Apparatus and method for reducing imaging errors in imaging systems having an extended depth of field

• US 6,097,856 A
• Filed: 07/10/1998
• Issued: 08/01/2000
• Est. Priority Date: 07/10/1998
• Status: Expired due to Term

• Current Assignee: Hand Held Products Incorporated (Honeywell International Inc.)
• Original Assignee: Welch Allyn Incorporated
• Inventors: Hammond, Charles Mitchell Jr.
• Primary Examiner(s): Couso, Jose L.
• Application Number:
  US09/113,523
• Time in Patent Office: 753 Days
• Field of Search: 382/100, 382/210, 382/212, 382/280, 382/255, 382/312, 382/321, 382/322, 382/323, 382/324, 356/394, 356/398, 359/11, 359/29, 359/559, 359/558, 359/563, 359/564, 250/201.2, 250/201.4
• US Class Current: 382/312
• CPC Class Codes: G02B 27/0075 with means for altering, e.g. increasing, the depth of field or depth of focus
• CPC Class Codes: G06T 2207/20192 Edge enhancement; Edge preservation
• CPC Class Codes: G06T 5/73 Deblurring; Sharpening
• CPC Class Codes: H04N 1/4092 Edge or detail enhancement

Abstract

An improved opto-electronic imaging system which is adapted for use with incoherently illuminated objects, and which produces final images having a reduced imaging error content. The imaging system includes an optical assembly for forming an intermediate image of the object to be imaged, an image sensor for receiving the intermediate image and producing an intermediate image signal, and processing means for processing the intermediate image signal to produce a final image signal having a reduced imaging error content. A reduction in imaging error content is achieved, in part, by including in the optical assembly a phase mask for causing the OTF of the optical assembly to be relatively invariant over a range of working distances, and an amplitude mask having a transmittance that decreases continuously as a function of distance from the center thereof. The reduction in imaging error content is also achieved, in part, by including in the processing means an improved generalized recovery function that varies in accordance with at least the non-ideal calculated IOTF of the optical assembly under a condition of approximately optimum focus.

Claims

1. An imaging system for imaging an object located in a target region, said imaging system including, in combination:

an optical assembly having an optical axis, said optical assembly including;

a.) amplitude masking means for receiving light and transmitting light having an attenuated amplitude that varies as a function of position within said amplitude masking means, said function being substantially free of discontinuities;

b.) imaging means for forming an intermediate image of said object, said image having imaging errors that include at least a misfocus error that is dependent upon the distance between the imaging means and the object; and

c.) phase masking means for receiving light and modifying the phase thereof as a function of position within said phase masking means;

image sensing means, including an array of photosensitive elements, for receiving said intermediate image and converting said image into an intermediate image signal that includes a plurality of elements which are distorted by said imaging errors; and

processing means for;

a.) providing a generalized recovery function that, in the frequency domain, includes at least the reciprocal of the calculated non-ideal Incoherent Optical Transfer Function (IOTF) of said optical assembly, under a condition of approximately optimum focus; and

b.) applying said recovery function to one of a frequency domain and a spatial domain representation of said intermediate image signal to produce a final image signal that includes a set of elements which together define a representation of said object which is largely free of those of said imaging errors which have been taken into account in the calculation of said calculated non-ideal IOTF.

2. The imaging system of claim 1 in which said amplitude masking means is characterized by a transmittance function which approximates one of a Gaussian function, an exponential function, a Bessel function and segments of cosine functions.

3. The imaging system of claim 1 in which said amplitude masking means is formed as a coating on a surface of one of said imaging means and said phase masking means.

4. The imaging system of claim 1 in which at least two of said amplitude masking means, said imaging means and said phase masking means comprise different parts of a single element.

5. The imaging system of claim 1 further including means for supporting said amplitude masking means, said imaging means, and said phase masking means in a predetermined spatial relationship to one another.

6. The imaging system of claim 1 in which said processing means applies said generalized recovery function by multiplying a frequency domain representation of said intermediate image signal by a frequency domain representation of said generalized recovery function.

7. The imaging system of claim 1 in which said processing means applies said generalized recovery function by convolving a spatial domain representation of said generalized recovery function with a spatial domain representation of said intermediate image signal.

8. The imaging system of claim 1 in which said generalized recovery function further includes a finishing function for one of (i) optimizing the spectral profile of the frequency domain representation of said intermediate image signal, and (ii) limiting the bandwidth of the frequency domain representation of the intermediate image signal.

9. The imaging system of claim 1 in which said generalized recovery function is a fraction the denominator of which comprises said calculated non-ideal IOTF, and the numerator of which comprises a calculated ideal IOTF of said optical assembly, without said phase masking means, under a condition of approximately optimum focus.

10. The imaging system of claim 9 in which said calculated non-ideal IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly with said phase masking means, and in which said calculated ideal IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly without said phase masking means.

