EP2329306A1

EP2329306A1 - Variable focus optical system

Info

Publication number: EP2329306A1
Application number: EP08799347A
Authority: EP
Inventors: Hiroshi Hayakawa; Masayoshi Yamanouchi; Toshio Sujino
Original assignee: Optoelectronics Co Ltd; Opticon Inc
Current assignee: Optoelectronics Co Ltd; Opticon Inc
Priority date: 2008-09-09
Filing date: 2008-09-09
Publication date: 2011-06-08
Also published as: WO2010030267A1; JP2012502349A; CN102150066A; EP2329306A4

Abstract

A variable focus illumination system includes a light source which projects light forwardly, in the path of the light from the source, a movable lens forward of the light source which is mounted for axial movement towards and away from the light source, and a stationary lens is mounted forward of the moving lens. By design, the lens combination creates an image of the light source which is rearward of the actual light source, and movement of the movable lens focuses the light projected forwardly from the image. In comparison to a single lens system, in this dual lens system, the positioning accuracy required in the moving lens in order to achieve a given positional accuracy of focus may be an order of magnitude lower. In accordance with another aspect of the invention, the movable lens is formed as a unit with an opposed, spaced aperture of fixed diameter.

Description

VARIABLE FOCUS OPTICAL SYSTEM

BACKGROUND ART

The present invention relates generally to optical systems and, more particularly, concerns an optical illumination system that provides increased depth of field through the use of a variable beam focus.

Optical scanners, such as barcode readers project a scanned laser beam, which is directed at a remote target containing a code. Illumination reflected from that code is then processed to detect the code. Accurate reading of the code requires that the laser beam remain in focus as it scans across the code. This requires a greater depth of field than is commonly available with CCD or CMOS image sensors. Therefore, variable focus is typically used with laser scanners in order to increase the effective depth of field.

A typical laser beam illumination system 10 is illustrated schematically in Fig.1. A laser light source, such as a laser diode 12 projects laser light forwardly. The light impinges upon and passes through a focusing lens 14, in this case, a fixed lens and, forward of the lens is passed through an aperture 16. A relatively narrow beam is projected from aperture 16 and exhibits a beam waist 18, or a minimum diameter, at a distance ZO from aperture 16, the actual value of ZO being determined, at a particular wavelength of light, by the focal length of lens 14 and the diameter of aperture 16.

Fig. 2 is a schematic representation of a variable focus laser beam illumination system 10', as disclosed in Japanese Patent No. 3730673. That is, the system 10' produces a laser beam in which the distance of the beam waist from the aperture maybe be adjusted. In this case, the light source 12 projects laser light forwardly onto and through a lens 20. Lens 20 is mounted for axial movement towards and away from light source 12. Light emitted forwardly from lens 20 impinges upon an aperture 22, which is variable in diameter. Through the movement of lens 20 and the simultaneous adjustment of aperture 22, the distance of the laser beam waist from aperture 22 can be adjusted through a range of values. By sensing the distance of a target from the aperture 22 and adjusting lens 20 and aperture 22 accordingly, the beam waist distance from the aperture 22 maybe adjusted through a range of values, so that it may be set at a distance corresponding to the distance of the target. As a result, the depth of field of the light source 10' is effectively increased. Although the increase in effective depth of field of the laser source is a desirable result, it is achieved at considerable expense. Control of lens position and aperture size are achieved to two separate control systems and actuators which must be coordinated. Thus, the light source becomes complex, and miniaturization becomes difficult.

DISCLOSURE OF INVENTION

In accordance with one aspect of the present invention a variable focus illumination system includes a light source which projects light forwardly, in the path of the light from the source, are a movable lens, forward of the light source, which is mounted for axial movement towards and away from the light source, and a stationary lens mounted forward of the light source. Preferably, the stationary lens is forward of the movable lens. By design, the lens combination creates an image of the light source which is rearward of the actual light source, and movement of the movable lens focuses the light projected forwardly from the image. In comparison to a single lens system, in this dual lens system, the positioning accuracy required in the moving lens in order to achieve a given positional accuracy of focus may be an order of magnitude lower.

