CA2020739C - Method of detecting a bar code - Google Patents

Abstract

The invention relates to a method and an apparatus for detecting a bar code from a bar code signal which essentially forms a cross-section of a bar code pattern which through irradiation luminesces from the background of a carrier under fluorescent action.
Detection is performed by testing the bar code signal F(t) within each area (ZG1, TIS) within which a bar may be expected, against a bar criterion (THR, MTHR) obtained by prediction with the aid of a prediction table (TABLE 1) from a local background signal value (AGR) locally derived from the bar code signal F(t).
In this method use is made of the fact that, first, between the bars background of the carrier is invariably present, making a periodical reliable background approximation from the bar code signal value possible, and, second, there is a certain correlation between a background and the additive signal contribution as a response of the bars luminescing from that background under irradiation.
The prediction table is compiled beforehand from series of values - obtained with the aid of a test set of letters - for the average background signal, the maximum variation thereof, and the corresponding minimum bar response. The properties of the bar ink used and the pickup means (5, 61) for obtaining the bar code signal are expressed in these values. The advantage is that background influence, notably as a result of the local or global luminescence of the background itself, no longer adversely affects the reliability of a 'bar/no bar' decision.

Description

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Title: Nethod of detecting a bar code A. BackarQtln~l of the i nvention 1. Field of the invention The invention relates to the reading of bar code patterns applied to carriers for the carriers to be automatically recognized. It cullCe~llS a method of detecting a bar code from a bar code signal which essentially forms a cross-section of a bar code pattern which, through irradiation, lllmi nP~cPC from the background of a carrier. The invention also comprises an apparatus for reading such a bar code pattern .

2. DescriPtion of the Prior ~rt In automatic postal processing systems, as is well known, bar coding is used for sorting according to destination, for inGtance. To that end, at the input of such a system, for instance by means of video coding, each letter to be processed in such a system is provided with a processing code in bar code form.
The processing code may be a destination code, as a postcode, derived from the destination address provided on the letter. At one or more decision points in the process the bar code is read. Reading the code basically comprises the following steps:
a. picking up an image signal of the physical bar pattern on the carrier by passing it along optical scAnn i ng means;
b. detecting the bar pattern from the image signal and indicating, for instance in digital form, "bar/no bar" and, if applicable, the type of bar ` ` 2~20~3~

(e.g. thick/thin), for each position in the bar pattern;
c. rleco~l i n~ the detected bar pattern .
On the one hand, a bar pattern provided on the 5 carrier should be as inconspicuous as possible, but on the other it should be readily distinguishable from any other printing when read automatically.
Accordingly, such bars are typically applied to a carrier in an ink that emits light under luminescent, 10 particularly fluorescent, effect. A bar code signal of a lllm;n~cc~nt bar code pattern can be read using transducing means such as known, for instance from Dutch patent specification NL 164g80. For the bar pattern on the carrier to luminesce, it is to be 15 subjected to focussed irradiation using W light, for instance. Here, a specific problem arises, namely that of bauk~Lull..d influence due to such irradiation.
This means that irradiation will not only cause the bars written in fluorescent ink to ll~m;n~cce, but 20 also their ba~ kyLuulld, wholly or locally, which is a fact to be taken account of. This is the case when envelopes used f or letters are made of paper containing so-called "whiteners", which have rluorescent properties. The same problem presents 25 itself when other writing or printing in f luorescent ink extends into the zone of the letter where the bar pattern is applied. Moreover, it has turned out that a lllm;n~cc~nt background may act as an amplifier of the lllmin~c~ nt effect of the bars themselves. Najor 30 variances may then arise in the signal amplitude of the image signal read, not only in bar code signals of successive letters, but even within one and the same bar code signal. This may weaken the reliability of the signal information used to make "bar/no bar"
35 decisions.

