WO2022153141A1

WO2022153141A1 - Flat scanner

Info

Publication number: WO2022153141A1
Application number: PCT/IB2022/050057
Authority: WO
Inventors: Silvia Colagrande; Massimo Colagrande; Lorenzo Colagrande
Original assignee: Silvia Colagrande; Massimo Colagrande; Lorenzo Colagrande
Priority date: 2021-01-15
Filing date: 2022-01-05
Publication date: 2022-07-21
Also published as: IT202100000170U1; CN214851410U; DE202021100877U1

Abstract

Described is a flat scanner (100) of the xy type configured to implement the Photometric Stereo technique and comprising a scanning surface (ɤ) on which an object to be scanned can be positioned and an acquisition device (1) positioned along a vertical optical axis (A) and configured for acquiring at least one image of the object to be scanned. The scanner (100) also comprises at least three light sources (2a, 2b, 2c) which can be selectively activated to emit a beam of light collimated on the scanning surface (ɤ). Each light source (2a, 2b, 2c) defines a tilt angle (δ) between a projection of the light source (2a, 2b, 2c) on the scanning surface (ɤ) and a reference axis (B) lying on the scanning surface (ɤ) and a slant angle (θ) between the light source (2a, 2b, 2c) and the optical axis (A). The light sources (2a, 2b, 2c) are angularly distributed about the optical axis (A) in such a way that each light source (2a, 2b, 2c) has a slant angle (θ) of between 20° and 80° and a tilt angle (δ) such as to differ from the tilt angles (δ) of the other light sources (2a, 2b, 2c) by a value greater than or equal to 180°/n where n corresponds to the number of light sources (2a, 2b, 2c).

Description

DESCRIPTION

“FLAT SCANNER”

This invention relates to a flat scanner of the xy type configured to implement the Photometric Stereo technique.

In particular, the invention is widely used in the field of image acquisition.

As is known, the Photometric Stereo technique allows 3D information to be obtained starting from a set of colour or greyscale images. Since this technique is particularly suitable for capturing fine details of mainly two-dimensional surfaces (such as, for example, a painting, a bas-relief, a marble slab and the like) and is not, on the other hand, able to capture the shape of complex three-dimensional objects (such as, for example, a bowl), it is designed to be implemented in a scanner for flat images in order to scan materials and surfaces and at the same time return colour and 3D digital information.

As is known, the Photometric Stereo technique is implemented by illuminating an object positioned on a scanning surface by means of a plurality of luminous sources which can be selectively activated and acquiring, at each activation, an image representing the object.

For the implementation of the Photometric Stereo technique, flat scanners are normally used comprising a scanning surface on which the object is positioned and also comprising a sensor for images configured for acquiring an image relative to the object. These scanners also comprise an optical system interposed between the image sensor and the scanning surface and aligned with the image sensor along a vertical axis perpendicular to the scanning surface.

In prior art flat scanners, therefore, the light sources are switched on one at a time and, upon each switching on, an image of the object and/or an image of a portion of it is acquired by the image sensor.

Although, unlike the other 3D image acquisition techniques, the Photometric Stereo technique does not require the use of dedicated 3D devices (such as, for example, confocal lasers and the like) whilst allowing information to be obtained in colour and with a high level of detail, it is necessary that the light sources are arranged inside the scanner in such a way as to obtain directions of illumination sufficiently different from each other and in such a way that the light produced by each light source illuminates the object to be scanned in a uniform manner.

It is also known that the greater the number of light sources the better is the result. In this regard, there are prior art flat or planetary scanners comprising a first and a second pair of luminous sources, an optical system and a linear image sensor, that is to say, a sensor which is able to acquire an image relative to a portion of the object lying along a scanning line defined on the scanning surface. In this situation, the light sources are switched on one at a time and, each time, an image of the portion of the object lying along the scanning line is acquired. Subsequently, the object (or the optical system, image sensor and light sources as a whole) is moved in such a way that adjacent portions lie progressively along the scanning line and are therefore captured.

In these prior art scanners, the light sources of the first pair are normally located at the sides of the scanning line and extend parallel to it, whilst the light sources of the second pair are positioned at the ends of the scanning line and extend along a direction transversal to it. Disadvantageously, in this situation, the light sources of the second pair are not able to illuminate the scanning line with a beam of light having rays with uniform angles of incidence on the scanning line, and thus on the portion of object lying there. The rays which strike the end of the scanner line closest to the light source switched on have an angle of incidence greater than the rays which strike the end of the scanning line opposite the light source.

