Choosing a system for electronic fingerprinting

One of indispensable conditions for improving the effectiveness of AFIS is the use of state-of-the-art digital technologies replacing traditional ink fingerprinting.

Criteria for choosing a fingerprint scanner for an AFIS
Scanner functionality and scanning window size
Compliance with effective requirements to image characteristics
Need for ‘nail-to-nail’ rolling and smear compensation
Scanning of ‘complicated’ fingers and excluding the influence of moisture
Friendly GUI and ergonomics

Digital fingerprinting technologies based on the use of live-scan devices allow the most accurate recording of fingerprint patterns and creation of high-quality fingerprint databases.

The high quality of fingerprints ensures high accuracy of automatic recognition and encoding of images in the AFIS, and as a result, an increase in its most important search characteristics -reliability, selectivity, and speed of operation.

In addition to radically improved quality of fingerprint data arrays, the use of fingerprint scanners solves a number of other important tasks that arise when creating a fingerprint identification system:

Wide use of AFISs for the needs of law enforcement agencies and the increased interest in application of fingerprint identification methods in civilian security systems have generated growth of offers of fingerprint scanners from manufacturers around the world.

Today, there are different types of finger and palm print readers on the market – optical, capacitance, thermal, ultrasonic, etc. In forensic AFISs, optical live-scan devices are mostly used.

Optical scanners first appeared more than 50 years ago.

The first optical device patented in the Soviet Union (1956) is a papillaroscope designed for examination of fingerprints.

The operation of optical scanners is based on the effect of disturbed reflection of light at the border of transparent mediums. A bright light illuminates the glass platen from beneath and is completely reflected from the valleys in papillary pattern, but not from ridges.

High-quality optical scanners form images with better characteristics in terms of resolution and dynamic range, compared to other types of devices.

Критерии выбора дактилоскопического сканера для работы в АДИС (AFIS)

Criteria for choosing a fingerprint scanner for an AFIS

When choosing a fingerprint scanner and a system for electronic fingerprinting, be guided by the following criteria:

Relative value of key characteristics of the scanner (by estimate of PAPILLON AO)

Функциональность сканера и размер окна сканирования

Scanner functionality and scanning window size

The choice of a functional type of the scanner depends on the nature of tasks to be performed with it.

  1. Creation of tenprint cards for AFIS databases

Since the AFIS searches involve not only enrolled fingerprints but also latent fingerprints lifted at crime scenes, as well as fragmented patterns (unidentified corpses, victims of disasters and military operations), the fingerprint scanner you select must provide the imaging of fingers rolled from nail to nail.

If the tenprint cards include images of plain impressions, the scanner must have a sufficient size of the sensitive area for simultaneous scanning of four fingers.

If the AFIS deals not only with finger impressions, but also with palm prints (both enrolled and latent), it is necessary to choose a live-scan device with the scanning area sufficient for capturing images of palms.

2. Identity checks against AFIS databases (Express ID Checks in PAPILLON AFIS):

To support this mode, it is enough to use a single-finger scanner with a sensitive area designed to scan a single motionless finger. The size of the scanning area must ensure the capture of a complete uncut fingerprint image. Failure to comply with this requirement leads to the loss of some of the features, and consequently, to a decrease in the AFIS efficiency. Also, the insufficient size of the scanning window complicates the work of an officer who positions the individual’s fingers on the scanning platen.

Minimum scanning area dimensions (W x H) depending on the type of images acquired:

Rolled fingerprints – 41 х 38 mm (1.6 х 1.5″)
Plain impressions of four fingers – 1 х 76 mm (3.2 x 3.0″)
Palmprints – 127 х 127 mm (5.0 х 5.0″)
Single-finger flat impression (FAP levels) – 20 х 20 mm

Соответствие характеристик изображения требованиям нормативных документов

Compliance with effective requirements to image characteristics

Flatbed and fingerprint scanners are certified in the US FBI according to the “Test Procedures for Verifying IAFIS Scanner Image Quality Requirements”, CJIS-TD-0010 v. 7. The requirements of this document are duplicated in a number of international standards in the field of biometrics, and in Russian national standards as well. In particular, GOST R ISO/IEC 19794-4-2006 “Automatic Identification. Biometric Identification. Formats for Exchange of Biometric Data. Part 4. Fingerprint Image Data.” explicitly demands all live-scan devices used for law enforcement purposes to be certified for compliance with the FBI’s requirements.