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When the bar code used is of the ' mark space ' type, the background influence also makes it more dif f icult to detect spaces in a bar pattern . In short, the problem is basically one of finding a 5 reliable signal threshold or another criterion for each "bar/no bar" decision to be taken.
B. ' ry of the invention The invention offers a solution to the problem stated hereinabove. It is based on the experimental 10 experience that, first, a reliable background approximation from the bar code signal values is always possible in virtue of the fact that the ba~,}.yl UUllli of the carrier is invariably ~re6ent between the respective bars, 15 and, second, there is a certain correlation between a ba. kyL uu.ld and the additive ~ uullse of the bars lllm;nPsrin~ from the bauk~uu-ld under irradiation.
Using this experience, the method according to the invention is characterised in that 20 the bar code signal within each signal area in which the bar code signal may be expected to have a bar signal value cu~ L,ul ~l;n~ to a bar, is tested against a bar criterion obtained through prediction from a local approximated background signal value 25 derived from the bar code signal in that signal area.
This means it is possible to make a reliable prediction for each ~ar area to be PYAmi nPd about what criterion the signal values within that area should meet f or a bar to be detected or not within 30 that area, on the basis of a priorly established correlation between the local baul~u,Luu-ld signal value and the additive L~ uullse of a l~lminP~cPnt bar pattern. Sinoe such a correlation is also a reflection of the properties of the ink used and the o~9 properties of the pickup used, the operation according to the invention is further characterized in that the prediction is carried out with the aid of a priorly compiled prediction table.
Further preferred features and r-' o~ 5 of the invention are summarized in the other 6ubclaims and described in detail with reference to the drawings.
C. References ( 1) Dutch patent specif ication NL 164980 Title: Optical reading head (2) Dutch patent specification NL 183790 Title: Method for character segmentation D. Brief description of the ~lrawings The invention will be further explained with reference to the drawing6, in which:
Fig. 1 shows an apparatus for obtaining an index signal F(x) and for detecting an index from this index signal and ~l~co~l;n~ an index;
Fig. 2 shows an ideal index signal F* (x);
20 Fig. 3 shows the transfer function (PSF) H(x) of the pickup used;
Fig. 4 shows the convolution F(x) of F*(x) with H(x), in theory;
Fig. 5 shows ditto, in practice;
25 Fig. 6 schematically shows the spectral distribution of the light emission of carriers containing f luorescent pigments;
Fig. 7 shows a part of the index zone of a carrier with the outermost positions of the f irst bar;
Fig. 8 shows an index signal of the part shown in Fig. 7, viewed in the time according to a convolution as shown in Fig. 4;

~2~3 Fig. 9 6hows the signal of an index bar.
E. Desription E . 1 Tn~rQduction For the purpose of automatic postal processing, a destination code on a letter, for example in the form Or a postcode, is translated into a bar code, called index,and applied to the letter in a fluorescent ink (printed, written, or sprayed). For the Netherlands, the postcode consists of four numerical and two alpha signs separated by a space. In a video coding operation, for instance, this information is encoded into a bar pattern consisting of 36 successive segments, 6 units of 6 segments per sign, with a nominal pitch of 1. 66 mm. In each of these segments a vertical bar may be flicpos~d with nominal ~ ionc of 0 . 5 mm width and 5 mm height. The encoding is such that each unit starts with a bar and, in addition, can be represented by a bit pattern of zeros (no bar) and ones (bar). The reading of the index is based on 2 o the f luorescent properties of the bar ink .
Fig. l schematically shows how a letter 1 with an index pattern 3, also called 'index' for short, provided in an index sone 2 specif ically intended for the purpose, is passed along a W light source 5 emitting W light of 365 nm, and a pickup 61 at a LL~nS~OL ~ rate of about 3 m/sec and a frequency of 8 letters/sec in a transport direction 4 for the index 3 to be read. Irradiated by the W light, the fluorescent bars of the index 3 light up from a ba~:kyL~u~-d formed by the material of the letter. Due to this ll~m;n~cc~n~e~ an optical signal is generated which is suhceqll~ntly picked up by the pickup 61 and converted into an electric index signal F(x). Then, in known manner, this signal is sampled, converted 20207~

