NBSIR 73 143 (l>)

Evaluation of Methods for Automatically

Determining the Fitness of Currency

D. P. Stokesberry and I. Philmon

Microwave & Mechanical Instrumentation Section

Measurement Engineering Division

Institute for Applied Technology

February 1, 1973

Progress Report covering the period

July 1 - December 31, 1972

Prepared for Bureau of Engraving and Printing U. S. Department of the Treasury

Washington, D. C. 20401

NBSIR 73-143

EVALUATIOf^ OF iETHODS FOR AUTOMATICALLY

DETERMIf^ING THE FITNESS OF CURRENCY

D. P. Stokesberry and I. Philmon

Microwave & Mechanical Instrumentation Section

Measurement Engineering Division

Institute for Applied Technology

February 1, 1973

Progress Report covering the period

July 1 - December 31, 1972

Note The results contained and the conclusions reached in this progress report are preliminary. Final results

and conclusions will be presented in the final report.

Prepared for Bureau of Engraving & Printing

U. S.'Department of the Treasury

Washington, D. C. 20401

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

IMATIOIMAL BUREAU OF STANDARDS, Richard W. Roberts, Director
II
Table of Contents

Page

1. Summary 1

2. Verification of Denomination 2

3. Tape Detection 5

4. Other Work 7

5. Plans For Future Work 8

6. Bibliography S

ii
I
EVALUATION OF METHODS FOR AUTCMATICALLY

DETERMNING THE FITNESS OF CURRENCY

1. Summary

The work covered by this report has been mainly directed toward

studying the feasibility of machine methods of verifying the denomination

of a note. Progress has also been made on the methodology of detecting

tape, tears, folds, and missing pieces. Hardware which demonstrates the

utility of each detection method is now being devised.

The denomination of a bill is specified by the ink pattern on that

bill, i.e., by the spatial relationship of the inked and non-inked areas. The ink pattern is low contrast; the inked areas are only partially

covered by dark ink and the clear areas are covered by a background tint.

Furthermore, the denomination is specified in a stylized font which does

not optimize the differences between characters. The information contained

in the ink pattern can be transformed into an electrical signal via optical

reflectivity measurements. The resolution of the signal is determined by

the size of the spot of light scanning the bill. A large spot will not

adequately resolve that pattern. A very small spot size will resolve the

fine detail of the printing, but this resolution is not necessary to

determine the denomination of the bill. Many of the features of the ink

pattern are independent of the denomination of a bill. Thus, much of the

information in the electrical signal is not pertinent and only obscures

the desired information.

Bills of different denominations have been scanned. We have demonstrated

that the electrical signals contain the desired information by converting

voltage levels to gray levels on a computer printout. The denomination of

the bill is apparent by visual examination of the printout, which is a crude

picture of the bill. A spot diameter in the range between 0.05 and 0.15 cm results in adequate resolution without overemphasizing the fine detail of

the printing. The large amount of data generated can be somewhat reduced

by restricting the examination of the note to specific areas of the bill.

Methods of analyzing similar waveforms electronically have proven successful

in the optical character recognition and pattern recognition fields. It is

quite likely that the same techniques could be applied in the development

of an automatic machine verification of denomination.

Both contacting and non-contacting methods of detecting the change in

thickness of a bill due to the presence of tape have been investigated.

We have found that it is possible to obtain adequate sensitivity and speed

of response from a simple contacting, moving-arm device. The contact with

the bill can be made in a way that will not interfere with many types of

transports, because the device looks like a idler roller to the transpoi't.
This method will work directly on either multiple-roller or spinningcylinder transports, and it would be unobtrusive to a moving-belt

transport. The measurement of thickness could be made through a moving

belt, but the belting material may cause both a loss in sensitivity

and an increase in noise. This approach looks more attractive than

non-contacting thickness measurement by detection of the mass absorption

of x-rays or beta particles. Commercially available detectors tend to

be slow or fragile. The contacting method is cheaper, simpler, and more effective than this non-contacting method.