In accordance with another aspect of the invention, the movable lens is formed as a unit with an opposed, spaced aperture of fixed diameter. The illumination system includes a light source which projects light forwardly, the aforementioned unit is mounted forward of the light source for axial movement towards and away from the light source, and a stationary lens is mounted forward of the light source. Not only does the movement of the movable lens change the focus of the emitted light beam, but the simultaneous movement of the aperture towards and away from the light source varies the maximum angle of the light passed on to the stationary lens. This results in particularly effective control of focus. Not only is beam waist diameter reduced in comparison to a fixed diameter aperture, resulting in a smaller light spot size produced by the illumination system, but the variation in spot size over the entire focus range of the illumination system is reduced. In addition, more consistent illumination intensity is achieved. BRIEF DESCRIPTION OF DRAWINGS

The foregoing brief description and further objects, features, and advantages of the present invention will be understood more completely from the following detailed description of presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention, with reference being had to the accompanying drawings, in which:

FIG. 1 is schematic illustration of a typical, existing laser beam illumination;

FIG. 2 is a schematic representation of a variable focus laser beam illumination system known in the art;

Fig. 3 is a sectional view of a first embodiment of a variable focus illumination system in accordance with the present invention;

Fig. 4 is a graph of system focal length F as a function of the position of moving lens 20 for the type of illumination system (30) represented by Fig. 2;

Fig. 5 is a graph of system focal length as a function of the distance between laser diode 34 and moving lens 38 in system 30, as illustrated in Fig. 3;

Fig. 6 is a schematic representation of the optical parameters of system 30 of Fig. 3;

Fig. 7 is a sectional view of a second embodiment 130 of a variable focus illumination system in accordance with the present invention;

Figs. 8(A) and 8(B) are schematic representations of the effect of aperture 156 in the extreme positions of frame 154 in system 130, with Fig. 8 (A) relating to frame 154 at its most forward position and Fig. 8 (A) relating to frame 154 at its most rearward position; and

Fig. 9 is a graph of spot size (beam waist diameter) as a function of distance from the aperture. BEST MODE OF CARRYING OUT INVENTION

Turning now to the drawings, Fig. 3 is a sectional view of a first embodiment of a variable focus illumination system 30 in accordance with the present invention. System 30 includes a housing 32 and broadly comprises a light source in the form of a laser diode 34 mounted at the rear of the housing, a stationary lens 36 mounted to the housing 32 forward of the laser diode 34 and a moving lens 38 mounted intermediate laser diode 34 and stationary lens 36 for movement towards and away from the laser diode. Movement of lens 38 is achieved by means of a linear actuator which will be described further below.

In operation, laser diode 34 emits light forwardly, towards lenses 36 and 38. Through the cooperation of lenses 36 and 38, a focused beam is projected forwardly of lens 36.

Movement of lens 38 causes the waist of the projected beam to move. Specifically, as lens 38 moves closer to laser diode 34, the beam waist moves forward, increasing the effective focal length of system 30.

Preferably, laser diode 34 produces light with a wave length of 650nm, although light of other wavelengths can also be used. Diode 34 is mounted in opening 32a at the rear of housing 32 to emit light in a forward direction.

Stationary lens 36 is mounted on a wall 40 inside housing 32 and projects into an opening 40a in wall 40. Preferably, lens 36 has a focal length of 2.33mm and is mounted at a fixed distance of 2.284mm from laser diode 34. However, it will be appreciated that these values may change, depending on the application. Moving lens 38 is preferably a spherical lens with a focal length of 20mm and is mounted to be movable over a distance of 1 to 1.5 mm from laser diode 34. However, other focal lengths and distances of movement may be used, depending upon the application. It is also contemplated that lens 38 may be cylindrical, a convex toroid, a concave toroid, or any other shape. With this construction, the system focal length can be varied from 100 to 800mm.