into a digital signal by means of A/D converting means 62, and under control of a proces60r 63 temporarily stored in a memory 64 accessible for further processing. The further processing comprises 5 the detection proper of the index pattern from the stored digital signal values, and is carried out by the abu~ ioned ~JLOCeSSUL 63 using ,uro~L -~based on the new metllod of detection according to the invention to be described hereinaf ter . The detected 10 index pattern, the bar code, is then decoded into index I, the destination code proper, with the aid of oll;n~ means 65, and used for further proc~cs;n~ of the carrier of the imdex pattern cuLL~ ; n~ to this index.
15 E . 2 AnalYsis o E the index siqnal F ~x) The electric index signal F(x) in fact repre6ents a cross-section of the index 3 on the letter 1 scanned in a direction x, opposite to the direction of transport 4. The pickup 61 i8 required to have a 20 distinctive power in the direction x. If its power were infinitely great, in such an ideal case F would look like the fictive signal F*(x). A part of the form of such a signal is shown in Fig. 2 as a function of x covering five segments, the signal in 25 each segment - the segment separation is designated by 7 - indicating either a space 8 or a bar 9. In practice, however, the pickup has a finite resolving power, on account of the fact that the index pattern 3 is picked up with a pickup provided with a vertical 30 slit (i.e. vertical to the direction of transport x) having a finite width, preferably chosen to be equal to the nominal width of an index bar, which is 0 . 5 mm in the present case. The pickup accordingly has a transfer function (Point Spread Function [PSF] ) 35 designated by H(x) in Fig. 3, which is uniform across ~` 2~207~9 the slit width 10 and zero outside of it. Ftx) can thus be repre6ented by the convolution of F* (x) with H (x):
F(x) = F*(x) x ~ H(x) (1) The theoretical form of F(x) i5 shown in Fig. 4 and a corr~cp~An~li nj signal in practice in Fig. 5, where 7 again indicates the segment separation, 8 a space and 9 an index bar.
The signal F(x) is built up from three signal Ants, the ~ t coming from the paper ba~:}.u,Luulldl the emission of the fluorescence pigments of the ink used for the index bars, and the noise in the pickup system.
F(x) = A(x) + I(x) + R(x) (2) 15 wherein A(x): Bauk~Luul.d ~ ,vl~ent I (x) : Index ~ Ant R(x): Noise ~ Ant The f irst two ~ -nts themselves are each 20 composite and will be subjected to further consideration. A substantial part of the noise - An~nt consists of paper noise, but also the pickup used for obtaining an electric index signal F (x) contributes to the noise . It will be shown that 25 by using the invention, the influence of the noise 1 on the detection result is implicitly taken into account, or rather, eliminated, and thus taking special measures is not re~iuired.
E.2.1. Ba~ uu-~d ~ Ant A(x~
30 Experiments have shown that the ba~;kyLuu--d ^ -t is mainly determined by the optical properties of the paper . In the f irst instance they are assumed to be ?ollcly present throughout the index zone 2.
The ba( ku,Luu-.d - l. in llu-.cu.. l.d-"inated" index zones can be def ined as:
A(x) = AP (3 AP: Ba.:k~L~,ul.d primary When the paper merely reflects (and does not 5 fluoresce), AP will only consist of the reflected W
light . This is f iltered out in the optical system by an optical low-pass filter (for wavelengths from about 580 nm). Therefore, reflected radiation with a wavelength of 365 nm does not contribute to A(x).
However, most types of paper used for envelopes contain so-called "whiteners". These are substances with a variety of fluorescent pigments which, together, have a whitening effect. When such paper is irradiated with W light, an emission occurs with a 15 spectral distribution as schematically shown in Fig.
6. Flg. 6 shows, on the one hand, the radiation energy SE (random scale) of the W source emission 11, the "whitener" emission 12, and the index emission 13, respectively, as a function of the 20 wavelength in nm, and, on the other, the passed quantity D in percentages of this radiation energy SE, limited by the sensitivity 14 of the photo multiplying tube used in the pickup 61 and the low-pass filter function 15 referred to hereinabove.
25 This spectral distribution has a non-negligible extension beyond 580 nm to be accordingly observed as a contribution to A(x) (schematically represented by the hatched area 16 of Fig. 6). However, when the index zone is "contaminated" by (non-fluorescent) 30 printing, variance will occur in the background contribution. Such printing brings with it a damping of the ba.:l.yL.,ul.d signal, which can accordingly be def ined as:
A(x) = al(x).AP (4) 3s wherein al(x): damping factor at the location of -- 202~73~ ~