This results in a low quality of the images acquired and therefore a poor precision of the scanning as well as in the digital reconstruction of the scanned object.

There are also prior art flat scanners comprising sensors for linear type images wherein the second pair of light sources is positioned in such a way that each light source lies along the direction defined by the scanning line and arranged in such a way as to emit collimated light rays in the scanning area.

In this situation, the four light sources are therefore all parallel to the scanning line, are arranged in a cross-like fashion and, since each light source of the second pair is collimated, the fact that the light rays are uniform in terms of angles of incidence and also light intensity of the beams of light emitted is guaranteed. Disadvantageously, even though this scanners complies with the uniformity constraints imposed by the Photometric Stereo technique, it is not able to be adapted to the introduction of a matrix and non-linear image sensor. More specifically, given the arrangement of the light sources, the scanner is not able to illuminate a two-dimensional surface in a suitably uniform manner.

This prior art scanner is therefore not very flexible and not very versatile since, changing the positioning of the light sources or changing the type of sensor for images used, the scanner is no longer able to provide the directional accuracy and the correct luminous uniformity required by a correct and optimum implementation of the Photometric Stereo technique.

Inother words, in this scanner, in order to obtain an acceptable implementation of the Photometric Stereo technique, the light sources must be necessarily positioned and arranged as described above and the image sensor must necessarily be of the linear type. Moreover, the scanner has numerous constraints (such as, for example, that relative to the fact that the light sources must also provide a high degree of uniformity of luminous intensity and not only a high uniformity of the angles of luminous incidence) and numerous limitations which do not make it adaptable, for example, to scanning the object for areas and no longer for lines.

The technical purpose of this invention is therefore to provide a flat scanner of the xy type which is able to overcome the drawbacks of the prior art.

The aim of this invention is therefore to provide a flat scanner of the xy type which is able to simplify the problem relative to the positioning and arranging of the light sources and which is at the same time able to provide good quality scanners using the Photometric Stereo technique.

A further aim of the invention is to provide a flat scanner of the xy type which is compact and economical.

A further aim of the invention is to provide a flat scanner of the xy type which has strict construction constraints in order to obtain a good scanning but at the same time be flexible in such a way as to make the scanner adaptable to various requirements.

The technical purpose indicated and the aims specified are substantially achieved by a flat scanner of the xy type comprising the technical features described in one or more of the accompanying claims. The dependent claims correspond to possible embodiments of the invention.

Further features and advantages of the invention are more apparent in the nonlimiting description which follows of a non-limiting embodiment of a flat scanner of the xy type.

The description is set out below with reference to the accompanying drawings which are provided solely for purposes of illustration without restricting the scope of the invention and in which:

Figure 1 shows a diagram of an example of a positioning of a light source inside the scanner according to the invention;

Figure 2 shows a schematic view of an embodiment of a flat scanner of the xy type according to the invention;

Figure 3 shows a possible embodiment of a flat scanner of the xy type according to the invention;

Figure 4 shows a further possible embodiment of a flat scanner of the xy type;

Figure 5 shows a further possible embodiment of a flat scanner of the xy type;

Figure 6 shows a further possible embodiment of a flat scanner of the xy type.

With reference to the accompanying drawings, the numeral 100 denotes a flat scanner of the xy type configured to implement the Photometric Stereo technique. The scanner 100 comprises a scanning surface on which an object to be scanned "O” can be positioned.

The expression "scanner of the xy type" is used to mean a scanner wherein the acquisition of images representing the object "O" is performed by scanning different separate portions of the scanning surface v.

The scanner 100, according to the invention, also comprises an acquisition device 1 positioned along a vertical optical axis "A" and perpendicular to the scanning surface v. The acquisition device 1 is configured for acquiring at least one image of the object "O" to be scanned or an image of a part of the object "O", in particular of the part lying in a portion of the scanning surface v located below the acquisition device 1 (Figure 3).

Preferably, the acquisition device 1 is configured for acquiring a succession of images representing the object "O" to be scanned in such a way as to obtain a digital reconstruction of the object "O" which is faithful and complete according to a process which will be described in detail below.

