Process for discriminating the color of a surface and apparatus for implementing the process

• US 6,157,453 A
• Filed: 04/08/1999
• Issued: 12/05/2000
• Est. Priority Date: 04/10/1998
• Status: Expired due to Fees

• Current Assignee: Datasensor SpA (Datalogic S.p.A.)
• Original Assignee: Datalogic S.p.A.
• Inventors: Tamanti, Maria Lisa, Righi, Gianfranco
• Primary Examiner(s): EVANS, FANNIE L
• Application Number:
  US09/288,319
• Time in Patent Office: 607 Days
• Field of Search: 356/402, 356/405, 356/406, 356/407, 356/420, 356/425, 250/226
• US Class Current: 356/406
• CPC Class Codes: G01J 1/32 adapted for automatic variation of the measured or reference value regulation of light intensity G05D25/00
• CPC Class Codes: G01J 3/46 Measurement of colour; Colour measuring devices, e.g. colorimeters measuring colour temperature G01J5/60
• CPC Class Codes: G01J 3/50 using electric radiation detectors
• CPC Class Codes: G01J 3/501 Colorimeters using spectrally-selective light sources, e.g. LEDs

Abstract

The invention relates to a process for discriminating the color of a surface, comprising the steps of acquiring at least a colored acquisition surface, comparing the color of a detecting surface with said at least one color previously acquired so that said color can be recognized. In particular, the acquisition step comprises the steps of sequentially illuminating the acquisition surface with each of three light beams corresponding to three primary colors, each of said light beams being generated by a respective emission source supplied with a supply current IR, IG, IB; picking up by means of a light-receiving element the light scattered from the illuminated acquisition surface; generating an electric signal VR, VG, VB proportional to the luminous intensity of the picked-up light, as the acquisition surface is being illuminated with each of the three beams and controlling the supply current IR, IG, IB of the emission sources such that at least two of the electric signals VR, VG, VB generated are identical with each other within a predetermined tolerance margin.

Claims

1. A process for discriminating the color of a surface, comprising the steps of:

a) acquiring at least one color of a colored acquisition surface;

b) comparing the color of a detecting surface with said at least one color previously acquired, so that said color can be recognized;

wherein the acquisition step a) comprises the following steps;

a1) sequentially illuminating the acquisition surface with each of three light beams corresponding to three primary colors, each of said light beams being generated by a respective emission source supplied with a supply current IR, IG, IB;

a2) picking up by means of a light-receiving element the light scattered from the illuminated acquisition surface;

a3) generating an electric signal VR, VG, VB proportional to the luminous intensity of the picked-up light, as the acquisition surface is being illuminated with each of the three beams;

a4) controlling the supply current IR, IG, IB of the emission sources such that at least two of the electric signals VR, VG, VB generated are identical with each other within a predetermined tolerance margin; and

wherein the recognition step b) includes the following steps;

b1) sequentially illuminating, with the aforesaid three light beams, the detecting surface bearing the color to be recognized;

b2) picking up, by means of the light-receiving element, the light scattered from said illuminated surface;

b3) generating an electric signal VR'"'"', VG'"'"', VB'"'"' proportional to the luminous intensity of the picked-up light, as the detecting surface is being illuminated with each of the three beams;

b41) checking that the electric signals VR'"'"', VG'"'"', VB'"'"' generated are lower than a predetermined saturation value and that their sum is higher a predetermined minimum value and;

b41a) if both of said checks give a positive result, checking that at least two of the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated are equal, within a predetermined tolerance margin, to those of the electric signals VR, VG, VB acquired during step a);

b41b) if at least one of said checks gives a negative result, indicating that the color of the surface being scanned is not recognized.

2. A process according to claim 1, wherein step b41a) further includes the following steps:

selecting, from the signals VR'"'"', VG'"'"', VB'"'"', the two signals having the highest values;

defining, for each of the selected signals, a reference variable R'"'"', G'"'"', B'"'"' equal to the ratio of the selected signal to the sum of the signals VR'"'"', VG'"'"', VB'"'"';

checking if each of the reference variables above defined falls within a respective predetermined range of values and;

if the above check gives a positive result for both said variables, indicating that the color is recognized;

if the above check gives a negative result for at least one of said variables, indicating that the color of the surface being scanned is not recognized.