the printing.
The following applies to the damping factor:
0 ~ al(x) ~ l, al (x) = 1 for x without printing al (x) < 1 for x with printing.
In practice the values of al (x) are between 10% and 100% .
- In practice there have also been instances of printing in "narrow-band" fluorescent ink, applied with a so-called "marker" pen, for example. They exhibit the same behaviour as the index bars, but have different dimensions.
A non-f luorescent printing can only dampen the reflected or the emitted radiation o~ the background and accordingly appears as a damping factor in the f ormula .
A fluorescent printing itself emits light (as does the index) and thus makes a contribution of its own to the ba.kyLuu..d signal. This leads to an additive ~ 1 AF(x).
A(x) = AP + AF(x) (5) wherein AF : background f 1U~L ~8.. ~ ^nt Therefore, the background . -nt can be generally 25 defined as:
A(x) = al(x).AP + AF(x) (6) E. 2 . 2 . Index ~ ?nt I (x~
Practice has shown that the conception of an index bar as lighting up from its ba~yL~ u..d under the 30 influence of W light is too simple. One of the most marked rh^n. - in f luorescent indexes is the great influence of the ba~:k~L~u-.d on the signal amplitude of the index bars. When the index signals of a dark letter and a white letter are compared, the index `-- 2~7~9 bars on the letters themselves do not turn out to differ Yery strongly, but they do in the signals: the index bar amplitude of the dark letter is approx. 400 mV, whereas that of the white letter >15 V!
When it is assumed that the bauhyluulld of the dark letter hardly contributes to the index bar amplitude, this amplitude is exclusively det~rm; necl by the W
radiation striking the bars directly. Accordingly, in this case the index c -nt is def ined a6:
I(x) = IP(x) (7) wherein IP (x) : index primary ~n~t This primary L then has an amplitude contribution of approx. 400 mV. However, when the signal comprises a clear ba~_h~Luulld contribution, the index bar amplitude is many times larger. Upon further examination, it turns out there is a fairly constant correlation between the index bar amplitude and the bachyLuull~ value.
Expressed in fuL l l~c form:
I(x) = IP(x) + IS(x) (8) = IP(x) + a2(x). A(x) IS(x) = a2(x). A(x) wherein IS (x) : index s~cnn~ry - -nt a2 (x) : correlation factor In practice it turns out that a2 (x) is roughly between 5 and 8. The bL~h~L~ul-d, therefore, seems to act on the index emission as an amplif ier. In other words, the index bar signal I (x) is determined as to a much greater part by 5ecl n~Ary excitation by the ba.hyL~,ul.d than by direct irradiation with W! This is an important conclusion, especially when contamination of the index zone is considered.
When non-fluorescent background with damp factor ~a~739 al(x) is involved, A(x) can be defined as [see (4) ]:
A(x) = al(x).AP
Formula ( 8 ), in turn, def ines:
I(x) = IP(x) + a2(x).A(x) (~) 5 theref ore:
I(x) = IP(x) + al(x).a2(x).AP (10) The contribution of IP (x) is small in comparison with a2 (x) . A(x), so that the index amplitude is virtually exclusively det~rm; ned by the latter component. In 10 the case of ba~;hyruulld printing, however, this term is weakened by a factor al(x), which may decrease to 10% or further! This means that such printing interfering with the index bars causes a very large variance in the index bar amplitude.
15 E . 3 . Statement of the~ T~roblem In summary it may be said that the relevant information in the index signal F(x) is represented by the ~ 1. I (x) . It comprises a primary component IP(x) making a fairly small amplitude 20 contribution of little variation, and a ser-~n~lAry c, _ --t IS(x), which may give ri6e to very large variations in the peaks of F (x) . Although the background amplitude may also vary strongly ( f luorescent contamination of the index zone 2 [ Fig .
25 1] ), it is invariably (amply) ~Yreed~d by a bar contribution in the amplitude signal (amplifier ef ~ect) . But precisely such possibly large variations in the index signal F(x) both of the ba.}.~Luul.d ~- ,r--t A(x) and of the additive index component 30 I(x) proper make it difficult to reliably establish the presence of a bar or a space in a part of the index signal under examination. A peak approximation using conventional peak follow methods is inadequate here, since such an approximation is sensitive to 12 2~Q~
successive spaces.
E. 4 . The detection alqorithm Starting from the fact that it has been experimentally est~hl 1 Rh-~d that 5 a) a reliable background approximation is always po66ible(''bauk~Luu..d'' is present between all the bars ), and h) there i6 a correlation between a given background and the additive re6pon6e of f luore6cent bars applied to it, an index detection algorithm has been developed in which the most critical a6pect of the method, namely the peak approximation, is replaced by a prediction of the index bar response. This prediction is made 15 with the aid of a prediction table (see Table 1) on the basis of a locally det~rmi n~d background signal amplitude. This table takes account of the properties of the W light source/signal pickup combination (5, 61) used and the ink used. Such a table is compiled 20 beforehand using the correctly detected index signals from a test set of letters. See under E. 4 . 4 . below.
The detection algorithm proper comprises two subalgorithms (i) the detection of a possible first bar, 2 5 and (ii) a segmentation and classification algorithm of the f irst bar and each successive bar.
Both the detection of the position of a possible 30 ~irst bar and the actual determination of the presence and the be6t po6ition of the f irst bar and each 6ucce6sive bar are carried out on the basis of the aLu.,O l.ioned prediction with the aid of the prediction table.

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With a view to a more detailed discussion of the subalgorithms mentioned, a further signal description will be given first.
E. 4 .1. Si~nal descri~tiQn in view of the alqorith~
Fig. 7 shows a part of the index zone 2 of a letter 1 moving in a direction 4 along the pickup 61 (Fig. 1), with the index pattern in the direction x being scanned from the letter edge 16. Of the index pattern the f irst bar is shown in two positions 17 and 18 at a minimum possible distance from the edge 16 and at a maximum possible distance from the edge 16, respectively, and a possible second bar 19 at pitch distance from position 18 of the f irst bar. A
broken line 20 desig~ates the position of the letter 1 relative to the centre line of the pickup 61 at the moment when edge det~ction occurs. Edge detection is carried out using for instance a photo cell arranged along the letter transport line.
Further references in Fig. 7 have the following 2 0 meaning:
LFC: position of the letter upon edge detection LPl: minimum position 17 of the f irst bar LAl: maximum deviation of the f irst bar relative to the minimum position referred to LIS: pitch LSD: bar width Fig. 8 shows a corrP~p~n~lin~ index signal F(t) viewed in time, picked up by a pickup provided with a vertical slit with a width OSB eslual to the nominal 3 o width of the index bar used in the index pattern .
COLLC: ~ollding first and second bar positions are indicated by 17 ', 18 ', 19 ', respectively. Further references in Fig. 8, now viewed in time, have the following meaning:

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TFC: moment of letter edge detection (t=0 TP1: minimum 'position' of the first bar TA1: maximum deviation of the f irst bar TIS: pitch 5TNSD: ' bar width ' TDSA: target area AGR: (approximated) ba.}.~L~ul.d amplitude THR: thre6hold value TOP: bar amplitude peak value The signal F (t) i8 stored chronologically - from the moment t=0 when the pickup i5 switched on after edge detection up to a moment T which, using a safety margin, i8 well beyond the moment when the last index bar has passed the pickup 61 - and digitally in an addressable memory, for instance at a sampling interval of 23 ~ sec and a sampling step of 15 mV.
Thus, the time differences in fact become address diferences and signal level differences become differences in address content. Hereinafter the digitized signal values for 0 ~ t S T will also be designated by F (t) since the chances of misunders~n~l;n~c arising are small and the ro~ hi~ity is thus ~romoted.
E. 4 . 2 . Detection of the f ~ rst bar Referring to Fig. 8 the subalgorithm in respect of the detection of the fir6t bar (Fig. 7: 17, 18) will be explained.
The f irst bar is located in a search area ZG1, where TP1 S t < TP1 + TA1 + TNSD (11) i . e. between the outermost positions of the f irst bar indicated by 17 ' and 18 ' . The detection of the ~irst bar comprises a first broad detection and a second, f iner detection . First the search area ZG1 is broadly ~ 2~2~73~

stepped through at a ~itep which is selected to be equal to the width of a target area TDSA = (l-ALPHA)*TNSD/2, (12) namely, half the width of that part of a theoretical 5 bar amplitude which exceeds a threshold value T~R.
THR is def ined as TiIR = AGR + VARAGR + ALPHA * CONTRAST ( 13 ) wherein AGR: approximated background amplitude VA~AGR: ba~ Luuild variation (in AGR from Table 1) ALPE~A: detection parameter (between O and 1), experimentally determined CONTRAST: difference between the expected minimum response and the maximum ba~ }~yLOulld variation VARAGR (also from Table 1) The approximated ba~_k~L~,u-ld amplitude AGR at the moment t, with each step TDSA carried out, is det~rm;n~ as the greatest value o~ LMIN and R~IN, 20 LMIN and RNIN ~. i.L~s~,ll ing the smallest siqnal amplitudes round in the time intervals t-TIS to t and t to t~TIS, respectively, i.e. in areas to the left and to the right of t with a size of the pitch.
When at a certain time t=tO F(tO) is greater than 25 the instantaneous threshold TE~R, then the second, f iner detection method is carried out which is in fact (selected to be) equal to the method for the detection of each successive bar. See the se tion and classif ication function under E. 4 . 3 .
30 to be described in greater detail hereinafter. This finer detection scans the area between tO-TIS/2 and tO with small steps, namely per sample (i.e. sampling interval), selects the best position of a segment possibly containing a bar (segmentation), and checks 35 wh--ther this segment actually contains a 'bar' 16 2~12a7~
(classif ication) . If this is not the case, the process continues with the f irst broader detection with tO as the new start position.
The detection of the first bar is terminated when:
5 a. the detected first segment is actually classified as a bar segment, b. no bar segment is found in the searching area ZGl.
After b. the detection is discontinued and a 'reject' code is generated. After a. the det~nm;nPcl position 10 of the first segment i8 used for segmenting and classifying the next segment.
E. 4 . 3 . Seqmentation and classif ication When the position of the f irst segment is det~rminP~ it seems easy to sequentially segment the 15 further signal F(t) at a fixed pitch TIS. However, this would only be the case if in practice, too, the bars could be applied at a constant pitch. In practice, however, a certain specified pitch tolerance should be taken into account. Moreover, the 20 time~ r~n~ont signal F(t) is also influenced by variations in the transport rate of the letter. For that reason, the best positions of the successive segments are perio~lic~lly determined by repeating in each segment the search for the best position within 25 a ,yl-- l.L~...isation area, which is defined by the pitch tolerance. The pitch TIS, however, is ~ Lessed in the number of samples and has a tolerance of 1 sample in the present ~ t. Such a method of segmentation, in which a pitch tolerance is taken 3 0 into account, is known per se as a special case (since only one value for the pitch size is used) from Dutch patent specification 183790.
Fig. 9 once again shows the theoretical signal of a segment with a bar. Such a segment generally has ` ~` 202~7~9 the following properties:
(i) the signal value of the index signal F in the middle area is greater than the signal values F (tL) or F (tR) at the left-hand edge tL or the right-hand edge tR of the segment.
(ii) the signal values F(tL) and F(tR) of left-hand edge tL and right-hand edge tR are not very different.
Starting from this, the signal value in the middle 10 area of a segment is defined as integrated value IMID
during a time interval TTOP
TTOP = GANMA * TNSD (14) wherein GAMMA: a detection parameter between 0 and 1, 15 TNSD: the bar width.
The extent to which ~LUIJ~L LY (i) is present is expressed in a first structural feature SMATCH = IMID -- ILEFT - IRIGHT (15) wherein 2 O IMID: the integrated value during TTOP, ILEFT: the signal value F(tL) on the left-hand edge of the segment, IRIGHT: the signal value F(tR) on the right-hand edge of the segment.
The extent to which both properties (i) and (ii) are present is summarized in a second structural feature SCORE = SMATCH -- [ILEFT -- IRIGHT] (16) The second structural feature SCORE is a measure of the balance between left and right. Within the ~,y~ r~ ization area that segment position is looked for in which the second _L u~;LuL~-l feature SCORE is largest .
The first structural feature SMATCH is used for classifying the segment as a bar or space segment. To ` ~ 2~2~739 that end it is tested against a threshold MTHR which i6 det~rmi n~d depending on an approximated background signal value AGR found in the segment in that position where SCORE is largest.
5 MTHR is def ined as:
MHTR = (TTOP--2) * AGR + TTOP * VARAGR + BETA * TTOP *
* CONTRAST ( 17 ) wherein:
AGR: approximated background signal value as the average of ILEFT and IRIGHT, TTOP: as (14), BETA: detection parameter for adjusting the extent of (lF-pPn~lr~nry on the bar response between 0 and 1, VARAGR: ba-_ky,~u-.d variation (at AGR from Table 1), CONTRAST: difference between the expected minimum response RESP and the maximum ba-_kyLvul,d variation VARAGR (also from Table 1).
This threshold is chosen such that the part that is in~r~n~r~nt of the bar response equals the maximum of the structural feature SMATCH for a space. SMATCH for a space is at a maximum when:
ILEFT = IRIGHT = AGR ( 18 ) IMID = TTOP * (AGR + VARAGR) ( 19 ) This means that for the same background signal value AGR, the SMATCH value of a bar should be greater than that of a space; ancl the extent by which it should be at least greater is det~rm-nPfl by the fraction BETA
of the bar response in the middle area predicted with the prediction table (Table 1) for the approximated background signal value found. A threshold MTHR thus chosen offers the following advantages:
a) the chance of a space being misclassif ied as a bar is small, because the minimum MTHR (when BETA = 0) equals the maximum of SMATCH of a space.