In a preferred embodiment, the acquisition device 1 comprises a telecentric optical system la. The telecentric optical system la is configured for directing a light beam from the scanning surface v to the acquisition device 1 in such a way that the light rays are substantially parallel to each other.

The expression "telecentric optical system" is used to mean a system comprising a telecentric lens (Figure 3) or a system comprising a lens which, located at a sufficiently large distance from the scanning surface v, is able to operate as a telecentric lens (Figure 4).

In effect, since the xy type scanners typically have a scanning area 3 with reduced dimensions relative to the maximum scanning format possible, by positioning a lens at a sufficiently high distance from the scanning area 3, it is possible to consider any lens to be able to operate as a telecentric lens.

In the preferred embodiment, the acquisition device 1 also comprises a sensor for linear images lb or a sensor for matrix images lb'. More specifically, the linear sensor lb is a sensor which is able to acquire an image of the object "O" relative to a portion of the object "O" lying along a scanning line 3 on the scanning surface v. In other words, the linear sensor lb performs a scanning of the object "O" progressively acquiring images relative to portions of it lying along the scanning line 3 (Figure 6).

The matrix sensor lb', on the other hand, is a sensor which is able to acquire an image of the object "O" relative to a portion of the object "O" lying below the matrix sensor lb' inside a scanning area 3 of the scanning surface v (Figures 3, 4 and 5).

As shown in the accompanying drawings, the telecentric optical system la and the image sensor lb, lb' are aligned along the vertical optical axis "A" in such a way that the telecentric optical system la is interposed between the scanner surface and the sensor lb, lb'. In this situation, the telecentric optical system la operates in such a way as to direct and “straighten" a beam of light deriving from the scanning surface v towards the image sensor lb, lb', as described in detail below.

The xy type scanner 100, according to the invention, also comprises at least three light sources 2a, 2b, 2c which can be selectively activated to emit a beam of light collimated on the scanning surface v.

Preferably, the flat scanner 100 may comprise any number of light sources 2a, 2b, 2c which can be selectively activated provided they are more than three.

In the preferred embodiment, the light sources 2a, 2b, 2c can be selected from among: arrays of LED lights; matrices of LED lights; telecentric lights.

Alternatively, the light sources 2a, 2b, 2c comprise respective mechanical baffles configured for directing the respective beam of light collimated on the scanning surface v.

According to a further embodiment, since the xy type scanners normally have a scanning area 3 with reduced dimensions, even diffused light sources (such as, for example, the point sources) 2a, 2b, 2c located at a large distance from the scanning surface v may be considered to be able to emit a collimated light beam (Figure 2).

Each light source 2a, 2b, 2c defines a tilt angle 6 between a projection of the light source 2a, 2b, 2c on the scanning surface v and a reference axis "B" lying on the scanning surface v. Each light source also defines a slant angle of 9 between the light source 2a, 2b, 2c and the optical axis "A".

As shown in the accompanying drawings, the light sources 2a, 2b, 2c are angularly distributed about the optical axis "A" in such a way that each light source 2a, 2b, 2c has a slant angle 9 of between 20° and 80° and a tilt angle 6 such as to differ from the tilt angles 6 of the other light sources 2a, 2b, 2c by a value greater than or equal to 1807n where n corresponds to the number of light sources 2a, 2b, 2c.

The positioning of the light sources 2a, 2b, 2c according to the above-mentioned angles 9, 6 makes it possible to guarantee sufficiently different directions of illumination for an optimum implementation of the Photometric Stereo technique. In more detail, it is precisely the very different tilt angles 6 which determine the best scanning results. Thanks to the fact that the light sources 2a, 2b, 2c have tilt angles 6 different to each other (and in some cases multiples of one another), a uniform distribution of the light sources 2a, 2b, 2c about the optical axis "A" is guaranteed, as is guaranteed an illumination of the object "O" to be scanned according to directions of illumination which are sufficiently different to each other.