3. A process for discriminating a color range of a surface, characterized in that it comprises the following steps:

i) performing, according to claim 1, a first acquisition of a first color establishing one extreme of a color range to be recognized;

ii) storing the supply current values determined during step i);

iii) performing, according to claim 1 and using said emission current values, a second acquisition of a second color establishing the other extreme of said color range;

iv) checking that the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated during the recognition step, according to claim 1, are the same, within a predetermined tolerance margin, as those of the electric signals VR, VG, VB acquired during said acquisition steps.

4. A process for discriminating a plurality of colors on a surface, characterized in that it comprises the following steps:

performing sequentially, according to claim 1, a plurality of acquisitions of a plurality of colors;

each time storing the supply current values determined during the preceding step;

checking that the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated during the recognition steps according to claim 1, are the same, within a predetermined tolerance margin, as those of the electric signals VR, VG, VB acquired during said acquisition steps.

5. A process according to claim 3, wherein the supply current IR, IG, IB of the emission sources is controlled such that only two of the electric signals VR, VG, VB generated are identical with each other, within a predetermined tolerance margin, and higher than a predetermined minimum value.

9. A process according to claim 5, wherein the supply current IR, IG, IB of the emission sources is controlled such that the electric signals VR, VG, VB generated are all equal, within a predetermined tolerance margin, to the value of the smallest of said signals.

10. A process according to claim 5, wherein step a3) further includes the following steps:

a31) generating, within a light-receiving element, an analog electric signal which is proportional to the intensity of the picked-up light;

a32) amplifying the analog electric signal generated by the light-receiving element;

a33) converting the amplified analog signal to a digital signal VR, VG, VB.

11. A process according to claims 10, wherein the supply current IR, IG, IB of the emission sources is controlled such that the electric signals VR, VG, VB generated are all equal, within a predetermined tolerance margin, to a predetermined reference value and wherein step a4) further includes the following steps:

a41) checking if each of the digital signals VR, VG, VB has its value within a predetermined range of values defined by said reference value plus/minus a predetermined tolerance margin and;

a41a) if so, storing the values of said digital signals VR, VG, VB

a41b) if not so, iteratively varying the supply current within a predetermined range of current values, and repeating step a41) at each iteration.

12. A process according to claim 11, wherein step a41) further includes the following steps:

a411) setting the value of the supply current IR, IG, IB of the emission source which is supplied, first at a actual value equal to the highest value in a predetermined range of supply current values;

a412) checking if the value of the first of the signals VR, VG, VB to be generated is lower than an upper limiting value of said predetermined range of values and;

a412a) if so, checking that the value of said signal VR, VG, VB is higher than a lower limiting value of said predetermined range of values, and if so, storing the actual value of the supply current IR, IG, IB and iteratively repeating the steps from a411) for the emission source coming next in the supply sequence;

a412b) if not so, reducing the value of said supply current IR, IG, IB by a predetermined factor, and iteratively repeating step a412).

13. A process according to claim 12, wherein, if the check of step a412a) gives a negative result, a step is carried out to check if the actual value of the supply current IR, IG, IB is different from the highest value in the predetermined range of current values and:

if so, taking as the supply current value a value equal to one half the difference between the highest value in the predetermined range of current values and the actual value of the supply current IR, IG, IB and repeating the steps from a412);

if not so, carrying out the following steps;

a412a1) iteratively repeating the steps from a411) for the emission source coming next in the supply sequence;

a412a2) setting, as the reference value, the value of the smallest of the digital signals VR, VG, VB and repeating the steps from a41).

14. A process according to claim 13, wherein step a412a2) is preceded by a step of checking if the amplification of the analog electric signal can be increased and, if so, increasing by a predetermined interval the amplification of the analog electric signal and iteratively repeating the steps from a411);

if not so, carrying out step a412a2).

15. A process according to claim 13, wherein step a412a2) further includes the following steps:

checking if the reference setting value is higher than a first predetermined minimum value and;

if so, repeating the steps from a41);

if not so, setting the smaller of the remaining two of the signals VR, VG, VB as the reference value, and repeating the steps from a41).

16. A process according to claim 5, wherein if the check of step a412a2) gives a negative result, the following steps are carried out:

checking if the reference setting value is higher than a second minimum value, and

if so, setting as the reference value for the smallest of the digital signals VR, VG, VB, the actual value of that signal, and as the reference value for the remaining two signals the value of the smaller of the two actual remaining signals, and repeating the steps from a41);

if not so, indicating an error.

