution Target.” This target is used as an example only, and its use here should not be construed as an endorsement. Other test targets can be used as long as they possess line pairs in the range of 9.8 to 13 cycles per millimeter.

Resolution test targets come in a variety of forms and styles. Horizontal and vertical multi-bar test targets are the focus of this procedure. Such multi-bar test targets consist of pairs of dark and light parallel lines (“bars”) of equal width (“line pair” or “cycle”), which repeat at a given frequency. The frequency is then defined in terms of cycles per unit distance. On the T-90 chart, spatial frequencies are reported in cycles per millimeter.

As an example, a set of line pairs in which the width of each individual line is 0.1 mm (i.e., dark line width = 0.1 mm and light line width = 0.1 mm) would have a combined line pair width of 0.2 mm, and would be described as having 5 cycles per millimeter (1/0.2 = 5).

Because a nominal resolution of 1000 ppi corresponds to an achievable resolution of approximately 9.8-13 cycles per millimeter. Any test target within this range would be sufficient. For the purpose of this guide, the 12.5 cycle per millimeter region of the T-90 chart is demonstrated.

Figure 1. Area of chart depicting 12.5 cycles per millimeter (Credit: SWGDE)

5. Procedure for Testing Digital Cameras

In order to consistently and reliably capture images with a digital camera at the required resolution, the camera shall be tested in the configuration(s) commonly used by the agency/organization. Resampling should not be used within this procedure in order to achieve 1000 ppi.

5.1 Field of View Determination to Achieve a Minimum of 1000 ppi

Prior to testing for resolution, it is necessary to determine the camera system’s maximum field of view that would record the target area at the equivalent of 1000 ppi. The pixel dimensions of the sensor define the area of maximum coverage for 1000 ppi. Refer to the specifications of the camera being tested to determine what values are appropriate. There may be several different sensor settings and file configurations that will allow 1000 ppi. The test should be completed using the same settings to be used during the capture of the friction ridge detail.

Determine the pixel resolution for the camera. See the manufacturer’s specifications for this value or obtain from the metadata of a previously recorded image captured by the camera being tested.
Divide the pixel resolution by 1000. The camera used in this example has a resolution of 3872 x 2592 pixels (See Figure 2). Dividing this by 1000 ppi results inches x 2.592 inches (3-3/4 inches x 2-1/2 inches). This represents the area of coverage in which the camera should be capable of capturing at 1000 ppi.

Figure 2. (Credit: SWGDE)

Make a template (or frame) to the exact dimensions of this area of coverage (3-3/4 x 2-1/2 inches).
Place the template on a flat surface.
Mount the camera on a tripod, or copy stand, above the template. Ensure the camera’s sensor plane is parallel to the template.
If using a fixed focal length lens, proceed to step 5.1.7. If using a zoom lens, proceed to step 5.1.8.
While looking through the viewfinder or live view display, adjust the height of the camera to fill the frame with the template and focus the image. If focus cannot be achieved, then 1000 ppi cannot be recorded and the test should be terminated for this lens. Otherwise, go to step 5.1.9.
When using a zoom lens, repeat step 5.1.8 for each of the zoom settings that will be used for photographing latent prints. This will result in different camera heights for different zoom settings. If focus cannot be achieved for some zoom settings, then 1000 ppi cannot be recorded with those settings. If focus cannot be achieved for this lens at all, then 1000 ppi cannot be recorded and the test should be terminated.
Record the height determined in step 5.1.7 or 5.1.8. This height is the maximum camera-to-subject distance to record 1000 ppi resolution.

The camera setup is ready to replace the template with the resolution test target and proceed to Section 5.2.

5.2 Camera Setup

Place the test chart on the flat surface below the camera so the test bars are in a vertical orientation (see Figure 1). The camera’s sensor plane must be parallel to this surface. Ensure that the camera is at the height determined in 5.1.7 or 5.1.8 and the region of the test chart depicting 12.5 cycles per millimeter is within the field of view.
Adjust the camera settings as needed for proper focus and exposure.
Adjust camera settings to use a file format with the highest resolution and least compression available.
Capture an image with the camera.
Open the file in an image processing application and zoom in on the region of the image that depicts 12.5 cycles per millimeter so that individual pixels are visible. If the camera has accurately captured 12.5 cycles per millimeter, it will be possible to distinguish the dark and light line pairs in this region. Do not use image post- processing to improve the visibility of the line pairs.
If it is not possible to distinguish the dark and light line pairs in this region, then this configuration will not record 1000 ppi. Resolution may be increased by decreasing the field of view (zoom in or get closer).
Rotate the chart 90-degrees and repeat steps 5.2.2 through 5.2.6 to measure vertical resolution. In some cases, the resolving power of the camera may be lower in the horizontal or vertical direction. Therefore, the shorter of the two distances determined should be recorded and used.
If resolution was modified by changing the field of view, the final distance from the target should be recorded for future use.

This process should be documented and repeated in accordance with organizational policy.

If the camera or lens was repaired or replaced, then this procedure should be performed prior to use in casework.

5.3 Capturing Friction Ridge Detail

For procedures to capture friction ridge detail, see SWGDE Guidelines for Capturing Friction Ridge Detail [7]

6. Procedure for Testing Scanners

6.1 Testing Scanner Resolution

For information on additional equipment, description of resolution test targets, and 1000 ppi resolution, as measured in cycles per millimeter, please reference Section 4 above.