In other words, the different directions of illumination for the purpose of an excellent implementation of the Photometric Stereo technique are determined mainly by the tilt angles 6 of the light sources 2a, 2b, 2c which are in turn determined by the number of light sources 2a, 2b, 2c used. In this situation, the addition or the removal of one or more light sources 2a, 2b, 2c is less difficult than what occurred in the prior art scanners since it is sufficient to vary the difference between the tilt angles 6 of the light sources which are to be used (by dividing 180° by the new number of light sources 2a, 2b, 2c of the flat scanner 100) to again obtain a flat scanner 100 which is able to guarantee a sufficient differentiation of the directions of illumination of the object "O".

The Applicant, thanks to numerous practical tests and experiments, has found that, although the ideal for an optimum implementation of the Photometric Stereo technique is that of distributing the light sources 2a, 2b, 2c about the optical axis "A" according to tilt angles 6 equal to 360°/n (with n is the number of light sources 2a, 2b, 2c of the scanner 100), this distribution is quite rigid and binding. For example, during the construction of the scanner 100 it might be desirable to arrange light sources 2a, 2b, 2c with tilt angles 6 which are different from angles equal to 360°/n to make space for further devices or in any case to implement technical solutions which would require a deviation of the tilt angles 6 of one or more light sources 2a, 2b, 2c relative to the ideal value.

The Applicant, carrying out numerous practical tests, has noted that, since in the Photometric Stereo technique the greater the number of light sources 2a, 2b, 2c (each having a direction of illumination different from the others) the more precise is the scanning, tilt angles 6 different from each other by a value greater than or equal to 180°/n allow very good scanning results to be obtained (very close to the results obtained with tilt angles 6 equal to 360°/n) and at the same time allow greater flexibility of the arrangement of the light sources 2a, 2b, 2c.

In other words, the fact of positioning the light sources 2a, 2b, 2c in such a way that the respective tilt angles 6 differ from each other by a value greater than or equal to 180°/n makes it possible to obtain scanning results close to those obtained with optimum tilt angles 6 (equal to 360°/n) but at the same time guarantee a good construction flexibility of the scanner 100. The Applicant has noticed that the addition of one or more light sources 2a, 2b, 2c, 2d substantially compensates for the choice of tilt angles 6 which are not ideal. For example, by introducing in the scanner 100, according to the invention, four light sources 2a, 2b, 2c, 2d having tilt angles 6 different by a value greater than or equal to 180°/n (Figure 2) a scanning is obtained which is better than that which could be obtained with three light sources 2a, 2b, 2c located at tilt angles 6 equal to 3607n (Figure 3).

In the preferred embodiment, the tilt angles 6 of each light source 2a, 2b, 2c, 2d are different from each other in such a way that they differ by a value greater than or equal to 1807n where n indicates the number of light sources 2a, 2b, 2c, 2d present in the flat scanner 100 whilst a value of the slant angle 9 is fixed for each light source 2a, 2b, 2c, 2d.

Preferably, the value of the slant angle 9 is fixed at 54.74°. This value is normally used for Lambert surfaces.

Alternatively, the slant angle 9 may vary, in particular, by varying the slant angle 9 of the light sources 2a, 2b, 2c, 2d it is possible to optimise the digitalization of materials according to their different surfaces (for example, shiny, extra-shiny or, on the other hand, super- matte).

Alternatively, the tilt angles 6 of each light source 2a, 2b, 2c are different from each other by a value greater than or equal to 1807n as the slant angles 9 of each light source 2a, 2b, 2c also differ from each other. Advantageously, the fact that the tilt angles 6 differ by a value greater than or equal to 180°/n guarantees a sufficient differentiation between the directions of illumination and therefore guarantees good results in the implementation of the Photometric Stereo technique allowing at the same time further and diversified forms of construction of the scanner 100 as well as a good flexibility in the arrangement of the light sources 2a, 2b, 2c.

Inother words, although the positioning of the light sources 2a, 2b, 2c according to the above-mentioned relationship of the tilt angles 6 is precise, it is sufficiently flexible and such as to allow a scanner 100 to be constructed with any number of light sources 2a, 2b, 2c greatly simplifying the design of the scanner 100 and, at the same time, leaving large margins of construction flexibility.

Moreover, since each light source 2a, 2b, 2c emits collimated light beams, the scanning surface v, and in more detail the scanning area or line 3, is illuminated with beams of light whose rays have a uniform angle of incidence.