11. The imaging system of claim 9 in which said generalized recovery function further includes a finishing function, and in which said finishing function is a function that takes into account at least one of (i) the cosine-fourth effect, (ii) the effect of non-uniform illumination across said target region, (iii) aliasing effects and (iv) diffusion effects.

12. The imaging system of claim 9 in which said generalized recovery function further includes a finishing function for one of (i) optimizing the spectral profile of the frequency domain representation of said intermediate image signal, and (ii) limiting the bandwidth of the frequency domain representation of the intermediate image signal.

13. The imaging system of claim 1 in which said phase masking means is a cubic phase mask which is shaped so that it corrects approximately only for said misfocus error.

14. The imaging system of claim 1 in which said processing means further includes means for storing said generalized recovery function in a form which approximates the form in which it is applied to said intermediate image signal.

15. An imaging system for imaging an object located in a target region, said imaging system including, in combination:

an optical assembly having an optical axis, said optical assembly including;

a.) aperture means for defining an aperture having a transmittance which decreases continuously as the distance from said optical axis increases, said aperture means being characterized by an aperture function;

b.) lens means for forming an intermediate image of said object, said image having imaging errors that include at least a misfocus error that is dependent upon the distance between said lens means and said object, said lens means being characterized by a lens function; and

c.) phase masking means for receiving light incident thereon and modifying the phase thereof to cause the Optical Transfer Function (OTF) of the optical assembly to be approximately invariant over a range of distances between said lens means and said object, said phase masking means being characterized by a phase function;

image sensing means, including an array of photosensitive elements, for receiving said intermediate image and converting said image into an intermediate image signal that includes a plurality of image elements which are distorted by said imaging errors;

a memory for storing a set of said image elements; and

processing means for;

a.) obtaining a generalized recovery function that varies in accordance with at least the inverse of the non-ideal Incoherent Optical Transfer Function (IOTF) of said optical assembly, as calculated from at least said aperture function, said lens function and said phase function, under an approximately in-focus condition; and

b.) combining one of a frequency and spatial domain representation of said intermediate image signal and said generalized recovery function to produce a recovered image signal that includes a set of image elements that together define a representation of said object in which said imaging errors have been reduced.

16. The imaging system of claim 15 in which said aperture means includes at least one amplitude mask characterized by a transmittance function that approximates one of a Gaussian function, an exponential function, a Bessel function and segments of cosine functions.

17. The imaging system of claim 15 in which said aperture means includes an amplitude mask, and in which said amplitude mask comprises a coating on a surface of one of said phase masking means and said lens means.

18. The imaging system of claim 15 in which at least two of said aperture means, said lens means and said phase masking means comprise different parts of a single article.

19. The optical reading apparatus of claim 15 further including means for supporting said aperture means, said lens means, and said phase masking means in a predetermined spatial relationship to one another.

20. The imaging system of claim 15 in which said combining includes the multiplying of frequency domain representations of said generalized recovery function and said intermediate image signal, and the taking of an Inverse Fast Fourier Transform (IDFFT) of the result of said multiplying to produce said recovered image signal.

21. The imaging system of claim 15 in which said combining includes a convolution integration of a spatial domain representation of said generalized recovery function with a spatial domain representation of said intermediate image signal.

22. The imaging system of claim 21 in which said calculated non-ideal IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly with said phase masking means, and in which said calculated ideal IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly without said phase masking means.

23. The imaging system of claim 15 in which said calculated non-ideal IOTF of said optical assembly is calculated as the product of a plurality of terms including:

said aperture function, said lens function, said phase function, and at least one additional term which mathematically describes an error producing effect not otherwise included in said aperture function, said lens function or said phase function.

24. The imaging system of claim 23 in which said error producing effect is spherical aberration in said lens means.

25. The imaging system of claim 15 in which said phase masking means is a cubic phase mask which is dimensioned so that it corrects approximately only for said misfocus error.

26. The imaging system of claim 15 in which said generalized recovery function is a fraction the denominator of which comprises said calculated non-ideal IOTF, and the numerator of which comprises a calculated ideal IOTF of said optical assembly, without said phase masking means, under an approximately in-focus condition.

27. The imaging system of claim 26 in which said numerator further includes a finishing function for one of (i) optimizing the spectral profile of the frequency domain representation of said intermediate image signal, and (ii) limiting the bandwidth of the frequency domain representation of said intermediate image signal.

28. The imaging system of claim 26 in which said numerator further includes a finishing friction that takes into account at least one of (i) the cosine-fourth effect, (ii) the effect of non-uniform illumination across said target region, (iii) aliasing effects, and (iv) diffusion effects.

29. The imaging system of claim 15 in which said generalized recovery function is calculated and stored prior to the imaging of said object.