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b) According as BETA ls chosen to be smaller, more forms of bars where the response exceeds the background variations can be classif ied as bars, which renders the present method more generally applicable.
Accordingly, the classification proper is as follows:
the segment is a ' bar ' segment when SMATCH > MTHR and it is a space segment when SMATCH S MTHR.
When a segment is classified as a 'bar' segment, the position of this segment where SCORE is greatest is used as a start position (by..~ u--isation) for a next segment to be ~Y~m;n~d.When a segment is classified as a ' space ' segment the startposition for the next segment is the position of the preceding segment plus the nominal pitch TIS. In both cases the start position of the next segment to be ~Y;~lnin~d i6 det~rminPd by the observed position of the present segment plus the nominal pitch TIS.
E. 4 . 4 . The };)rediction table 2 o For each pickup a separate prediction table is to be compiled, Table 1 ls an example. For the compilation of such a table a random known index detection method may be started from, or the index detection method according to the invention with a table for another pickup. A test set is selected of index signals properly detectable by such a method, of index patterns written in the same ink on random letters. 8y the same method, or possibly by hand, these signals are (again) segmented and classified as 3 O space or bar segments . Of each classif ied segment a background signal value, for instance the minimum signal value, and the maximum signal value are det~nmin~d. Of each index signal - both of the space segments and of the bar segments - a histogram of the 20207~9 background 6ignal values and a histogram of the maximum signal values are drawn up. On the basis of these histograms, for each background signal value found, maximum ba~;l~r~ul~d variation and the minimum 5 re6ponse of a bar are ~pt~rm; n-~d. The values thus found form three series, one of ba~ oul.d signal values, one of maximum ba- hyL~,ul-d variations and one of minimum bar responses. These series generally exhibit gaps in their sequence and therefore are 10 supplemented with values CULL-~L,~, i;ng with intermediate missing baci~L~.u~ld signal values, for instance up to the sampling step of the digitized signal, and adjusted so that the whole shows a fluent course .
Table 1 shows the results for a test set of 80 letters. For each signal step of 40 mV for the background signal AGR (column 1) up to a certain maximum, the maximum ba~ k~L~,ul-d variations VARAGR
(column 2) and the minimum additive response RESP
20 (column 3) of an index bar are specified. Column 4, - furth lc:, lists the cULr~ ;ng contrast CONTRAST, which is the difference in value between the minimum additive response RESP and the maximum bac3~r ~UIId variation VARAGR for the same background 25 signal value AGR. All values are .:x~Lessed in mV.
In a table compiled in this way, the maximum possible contribution in a positive sense of the aL~ ioned noise _ ~ [R(n) in formula (2) ]
is also taken into account in the values for the 30 maximum ba~ky- ~JUI-d variation (column 2); and that same contribution in a negative sense is taken into account in the values of the minimum additive response of the bars (column 3 ), so that each of the CONTRAST values in column 4 in fact represents the 35 minimum noise-;n~l~r~n~9~nt part of a bar response, which may occur with the background signal value in column 1 cULL ~y~n~lin~ with that CONTRAST value. It i6 precisely this measure CONTRAST which is used in the two bar criteria de6cribed hereinabove, namely 5 the thresholds THR t~ormula (13) ] and MTHR [formula (17) ] for the provisional and definitive decision, respectively, on the presence of a bar or a space.
Any inf luence of the noise - t on this decision is theref ore no longer present .
For the "on line" operation of the detection algorithm, this table is converted into a new table in the compilation/a6sembly phase of the detection ~)L~:IyL ~-, at given values for the detection parameters ALPHA, BETA and GAMMA, by carrying out the operations according to the formulae (13), (14), and ( 17 ), in which new table during the on line operation, for an observed ba~}.gLuul.d signal value AGR, the values for THR and MTHR are directly found.
E. 4 . 5 . Parameter adiustment The results of the new detection algorithm are only influenced by the parameter choice of ALPHA, BETA and GAMMA.
The parameter ALPHA mainly inf luences the processing time. Its influence on the detection results, however, is limited, since the detection of the first bar in~oLyuLates the possibility of synchronising again when a false synchronisation is registered .
BETA indicates the reguired guality of the 3 0 segments of the indeY bars . Too high a BETA may cause an in~ULLe:~;l classification, for a bar may be classified as a space. The reverse applies when BETA
is too low. However, in virtue of the choice of the threshold value MTHR, the chance of a space being ` ~i 2~2~73~