A generally cylindrical stationary yoke 45 is mounted to the interior of housing 32 so as to project rearwardly. A sleeve-like moveable yoke 50 is mounted over yoke 45 so as to be axially slideable therealong. The forward end of yoke 50 is mounted to housing 32 by means of a flexible suspension element 52. At the rear of yoke 50, there is mounted a frame 54 which supports moving lens 38. Frame 54 is, in turn, mounted to housing 32 by means of a flexible suspension element 56. The suspension elements 52 and 56 retain yoke 50 that it slides axially on yoke 45. This results in lens 38 moving towards and away from laser diode 34.

A linear actuator is defined by a stationary magnet 60 mounted inside housing 32 and an electric coil 65 formed around yoke 50. As will be appreciated by those skilled in the art, when an electric current is applied to coil 65, the induced magnetic field will interact with magnet 60, causing yoke 50 to slide axially over stationary yoke 45. By controlling the amplitude and direction of the current applied to coil 65, the position of yoke 50 can be controlled, controlling the position of lens 38 relative to laser diode 34. In practice, the distance of a target would be detected and the position of lens 38 controlled so as to place the beam waist at the location of the target.

Fig. 4 is a graph of system focal length F as a function of the position of moving lens 20 for the type of illumination system (30) represented by Fig. 2. As maybe seen, the curve shown in the graph is so steep that system focal length variation from 100mm to 800mm is obtained by moving the movable lens approximately .05mm. For comparison, Fig. 5 is a graph of system focal length as a function of the distance between laser diode 34 and moving lens 38 in system 30, as illustrated in Fig. 3. As may be seen, variation in system focal length from 100mm to 800mm is achieved by moving lens 38 over a distance of .5mm. In other words, the amount of movement of lens 38 is an order of magnitude greater than the amount of movement of lens 20. Thus, if it were desirable to control system focal length in discrete steps, say 5mm steps, this would be far more difficult in system 10' of Fig.2 than in system 30 of Fig.3.

Although, in the preferred embodiment, moving lens 38 is disposed between laser diode 34 and stationary lens 36, such construction is not required. The invention contemplates that stationary lens 36 could be disposed between laser diode 34 and moving lens 38.

Fig. 6 is a schematic representation of the optical parameters of system 30. Illustrated are lenses 36 and 38 and the position O of ^'the laser diode 34. One of the effects of the dual lens system of the invention is to create an image of laser diode 34 at a position O_imag which is shifted backwards from position O by an amount SIFT. The shifted image is then focused through the movement of lens 38. In effect, operation is shifted to a less steep portion of the characteristic of graph of Fig.4, resulting in the characteristics graph of Fig.5. Fig. 7 is a sectional view of a second embodiment 130 of a variable focus illumination system in accordance with the present invention. Most elements of system 130 are identical in construction and operation to corresponding elements in system 30 and have been indicated with the same reference characters. The primary difference is in the construction of frame 154, which differs from frame 54 in that an integral aperture structure 158 is mounted rearwardly at a distance from moving lens 38. In the preferred embodiment, this distance is .3 mm and the diameter of aperture 156 is .25 mm. However, those skilled in the art will appreciate that these values may be varied depending upon the application. Otherwise, the distances, sizes and amount of movement of the various components remain the same. It is also contemplated that aperture 158 may be forward of lens 38 and that the fixed lens 36 may be between laser diode 34 and frame 154.

Figs. 8(A) and 8(B) are schematic representations of the effect of aperture 156 in the extreme positions of frame 154. In Fig. 8 (A), frame 154 is at its most forward position. In this position, aperture 158 masks a substantial portion of the light emitted from diode 34. This results in an effective aperture diameter (pi at the forward end of lens 36. On the other hand, with frame 154 in its rearmost position, much more of the light emitted from diode 34 passes through aperture 158, resulting in an effective aperture diameter Cp₁' which is substantially greater than Cp₁. In the preferred embodiment, (pi is .5mm and Cp₁' is .8 mm. It will be appreciated that these may vary, depending upon the particular application. However, it is clear that the effective aperture increases substantially with increased distance of the beam waist. Therefore, the amount of illumination delivered to the target increases, desirably, with distance of the target, resulting in more consistent intensity of illumination.