2. Verification of Denomination

The manual sorting process currently practiced in the Federal Reserve

Banking System keeps an accurate account of the value of each packet of

notes sorted by (l) counting the notes and (2) verifying that all the notes

in a packet are the same denomination. This verification is done by visual

inspection as each note is separated from the rest during the sorting. In

an automatic currency-handling system, the notes will be separated only

inside the transport, and there will be no opportunity to inspect each

note separately. Therefore, the currency-handling system must include some method of verifying that the denomination of each note is what is expected.

Notes of the wrong denomination should be separated from the rest of the

bills. The machine need only recognize that the denomination of a bill

is incorrect, but, since the currency-handling system will undoubtedly be

capable of handling all denominations, each denomination must be uniquely

identifiable.

The Federal Reserve Banking System is now considering proposals on

various methods of determining the authenticity of a note. Some, but not

all, of these proposed methods will also uniquely determine the denomination

of the note. There is a need for a separate means of verifying the denomination during the interim period until these authentication methods

are developed. We are currently studying methods of verifying the denomination of a note while it is in motion, as it would be in a currency-handling

system.

The verification method is, in general, subject to two types of error:

(1) it may incorrectly separate out bills of the correct denomii\ation and

(2) it may fail to separate out bills of the wrong denomination. Errors

of the first type will cause an increase in activity for the operator and

will reduce throughput and efficiency. If this type of error occurs very

often, then almost all the bills sorted out will be false alarms. For

example, if the machine verification method makes type-one errors one percent

of the time and if less than one bill in a thousand is of the wrong denominatic

then almost all the bills sorted out would be false alarms and a false alarm

would occur on just about every strap of 100 bills. Type-two errors must also

2
bo ininirnized because these errors cause erroneous accounting. For

example, if the machine verification method makes type-two errors

one percent of the time and if about one bill in a thousand is of

the wrong denomination, then an accounting error occurs on about one of a thousand straps. For a machine throughput rate of 50 000 bills

per hour, this represents one error every two hours. This type of

error may be unrecoverable since the verification method gives no indication of error. In a well designed machine, the suppression of one type of error will lead to an increase in the other type of error

and it should be possible to change the ratio of the two types of errors by adjusting the machine.

Since the denomination of a note is specified by the pattern of the

printing, optical sensing of the ink pattern is the most obvious approach,

and it is the only method under study. The spatial pattern of the printing

can be sensed by scanning the bill with a small spot of light and monitoring the light reflected from the bill with a photo-sensitive detector.

The motion of the bill provides scanning in one direction; multiple spots

of light can provide the effect of scanning in the other direction. The

result is a time-varying signal at the output of the detectors. The

denomination of a bill can be verified by comparing the signal with that

expected for a bill of the correct denomination. If the signal does not

compare adequately, the bill can be separated from the rest of the bills

into a separate sort category. An automatic currency-handling system

will probably require some type of operator procedure to keep the accounting

correct whenever this occurs.

Our work on reflectivity measurements of currency, reported on previously,! indicates that adequate sensitivity can be obtained with a simple light source and photo-detector combination. Reflectivity profiles

of a clean bill, Fig. l(a), and a dirty bill, Fig. l(b) show the output of

a photo-detector as a 0.16 cm spot scans along a track parallel to the

long dimension of the note and approximately equidistant from the top and

bottom of the bill. The valleys in the waveforms occur as the spot scans

across inked areas of the bill. These areas are readily discernible even

on a dirty bill. This spot size is approximately equal to the dimensions

of the printed denomination numbers on the note, and it is much larger than

the fine detailed printing on the more heavily inked areas. Use of a smaller

spot size would result in better definition of the ink pattern, but it would

require more critical positioning for the scanning.

Entire notes have been scanned with a document scanner which moves a small spot of light over a stationary photographic negative of the dociunent.

The scan consists of horizontal sweeps across the document wherf^ the first

sweep is across the top edge of the document and each succeeding sweep is

moved down toward the bottom of the document. The results of these scans have been recorded in a form suitable for entry into a computer so that

3
different comparison techniques can be analyzed. The results of these

scans are two-dimensional arrays of numbers. One array coordinate

represents distance along the long dimension of the bill and the other

coordinate represents distance across the short dimension of the bill.