17. A process according to claim 10, wherein the supply current IR, IG, IB of the emission sources is controlled such that the electric signals VR, VG, VB generated are all equal, within a predetermined tolerance margin, to a predetermined reference value and wherein the reference value is defined as one half the reference voltage utilized by an analog-to-digital converter employed.

18. A process according to claim 15, wherein the first predetermined minimum value is one third the value of the smallest acceptable sum of the signals VR, VG, VB values.

19. A process according to claim 16, wherein the second predetermined minimum value is one half the difference between the value of the smallest acceptable sum of the signal VR, VG, VB values and the value of the smallest of the signals VR, VG, VB.

20. A process according to claim 12, further comprising the following steps:

making a plurality of measurements of VR, VG, VB using the stored supply current values IR, IG, IB;

calculating the arithmetical mean of the values of the signals VR, VG, VB corresponding to each stored supply current IR, IG, IB;

calculating the minimum and maximum limiting values of the digital signals VR, VG, VB by respectively adding and subtracting to/from said arithmetical mean a predetermined error, selected to suit specific application requirements.

21. A process according to claim 20, further comprising the following steps:

defining reference parameters R, G, B respectively as the ratio of the value of each signal VR, VG, VB to their sum;

calculating, for each reference parameter, an upper limit respectively defined as the ratio of the maximum value of the signal VR, VG, VB considered to the sum of that maximum value plus the minimum values of the other two signals VR, VG, VB.

22. A process according to claim 20, further comprising the following steps:

defining reference parameters R, G, B respectively as the ratio of the value of each signal VR, VG, VB to their sum;

calculating, for each reference parameter, a lower limit respectively defined as the ratio of the minimum value of the signal VR, VG, VB considered to the sum of that minimum value plus the maximum values of the other two signals VR, VG, VB.

6. A process for discriminating the color of a colored surface, comprising the following steps:

a1) sequentially illuminating an acquisition surface having the color to be discriminated with each of three light beams corresponding to three primary colors, each of said light beams being generated by a respective emission source supplied with a supply current IR, IG, IB;

a2) picking up by means of a light-receiving element the light scattered from the illuminated acquisition surface;

a3) generating an electric signal VR, VG, VB proportional to the luminous intensity of the picked-up light, as the acquisition surface is being illuminated with each of the three beams;

a4) controlling the supply current IR, IG, IB of the emission sources such that at least two of the electric signals VR, VG, VB generated are identical with each other, within a predetermined tolerance margin;

a5) acquiring the color of the acquisition surface by memorizing the supply current values IR, IG, IB thus controlled;

b) comparing the color of a detecting surface having the color to be discriminated with said color previously acquired, so that said color of the detecting surface can be recognized.

7. A process according to claim 6, wherein the supply current IR, IG, IB of the emission sources is controlled such that the electric signals VR, VG, VB generated are all identical with one another, within a predetermined tolerance margin.

8. A process according to claim 7, wherein the supply current IR, IG, IB of the emission sources is controlled such that the electric signals VR, VG, VB generated are all equal, within a predetermined tolerance margin, to a predetermined reference value.

23. A process according to claim 6, wherein the recognition step b) includes the following steps:

b1) sequentially illuminating, with the aforesaid three light beams, the detecting surface bearing the color to be recognized;

b2) picking up, by means of the light-receiving element, the light scattered from said illuminated surface;

b3) generating an electric signal VR'"'"', VG'"'"', VB'"'"' proportional to the luminous intensity of the picked-up light, as the detecting surface is being illuminated with each of the three beams;

b4) checking that at least two of the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated are equal, within a predetermined tolerance margin, to those of the electric signals VR, VG, VB acquired during step all to a5).

24. A process according to claim 23, wherein step b4) includes the following steps:

b41) checking that the electric signals VR'"'"', VG'"'"', VB'"'"' generated are lower than a predetermined saturation value and that their sum is higher a predetermined minimum value and;

b41a) if both of said checks give a positive result, carrying out step b4);

b41b) if at least one of said checks gives a negative result, indicating that the color of the surface being scanned is not recognized.

25. A process according to claim 24, wherein the electric signal are amplified to an amplification level and if the first of the checks of step b41) gives a negative result, a step is carried out to check if the amplification level can be lowered and:

if so, lowering said amplification level by a predetermined interval and iteratively repeating the steps from b41);

it not so, indicating that the color of the surface being scanned is not recognized.

