An optical system
The invention relates to an optical system for imaging an object on an image sensor, said system having an optical axis extending through an aperture and through image-forming means that comprise an aspheric light-beam-refracting surface.
The invention is directed in particular, but not exclusively, to reading of information on vials that may contain eg insulin. Such indication is to contain information regarding the contents of the container, their expiry date, production date, batch number, etc.
Various kinds of bar codes, matrix codes and code readers are known for this purpose. The code readers are constructed primarily on the basis of two principles. Firstly, a principle in which a scanning collimated beam that has a small cross section is used for lighting the code point by point, which is typically accomplished by a laser beam being reflected in mirrors with cyclical movements in combination with a photodetector to determine the intensity of reflected light from a lighted point on the code.
Another principle is the use of a diffuse light source and an optical system that is able to depict the diffusely lighted code clearly on an image sensor. Scanning constructions for matrix codes are more complex than those intended for bar codes, since in general lighting of a matrix code requires a two-dimensional movement of the beam. Generally, it may be difficult to ensure the reliability of the construction with movable parts in the apparatus that is worn by a user in his/her everyday life, and wherein the requirements to miniaturisation are high. This circumstance in combination with decreasing prices on image sensors makes systems with image sensors increasingly attractive for the reading of matrix codes.
Systems based on reading of bar codes are known ia from US patent No. 4,978,335 and US patent No. 5,821,524, while systems with a two-dimensional matrix code are described in Japanese patent application Nos 2001-075480 and 11-3167877. The former also mentions use of CCD-cameras for reading of the code, but no technique is available by which it is possible in practice to image a two-dimensional information matrix on a CCD camera when the below conditions are to be taken into account.
The prior art optics for image sensors intended for code readers have a work range that extends from no less than 30 to typically 500 mm focal length. A very important aspect of the invention is that the matrix code can be read by means of an optical system that is so compact that it can be built integrally with eg a drug administration device for dispensing insulin. Thereby the information on the vial can be used for calculations within the drug administration device, and albeit the invention also lends itself for use in connection with conventional bar codes, their information density is typically too small for efficiency advantages to be obtained in connection with a calculation unit in the drug administration device. The invention will now be described in connection with two-dimensional matrix coding and two problems arise from this; on the one hand the problem of positioning the matrix code correctly relative to the optical system and on the other hand the problem of having the matrix code imaged correctly on the sensor so as to ensure that only little distortion in lack of focus occurs even in case the focal length is desired to be considerably less than 30 mm.
The problem is due to the overall limited depth of field and increasing optical distortion in case of small focal lengths. Deficient focusing in combination with the position on the code being depicted on the image sensor with a nonlinear positional transformation makes the decoding algorithm unstable or entirely prevents decoding. On top of this problem there is the cylindrical surface of the carpule that makes demands to the tolerance of the system
towards precisely depth of field and nonlinear positional transformation. The major problem is that beams coming from the extreme comers of the object have to travel a considerably longer path from the image plane than beams coming from the centre of the matrix code.
GB 2 287 551, US 5 305 147, and US 5 311 364 teach optical systems with aspheric lenses wherein short focal length is aimed at. These prior art systems are very complex and expensive since they aim at large collimating apertures and low distortion in general.
The object of the invention is to provide an optical system of the above-described kind, wherein it is accomplished to have very little distortion in terms of, lack of focus by means of inexpensive and operationally reliable optical elements.
This object is obtained in that the system comprises an aperture configured and located such in relation to an aspheric light-beam-refracting surface that beams of light from two image points having dissimilar distances from the optical axis are deflected by mutually different surface portions of the aspheric surface.
In this document aspheric surfaces refers to non-planar and non-spherical surfaces that according to the invention enable compensation of optical distortion by means of the disclosed use of aperture size and position. The aspheres can be bicurvatured, for example a paraboloide, as well as predominantly elongated and single-curvatured, for example a cylinder. The asphere can be used as the only focussing element in the system, or it can be used as a predominantly corrective element in combination with other optical elements.
According to the invention the diameter of the aperture is such that beams coming from the edge of the object are refracted by points on the surface of the asphere, not comprising the apex of the asphere. This makes it possible to compensate for the difference in distance between object and image plane, depending on whether beams are viewed that originate from the comer or the centre of the object. The varying distance makes it necessary to have different focal distances or focal lengths for the two extremes, which is precisely obtained with the optical system according to the invention.
The opening size of the aperture to object and image planes, respectively; is dimensioned such that the part of the aspheric surface that refracts beams from the edge of the object does not coincide with that part of the aspheric surface that refracts beams coming from the centre of the object.