Each number in the array represents the reflectance value found for the

area of the bill defined by its coordinates. If shades of gray are substituted for the numerical values, the array becomes a crude picture

of the bill. The resolution of the picture is determined by the number

of points in the array. Hence, the scanning process has generated

electrical signals which represent, in different form, the same information

as a picture of the bill.

The desired response from a comparison is a simple "yes" or "no" answer. The signal from the photo-de tector, however, contains a great

deal of information about the bill. Much of this information is not

required to determine the denomination of a note and, indeed, it tends

to mask the essential information. The problem, therefore, is to devise

a system which will reliably determine whether or not the denomination

of a note is correct in spite of expected signal variations. This system

must operate on the essential information and it must be insensitive to

the extraneous information gathered in the scanning process. This will

require a comparison technique which will allow variations in the signals

received from notes of the correct denomination but will recognize the

difference between these signals and signals from bills of other denominations.

One method of distilling the information to a single value is to count

the number of peaks in the waveform of a scan of the bill. If the number,

of peaks is particularly sensitive to the denomination of a note, this

would constitute a simple and effective means of determining or verifying

denomination. The number of peaks in longitudinal scans of bills of

different denomination have been computed. (A longitudinal scan is one that parallels the long dimension of the note and extends over the entire

length of the note.) The results are summarized graphically in Fig. 2

through Fig. 9. In these figures the horizontal axis is a scan numbeiv which

indicates the position of the scan on the note. The first scan is across

the top edge of the note, and increasing numbers indicate movement of the scan down the note. The last scan is across the bottom edge of the note.

The vertical axis is the number of peaks in a scan.

Fig. 2, 3, 4, and 5 indicate the number of peaks in scans of one,

five, ten, and twenty dollar notes respectively. Each scan was digitized

into 160 points and 53 scans of the bill were recorded. This is equivalent

to using a 0.1 cm diameter spot to scan the bills. The four figures are quite similar. The highest number of peaks occurs near the top and bottom

edges of the bills. Scans of the middle sections of the four bills contain

a lower number of peaks, and the number is relatively independent of

denomination. This concurs with visual examination of the notes. The

4
denomination of a note is most readily determined by the information in

the top and bottom sections of the note. The middle section contains

little information on the denomination except for the portraits, and a much finer spot would be required to resolve the portrait. The difference

in number of peaks in either the top or bottom sections of the notes is

not sufficient to identify denomination unequivocally under less than

ideal conditions. If the notes are not in mint condition and if allowance

is made for less than perfect alignment of the scan due to the transport

or variability in the margins of the bill, there is a great deal of overlap between the number of peaks in the sean of notes of different denomination.

Higher resolution scans have also been recorded. Figures 6, 7, 8,

and 9 indicate the number of peaks for selected scans of one, five, ten,

and twenty dollar notes respectively. Each scan was digitized into 360

points, and the spacing between adjacent scans was approximately 0.05 cm. This is equivalent to using a 0.05 cm diameter spot to scan the bill. The

first 24. scans are across the top section of the bill, starting at the

top edge and proceeding down about 1.2 cm. The last 24 scans are across

the bottom section of the bill, starting about 1.2 cm from the bottom edge

and proceeding down to the bottom edge. These scans examine in greater

detail those portions of the notes which explicitly specify their denomination.

The number of peaks in a scan is increased as the spot size is reduced.

The scans across the bottom section include the area of the bill which has

the denomination of the bill spelled out on it. The number of peaks in this

region varies from 27 to 45 for the one dollar note, 31 to 36 for the five

dollar note, 31 to 40 for the ten dollar note and 36 to 47 for the twenty

dollar note. All in all, althoijgh the number of peaks in a scan is affected

by the denomination of a note, this method of analysis is not sufficient

to determine denomination.

The identification of denomination by optical scanning is very similar

in nature to optical character recognition and pattern recognition.

Techniques which have been developed in these fields can be quite sophisticated.

Further investigation should attempt to fit these techniques to the problem

of denomination verification.

3. Tape Detection

Many different types of tape may be found on bills in circulation.