26. A process according to claim 24, wherein the electric signal are amplified to an amplification level and if the second of the checks of step b41) gives a negative result, a step is carried out to check if the amplification level can be raised and:

if so, raising said amplification level by a predetermined interval and iteratively repeating the steps from b41);

it not so, indicating that the color of the surface being scanned is not recognized.

27. A process according to claim 24, wherein step b41a) further includes the following steps:

selecting, from the signals VR'"'"', VG'"'"', VB'"'"', the two signals having the highest values;

defining, for each of the selected signals, a reference variable R'"'"', G'"'"', B'"'"' equal to the ratio of the selected signal to the sum of the signals VR'"'"', VG'"'"', VB'"'"';

checking if each of the reference variables above defined falls within a respective predetermined range of values and;

if the above check gives a positive result for both said variables, indicating that the color is recognized;

if the above check gives a negative result for at least one of said variables, indicating that the color of the surface being scanned is not recognized.

28. A process for discriminating a color range of a surface, characterized in that it comprises the following steps:

i) performing, according to claim 6, a first acquisition of a first color establishing one extreme of a color range to be recognized;

ii) storing the supply current values determined during step i);

iii) performing, according to claim 6 and using said emission current values, a second acquisition of a second color establishing the other extreme of said color range;

iv) checking that the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated during the recognition step, are the same, within a predetermined tolerance margin, as those of the electric signals VR, VG, VB acquired during said acquisition steps a1) to a5).

29. A process for discriminating a plurality of colors on a surface, characterized in that it comprises the following steps:

performing sequentially, according to claim 6, a plurality of acquisitions of a plurality of colors;

each time storing the supply current values determined during the preceding step;

checking that the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated during the recognition steps are the same, within a predetermined tolerance margin, as those of the electric signals VR, VG, VB acquired during said acquisition steps a1) to a5).

30. An apparatus for discriminating the color of a colored surface, comprising:

three emission sources of three light beams corresponding to three primary colors being respectively supplied with a supply current IR, IG, IB;

a light-receiving element adapted to receive light scattered from the illuminated surface and generate an electric signal VR, VG, VB which is proportional to the luminous intensity of the picked-up light;

a microcontroller assembly for controlling the supply current IR, IG, IB of the emission sources such that when an acquisition surface having the color to be discriminated is illuminated at least two of the electric signals VR, VG, VB generated are identical with each other, within a predetermined tolerance margin.

31. An apparatus according to claim 30, wherein the emission sources (4a, 4b, 4c) are either LEDs or1aser diodes.

32. An apparatus according to claim 30, further comprising an amplifier (8) for amplifying the analog signals generated by the light-receiving element, and an analog-to-digital converter (9) for converting said amplified signals.

33. An apparatus according to claim 32, wherein said amplifier (8) comprises three separate amplification channels, each for amplifying one of the signals VR, VG, VB.

34. An apparatus according to claim 32, wherein the amplifier (8) is operatively associated with the microcontroller assembly (10) so that the amplification level of the signals VR, VG, VB can be varied to suit specific application requirements.

35. A process for discriminating the color of a colored surface, comprising the following steps:

a1) sequentially illuminating an acquisition surface having the color to be discriminated with each of three light beams corresponding to three primary colors, each of said light beams being generated by a respective emission source supplied with a supply current IR, IG, IB;

a2) picking up by means of a light-receiving element the light scattered from the illuminated acquisition surface;

a3) generating an electric signal proportional to the luminous intensity of the picked-up light, as the acquisition surface is being illuminated with each of the three beams and converting said electric signal into a digital signal VR, VG, VB;

a41) checking if each of the digital signals VR, VG, VB has its values within a predetermined range of values defined by a reference value plus/minus a predetermined tolerance margin, and;

a41a) if so, acquiring the color of the acquisition surface by memorizing the supply current values IR, IG, IB thus controlled and

b) comparing the color of a detecting surface having the color to be discriminated with said color previously acquired, so that said color of the detecting surface can be recognized;

a41b) if not so, iteratively varying the supply current within a predetermined range of current values, and repeating step a41) at each iteration.

36. A process according to claim 35, wherein step a41) further includes the following steps:

a411) setting the value of the supply current IR, IG, IB of the emission source whiqh is supplied first at a actual value equal to the highest value in a predetermined range of supply current values;

a412) checking if the value of the first of the signals VR, VG, VB to be generated is lower than an upper limiting value of said predetermined range of values and;

a412a) if so, checking that the value of said signal VR, VG, VB is higher than a lower limiting value of said predetermined range of values, and if so, storing the actual value of the supply current IR, IG, IB and iteratively repeating the steps from a411) for the emission source coming next in the supply sequence;

a412b) if not so, reducing the value of said supply current IR, IG, IB by a predetermined factor, and iteratively repeating step a412).