Preferably an aspheric lens element is used, but the principle of the invention also comprises use of an aspheric mirror.
Light-beam refraction surfaces refer to refractive and reflective optical elements with properties that can be described by geometrical optics (ray tracing models). Light-beam refracting surfaces furthermore refer to optical elements that are partially or fully based on diffraction phenomena.
According to a preferred embodiment the aspheric lens is injection-moulded and the aperture is circular.
According to an alternative embodiment the aperture may be elongate, thereby allowing that the light beams in one single dimension, ie planes that contain the optical axis, can be refracted by both the centre and the edge area of the lens. The optical error will not be particularly large due to the matrix code usually being single-curved and not double-curved. Preferably an elongate aperture opening will be used in combination with an elongate lens,
whose surface has an aspheric curvature seen transversally to the longitudinal direction of the lens.
According to a preferred embodiment the system comprises a moulded housing provided with an aperture that is integral with the housing and comprises fixation means for the image-forming means. Several lenses may be provided in the housing that preferably comprises fixation means for one or more light sources or light conductors. Since the housing may also comprise fixation means for the image sensor, an optical system is obtained, wherein it is possible to accomplish, by inexpensive means, an accurate positioning of simple optical elements configured such that an approximately, distortion-free imaging of a curved object can be accomplished on a plane object sensor, while simultaneously the optical distance between object and the image sensor is very small, eg less than 30 mm.
The small distortion that still occurs with the optical system according to the invention can be further reduced by means of image-correcting circuits that are known per se and receive digital image data from an image sensor. The correcting circuit can be based on a digital polynomium function.
The invention is especially useful when incorporated in a doser for drug administration or a BGM due to its very compact and reliable design.
List of Figures
In the following the invention will be described with reference to the accompanying drawing, wherein:
Figure 1 shows the technical principle according to the invention; Figure 2 shows a two-dimensional matrix code known per se;
Figure 3 is a sectional view through an optical sensor according to the invention; while
Figure 4 is an exploded view of the embodiment shown in Figure 3.
Reference is now being made to Figure 1, wherein the principle on which the present invention relies Is first explained.
In the figure a part of the surface 1 of a cylindrical vial is shown, which is provided with a label 2 (see Figure 2). Furthermore an optical sensor or camera chip 3, a lens 4 and an aperture 5 are shown. From the figure it will clearly appear that the path to be travelled by a light beam from point A to point B is longer that the path to be travelled by a light beam from point C to point D. The deviation in path lengths can for example reach 12% when triangulation is used on the optical system. The effect of the present invention is that the optical system can be made very compact and if a length of the optics of eg 12 mm is imagined, said 12% will correspond to 1.4 mm, which is approaching the theoretical depth of field for such system when the aperture opening is 0.5 mm. The depth of field of eg 1.4 mm is to suffice for said effects with the path length plus all uncertainties in the mechanical joints. This means that, in practice, it is not possible to construct an optical system with lengths of eg 12 mm of a sufficiently high quality by means of prior art constructional principles. Albeit the aperture opening is very important to the depth of field, once it is less than approximately 0.5 mm, the undesired diffraction phenomena increase and the light intensity of the sensor drops to levels that are useless to relevant lights sources and"sensors.
According to the invention an aspheric image-forming element, such as the lens 4, is combined with a small aperture, eg 0.5 mm. The aperture and its location in relation to the lens 4 is to be configured such that the beam path is
accomplished that is shown in Figure 1, from where it will appear that light beams from the point A is refracted through such portions of the aspheric surface that do not overlap the portions of the aspheric surface where the light beams from C to D are refracted. Therefore the two said areas of the lens surface may be configured such that they each take into account the different focal distances to the respective parts of the label 2.
The actual design of a lens working according to the principle according to the invention can be done by means of well known techniques, eg the formula
(Formula removed)

where
1/c =-1.24042
k = -0.786851
r is the radial coordinator in a coordinate system having its origin in the apex
of the lens.
Figure 2 shows an example of a two-dimensional matrix code 6 known per se and comprising a number of dots, eg a dark dot 7 and a light dot 8. Such matrix codes are known, see eg US patent No. 5,126,542, wherein the light and dark dots, respectively, represent 0 and 1, respectively, in digital information. Such two-dimensional matrix code has considerably higher information density than can be obtained In connection with a bar code, and albeit the invention also lends itself for use in connection with the bar code, the advantages of the invention are particularly obvious when a two-dimensional matrix code is used since, by imaging of such code imaging problems will occur when the matrix code is located on a round surface, eg a label on an insulin ampoule.
