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Abstract

A bar code signal 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 the 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.

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First Claim