Locate the portion of the test chart that depicts 12.5 cycles per millimeter (see Figure 1).
Set the scanner’s nominal resolution to 1000 ppi.
Place the test chart on the scanner plate with the top of the chart at the top of the scanning region. This will allow the user to measure the resolution in the horizontal aspect (as depicted in the figures above).
Scan the chart and and save the file using either lossless or no compression (such as TIFF or Bitmap).
Open the file in an image processing application and zoom in on the region of the image that depicts 12.5 cycles per millimeter so that individual pixels are visible. If the scanner has accurately captured 12.5 cycles per millimeter, then it should be possible to distinguish the dark and light line pairs in this region. Do not use image post-processing to improve the visibility of the line pairs.
If it is possible to distinguish the dark and light line pairs in the region of interest, then you have verified that your scanner can sample at 12.5 cycles per millimeter in the horizontal direction. If not, then your scanner does not meet the necessary resolving power at the 1000 ppi setting.
The scanner should be set to a higher nominal resolution (up to the limits of the scanner’s optical or machine resolution) and retested (step 6.1.3). Some scanners exhibit higher achievable resolution in the center of the scan area. Thus, it may be appropriate to retest at different locations on the scanner.
Rotate the chart 90-degrees and repeat steps 6.1.3 through 6.1.7 to measure the vertical resolution.

This process should be documented and repeated in accordance with organizational policy.

As scanners age or are used frequently, it may be necessary to retest on a more frequent basis. Likewise, if the scanner was repaired, then this procedure should be performed prior to use in casework.

7. References

[1] Scientific Working Group on Friction Ridge Analysis, Study and Technology, “Standard for Friction Ridge Impression Digital Imaging”. [Online] https://www.nist.gov/system/files/documents/2016/10/26/swgfast_standard_imaging_final_2.0_130427.pdf

[2]Mangold, K. (2016), Data Format for the Interchange of Fingerprint, Facial & Other Biometric Information ANSI/NIST-ITL 1-2011 NIST Special Publication 500-290 Edition 3, Special Publication (NIST SP), National Institute of Standards and Technology, Gaithersburg, MD, [online] https://nist.gov/publications/data-format-interchange-fingerprint-facial-other-biometric-information-ansinist-itl-1-1

[3] K. Jain, Y. Chen, and M. Demirkus, “Pores and Ridges: High-Resolution Fingerprint Matching Using Level 3 Features,” IEEE Transactions on Pattern Analysis & Machine Intelligence, vol. 29, pp. 15-27, January 2007.

[4] Olshausen. “Aliasing” (Handout), PSC 129 – Sensory Processes. Bruno Olshausen Homepage, Professor, Redwood Center for Theoretical Neuroscience (University of California, Berkeley). October 10, 2000. [Online]. http://redwood.berkeley.edu/bruno/npb261/aliasing.pdf

[5] Zhang, F. Liu, Q. Zhao, G. Lu, and N. Luo, “Selecting a Reference High Resolution for Fingerprint Recognition Using,” IEEE Transactions On Instrumentation And Measurement, vol. 60, no. 3, pp. 863-871, March 2011.

[6] Scientific Working Group on Digital Evidence, “SWGDE Digital Image Compression and File Formats Guidelines,”. [Online].https://www.swgde.org/documents/published-complete-listing/16-m-001-digital-image-compression-and-file-format-guidelines/

8. History

Revision	Issue Date	History
1.0 DRAFT	9/15/2016	Initial draft created (titled “SWGDE Guidelines for Capturing Latent Impressions Using an Image Capture Device”) and SWGDE voted to release as a Draft for Public Comment.
1.0 DRAFT	10/8/2016	Formatting and technical edit performed for release as a Draft for Public Comment.
1.0 DRAFT	1/12/2017	Full rewrite performed on the initial draft; title changed to SWGDE Guidelines for the Testing and Capture of Latent Impressions Using a Camera or Scanner.” SWGDE voted to re-release as a Draft for Public Comment.
1.0 DRAFT	2/21/2017	Formatting and technical edit performed for re- release as a Draft for Public Comment.
1.0 DRAFT	6/22/2017	Full rewrite performed on the second draft. SWGDE voted to re-release as a Draft for Public Comment.
1.0 DRAFT	7/11/2017	Formatting and technical edit performed for re- release as a Draft for Public Comment.
1.0	8/24/2017	SWGDE voted to publish as an Approved document (Version 1.0).
1.0	9/25/2017	Formatted and published as Approved Version 1.0.
2.0 DRAFT	1/10/2023	Full re-write on v1.0; title changed to “SWGDE Guidelines for Testing a Camera or Scanner for the Capture of Friction Ridge Detail”; removed section on capturing images and referenced companion document “SWGDE Guidelines for Capturing Friction Ridge Details”. Submitted for SWGDE vote to release as a Draft for Public Comment.
2.0 DRAFT	3/31/2023	SWGDE voted to release as a Draft for Public Comment; formatted for release for public comment.
2.0	6/15/2023	No public comments received; forwarded for SWGDE membership vote to release as a Final Approved document.
2.0	7/14/2023	SWGDE membership voted to release as a Final Approved document pending minor editorial and formatting changes.
2.0	8/19/2025	Minor editorial and formatting changes based on SWGDE membership vote completed July 14, 2023. Updated to new template and formatted for release as a Final Approved Document.

Version: 2.0 (8/19/2025)