37. A process according to claim 36, wherein, if the check of step a412a) gives a negative result, a step is carried out to check if the actual value of the supply current IR, IG, IB is different from the highest value in the predetermined range of current values and:

if so, taking as the supply current value a value equal to one half the difference between the highest value in the predetermined range of current values and the actual value of the supply current IR, IG, IB and repeating the steps from a412);

if not so, carrying out the following steps;

a412a1) iteratively repeating the steps from a411) for the emission source coming next in the supply sequence;

a412a2) setting, as the reference value, the value of the smallest of the digital signals VR, VG, VB and repeating the steps from a41).

38. A process according to claim 37, wherein step a412a2) is preceded by a step of checking if the amplification of the analog electric signal can be increased and:

if so, increasing by a predetermined interval the amplification of the analog electric signal and iteratively repeating the steps from a411);

if not so, carrying out step a412a2).

39. A process according to claim 37, wherein step a412a2) further includes the following steps:

checking if the reference setting value is higher than a first predetermined minimum value and;

if so, repeating the steps from a41);

if not so, setting the smaller of the remaining two of the signals VR, VG, VB as the reference value, and repeating the steps from a41).

40. A process according to claim 39, wherein if the check of step a412a2) gives a negative result, the following steps are carried out:

checking if the reference setting value higher than a second minimum value, and

if so, setting as the reference value for the smallest of the digital signals VR, VG, VB , the actual value of that signal, and as the reference value for the remaining two signals the value of the smaller of the two actual remaining signals, and repeating the steps from a41);

if not so, indicating an error.

41. A process according to claims 35, wherein the supply current IR, IG, IB of the emission sources is controlled such that the digital signals VR, VG, VB generated are all equal, within a predetermined tolerance margin, to a predetermined reference value and wherein the reference value is defined as one half the reference voltage utilized by an analog-to-digital converter employed.

42. A process according to claim 39, wherein the first predetermined minimum value is one third the value of the smallest acceptable sum of the signals VR, VG, VB values.

43. A process according to claim 40, wherein the second predetermined minimum value is one half the difference between the value of the smallest acceptable sum of the signal VR, VG, VB values and the value of the smallest of the signals VR, VG, VB.

44. A process according to claim 36, further comprising the following steps:

making a plurality of measurements of VR, VG, VB , using the stored supply current values IR, IG, IB;

calculating the arithmetical mean of the values of the signals VR, VG, VB corresponding to each stored supply current IR, IG, IB;

calculating the minimum and maximum limiting values of the digital signals VR, VG, VB by respectively adding and subtracting to/from said arithmetical mean a predetermined error, selected to suit specific application requirements.

45. A process according to claim 44, further comprising the following steps:

defining reference parameters R, G, B respectively as the ratio of the value of each signal VR, VG, VB to their sum;

calculating, for each reference parameter, an upper limit respectively defined as the ratio of the maximum value of the signal VR, VG, VB considered to the sum of that maximum value plus the minimum values of the other two signals VR, VG, VB.

46. A process according to claim 44, further comprising the following steps:

defining reference parameters R, G, B respectively as the ratio of the value of each signal VR, VG, VB to their sum;

calculating, for each reference parameter, a lower limit respectively defined as the ratio of the minimum value of the signal VR, VG, VB considered to the sum of that minimum value plus the maximum values of the other two signals VR, VG, VB.

47. A process according to claim 35, wherein the recognition step b) includes the following steps:

b1) sequentially illuminating, with the aforesaid three light beams, the detecting surface bearing the color to be recognized;

b2) picking up, by means of the light-receiving element, the light scattered from said illuminated surface;

b3) generating an electric signal proportional to the luminous intensity of the picked-up light, as the detecting surface is being illuminated with each of the three beams and converting said electric signal into digital signal VR'"'"', VG'"'"', VB'"'"';

b4) checking that at least two of the values of the digital signals VR'"'"', VG'"'"', VB'"'"' generated are equal, within a predetermined tolerance margin, to those of the signals VR, VG, VB acquired during step a1)-a41).

