Optical scanner with fixed focus optics

• US 5,777,314 A
• Filed: 11/05/1996
• Issued: 07/07/1998
• Est. Priority Date: 02/27/1992
• Status: Expired due to Fees

• Current Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Original Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Inventors: Roustaei, Alexander
• Primary Examiner(s): Hajes, Donald T.
• Assistant Examiner(s): Le, Thien Minh
• Application Number:
  US08/743,977
• Time in Patent Office: 609 Days
• Field of Search: 235/462, 235/472, 235/454, 235/467
• US Class Current: 235/462.42
• CPC Class Codes: G03F 7/705 Modelling or simulating from physical phenomena up to complete wafer processes or whole workflow in wafer productions
• CPC Class Codes: G06K 7/10722 Photodetector array or CCD scanning
• CPC Class Codes: G06K 7/10732 Light sources
• CPC Class Codes: G06K 7/10742 including a diffuser for diffusing the light from the light source to create substantially uniform illumination of the target record carrier
• CPC Class Codes: G06K 7/10811 Focalisation
• CPC Class Codes: G06K 7/12 using a selected wavelength, e.g. to sense red marks and ignore blue marks

Abstract

Methods are provided to assist in selection of appropriate optical components for optimizing performance of the optical scanner for reading one-, two- and three-dimensional optically encoded information at a wide range of field depths and angles. The optical components are fixed in position relative to each other so that no position adjustments are required to obtain a reading at a desired distance once the optical components are optimized. Among the parameters that are considered are LEDs for emitting light within specific wavelength ranges, optical filters, lens focal lengths, magnification, etc. The Fourier transform of an input square wave signal generated by light reflected from a bar code is used to generate the appropriate resolution requirements. Selection of appropriate optical components is dependent upon the intended operation of the scanner. For example, depth of field is considered, as is scan angle and density of the code to be read. The performance models are based upon the modulation transfer function (MTF) of the optical system, comprising the encoded information, the lens system and the CCD array.

Claims

1. An optical assembly within an optical scanner for scanning a symbol comprising a plurality of contrasting regions representative of encoded information, each contrasting region having a bar width, wherein said symbol is illuminated by incident light to produce modulated reflected light providing a square wave input representative of said plurality of contrasting regions, said optical assembly comprising:

at least two LEDs for generating the incident light;

a detector array for detecting the square wave input and generating an electrical signal representative of the square wave input, said detector array comprising a plurality of CCDs and having an array area;

a fixed focus multi-element lens system having a pre-selected magnification for receiving the modulated reflected light and providing focused reflected light to said detector array, the pre-selected magnification being selected so that the focused reflected light impinges upon a substantial portion of said array area over a range of distances to said symbol;

wherein said at least two LEDs, said detector array, and elements of said multi-element lens system are selected by determining a spatial frequency of said plurality of contrasting regions, calculating a square wave modulation transfer function using the Fourier transform of the square wave input to determine an initial optical resolution, comparing the initial optical resolution to a minimum required optical resolution for a given density of the plurality of contrasting regions to determine whether a particular symbol with the given density can be read at a given distance from the symbol, and varying the elements of said multi-element lens system to provide an adjusted magnification and an adjusted optical resolution greater than the minimum required optical resolution at the given distance and to obtain equal resolvability for an entire field of view.

2. An optical assembly within an optical scanner as in claim 1 wherein each CCD within said detector array is assumed to have a modulation transfer function which is approximately 1.0 over a standard range of spatial frequencies for said plurality of contrasting regions.

3. An optical assembly within an optical scanner as in claim 1 wherein the minimum required optical resolution is determined according to the relationship:

space="preserve" listing-type="equation">F.sub.s =1/(magnification)(bar width).

