Patent ID: 7325738

Claim:
A method of determining the lower limit of decoding resolution in a digital imaging-based bar code symbol reader having image formation optics for forming an image of an object and an image sensor having a plurality of sensor pixels, said method comprising the steps of: (a) using a software-based optical design program to generate a composite depth of field (DOF) chart of the image formation optics employed in said 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 the image of the object drops to 0.3; (c) converting the spatial-frequency of the object to code millimeter size, and then plotting this converted data against object distance, wherein the resulting plot includes an optical performance curve 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 1.4 and 1.6 sampling limit lines, 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 one of the sampling limit lines; (h) following said sampling limit lines until one of said lines re-intersects said 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 in said digital imaging-based bar code symbol reader.