NBSIR 74-627

The Influence of Ink on the

Quality of Fingerprint Impressions

Raymond T. Moore

Computer Systems Section

Computer Systems Engineering Division

Institute for Computer Sciences

and Technology

National Bureau of Standards

Washington, D. C. 20234

December 12, 1974

Final

Prepared for Federal Bureau of Investigation

U. S. Department of Justice

Washington, D. C. 20537
\
NBSIR 74-627

THE INFLUENCE OF INK ON THE

QUALITY OF FINGERPRINT IMPRESSIONS

Raymond T. Moore

Computer Systems Section

Computer Systems Engineering Division

Institute for Computer Sciences

and Tectinology

National Bureau of Standards

Washington, D. C. 20234

December 12, 1974

Final

Prepared for Federal Bureau of Investigation

U. S. Department of Justice

Washington, D. C. 20 537

U. S. DEPARTMENT OF COMMERCE, Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

ILLUSTRATIONS

Figure Title

1 Ink testing machine

2 Typical test tracks

3 Vernier measurement of incremental

slippage

4 Four ink film thicknesses

5 Four ink film thicknesses with a different ink

6 38 mm (1-1/2 inch) bead of moderately

stiff ink 10900

7 38 ram (1-1/2 inch) bead of fairly

free flowing ink S

8 38 ram (1-1/2 inch) bead of very stiff

ink 10880

9 63 ram (2-1/2 inch) bead of 10880 ink

10 38 mm (1-1/2 inch) bead of moderately

stiff 10940 ink

11 38 mm (1-1/2 inch) bead of free

flowing type U ink

12 63 ram (2-1/2 inch) bead of ink U

13 Tests from a porous inked pad

1
THE INFLUENCE OF INK ON THE

QUALITY OF FINGERPRINT IMPRESSIONS

ABSTRACT

Tests were conducted on several types of ink to determine their influence

on the quality of fingerprint impressions which they could produce. The i thickness and uniformity of the film used to ink the fingers were found to

be the most significant factor in providing high quality impressions. A

method is described for metering out printer's ink and estimating whether > or not a uniform film of near optimum thickness has been rolled out on a glass Inking plate.

Key Words: Film thickness; film uniformity; fingerprint impressions;

fingerprint readers; image quality; ink films; lubricity.

INTRODUCTION

The performance of automatic fingerprint reading equipment, such as the

FINDER developed by the Federal Bureau of Investigation, is strongly

dependent upon the quality of inked fingerprint impressions. Blurs,

smudges, over-inked or badly under-inked Impressions degrade the output

data from the reader, even though sophisticated calibration and digital

filtering techniques are employed to compensate for variations in image

quality. This degradation takes the form of missed true minutiae and

increased numbers of false mlnutla detections, together with erroneous or missing ridge contour data. While matching minutia data can be

successfully .accomplished with moderate numbers of missing or false

minutiae, inadequate ridge contour data severely Impedes the automatic

registration and classification processes. Any single finger which

cannot be classified multiplies the number of file fingerprint reference

classes which must be considered as match candidates. Faulty registration adversely affects the matching process and may, under certain

circumstances, greatly increase the computation time that is required.

These considerations support the importance of high image quality in

inked fingerprint Impressions and have led to an investigation of the

factors which Influence image quality. The initial results of this

investigation suggest that control of the thickness and uniformity of

the ink film rolled on the glass plate for the purpose of inking the

fingers is a major factor in producing impressions of high image quality.

2
DESCRIPTION OF TESTS

Current Federal Bureau of Investigation Instructions for taking Inked

fingerprints stipulate the use of "heavy black printer's ink." No other

physical attributes of the ink are specified. Contacts with suppliers

of printing inks have revealed that there were many formulations of

black inks with widely differing viscosities, and presumably differing

in other characteristics. Three samples of these were obtained for

testing, together with three samples of inks specially packaged as "fingerprint inks" and distributed by organizations specializing in

law enforcement supplies.

The plan was to test the relative slipperiness or lubricity of these

inks to see whether there were significant differences. If so, this

factor would be expected to influence the likelihood of smudging an impression. This approach was suggested because of the empirical

observation that some of the non-inked processes for recording fingerprints employ chemical agents which do not feel as slippery as conventional inks.

A simple Instrument was constructed to test the relative slipperiness of

the ink samples. As shown in figure 1, it consisted of a wheel with

fixed loading established by weights which were mounted on a carriage

that could be rolled along a surface in a straight line. Cam

mechanisms permitted the wheel to be lowered to, or raised from, the

rolling surface. A thin solid rubber tire with a large number of

uniformly spaced transverse ridges was mounted on the wheel. The wheel

was also equipped with a torsion spring and a locking dog mechanism.