48. A process according to claim 47, wherein step b4) includes the following steps:

b41) checking that the digital signals VR'"'"', VG'"'"', VB'"'"' generated are lower than a predetermined saturation value and that their sum is higher a predetermired minimum value and;

b41a) if both of said checks give a positive result, carrying out step b4);

b41b) if at least one of said checks gives a negative result, indicating that the color of the surface being scanned is not recognized.

49. A process according to claim 48, wherein the electric signal are amplified to an amplification level and if the first of the checks of step b41) gives a negative result, a step is carried out to check if the amplification level can be lowered and:

if so, lowering said amplification level by a predetermined interval and iteratively repeating the steps from b41);

if not so, indicating that the color of the surface being scanned is not recognized.

50. A process according to claim 48, wherein the electric signal are amplified to an amplification level and if the second of the checks of step b41) gives a negative result, a step is carried out to check if the amplification level can be raised and:

if so, raising said amplification level by a predetermined interval and iteratively repeating the steps from b41);

if not so, indicating that the color of the surface being scanned is not recognized.

51. A process according to claim 48, wherein step b41a) further includes the following steps:

selecting, from the signals VR'"'"', VG'"'"', VB'"'"', the two signals having the highest values;

defining, for each of the selected signals, a reference variable R'"'"', G'"'"', B'"'"' equal to the ratio of the selected signal to the sum of the signals VR'"'"', VG'"'"', VB'"'"';

checking if each of the reference variables above defined falls within a respective predetermined range of values and;

if the above check gives a positive result for both said variables, indicating that the color is recognized;

if the above check gives a negative result for at least one of said variables, indicating that the color of the surface being scanned is not recognized.

52. A process for discriminating a color range of a surface, characterized in that it comprises the following steps:

i) performing, according to claim 35, a first acquisition of a first color establishing one extreme of a color range to be recognized;

ii) storing the supply current values determined during step i);

iii) performing, according to claim 35 and using said emission current values, a second acquisition of a second color establishing the other extreme of said color range;

iv) checking that the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated during the recognition step are the same, within a predetermined tolerance margin, as those of the electric signals VR, VG, VB acquired during said acquisition steps.

53. A process for discriminating a plurality of colors on a surface, characterized in that it comprises the following steps:

performing sequentially, according to claim 35, a plurality of acquisitions of a plurality of colors;

each time storing the supply current values determined during the preceding step;

checking that the values of the electric signals VR'"'"', VG'"'"', VB'"'"' generated during the recognition steps are the same, within a predetermined tolerance margin, as those of the electric signals VR, VG, VB acquired during said acquisition steps.

54. A process for discriminating the color of a colored surface, comprising the following steps:

a1) sequentially illuminating an acquisition surface having the color to be discriminated with each of three light beams corresponding to three primary colors, each of said light beams being generated by a respective emission source supplied with a supply current IR, IG, IB;

a2) picking up by means of a light-receiving element the light scattered from the illuminated acquisition surface;

a3) generating an electric signal VR, VG, VB proportional to the luminous intensity of the picked-up light, as the acquisition surface is being illuminated with each of the three beams;

a4) controlling the supply current IR, IG, IB of the emission sources such that at least two of the electric signals VR, VG, VB generated are identical with each other, within a predetermined tolerance margin;

defining, for each of the selected signals, a reference variable R, G, B equal to the ratio of the selected signal to the sum of the signals VR, VG, VB;

a5) acquiring the color of the acquisition surface by memorizing the supply current values IR, IG, IB thus controlled;

b) comparing the color of a detecting surface having the color to be discriminated with said color previously acquired, so that said color of the detecting surface can be recognized.

55. A process according to claim 54, wherein the recognition step b) includes the following steps:

b1) sequentially illuminating, with the aforesaid three light beams, the detecting surface bearing the color to be recognized;

b2) picking up, by means of the light-receiving element, the light scattered from said illuminated surface;

b3) generating an electric signal VR'"'"', VG'"'"', VB'"'"' proportional to the luminous intensity of the picked-up light, as the detecting surface is being illuminated with each of the three beams;

defining, for each of the selected signals, a reference variable R'"'"', G'"'"', B'"'"' equal to the ratio of the selected signal to the sum of the signals VR'"'"', VG'"'"', VB'"'"';

b4) checking that at least two of the values of the reference variable R'"'"', G'"'"', B'"'"' defined are equal, within a predetermined tolerance margin, to those of the reference variable R, G, B acquired during step a1) to a5).