Reader device or laser scanner for reading characters with different reflectance values, particularly bar codes

Claim 1

Laser scanner for reading characters printed with different reflectance values, particularly bar codes, characterized in that it comprises: -- at least two laser beam source units (1,2), each one provided with optical means (1d,2d) for focusing the beam (3) of one unit (1) onto a near region and the beam (5) of the other unit (2) onto a distant region, said optical means being such that the beam generated by each unit coincides, in terms of direction and position, with the beam generated by the other unit, said near and distant regions forming said reading area (26) and overlapping in an intermediate region; -- optical scanning means (9) for periodically directing the coincident beam (8) onto a reading area (26) that contains said code; -- optical receiving means (10) for collecting the light that returns from the scanning of the code and for providing an optical signal; -- sensor means (11) for converting the optical signal into an electric signal; -- amplifier means (12) connected to said sensor means (11) and suitable to provide, in output, an amplitude signal that is proportional to the light received by the sensor means (11); -- two independent power supply (1a,2a) and modulation circuits (1b,2b), each one coupled to one of said laser units (1,2) and having an input for a modulating signal of preset frequency; -- a local oscillator (14) for generating said modulating signal of preset frequency; -- a selector (17) for sending said modulating signal to one or the other of said power supply circuits (1a,2a) so that it emits a signal modulated at the frequency of said local oscillator (14); -- a phase demodulator (13) for measuring the phase difference between the signal received from said amplifier means (12) and the signal of said local oscillator (14), the result of this phase demodulation being a signal that indicates the distance between the reading field scanned by the laser beam at that exact instant and said scanner; -- comparator means (19) for comparing said distance signal to a reference signal that is equivalent to the distance of said intermediate region from said scanner and for providing signals suitable to activate, in real time, said selector (17) into the position for activating the laser unit (1,2) that is focused on the region containing the code; -- amplitude demodulation means (21) connected to said amplifier means (12) and providing a signal that is equivalent to the code that has been read; -- digitizer means (23) connected to said amplitude demodulation means (21) and providing pulses the duration of which matches the symbols of the code that has been read; -- decoder means (24) connected to said digitizer means (23) and to said phase demodulator (13) and providing data that are equivalent to the content of the code that has been read and to its distance from the scanner.