With the dog unlocked, the wheel could rotate freely on the pivot

bearings supporting the needle point axle. Locked, the torsion spring

was engaged and drag against the rolling surface Increased in

approximately linear proportion to forward motion. The wheel would

begin to skid when the frlctlonal forces at the rolling surface were

exceeded by the force developed by winding up the torsion spring.

To operate the device, the wheel is cammed away from the rolling surface

and unlocked. An inked glass plate is placed on the rolling surface

and the wheel is lowered onto the plate, rolled across it once, and

then raised. The glass plate is then replaced by a fingerprint card

which is clamped in position. The wheel is then locked to its torsion

spring and cammed down to a fixed starting point at the right-hand edge

I of the fingerprint card and rolled across the card past the point at

which continuous skidding occurs. Five such tracks are shown in

figure 2. All were made using one of the sample printer's inks. For

the top, middle, and bottom track, the wheel was cleaned and re-inked.

For the second and fourth track, the card was repositioned vertically,

and the wheel was not unlocked or re-inked so that these tracks were

produced by ink left on the wheel after making the previous track.

3
Examination of this figure reveals that full skidding occurs about

twenty percent farther from the starting point on the tracks made when

the wheel was not re-inked, compared with the tracks made by a cleaned

and freshly re- inked wheel. In other words, with less ink the skid

resistance was greater. In addition, these tracks show that partial

skidding increases as the torsional force on the wheel increases. This

is evidenced by an increase in both the width and the center-to-center

spacing of the transverse tread lines as the track progresses from

right to left. As each new tread ridge contacts the fingerprint card on

the rolling surface, it appears to skid slightly. This removes some of

the ink and causes the coefficient of friction to increase sufficiently

to stop the skid. Ink removal becomes progressively greater until a full skid condition is developed at the point where the (nearly) dry

frictional forces are less than the torsional forces.

Similar tests with the other ink samples yielded comparable results. On

a cleaned and freshly re- inked wheel, the point at which a full skid

condition was reached did not vary significantly with the different ink

samples, and with all samples a higher skid resistance was developed

when the wheel was used to lay down a second track without being

re-inked.

Measurement of the incremental slippage that occurred prior to the

point of full slippage of the wheel was accomplished with the aid of a very simple scheme. An inked track was laid down across the width of a fingerprint card with the wheel unlocked from the torsion spring. This

produced a set of uniformly spaced tread marks showing no evidence of

slippage. The card was then cut down the middle of this track so that

it could be laid along other test tracks and used like a vernier. The

right-hand tread marks of both the test track and the measurement track were aligned. The incremental slippage in the test track tread marks

caused a progressive displacement of this alignment moving to the left,

and alignment again occurred each time the cumulative slippage in the

test track increased by one increment of tread-to-tread spacing in the

measurement track. This is illustrated in figure 3 where the points of

cumulative slippage of the first eight units have been marked.

Up to this point, the test results had suggested that a thinner film of

ink on the glass inking plate might yield better results by more nearly

approximating the residual film left on the wheel when it was not

re- inked. If the right amount of ink could be metered out on the plate,

a film of optimum thickness could be rolled out. Unfortunately,

printer's ink is an especially difficult material to dispense in

precisely measured volumes of the order of a few hundredths of a cubic

centimeter without the use of special laboratory equipment, not normally

available to persons taking fingerprints. Our solution to this problem

was to use an approximation technique to measure out the ink. The ink

samples were repackaged in collabsible metal tubes with eye tips having

an orifice diameter of one millimeter (3/46 inch). This permitted

laying a bead of ink for a measured length on the glass. An approximate

volume could then be calculated as the product of the orifice area and

4
the bead length. The accuracy of the volume is dependent upon the

viscosity of a given ink sample. It is reasonably good with a stiff,

highly viscous ink, whereas, with a more freely flowing ink, the

calculated volume tends to be smaller than the true value.

The combined area of the glass plate and rubber roller used in these

tests is 216 square centimeters. An ink volume of 0.0216 milliliter

distributed evenly over this area would make a film one micrometer

thick. A cylinder of ink the size of the eye tip orifice and 2.66

centimeters long would supply this volume. Thus, to a first approximation, each inch of ink bead laid out on the glass should make an ink

film one mocrometer thick.

Figure 4 shows four tracks and four fingerprints prepared from ink films

made from beads ranging from 19 to 38 millimeters (3/4 to 1-1/2 inches)

in length. Figure 5 is a similar presentation using a different ink.