These tapes are made of a variety of materials and they have a wide range

of physical characteristics, but the presence of tape on a bill invariably

causes a discernible increase in thickness. The thickness of a bill is

approximately O.OO4 inches. This value varies about 0.001 inches from

point to point on the bill due to the engraving of the paper by the printing

process and variations in thickness of the paper. The engraving is much

less noticeable on worn currency. Commercially available tapes of the type

5
found on bills are at least 0.002 inches thick. This additional thickness

is not difficult to sense statically with simple instruments such as a micrometer. At low bill speeds, the additional thickness can he detected

with a simple pivot arm and a micro switch. One end of the pivot arm is

held in contact with the bill. The other end presses on the tripping

mechanism of a small switch. The arm is designed so that a small change

in thickness causes a proportionately lorger change in the deflection of

the end of the pivot arm. The position of the microswitch is adjusted so that at a predetermined deflection of the arm, the switch is tripped.

This generates an electrical signal which indicates that the m.aximum

acceptable thickness limit has been exceeded. This type of detector is

common on low-speed currency-handling machinery to detect tape, folds or double feeds.

Both contacting and non-contacting methods of high-speed thickness

detection have been investigated. Theoretically, non-contacting methods

of detection are preferable because they do not interfere with the bill

transport mechanism and they tend to be capable of more rapid response.

One possible non-contacting method of detecting tape is a measurement of

the absorption of either soft x-rays or beta particles. The absorption

is proportional to the amount of material in the path of the radiation.

Either one-sided or two-sided measurements are possible. In other words,

the bill can either pass between the source and the detector, or the source

and the detector can be on one side of the bill and the radiation can be

reflected back with a suitable backing material. In either case, the

position of the bill in the path is not critical. Thus, tight control of

the bill by the transport is unnecessary. Experiment has shown that the

presence of tape will cause a two-to-one change in detector output for

3 keV x-rays. The radiation detectors are of the ionization chamber type.

The output of the detector is a pulse for each detected quantum of radiation.

After each pulse the detector has a characteristic dead time before it is

ready to sense another quantum of radiation. This dead time defines the

speed of response of the system. The number of pulses per unit time is

a measure of the intensity of the radiation. The accuracy of the measurement can be shown to be inversely proportional to the square root of the

number of pulses counted in a time period. Hence, accuracy is obtained

at the expense of speed of response. For bills moving at the rate of

twenty bills per second the detection scheme must have a response time in

the order of a few milliseconds in order to sense the presence of tape.

A measurement accuracy of five percent requires that the detector output

be about 400 counts in this time span. Thus the detector dead time must

be less than 5|J.s. A Geiger-Mueller tube has a dead time of about 270|j.s.

Proportional counters and scintillation tubes have fast enough response,

but they tend to be fragile and have limited useful lifetimes for the

measurement of soft x-rays. This method requires special high-voltage

supplies and, in general, it is bulky and expensive. We do not feel that

it is worthwhile to develop it at this time.

6
Contaoting methods of tape detection require special care to obtain

i'a.'rt. response and to avoid interference with the transport mechanism.

The response time of a mechanical system depends upon the mass of the

moving member and the spring constant which opposes the movement. If

the movement is rotational the moment of inertia, rather than the mass,

is used to characterize the system. Fast response is obtained by using

a strong spring and a low mass. A strong spring implies that considerable

force is required to deflect the moving member. This force must be applied

through the paper in such a manner that the movement of the paper through

the transport system is not impeded.

A laboratory model of a contacting thickness measuring device has

been constructed. This device consists of a small narrow roller bearing

mounted on a short shaft. The shaft is free to pivot in its mount. The

roller bearing is spring loaded against a driving wheel of a mechanism

which transports the bill. This device replaces one of the idler wheels

of the transport mechanism and is unobtrusive to the type of transport

that we are using. As a bill passes between the roller bearing and the

drive wheel, the shaft must pivot to accommodate the thickness of the bill.

Motion of the shaft is sensed optically. A flag attached to the shaft

interrupts the beam of light between a light source and a photo-detector.

Movement of the flag causes a difference in output from the photo-detector.

Typical waveforms are shown in Fig. 10. Both waveforms are the response

of the detector as a bill with tape on it moved through the device. The

tape was 0.75 in. wide and it was placed on the portrait area of the bill.

In Fig. 10(a), the bill was moving at the rate of two bills per second.

