Optical scanner using an axicon and an aperture to aspherically form the scanning beam

• US 5,331,143 A
• Filed: 08/28/1992
• Issued: 07/19/1994
• Est. Priority Date: 08/28/1992
• Status: Expired due to Term

• Current Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Original Assignee: Symbol Technologies, Inc. (Zebra Technologies Corporation)
• Inventors: Katz, Joseph, Marom, Emanuel
• Primary Examiner(s): Hajec, Donald
• Assistant Examiner(s): Young, Adrian
• Application Number:
  US07/936,472
• Time in Patent Office: 690 Days
• Field of Search: 235/472, 235/462, 250/236, 250/235, 250/234, 359/196, 359/213, 359/214
• US Class Current: 235/462.38
• CPC Class Codes: G06K 2207/1013 Multi-focal
• CPC Class Codes: G06K 7/10811 Focalisation

Abstract

The use of linear axicons or similar optical elements in a bar code scanner produces a scanning spot of a size correlated generally with dimensions of features of the scanned information, wherein the spot size remains substantially constant for varying distances between the scanner and the symbol over a substantial range of distances. Optical elements of this type produce a diffraction pattern comprising a central lobe and a number of rings surrounding the central lobe. An aperture is provided to limit the beam and thus the number of rings in the pattern which actually reach the symbol during scanning. The extent of the limited beam and the phase front tilt angle produced by the optical element are chosen to produce a desired resolution, which relates to the density of symbols the scanner is to read, and to produce a desired working range. The invention also provides a modular light emitting device including a universal light emitting module and an optical assembly, including the axicon and aperture, mounted on the module to adapt the optical characteristics of light from the module to conform to the requirements of a particular scanning application.

Claims

1. A scanner for scanning optically encoded information of varying light reflectivity, said scanner comprising:

(a) a light source for directing a collimated beam of light in a path toward information to be scanned, and means for causing the beam of light to move along a scan line;

(b) an optical element having a substantially flat first surface perpendicular to an axis of the collimated beam of light and a second surface defined by a figure of rotation revolved about said axis, said figure of rotation being at an angle with respect to said first surface, said optical element causing a phase tilt of the collimated beam of light inward toward said axis;

(c) means for forming an aperture for limiting the extent of the collimated beam of light passing through the optical element, wherein the extent of the collimated light passing through said optical element establishes a predetermined working range of the scanner for a particular phase tilt caused by the optical element; and

(d) a light detector positioned to receive light reflected from said information.

2. A scanner as in claim 1, wherein said light source is a laser.

3. A scanner as in claim 1, wherein the means for forming an aperture form a circular aperture or an elliptical aperture.

4. A scanner as in claim 1, wherein the phase tilt produced by the optical element relates to a minimum symbol width m of the information to be scanned as follows ##EQU19## where β

is the phase tilt caused by the optical element and λ

is the wavelength of the beam emitted by the light source.

5. A scanner as in claim 4, wherein the optical element defines the phase tilt β

in accord with the following relationship;

space="preserve" listing-type="equation">β

=(n-1)α

where α

is the angle between the figure of rotation and the first surface of the optical element, and n is the index of refraction of the optical element.

6. A scanner according to claim 4, wherein said optical element is a refractive linear axicon.

7. A scanner as in claim 1, wherein the phase tilt caused by the optical element relates to a spatial frequency f_x of the information to be scanned as follows ##EQU20## where β

is the phase tilt caused by the optical element and λ

is the wavelength of the beam emitted by the light source.

8. A scanner as in claim 7, wherein the optical element defines the phase tilt β

in accord with the following relationship;

space="preserve" listing-type="equation">β

=(n-1)α

where α

is the angle between the figure of rotation and the second surface of the optical element, and n is the index of refraction of the optical element.

9. A scanner according to claim 7, wherein said figure is a refractive linear axicon.

10. A scanner as in claim 1, wherein the optical element and aperture are dimensioned in accord with the following relationship:

##EQU21## where β

is the phase tilt caused by the optical element, the aperture limits the light passing through said optical element to a radius of R_o, and λ

is the wavelength of the beam of light emitted by the laser source.

11. A scanner as in claim 10, wherein the optical element defines the phase tilt β

in accord with the following relationship;

space="preserve" listing-type="equation">β

=(n-1)α

where α

is the angle between the figure and the light receiving surface of the optical element, and n is the index of refraction of the optical element.

12. A scanner as in claim 10, wherein the optical element and aperture are further dimensioned in accord with the following relationship:

##EQU22##

13. A scanner as in claim 10, wherein said aperture is a circular aperture of radius R_o.

14. A method for scanning a symbol, comprising the steps of:

(a) generating a collimated light beam and directing said light beam along a path toward a symbol to be read;

(b) moving said light beam to generate a scan line across said symbol to be read;

(c) modifying said light beam in the path toward said symbol to create a spot of light of a size correlated generally with the size of features of said symbol, said spot maintaining a substantially constant size for varying distances to said symbol, over a substantial range of said distances on an optical axis, the step of modifying including;

(i) producing a phase tilt of the collimated beam of light inward toward the optical axis, and

(ii) limiting the number of Bessel rings which surround a central lobe of the phase tilted beam of light.