4. An optical assembly within an optical scanner as in claim 1 wherein a combination of said CCDs and said at least two LEDs is selected according to the relationship:

space="preserve" listing-type="equation">V.sub.out =∫

V(λ

)F(λ

)d(λ

),

where V(λ

) is the CCD spectral sensitivity in volts/lumen/micron and F(λ

) is the flux output in lumens/micron of each LED of said at least two LEDs so that an output voltage V_out of the CCDs is maximized.

5. An optical assembly within an optical scanner as in claim 1 wherein said multi-element lens system includes an optical bandpass filter.

6. An optical assembly within an optical scanner as in claim 1 wherein the range of distances to said symbol is 0.5 inches to 20 inches.

7. An optical assembly within an optical scanner as in claim 6 wherein the range of distances covers a field of view of up to 30°

.

8. An optical assembly within an optical scanner as in claim 1 wherein said multi-element lens system is disposed at a fixed distance from said detector array.

9. An optical assembly within an optical scanner as in claim 1 wherein the magnification of said multi-element lens systems is optimized using ray tracing.

10. An optical assembly within an optical scanner as in claim 1 wherein said at least two LEDs produce sufficient illumination to obtain an output signal greater than the dark voltage noise from said detector array over the entire said range of distances.

17. A CCD optical scanner as in claim 10 wherein the range of distances covers a field of view of up to 30°

.

11. A CCD optical scanner for scanning a symbol comprising a plurality of contrasting regions representative of encoded information, each contrasting region having a bar width, wherein the symbol is illuminated by incident light from at least two LEDs to produce modulated reflected light providing a square wave input representative of the plurality of contrasting regions, the square wave input being detected by a detector array comprising at plurality of CCDs and having an array area, the improvement comprising:

a multi-element lens system having a fixed focus with a pre-selected magnification for receiving the modulated reflected light and providing focused reflected light to the detector array, the pre-selected magnification being selected so that the focused reflected light impinges upon a substantial portion of the array area over a range of distances to the symbol;

wherein elements of said multi-element lens system are selected in combination with the at least two LEDs and the detector array by determining a spatial frequency of the plurality of contrasting regions, calculating a square wave modulation transfer function using the Fourier transform of the square wave input to determine an initial optical resolution, comparing the initial optical resolution to a minimum required optical resolution for a given density of said plurality of contrasting regions to determine whether a particular symbol with the given density can be read at a given distance from said symbol, and varying the elements of said multi-element lens system to provide an adjusted magnification and an adjusted optical resolution greater than the minimum required optical resolution at the given distance and to obtain equal resolvability for an entire field of view.

12. A CCD optical scanner as in claim 11 further comprising the improvement that each CCD within the detector array is assumed to have a modulation transfer function which is approximately 1.0 over a standard range of spatial frequencies for said plurality of contrasting regions.

13. A CCD optical scanner as in claim 11 wherein the minimum required optical resolution is determined according to the relationship:

space="preserve" listing-type="equation">F.sub.s =1/(magnification)(bar width).

14. A CCD optical scanner as in claim 11 further comprising the improvement that a combination of the CCDs and said at least two LEDs is selected according to the relationship:

space="preserve" listing-type="equation">V.sub.out =∫

V(λ

)F(λ

)d(λ

),

where V(λ

) is the CCD spectral sensitivity in volts/lumen/micron and F(λ

) is the flux output in lumens/micron of each LED of the at least two LEDs so that an output voltage V_out of the CCDs is maximized.

15. A CCD optical scanner as in claim 11 wherein said multi-element lens system includes an optical bandpass filter.

16. A CCD optical scanner as in claim 11 wherein the range of distances to said symbol is 0.5 inches to 20 inches.

18. A CCD optical scanner as in claim 11 wherein said multi-element lens system is disposed at a fixed distance from the detector array.

19. A CCD optical scanner as in claim 11 wherein the magnification of said multi-element lens systems is optimized using ray tracing.

20. A CCD optical scanner as in claim 11 wherein the at least two LEDs produce sufficient illumination to obtain an output signal greater than the dark voltage noise from the detector array over the entire said range of distances.