classif led as a bar is small .
GAr~lA influences the processing time of the segmentation and the classif ication . Together GA2~A
and BETA influence the final results. The smaller 5 ALPEIA and BETA are, the less sensitive the algorithm will be to variations of the bars. Experimentally, ALPEIA = BETA = GANMA = 0.1 is a good choice with the limit set for the prscP~inq time (< 50 msec), a quantizing resolution of 15 mV and a sampling 10 frequency of 43 kHz.

. ~ 2Q2~7~
` ~ _ AGR VARAGR RESP CONTRAST
0 mV 80 mV 150 mV 70 mV
40 mV 100 mV 200 mV 100 mV
80 mV 120 mV 270 mV 150 mV
120 mV 140 mV 315 mV 175 mV
160 mV 160 mV 350 mV 190 mV
200 mV 180 mV 380 mV 200 mV
240 mV 190 mV 400 mV 210 mV
280 mV 200 mV 420 mV 220 mV
320 mV 210 mV 440 mV 230 mV
360 mV 225 mV 465 mV 240 mV
400 mV 250 mV 500 mV 250 mV
440 mV 275 mV 550 mV 275 mV
480 mV 300 mV 600 mV 300 mV
520 mV 325 mV 650 mV 325 mV
560 mV 350 mV 700 mV 350 mV
600 mV 375 mV 750 mV 375 mV
640 mV 400 mV 800 mV 400 mV
680 mV 425 mV 850 mV 425 mV
720 mV 450 mV 900 mV 450 mV
760 mV 475 mV 950 mV 475 mV
800 mV 500 mV 1000 mV 500 mV
840 mV 525 mV 1200 mV 675 mV
880 mV 550 mV 1400 mV 850 mV
920 mV 575 mV 1600 mV 1025 mV
960 mV 600 mV 1800 mV 1200 mV
1000 mV 625 mV 2000 mV 1375 mV
1040 mV 650 mV 2200 mV 1550 mV
1080 mV 675 mV 2400 mV 1725 mV
1120 mV 700 mV 2600 mV lg00 mV
1160 mV 725 mV 2800 mV 2075 mV
1200 mV 750 mV 3000 mV 2250 mV
1240 mV 775 mV 3050 mV 2275 mV
1280 mV 800 mV 3100 mV 2300 mV
1320 mV 825 mV 3150 mV 2325 mV
1360 mV 850 mV 3200 mV 2350 mV
1400 mV 875 mV 3250 mV 2375 mV
1440 mV 900 mV 3300 mV 2400 mV
1480 mV 925 mV 3350 mV 2425 mV
1520 mV 950 mV 3400 mV 2450 mV
1560 mV 975 mV 3450 mV 2475 mV
1600 mV 1000 mV 3500 mV 2500 mV
1640 mV 1025 mV 3550 mV 2525 mV
1680 mV 1050 mV 3600 mV 2550 mV
1720 mV 1075 mV 3650 mV 2575 mV
1760 mV 1100 mV 3700 mV 2600 mV
1800 mV 1125 mV 3750 mV 2625 mV
1840 mV 1150 mV 3800 mV 2650 mV
1880 mV 1175 mV 3850 mV 2675 mV
1920 mV 1200 mV 3900 mV 2700 mV
1960 mV 1225 mV 3950 mV 2725 mV
>2000 mV 1250 mV 4000 mV 2750 mV
TABLE I
_~3- .