Fig. 9 is a graph of spot size (beam waist diameter) as a function of distance from the aperture. Ideally, the spot size should be approximately .2mm and should not change with distance, as represented by curve 110. With the aperture in a fixed position, as represented by curve 120, spot size can be approximately equal to the ideal value at 100mm, but it increases linearly with distance reaching approximately .7 mm at a distance of 500mm from the aperture. With the moving aperture system illustrated in Fig. 7, distance from the aperture is measured from the effective aperture at the forward end of lens 36. As illustrated by curve 130 the spot size is approximately equal to the ideal value at 100mm from lens 36 and increases linearly with increasing distance, but at a substaintally lesser rate than curve 120. In effect, using a moving aperture decreases the average spot size, and the spot size increases more gradually with distance.

Although preferred embodiments of the invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that many additions, modifications and substitutions are possible, without departing from the disclosed invention as defined by the accompanying claims.

Claims

WHAT IS CLAIMED:

1. An optical illumination system comprising: a source of illumination projecting light forwardly; a first lens mounted in a fixed position in the path of the projected light; and a second lens mounted in the path of the projected light for movement towards and away from the light source.

2. The system of claim 1 wherein the first and second lenses are constructed to produce an image of the light source rearward of the light source, the lenses being further constructed to cause this image to be projected forward of the system with a beam waist that varies in position with the position of the second lens.

3. The system of claim 1 wherein the light source emits light in the range of wavelengths defined as laser light.

4. The system of claim 3 wherein the light source emits light having a wavelength of approximately 650 nm.

5. The system of claim 1 or 3 wherein the second lens is mount intermediate the light source and the first lens.

6. The system of claim 5 wherein the second lens is mounted to be moveable from a distance of about 1 mm from the light source to a distance of about 1.5 mm from the light source.

7. The system of claim 5 wherein the first lens is mounted at a distance of about 2.284 mm from the light source.

8. The system of claim 3 or 4 wherein the first lens has a focal length of about 2.33 mm.

9. The system of claim 8 wherein the second lens has a focal length of about 20 mm.

10. The system of claim 8 or 9 wherein the second lens is mounted to be moveable over a distance of about .5 mm relative to the light source.

11. The system of claim 3 or 4 wherein the second lens has a focal length of about 20 mm.

12. The system of claim 11 wherein the second lens is mounted to be moveable over a distance of about .5 mm relative to the light source.

13. The system of claim 3 or 4 wherein the second lens is mounted to be moveable over a distance of about .5 mm relative to the light source.

14. The system of claim 1 further comprising means defining an aperture mounted in opposed relationship to said second lens for movement therewith.

15. The system of claim 14 wherein the aperture is mounted intermediate the second lens and the light source.

16. The system of claim 14 wherein the light source emits light in the range of wavelengths defined as laser light.

17. The system of claim 15 wherein the light source emits light having a wavelength of approximately 650 nm.

18. The system of claim 16 or 17 wherein the aperture is mounted at a distance of approximately .3 mm from the second lens.

19. The system of claim 16 or 17 wherein the aperture has a diameter of about .25 mm.

20. A method for operating an optical illumination system for a target, the system including a source of illumination projecting light forwardly, a first lens mounted in a fixed position in the path of the projected light, and a second lens mounted in the path of the projected light for movement towards and away from the light source, said method comprising the steps of : detecting the distance of the target; and moving the second lens until the focal length of the system is approximately equal to the detected distance of the target.

21. The method of claim 20 performed with an illumination system further comprising means defining an aperture mounted in opposed relationship to said second lens for movement therewith.

22. The method of claim 20 performed with an illumination system wherein the first and second lenses are constructed to produce an image of the light source rearward of the light source, the lenses being further constructed to cause this image to be projected forward of the system with a beam waist that varies in position with the position of the second lens.

23. A method comprising mounting a fixed lens in front of a light source, installing a movable lens and an aperture at a fixed spacial relationship to one another, said movable lens having an optical axis, and causing the movable lens and aperture to jointly move along said optical axis.

24. The method of claim 23 wherein said causing comprises providing an electromagnetic field that jointly moves said aperture and said movable lens.