These requirements regulate the main quantitative characteristics of the image quality obtained with a fingerprint scanner, including:

  • Fingerprint reader resolution – 500±5 ppi
  • Geometrical accuracy of the image (amount of optical distortions)
  • Characteristics of the transfer function (the scanner’s ability to precisely reproduce the spatially periodic picture of the image)
  • Signal-to-noise ratio (resistance of the output to hardware noise and electromagnetic disturbance, and immunity to any change in power supply parameters, etc.)
  • Dynamic range of the grayscale (ability to create sufficiently contrasting images)
  • Linearity of the grayscale (high fidelity of grayscale images of reproduced fingerprints)
  • Uniformity of the grayscale (the identity of multiple images of the same finger obtained in different parts of the scanning platen)

A reliable proof of the high professionalism of the developers, the certification is necessary, of course. However, the FBI certificate is not a guarantee of high efficiency of a fingerprint device when used in an AFIS. The explanation for this is simple: the requirements for flatbed scanners have been applied to fingerprint scanners, though their purpose is to convert a previously created stationary, flat, paper original (tenprint slip) into an electronic format without distortion.

For fingerprint scanners, which are not tools for conversion, but for the initial registration of fingerprint information, it is necessary to control additional requirements of a qualitative nature that have not yet been taken into account in any regulatory documents, the fulfillment of which is critically important when forming images, directly affecting the quality of created data arrays and performance of the AFIS:

  • Ability to compensate for smears at taking rolled fingerprints
  • Ability to acquire high-quality images of ‘complicated’ fingers (too dry, with low relief of ridges, erased pattern (so-called ‘housewife’s fingers’), etc.)
  • Ability to acquire high-quality images of wet fingers (e.g. cases of excessive sweating)

Необходимость полной прокатки пальцев и компенсации смаза изображений

Need for ‘nail-to-nail’ rolling and smear compensation

Impact of the captured area of fingerprints on the AFIS reliability

PAPILLON AO has conducted statistical studies on the AFIS database of 2 million tenprint records and 130 thousand latent prints in one of the biggest cities of Russia:

Mode of operation
1. Standard mode of operation that involves verified rolled fingerprints

Result 11,000 LT-TP matches

Mode of operation
2. Rolled fingerprints were replaced with plain impressions (55% of the area of rolled prints)

Result 35% reduction of search reliability

Mode of operation

3. Rolled fingerprints were replaced with flat impressions of inch to inch in size (this size is accepted in some foreign AFISs). The area of cropped images was less than the area of original images by an average of 9%.

Result 7% reduction of search reliability

Mode of operation

4. Some image parameters accountable at scanner certification (as discussed above) were changed:

– the image resolution reduced from 500 ppi to 400 ppi
Result 1% reduction of search reliability

– the image resolution reduced from 500 ppi to 300 ppi
Result 3% reduction of search reliability

– artificial narrowing of the dynamic range from 128 to 16 gray levels

Result 2% reduction of search reliability

– artificial geometric distortions of fingerprints, on average ±5%
Result 0.5% reduction of search reliability

The results of the studies show that:

Conclusion: An AFIS requires the most complete, nail-to-nail, fingerprint images. If incomplete images are entered into the AFIS, the latent prints left by unrecorded parts of the fingers will never be identified, even if the tenprint cards of individuals who left those latent are stored in the database.

In the examples below, the latents are identified in the AFIS against rolled fingerprints, but not identified against inch-to-inch finger impressions.

Latent print (11 minutiae coded)

Rolled fingerprint

Inch-to-inch fingerprint images (framed) contain only 4 minutiae

Latent print (18 minutiae coded)

Rolled fingerprint

Inch-to-inch fingerprint images (framed) contain only 4-5 minutiae

 

Smears at taking rolled fingerprints and their compensation

A rolled fingerprint is an image of the papillary pattern of a finger on the plane of a paper slip or sensitive prism of a live-scan device, depending on the method of fingerprinting used.

The geometric shape of the nail phalanx of the finger is not a regular cylinder. When rolling, the finger slips along the prism (or paper) causing the emergence of smears. The more fully the finger is rolled, the more likely that smears will appear. Besides, the degree of manifestation of this image defect depends on the anatomical features of the finger and the skill of the fingerprinting operator.

On tenprint cards with nail-to-nail rolled finger impressions obtained with the ink method, there are areas that are little or completely unsuitable for recognition as a result of smears. For ink technology, this image defect is almost unavoidable.

Smeared fingerprint on a paper tenprint slip

 

When taking rolls with fingerprint scanners, the situation is different.

There are hardware and software methods that allow, if they are successfully implemented, to almost completely eliminate the phenomenon of smudging on a generated image and, all other conditions being equal, to increase its useful area by 5-10% (it is that part of fingerprints that on average turns out to be unsuitable on paper slips).

