Registers – Coded record sensors – Particular sensor structure
Reexamination Certificate
2001-04-04
2002-09-03
Le, Thien M. (Department: 2876)
Registers
Coded record sensors
Particular sensor structure
C235S462010
Reexamination Certificate
active
06443360
ABSTRACT:
The present invention relates to a method and a device for focusing an electrical signal representative of an optical code. The invention may advantageously be used in an optical code reader.
In the present description and following claims, the expression “optical code” indicates any graphical representation having the function of storing a coded information. A particular example of optical code consists of linear or bidimensional codes, wherein the information is coded through suitable combinations of elements having predetermined shape, such as for example square, rectangular or hexagonal, dark-coloured (usually black) separated by light elements (spaces, usually white), such as bar codes, stacked codes and bidimensional codes in general, colour codes, etc. The expression “optical code” also comprises, more in general, other graphic shapes with the function of coding information, including light printed characters (letters, numbers, etc.) and particular patterns (such as for example, stamps, logos, signatures, fingerprints etc.). The expression “optical code” also comprises graphical representations detectable not only in the field of visible light, but also in the wavelength range comprised between infrared and ultraviolet.
By way of an example and for the purpose of making the following description clearer, explicit reference shall be made to a barcode reader; of course, a man skilled in the art will recognise that what said is applicable to different types of optical code readers, such as those indicated above.
Typically, an optical code reader, schematically illustrated in FIG.
1
and indicated with reference numeral
1
, comprises an illumination unit
2
, adapted to emit a luminous beam towards the support containing the optical code
3
to be read, and an opto-electric reception unit
4
, adapted to collect the luminous signal diffused by the illuminated optical code. In particular, such luminous signal is collected, through an optical reception system, on suitable photo-receiving means, which convert the luminous signal into an electrical signal.
The following processing for reading the optical code can occur directly on the electrical signal, or before such successive processing, the electrical signal can be pre-processed by a pre-processing unit
5
, which can comprise amplifying means and/or other means adapted to suitably change the electrical signal for the particular application. Then the electrical signal, pre-processed or not, is sent to a digitising and decoding unit
6
, so as to extract its information content.
The digitising and decoding unit
6
includes a un digitiser and a decoder, not shown, which respectively have the purpose of digitising the received electrical signal, transforming it into a succession of digital pulses, and of extracting the coded information from the optical code, providing it in output in a form usable by processing devices connected downstream, or to displaying devices.
In a typical embodiment of a conventional reader, the illumination unit
2
comprises a luminous source, consisting of one or more LEDs and/or one or more diaphragms, and one or more focusing lenses, which focus the emitted luminous beam.
As an alternative, the luminous source can consist of one or more lasers, and the illumination unit
2
can comprise, besides the lens and/or the diaphragm, further scan means (for example, an oscillating mirror or a polygonal mirror rotor) for generating one or more scan lines of the luminous beam on the optical code.
Besides the photo-receiving means, the opto-electric reception unit
4
typically comprises one or more lenses and/or diaphragms adapted to collect the light beam diffused by the optical code and focus it.
In the following description, the expression “receiving optics” shall be used to indicate all of the lenses and/or diaphragms contained in the opto-electric reception unit
4
adapted to collect the light diffused by the optical code.
The photo-receiving means can consist of a linear or matrix sensor, for example a CCD (Charged Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), or of a photodiode.
Reader
1
can be of the fixed or portable type, and it is associated with a depth of field that is definable as the range of optical code-reader distances in which the reader reads correctly.
In all of the above-mentioned types of readers, an analogue signal is obtained at the output of the photo-receiving means, which is an electrical signal reproducing the reflectance modulations of the elements forming the optical code along a scan line of the same code. In fact, the light with which the optical code is illuminated is absorbed by the bars and reflected by the spaces. Thus, the analogue signal has an alternate pattern, with relative maximums (called peaks in the following description) at the spaces, relative minimums (called valleys in the following description) at the bars, and leading and trailing fronts at the transitions between bars and spaces, and vice versa, as illustrated in FIG.
2
.
In the following description, also the expression “negative peak” shall be used, where appropriate, to indicate a valley of the analogue signal, whereas a peak of the analogue signal may also be indicated with the expression “positive peak”.
Moreover, the analogue signal comprises a frequency band containing all the information relating to the detected optical code. The frequency band can be very variable, since it depends on various factors among which the print resolution of the optical code, the reader-optical code distance, and the properties of the photo-receiving means. For example, for a high-depth of field CCD reader, the frequency band can be comprised between 100 Hz and 50 KHz.
To allow a correct reading of the optical code, the analogue signal in input to the digitising and decoding unit
6
must reproduce as accurately as possible the alternation of the code bars and spaces, that is, it must exhibit minimum alteration with respect to the ideal alternate pattern.
In fact, the alteration of the analogue signal can cause errors during the digitisation, thus impairing the successive decoding. In particular, the analogue signal alteration mainly concerns the following factors:
1) amplitude of the analogue signal and relevant dynamics
2) signal
oise ratio (SNR) associated to the same analogue signal.
With reference to item 1), the analogue signal amplitude can be too little to allow, during digitisation, the recognition of some elements of the optical code. Typically, a high resolution or a low contrast of the optical code can be the causes of an analogue signal in which peaks and valleys are not very marked.
With reference to item 2), noise can be of electronic nature, due to the components of the reader, and of ambient nature, due for example to fluctuations in the illumination of the optical code. Noise introduces high-frequency portions in the analogue signal, and during digitisation and decoding, this may cause peaks and/or valleys due to noise and not to the optical code to be recognised as elements of the optical code.
A parameter of the “good quality” of the analogue signal is its focusing. In the case of a barcode reader, “focusing” refers to the ratio between the peak-valley amplitudes of the analogue signal of narrow elements, and the peak-valley amplitudes of the analogue signal of wide elements of the barcode, where elements indicates both a bar and a space. To this purpose, reference shall be made to
FIG. 2
, showing the temporal pattern of an analogue signal relating to a fragment of barcode, wherein Vp
1
indicates the peak value relating to a wide space, Vp
2
indicates the valley value relating to a wide bar, Vp
3
indicates the peak value relating to a narrow space, and Vp
4
indicates the valley value relating to a narrow band. By indicating with Vppw the peak-valley amplitude relating to the wide elements of the barcode (Vp
1
-Vp
2
) and with Vppn the peak-valley amplitude relating to the narrow elements of the barcode (Vp
3
-Vp
4
), focusing F is defined by the non-dimensional
Marchi Paolo
Piva Marco
Datalogic S.p.A.
Le Thien M.
Nixon & Vanderhye P.C.
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