Claims (9)

1. A method of detecting a bar code from a bar code signal which essentially forms a cross-section of a bar code pattern luminescing from the background of a carrier under radiation, characterized in that the bar code signal within each signal area in which the bar code signal may be expected to have a bar signal value corresponding to a bar, is tested against a bar criterion obtained through prediction from a local approximated background signal value derived from the bar code signal in that signal area.

2. A method according to claim 1 characterized in that the prediction is performed with the aid of a priorly compiled prediction table.

3. A method according to claim 2, characterized in that the prediction table comprises table values from which, directly or indirectly, for each of a plurality of background signal values in a range of possible background signal values a bar criterion value can be determined and that the prediction table is compiled on the basis of values for the maximum background variation and the minimum additive response of a bar, which values correspond to said possible background signal values and are obtained with the aid of a test set of carriers provided with a bar code applied in substantially the same ink which luminesces under irradiation.

4. A method according to one of the claims 1-3, characterized in that the bar code signal comes from a bar code with a substantially fixed pitch and of the 'mark-space' type.

5. A method according to claim 4, characterized in that the bar criterion is a threshold value for a bar code signal value within said signal area.

6. A method according to claim 4, characterized in that the bar criterion is a threshold value for a structural feature of the bar code signal value within said signal area.

7. A method according to claim 2, 3, 5 or 6 character-ized in that the bar code signal at least for the duration of the detection is recorded as a chronological series of digit-ized signal values [F(t)] in storing means (64) accessible for processing, in which also the prediction table is recorded, and that the method further comprises the following steps, St. 1: determining within said series [F(t)] of signal values a consecutive subseries of signal values, called bar segment, within which signal values corresponding to a bar may be expected;
St. 2: determining an approximated local background signal value (AGR) from the signal values within that bar segment;
St. 3: determining at least one bar criterion (MTHR) using the prediction table for the background signal value (AGR) determined in step St. 2;
St. 4: deciding if, and if so, where the signal values that correspond to a bar are found within the bar segment by testing the signal values within the bar segment against a bar criterion (MTHR) determined in step St. 3;
St. 5: determining a start position of a next bar segment depending on the result of step St. 4 when the series of signal values [F(t)] has not yet been stepped through completely, and repeating the preceding steps from step St. 1;
St. 6: generating the bar code found in a form suitable for further use.

8. A method according to claim 7, characterized in that the step St. 1 for determining a first bar segment comprises the following substeps:
St. 11: determining a search area (ZG1) depending on a priorly determined value for the first possible start position (TP1) of the first bar (17') from the beginning of the series of digitized signal values [F(t)];
St. 12: successively stepping through the search area (ZG1) at a second step adjusted to finding a target area (TDSA);
St. 13: determining at each step an approximated local background signal value (AGR) from the signal values [F(t)] in a local area covered by this step (t-TIS t t + TIS);
St. 14: determining a threshold value (THR) corresponding to said approximated local background value (AGR) using the prediction table;
St. 15: selecting the target area (TDSA) by testing whether in the local area (t-TIS t t + TIS) the signal values [F(t)] exceed the threshold value;
St. 16: determining a possible start position of a first bar segment depending on the target area found (TDSA);
St. 17: examining whether in the target area found (TDSA) the presence of a bar can be established by succes-sively performing the steps St. 1 through St. 4;
St. 18: performing step St. 5 if a bar can be determined;
St. 19: repeating the substeps from St. 12 for the remainder of the search area (ZG1) if no bar can be determined.

9. Apparatus for reading a bar code pattern applied to a carrier and under irradiation luminescing from the background of said carrier, comprising - irradiating and pickup means for picking up under irradiation an image signal of the bar code pattern and converting said image signal into an electric bar code signal;
- detection means for detecting the bar code from the bar code signal by the method according to claim 2; and - decoding means for decoding the bar code, the detection means comprising signal processing means, and storing means accessible to the signal processing means, in which storing means the bar code signal is stored for the duration of the detection and the prediction table is stored semi-permanently, the table values of the prediction table being related to said irradiating and pickup means.