Method of and system for determining the lower limit of decoding resolution in an imaging-based bar code symbol reader

• US 20050103863A1
• Filed: 07/29/2004
• Published: 05/19/2005
• Est. Priority Date: 11/13/2003
• Status: Active Grant

• Current Assignee: Metrologic Instruments Incorporated (Honeywell International Inc.)
• Original Assignee: Metrologic Instruments Incorporated (Honeywell International Inc.)
• Inventors: Hou, Rui, Zhu, Xiaoxun, Knowles, C. Harry, Au, Ka Man, Liu, Yong, Liu, Liang, Schmidt, Mark, Ghosh, Sankar, Kotlarsky, Anatoly, Tao, Xi, Zhang, Wenhua, Amundsen, Thomas, Russell, Garrett, Byun, Sung, Yu, Hongpeng, Bonanno, John, Schnee, Michael, Spatafore, Pasqual
• Granted Patent: US 7,325,738 B2
• Time in Patent Office: Days
• US Class Current: 235/462.410
• CPC Class Codes: G06K 7/10732 Light sources
• CPC Class Codes: G06K 7/10851 Circuits for pulse shaping, amplifying, eliminating noise signals, checking the function of the sensing device basic electronic circuitry H03
• CPC Class Codes: G06K 7/10881 constructional details of hand-held scanners

Abstract

A method of determining the lower limit of decoding resolution in a digital imaging-based bar code symbol reader. A software-based optical design program is used to generate the composite DOF chart of image formation optics employed in the digital imaging-based bar code symbol reader. The modulation transfer function (MTF) of the object image is used to determine the smallest mil size code that can be decoded, at a given object distance, to produce an optical performance curve. The program calculates the size of the field of view (FOV) of a single sensor pixel, when projected through the image formation optics and out into object space. For both the 1.4 and 1.6 sampling limit pixel rules, the calculated FOV values are plotted on the same axes as the optical performance curve. Using graphical solution techniques, the sampling limit line of choice determines the lower limit of the decoding resolution in the imaging-based bar code symbol reader.

Claims

1-56. -56. (canceled)

57. A method of determining the lower limit of decoding resolution in a digital imaging-based bar code symbol reader, said method comprising the steps of:

(a) using a software-based optical design program to generate a composite DOF chart of the image formation optics employed in a digital imaging-based bar code symbol reader;

(b) determining, as a function of object distance, the spatial-frequency of the object at which the modulation transfer function (MTF) of its image drops to 0.3;

(c) convert the spatial-frequency of the object to code millimeter size, and then plot this converted data against object distance, wherein the resulting plot includes an optical performance portion illustrating the performance of said image formation optics in terms of the smallest millimeter size code that can be decoded, at a given object distance;

(d) reading the DOF from said plot produced in step (c), by finding the intersections of the millimeter size with the optical performance curve;

(e) calculating, as a function of object distance, the size of the field of view of a single sensor pixel when projected through said image formation optics and out into object space, while accounting for the optical magnification of said image formation optics;

(f) for both the 1.4 and 1.6 sampling limit pixel rules, plotting the values calculated in step (e) on the same axes as the optical performance curve;

(g) following the optical performance curve until it intersects the sampling limit line;

(h) following said sampling limit line until it re-intersects said optical performance curve; and

(i) at said point of intersection, follow the optical performance curve whereby, the sampling limit line of choice represents the lower limit of the decoding resolution in said digital imaging-based bar code symbol reader.

58. A method of theoretically characterizing the depth of field (DOF) of image formation optics employed in an imaging-based bar code reader, comprising the steps of:

(a) using a software-based optical design program to generate a composite DOF chart of the image formation optics employed in a digital imaging-based bar code symbol reader;

(b) determining, as a function of object distance, the spatial-frequency of the object at which the modulation transfer function (MTF) of its image drops to 0.3;

(c) convert the spatial-frequency of the object to code millimeter size, and then plot this converted data against object distance, wherein the resulting plot includes an optical performance portion illustrating the performance of said image formation optics, in terms of the smallest mil size code that can be decoded at a given object distance;

(d) reading the DOF from said plot, by finding the intersections of the millimeter size with said optical performance curve;

(e) calculating, as a function of object distance, the size of the field of view of a single sensor pixel when projected through said image formation optics and out into object space, while accounting for the optical magnification of said image formation optics; and

(f) for both the 1.4 and 1.6 sampling limit pixel rules, plotting the values calculated in step (e) on the same axes as the optical performance curve.

59. The method of claim 58, which further comprises:

(g) following the optical performance curve until it intersects the sampling limit line;

(h) following the sampling limit line until it re-intersects the optical performance curve; and

(i) at said point of intersection, following the optical performance curve

whereby, the sampling limit line of choice represents the lower limit of the decoding resolution of said imaging-based bar code symbol reader.

60. The method of claim 59, wherein step (a) comprises:

launching a graphical user interface (GUI) for supporting pop-up windows to enable the user to easily type numbers into said software-based optical design program.

61. A method of designing image formation optics for use in a digital imaging-based bar code symbol reader having application requirements, said method comprising the steps of:

(a) calculating the modulation transfer function (MTF) of an image of an object formed by the image formation optics used in an imaging-based bar code symbol reader design having application requirements (e.g. decoding resolution);

(b) determining, as a function of object distance, the code element size of the bar code symbol (in millimeters) at which the modulation transfer function (MTF) of the resulting image falls to 0.3;

(c) generating a plot of minimum code element size against object distance; and

(d) using said plot, to determine whether or not the resulting performance of said imaging-based bar code symbol reader satisfies said application requirements.