Hardware methods of blurring compensation include:

The disadvantage of the first method is that it requires the use of cameras with a high frame rate. High-frequency and high-resolution cameras are expensive. Their use dramatically increases the cost of the scanner and, as a matter of fact, solves the problem at the expense of the consumer.

Attempts to use cameras with a lower resolution (and, accordingly, cheaper) result in a less size of the scanning area and a decrease in the amount of fingerprint data obtained.

The disadvantage of the second method is the presence of movable mechanical parts and automation tools that complicate the device, reducing its reliability and requiring scheduled maintenance or replacement (e.g. windings and moving parts of the step motor, the carriage drive, limit switches, etc.).

In case of software compensation for smears, the resulting image of the rolled finger is formed by programmatic gluing of fragments of sequentially captured frames. This approach is devoid of disadvantages of the hardware methods, but, in its turn, imposes some high requirements on the developers of software for scanners.

Thus, creating a scanner that could eliminate the problem of smears and provide a high-resolution imaging, and at a price acceptable to consumers at that, is a controversial and complicated task. Not all developers manage to successfully solve it, which means that not all devices used for taking rolled fingerprints equally reliably cope with the problem of smeared images.

How to check the scanner’s ability to compensate for smears at rolling

Since there are no yet methods that control the ability of a fingerprint scanner to compensate for smears during rolling, we can recommend that consumers, when choosing a device, visually assess the quality of generated images. To do this, it is enough to roll thumbs (which usually produce maximum smears) of several people and, having printed out the received images, carefully review them.

‘Undisguised’ smear – there are sharp shifts and breaks on the fingerprint image

‘Modest’ smear – shifts and gaps on the fingerprint image are shaded

High-quality compensation of smears on the fingerprint image

 

Сканирование «плохих» пальцев и исключение влияния влаги

Scanning of ‘complicated’ fingers and excluding the influence of moisture

Excessively dry or sweaty skin, papillary patterns with erased or small relief by nature – all these factors distort the true picture of a formed fingerprint.

Images of dry and erased fingers are obtained with low contrast, with a poorly distinguishable ridge pattern. Some significant details are lost due to the lack of contact of the substance naturally moisturizing the finger and the surface of the scanner prism.

Images of excessively sweaty fingers, on the contrary, are very dark. Some areas can merge into bright black spots due to the filling of valleys of the papillary pattern with moisture.

A high-quality fingerprint scanner should reduce the negative impact of these factors and form an informative image of a fingerprint, regardless of the skin condition.

The problem of dry skin, for example, is largely eliminated by the use of an elastic coating (membrane) on the contact surface of the scanning prism. Some manufacturers have found effective solutions to eliminate the influence of moisture.

To check how the device works with dry fingers, you need to dry the skin with chalk or alcohol before scanning.

You can simulate sweaty skin by simply dipping your fingers in water or moistening them with a damp cloth.

The analysis of the obtained images will allow you to make conclusions about the effectiveness of the scanner when working with ‘complicated’ fingers.

 

Low informative image due to excessively dry skin or insufficient contact with the scanning prism

 

 

A fingerprint of a very sweaty finger

 

Удобство программного интерфейса и эргономические характеристики

Friendly GUI and ergonomics

The quality indicators of an electronic fingerprinting system include the presence of a number of important functions that ensure the automation of the scanning process and minimize the influence of the human factor on the quality of generated images, as well as simplicity and convenience of operation.

For example:

  • Automatic control of the sequence of fingerprint images and the sequence of rolling. The slap-to-roll checks exclude frequent errors when the left hand is scanned instead of the right one, or when a wrong finger is scanned.
  • Automatic quality control. The system independently evaluates the size and quality of the received image, and in case of an unsatisfactory result, it issues messages about the alleged cause and makes it possible to repeatedly re-scan to achieve the required quality.
  • Automatic recording of the current scanned image in a tenprint file when switching to scanning the next one.
  • Support for an embedded database of tenprints with ability to search for records by a live-scan fingerprint or by biographic descriptors.
  • Independent control over the scanning process, without using a PC monitor and keyboard.
  • Standard interface connecting the scanner with a computer (USB, IEEE1394).

It stands to reason, it is impossible to make an exhaustive list of functions that a certain ‘ideal’ system for electronic fingerprinting should have. Well-developed software, as a rule, ensures the system flexibility, allowing it to adapt to a specific customer’s requirements and to perform a specific task.

The ergonomic requirements for fingerprint scanners are not unusual and do not require special explanations. This includes indicators that characterize, in addition to convenience of use, ease of installation, configuration, maintenance of the device, etc. It is also important that the outer dimensions and design of the scanner correspond to the operating conditions (e.g. a ruggedized housing of the device for use in adverse environment).

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