The curvature of label and the very short focal lengths that are relevant to such use of the invention bring about so comprehensive optical errors that they cannot be corrected in practice by means of known digital image-forming software programs. This problem is solved by means of the invention that can also be supplemented by digital image correction known per se, eg for counteracting cushion distortion and/or barrel distortion.
Figure 3 is a sectional view through a preferred embodiment of the invention.
The figure shows a vial 11 provided with a label 12 and supported in a seat 28. The information on the label is read by means of an optical system mounted on an electrical circuit board PCB 14~ On the circuit board light diodes 15 and 16 are also mounted as well as an image sensor 17. The opticaj system is also shown in Figure 4.
Reference numeral 18 is used to designate a light-conductor component for receiving light from the light diodes 15 and 16. The light-conductor component has polished surfaces, eg shown by 19-21, such that the light leaves through a dull surface 22, whereby homogenous and diffusely distributed light is emitted towards the label 12. Just outside the dull surface of the light-conductor component 18, a thin, transparent, protective barrier 23 is provided which allows for cleaning of the optical system.
A part 24 of the light-conductor component 18 is configured as an aspheric
lens 24 and by 25 an aperture is shown; see the aperture 5 shown in Figure 1. Furthermore a non-transparent shield 26 is provided to ensure that no intrusive light is allowed to enter the image sensor that is only to receive light from the label via the lens 24. The components just mentioned are surrounded by a housing 27 which will be explained in more detail with Figure 4.
Figure 4 is an exploded view of the embodiment shown in Figure 3. The housing 27 is a separate injection-moulded component containing a not shown, thin transverse beam 29 in which the aperture 25 is configured. The light-conducting component 18 comprises, as mentioned above, the lens 24, and as will appear from the above explanation, it is important according to the invention that the lens is arranged very well-defined in relation to the aperture 25. In the embodiment shown, this is accomplished by means of guide faces 30, 31 on the component 18, which faces fit into corresponding recesses in the moulded component 27, as will appear clearly from Figure 4. In this manner the lens 24 and the aperture 25 are positioned perfectly accurately in relation to each other. The positioning of the image sensor 17 in relation to the optical elements can be made in a conventional way, e.g. by cooperating projections and depression on the two parts to be assembled.

Claims
1. An optical system for Imaging an object on an image sensor, said system
having an optical axis extending through an aperture and through image-
forming means that comprise an aspheric light-beam refracting surface,
characterised In that the aperture Is configured such and has such location
in the optical system that light beams from two image points having dissimilar
distances to the optical axis are refracted essentially by mutually different
surface portions of the aspheric surface.
2. An optical system according to claim 1, characterised in that the image-
forming means comprise an aspheric lens.
3. An optical system according to claim 1, characterised in that the image-
forming means comprise an aspheric mirror.
4. An optical system according to claim 1, characterised in that the image-
forming means comprise a diffraction lens.
5. An optical system according to claim 2, characterised in that the lens is
injection moulded.
6. An optical system according to claim 1, characterised in that the aperture
is circular.
7. An optical system according to claim 1, characterised in that the aperture
is elongate.
8. An optical system according to claim 7, characterised in that the lens is
elongate with an aspheric surface that is defined by a conveyor that is
parallel with the longitudinal direction of the lens.
9. An optical system according to any one of claims 1-7, characterised in
that it comprises a moulded housing provided with an aperture that is integral
with the housing and comprises fixation means for the image-forming means.

10. An optical system according to claim 4, characterised in that several
lenses are provided in the housing.
11. An optical system according to claim 9 or 10, characterised in that a
number of mirrors are provided in the housing.
12. An optical system according to claims 9-11, characterised in that the
housing comprises fixation means for one or more light sources.
13. An optical system according to claim 12, characterised in that the
housing comprises light conductors.
14. An optical system according to claim 12 or 13, characterised in that the
housing comprises fixation means for an image sensor.
15. An optical system according to claims 8-14, characterised in that the
optical distance between the object and the Image sensor is less than 30
mm.
16. An optical system according to claims 1-13, characterised in that the
image sensor enables the production of digital image data.
17. An optical system according to claim 16, characterised in that an image-
correcting circuit is provided that is configured for adjusting image data such
that optical imaging errors are counteracted.
18. An optical system according to claim 15, characterised in that the
adjustment is provided by means of a digital polynomlum function.
19. An optical system according to claims 1-18, characterised in that it is
integrated In a drug administration device for medical treatment.
20. An optical system according to claims 1-14, characterised in that it is
integrated in a Blood Glucose Measure device for reading information on
strips used in such devices.