30. An imaging system for imaging an object located in a target region, said imaging system including, in combination:

an optical assembly having an optical axis, said optical assembly including;

a.) an aperture defining element having a transmittance which decreases continuously as the distance from the center of said aperture defining element increases in at least one direction;

b.) at least one image forming element for forming an image of said object, said image including imaging errors that include at least a misfocus error that is dependent upon the distance between said at least one image forming element and said object; and

c.) at least one phase mask for receiving light incident thereon and modifying the phase thereof to cause the Optical Transfer Function (OTF) of the optical assembly to be approximately invariant over a predetermined depth of field;

image sensing means, including an array of photosensitive elements, for receiving said image and converting said image into an intermediate image signal;

digital signal processing means for;

a.) obtaining a generalized recovery function having a frequency domain representation that includes the calculated ideal Incoherent OTF (IOTF) of said optical assembly, without said at least one phase mask, under an approximately in-focus condition, divided by the calculated non-ideal IOTF of said optical assembly, with said at least one phase mask, under an approximately in-focus condition; and

b.) processing said intermediate image signal and said generalized recovery function to produce a final image signal which is a better representation of said object than the intermediate image signal that would be produced by said image sensing means in the absence of said at least one phase mask.

31. The imaging system of claim 30 in which said aperture defining element includes an amplitude mask characterized by a transmittance function that approximates one of a Gaussian function, an exponential function, a Bessel function, and segments of cosine functions.

32. The imaging system of claim 30 in which said aperture defining element is supported on one of said at least one image forming element and said at least one phase mask.

33. The imaging system of claim 30 in which said at aperture defining element, said at least one image forming element, and said at least one phase mask comprise different parts of a single structure.

34. The imaging system of claim 30 in which said at least one phase mask is a cubic phase mask having a shape which cause it to correct approximately only for said misfocus error.

35. The imaging system of claim 30 in which said digital signal processing means multiplies a frequency domain representations of said intermediate image signal and said generalized recovery function.

36. The imaging system of claim 30 in which said digital signal processing means convolves spatial domain representations of said generalized recovery function and said intermediate image signal.

37. The imaging system of claim 30 in which said calculated ideal IOTF is calculated by calculating the autocorrelation of an approximately in-focus generalized pupil function of said optical assembly without said at least one phase mask, and in which said calculated non-ideal IOTF is calculated by calculating the autocorrelation of an approximately in-focus generalized pupil function of said optical assembly with said at least one phase mask.

38. The imaging system of claim 30 in which said calculated non-ideal IOTF takes into account at least one aberration producing feature of said at least one image forming element.

39. The imaging system of claim 30 in which said generalized recovery function further includes a finishing function for one of (i) optimizing the spectral profile of said intermediate image signal, and (ii) limiting the bandwidth of said intermediate image signal.

40. The imaging system of claim 30 in which said generalized recovery function further includes a finishing function which reduces the magnitude of imaging errors that result from at least one of (i) the cosine-fourth effect, (ii) the effect of non-uniformities in the illumination of said target region, (iii) aliasing effects, and (iv) diffusion effects.

41. The imaging system of claim 30 in which said generalized recovery function is produced and stored prior to the time that said object is imaged by said imaging system.

42. The imaging system of claim 30 in which said digital signal processing means is adapted to calculate a transform and inverse transform of signals generated during the operation of said imaging system, and in which said transform and inverse transform include at least a Discrete Fast Fourier Transform and Inverse Discrete Fast Fourier Transform.

43. In an optical reading apparatus for reading optically encoded data from an indicia located in a target region, in combination:

an optical assembly having an optical axis, said optical assembly including;

a.) at least one amplitude mask for receiving and transmitting light and for modifying said light in accordance with a transmittance function characterized by a transmittance that decreases smoothly as the distance from the center of said at least one amplitude mask increases;

b.) image forming means for forming an intermediate image of said indicia, said intermediate image including imaging errors that include at least a misfocus error that is dependent on the distance between said image forming means and said indicia; and

c.) at least one phase mask for receiving light incident thereon and modifying the phase thereof as a function of distance from the center of said at least one phase mask;

an image sensor for receiving said intermediate image and converting said image into an intermediate image signal; and

digital signal processing means for;

a.) obtaining a generalized recovery function that varies in accordance with at least the reciprocal of the calculated non-ideal Incoherent Optical Transfer Function (IOTF) of said optical assembly, under a condition of approximately optimum focus; and

b.) processing one of a frequency domain and a spatial domain representation of said generalized recovery function and said intermediate image signal to produce a final image signal having a reduced quantity of those imaging errors which have been taken into account in the calculation of said calculated non-ideal IOTF.

44. The optical reading apparatus of claim 43 further including decoding means for decoding said final image signal to produce said optically encoded data.

45. The optical reading apparatus of claim 43 further including means for supporting said at least one amplitude mask, said image forming means, and said at least one phase mask in a predetermined spatial relationship to one another.