15. A method according to claim 14, wherein said light beam is a laser light beam.

16. A method according to claim 14, wherein said step of producing a phase tilt includes directing said collimated light beam through a solid optical element having a substantially flat first surface and a second surface defined by a figure of rotation revolved about the optical axis, said figure of rotation being at an angle to said first surface.

17. A method according to claim 16, wherein said solid optical element is a refractive linear axicon.

18. A method according to claim 16, wherein said step of limiting the number of Bessel rings comprises limiting the radius of the collimated light beam passing through said solid optical element.

19. A method of scanning a symbol, comprising the steps of:

(a) generating a substantially monochromatic beam of collimated light;

(b) modifying the beam of light to create a beam spot of substantially constant diameter which extends along a predetermined distance along the path of said beam and which exhibits a predetermined diffraction pattern having a central lobe and a plurality of rings surrounding said central lobe;

(c) limiting the beam diffraction pattern to reduce the number of said rings surrounding said central lobe;

(d) directing the reduced beam diffraction pattern onto a symbol to be read; and

(e) moving the reduced beam diffraction pattern across the symbol to be read.

20. A method as defined in claim 19, wherein:

the step of modifying the beam of light comprises passing the beam of light through an optical element which produces a phase tilt of the collimated beam of light inward toward an axis of the optical element, and

the step of limiting the beam diffraction pattern comprises limiting the radius of the collimated beam of light passing through said optical element.

21. A method as defined in claim 20, wherein:

said optical element comprises a solid optical element having a substantially flat first surface and a second surface defined by a figure of rotation revolved about an axis perpendicular to said first surface, said figure being at an angle with respect to said first surface, and

the step of limiting the radius of the collimated beam of light passing through said optical element comprises passing the beam through an aperture to limit the beam to a predetermined radius.

22. A device for use in a system for reading optically encoded information, comprising:

(A) a light emitting module having a fore end portion from which light is emitted, said light emitting module comprising;

(i) a light emitting element emitting light in a direction toward the fore end portion of the light emitting module,

(ii) a first optical element for collimating and focusing the light from the light emitting element substantially to infinity, and

(iii) first mounting means for mounting the first optical element along an axis of light emitted from the light emitting element at a point adjacent the fore end portion of the light emitting module; and

(B) an assembly for mounting over the fore end portion of the light emitting module from which light is emitted, said assembly comprising;

(i) a second optical element that will produce a phase tilt of the collimated light from the first optical element so as to cross the axis along a continuous line of points along a substantial extent of the axis,

(ii) means for forming an aperture for limiting the light passing through the second optical element, and

(iii) second mounting means for mounting the second optical element and the means for forming an aperture over the fore end portion of the light emitting module from which light is emitted.

23. A device as in claim 22, wherein said light emitting element is a laser.

24. A device as in claim 22, wherein said second optical element is a solid refractive optical element having a substantially flat surface and a surface defined by a figure of rotation revolved about an axis perpendicular to said substantially flat surface, said figure being at an angle with respect to said substantially flat surface.

25. A device as in claim 24, wherein said solid optical element is a linear axicon.

26. A device as in claim 22, wherein the second mounting means positions the aperture between the first optical elements and the fore end portion of the light emitting module.

27. A device as in claim 22, wherein the second mounting means positions the second optical element between the first optical element and the aperture and positions the aperture adjacent a surface of the second optical element from which light emerges.

28. A device as in claim 22, wherein the second means for mounting comprise a detachable mounting structure for removably supporting the second optical element and the means for forming an aperture over the fore potion of the light emitting module, said detachable mounting structure facilitating removal and replacement of said assembly.

29. A device for reading optically encoded information having varying light reflectivity, comprising:

(A) a light emitting module having a fore end portion from which light is emitted, said light emitting module comprising;

(i) a light emitting element emitting light in a direction toward the fore end portion of the light emitting module,

(ii) a first optical element for collimating and focusing the light from the light emitting element substantially to infinity, and

(iii) first mounting means for mounting the first optical element along an axis of light emitted from the light emitting element at a point adjacent the fore end portion of the light emitting module;

(B) an assembly for mounting over the fore end portion of the light emitting module from which light is emitted, said assembly comprising;

(i) a second optical element that will produce a phase tilt of the collimated light from the first optical element so as to cross the axis along a continuous line of points along a substantial extent of the axis,

(ii) means for forming an aperture for limiting the light passing through the second optical element, and

(iii) second mounting means for mounting the second optical element and the means for forming an aperture over the fore end portion of the light emitting module from which light is emitted;

means for producing a scanning motion of said beam of light across a portion of the information; and

means for receiving light reflected back from the optically encoded information and producing electrical signals corresponding to the varying light reflectivity.