The waveform is an accurate indication of the point-to-point thiclmess

along the length of the bill. In Fig. 10(b), the bill was moving at the

rate of twenty bills per second. The peaks in the waveform are due to

the limited speed of response of the system and the waveform is not an

accurate indication of the point-to-point thickness along the length of

the bill. The presence of tape on the bill is, however, clearly indicated

in both waveforms. This device is sensitive enough and fast enough to

indicate the difference in thickness due to tape on bills' moving at the

rate of twenty bills per second,.

4. Other Work

Tears and missing pieces can be detected optically, and folds can be

detected by a combination of an optical measurement of bill size and a thickness measurement. Tears with a folded edge and missing pieces are relatively easy to detect optically, but tears where the edges tend to

stay together are difficult to sense

.

Optical methods of detecting torn bills require some type of mechanical

deflection of the bill to separate the edges of the tear. We have found

that light transmission measurements are quite sensitive to cuts in the

7
paper because the edges do not tend to overlap. A tear, however, does

not have sharp, straight edges, so there is no light path directlythrough the bill. Even if the edge of the bill is deflected perpendicularly to the plane of the bill, the overlap at the edges of the tear

still tends to mask the transmission of light through the bill. Best

results are obtained by looking for light transmission along a path

parallel to the edges of the tear instead of perpendicular to the plane

of the bill. This would imply that two transmission paths are needed

because the optimum path may be slanted either to the left or the right,

depending upon the direction of overlap of the tear. This method will

not be sensitive to tears parallel to the direction of motion of the bill,

since separation of the edges of a tear along the leading edge is difficult

to implement. Unobtrusive methods of deflecting the bill are highly

dependent upon the characteristics of the transport. Therefore, an optical

tear detector with mechanical or pneumatic means to "open" the tear must

be specifically designed to fit on a particular type of transport.

5. Plans For Future Work

1. Construct a measurement device which demonstrates the contacting

thickness measiarement proposed for tape detection and investigate

any promising new methods for the detection of tape or tears.

2. Design optical sensing methods which are sensitive to tears, folds

and missing pieces.

3. Determine whether well established techniques of optical character

recognition and pattern recognition are directly applicable to the

verification of the denomination of a bill.

6. Bibliography

[l] Stokesberry, D. P. and Philmon, I., Evaluation of Methods of

Automatically Determining the Fitness of Currency, NBS

Report 10 602, July 15, 1971.

8
1.0-

(fl) REFLECTIVITY PROFILE OF A CLEAN BILL

(b) REFLECTIVITY PROFILE OF A DIRTY BILL

0.5 SECOND

Figure 1. Reflectivity profile of a clean and a dirty bill

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mm N0U08

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16

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(o) OUTPUT FOR THE BILL MOVING AT THE RATE OF 2 BILLS /SEC

CO

0.5

0.4

0.3

0.2

0.1

0.05 sec A

TIME

(b) OUTPUT FOR THE BILL MOVING AT THE RATE OF 20 BILLS /SEC

Figure 10. Thickness detector output as a bill with tape on it passes

through the detector.

13
FORM NBS-114A (1.71) U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accessicm

BIBLIOGRAPHIC DATA -, ^ , , o SHEET NBSIR 73-143

3. Recipient's Accession No.

4. TITLH AND SUBTITLE

Evaluation of Methods For Automatically

Determining the Fitness of Currency

5. Publication Date

6. Performing Organization Code

7. AUIIIOR(S)

D. P. Stoke sherry & I. Philmon

8. Performing Organization

NBSIR 73-143

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4460443

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Bureau of Engraving and Printing

Department of the Treasury

Washington, D. C,

13. Type of Report & Period

Covered

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200--word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report summarizes the progress on developing fitness measurement

methods necessary for the automatic handling of currency. This work

shows the feasibility of determining the denomination of a bill by

making measurements of the optical reflectivity of the portrait side

of the bill. It also describes a method of measuring the thickness

of a bill while it is in motion. This method has adequate sensitivity

and speed of response to detect the presence of tape on a bill which

is moving through the detector at the rate of twenty bills per second.

17. KEY WORDS (Alphabetical order, separated by semicolons)

Fitness of currency; pattern recognition; thickness measurement

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