Further tests with the other ink samples showed that the eight unit

cumulative slip (refer to fig. 3) point always fell between the range

of values shown in figures 4 and 5 for ink film thicknesses of the order

of one and one-half to two and one-half micrometers calculated thickness.

These results are shown in figures 6 through 12, inclusive.

Figure 13 shows a sample track and fingerprint made from a newly inkimpregnated porous pad. The results indicate that the slipperiness of

ink from this pad was comparable to that of about two micrometer

thick films of the other inks tested. The inked ridge impressions of a fingerprint made from this pad were not quite as dark as some of the

others, but still provided an adequate contrast ratio. No tests were

made to determine the period of use necessary to deplete the ink supply

to the point where impressions with insufficient contrast would be

produced.

The percentage marks shown in several of these figures represent the

typical values of absolute diffuse reflectance as measured with an eight mil spot size with the ref lectometer spot centered over the inked

ridge. Values of the order of 15 to 25 percent are representative of

good quality prints and will usually be obtained from ink films about

two micrometers thick.

A useful way of estimating whether or not an ink film is the proper

thickness is to view a fluorescent lamp through the inked plate,

un-ined side facing the observer, at a distance of about six feet, in a normally lighted room. The outline of the lamp should just barely

discernible. This procedure will also dramatically reveal any unevenness in the film thickness.

5
CONCLUSIONS

From these tests > we conclude that apparent slipperiness of an Ink is

more Influenced by the thickness of the ink film that is rolled out on

the plate than on the type of printer's ink that is employed. The heavy

stiff inks are more difficult to roll out and distribute in a thin

uniform film, but if this is done carefully, good quality fingerprints

can be obtained. The inks that are commercially packaged as fingerprint

inks have a viscosity that makes rolling a uniform film somewhat easier,

but even with these, care is necessary. They are usually packaged in

collapsible metal tubes, but the orifice is much too large to permit

convenient metering of the ink. It is suggested that the distributors

of these products could perform a valuable service by packaging their

inks in tubes equipped with eye tips to facilitate metering the proper

amount of ink onto the glass rolling plate. A film thickness of about

two micrometers appears to be near optimum. This is obtained when 0.2

milliliter of ink is spread over each 1000 square centimeters of

combined area of plate and roller.

High quality fingerprint impressions are characterized by crisp, clear

ridge lines having few breaks and the absolute minimum of smudged areas. They should be well centered in the finger boxes and not overlay any of

the finger box outline marks to avoid masking useful minutiae or distorting ridge contour information. Officials taking fingerprints are urged to make every effort to obtain the highest quality of impressions

as the performance of an automated fingerprint identification system is

strongly dependent upon the quality of the input data.

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U.-S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. I'Um.K A I ION OR Ri;i^OR 1" NO.

NBSIR 74-627

2. Ciov't Accession

No.

3. Recipient's Accession No.

4, TITLH AND SLJHri ri.l-; The Influence of Ink on the

Quality of Fingerprint Impressions

5. Pub! icai ion Date December 12, 1974

6. Performing Organ i 7.al ion Code

650.01

7. AU'I'IIOR(S)

Raymond T. Moore

8. Performing Organ. Report No. NBSIR 74-627

9. PliRl- ORMlNc; ORGANIZATION NAMH ANH AOnRICSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Projeti/I'ask/Work Unit No. 6504400

11. C-ontract /(>rant No. FBI 34579

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Federal Bureau of Investigation

U. S. Department of Justice

Washington, D. C. 20537

13. Type of I^eport & Period Covered

Final

14» Sp(.)n s or in^ Agency Oodc

FBI

15. SUPP1J-:M1-:N l ARY NOTHS

16. AnSTRACn' (A 200-word or less {actual summary ol most signidcant information. If document includes a significant

bibliography or literature survey, mention it here.)

Tests were conducted on several types of ink to determine their influence on

the quality of fingerprint impressions which they could produce. The thickness

and uniformity of the film used to ink the fingers were found to be the most

significant factor in providing high quality impressions. A method is described

for metering out printer's ink and estimating whether or not a uniform film of near optimum thickness has been rolled out on a glass inking plate.

17. KE^' WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name: separated by semicolons

)

Film thickness; film uniformity; fingerprint impressions; fingerprint readers;

image quality; ink films; lubricity.

18. AVAILABILITY ^ Unlimited

^ °^ Official Distribution. Do Not Release to NTIS

Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20 i02. SD Cat. No. C13

Order 1- rom National Technical Information Service (NTIS)

Springfield, Virginia 221 'SI

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(THIS REPORT)

X

UNCLASSIFIED

21. NO. OF PAGES

15

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