46. The optical reading apparatus of claim 43 in which said transmittance function approximates one of a Gaussian function, an exponential function, a Bessel function, and segments of cosine functions.

47. The optical reading apparatus of claim 43 in which said generalized recovery function is a fraction the denominator of which comprises said calculated non-ideal IOTF of said optical assembly, and the numerator of which comprises the calculated ideal IOTF of said optical assembly, without said at least one phase mask, under a condition of approximately optimum focus.

48. The optical reading apparatus of claim 47 in which said calculated non-ideal IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly with said at least one phase mask, and in which said calculated ideal IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly without said at least one phase mask.

49. The optical reading apparatus of claim 43 in which said generalized recovery function further includes a finishing function which is adapted to optimize the performance of said reading apparatus for bar code reading applications.

50. The optical reading apparatus of claim 43 in which said generalized recovery function further includes a finishing function which is adapted to reduced imaging errors that result from at least one of (i) the cosine-fourth effect, (ii) the effect of non-uniformities in the illumination of said indicia, (iii) aliasing effects, and (iv) diffusion effects.

51. The optical reading apparatus of claim 43 in which said image sensor is a 1D image sensor, and in which said generalized recovery function is a generalized 1D recovery function.

52. An optical reading apparatus as set forth in claim 43 in which said image sensor is a 2D image sensor, and in which said generalized recovery function is a generalized 2D recovery friction.

53. The optical reading apparatus of claim 43 in which said digital signal processing means is adapted to calculate a transform and an inverse transform of signals generated during the operation of said reading apparatus, and in which said transform and inverse transform include at least a Discrete Fast Fourier Transform and an Inverse Discrete Fast Fourier Transform.

54. In an optical reading apparatus for reading optically encoded data from an indicia located in a target region, in combination:

an optical assembly having an optical axis, said optical assembly including;

a.) aperture means for defining a soft aperture for said optical assembly, said aperture means being characterized by an aperture function;

b.) lens means for forming an intermediate image of said indicia, said image having imaging errors that include at least a misfocus error that is dependent upon the distance between said lens means and said indicia, said lens means being characterized by a lens function; and

c.) phase masking means for receiving light incident thereon and modifying the phase thereof to cause the Optical Transfer Function (OTF) of the optical assembly to remain approximately constant over a range of distances between said lens means and said indicia, said phase masking means being characterized by a phase function;

image converting means for receiving said intermediate image and converting said image into an intermediate image signal that includes a plurality of image elements which are distorted by said imaging errors;

memory means for storing a set of said image elements; and

processing means for;

a.) obtaining a generalized recovery flnction that varies in accordance with at least the inverse of the calculated actual Incoherent Optical Transfer Function (IOTF) of said optical assembly, including at least said aperture function, said lens function and said phase function, under an approximately in-focus condition; and

b.) combining one of a frequency and a spatial domain representation of said recovery function and said set of image elements to produce a final image signal that includes image elements which have a reduced imaging error content.

55. The optical reading apparatus of claim 54 further including means for decoding said final image signal.

56. The optical reading apparatus of claim 54 in which said aperture function is characterized by a transmittance that decreases continuously as the distance from the center of said aperture means increases in at least one direction.

57. The optical reading apparatus of claim 54 in which said aperture function approximates one of a Gaussian function, an exponential function, a Bessel function, and segments of cosine functions.

58. The Optical reading apparatus of claim 54 in which said generalized recovery function is a fraction the denominator of which comprises said calculated actual IOTF of said optical assembly, and the numerator of which comprises the calculated ideal IOTF of said optical assembly, without said phase masking means, under an approximately in-focus condition.

59. The optical reading apparatus of claim 58 in which said calculated actual IOTF is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly, with said phase masking means, and in which said calculated ideal IOTF of said optical assembly is calculated by calculating the autocorrelation of the approximately in-focus generalized pupil function of said optical assembly, without said phase masking means.

60. The optical reading apparatus of claim 54 in which said phase masking means is a cubic phase mask which is shaped so that it corrects approximately only for said misfocus error.

61. The optical reading apparatus of claim 54 in which said recovery function further includes a finishing function for minimizing the imaging error content of said final image signal when said reading apparatus is used to read bar code symbols.

62. The optical reading apparatus of claim 54 in which said generalized recovery function further includes a finishing flnction which takes into account at least one of (i) the cosine-fourth effect, (ii) the effect of non-uniform illumination across said target region, (iii) aliasing effects and (iv) diffusion effects.

63. The optical reading apparatus of claim 54 in which said image converting means is a 1D image sensor, and in which said generalized recovery function is a generalized 1D recovery function.

64. The optical reading apparatus of claim 54 in which said image converting means is a 2D image sensor, and in which said generalized recovery function is a generalized 2D recovery function.