The flat scanner 100, according to this invention, is versatile, reliable and therefore able to optimise the implementation of the Photometric Stereo technique. In fact, in this flat scanner 100 the reduced dimensions of the scanning area 3 which are typical of xy scanners considerably simplify the problem of uniformity of lighting since they guarantee that, if a matrix type sensor lb' is used, the beams of light produced by each light source 2a, 2b, 2c strike the scanning area 3 in a substantially uniform manner.

In the flat scanner 100 according to the invention, the problem of positioning the light sources 2a, 2b, 2c is thus considerably simplified since a different directionality of the light beams emitted by each light source 2a, 2b, 2c is guaranteed by the difference of the values of the tilt angles 6 and it becomes easy to add further light sources 2a, 2b, 2c. Moreover, in the flat scanner 100 according to the invention, it is possible to use both a linear image sensor lb and a matrix sensor lb' without substantial modifications to the scanner 100 being requested, which, on the other hand, were necessary in the prior art.

A further advantage derives from the fact that, unlike the prior art scanners, there is no need to develop particularly complex and costly solutions to resolve the problems of uniformity of illumination in the scanning area 3.

In effect, with regard to the uniformity of the angles of incidence of the light beams emitted by the light sources 2a, 2b, 2c in the scanning area 3, the problem is resolved using light sources 2a, 2b, 2c which are able to emit collimated light beams, or as mentioned above substantially collimated.

Whilst with regard to the uniformity of intensity of the light beams emitted by the light sources 2a, 2b, 2c in the scanning area 3, the problem may be resolved with a compensation achieved by software and therefore without imposing further constraints on the arrangement of the light sources 2a, 2b, 2c as, on the other hand, occurs in the prior art. I has in fact been shown from experience that when the differences in the lighting intensity in the scanning area 3 are not particularly strong, these may be perfectly compensated with a "flat field" operation performed by acquisition software or directly by the optical system la. In effect, it is precisely the reduced dimensions of the scanning area 3, which are typical of an x,y scanner, to guarantee that the differences in lighting intensity are in any case limited.

The fact that the scanner 100 is of the xy type and the fact that a telecentric optical system la or substantially telecentric is used makes it possible to further reduce light uniformity problems since it guarantees both a reduced scanning area, and therefore easier to illuminate, and the rays of light from the scanning area 3 to the image sensor lb, lb' are collimated.

In other words, the light sources 2a, 2b, 2c of the scanner 100 emit light rays substantially collimated in the scanning area 3 and the telecentric optical system la guarantees that the rays of light are also substantially collimated from the scanning area to the image sensor lb, lb' providing the conditions for an optimum implementation of the Photometric Stereo technique.

In a preferred embodiment, the flat scanner 100 may also comprise at least one auxiliary light source configured to illuminate the scanning surface v. The auxiliary source may be positioned at any angle relative to the scanning surface movably v and relative to the optical axis "A".

In use, in order to obtain a reconstruction of an object "O" using a scanner 100 of the xy type, according to the invention, at least three light sources 2a, 2b, 2c are positioned around the optical axis "A" according to the angles described above. Following the positioning of the light sources 2a, 2b, 2c, an object "O" to be scanned is positioned on the scanning surface v under the acquisition device 1. Subsequently, a first light source 2a is activated in such a way as to illuminate the object "O". In this situation, the collimated light beam emitted by the light source 2a activated strikes the scanning surface v, and thus the portion of object "O" lying along the scanning line or area 3, and is then reflected towards the telecentric optical system la. In this situation, the telecentric optical system la diverts the light rays of the light beam deriving from the scanning surface v towards the sensor lb, lb' which performs a true acquisition of the image.

In the case of a linear sensor lb, the image acquired will correspond to an image representing a strip of the object "O", whilst in the case of a matrix sensor lb', the image acquired will correspond to a surface portion of the object "O".

Subsequently, the first light source 2a is switched off to favour the activation of a second light source 2b which, having a tilt angle 6 (and, if necessary, a slant angle 9) different from that of the first light source 2a, illuminates the object "O" from a further direction. In this situation, the collimated light beam emitted by the light source 2b activated strikes the scanning surface v, and thus the portion of the object "O" lying on the scanning line or area 3, and is then reflected towards the telecentric optical system la which diverts the light rays of the light beam deriving from the scanning surface v towards the image sensor lb, lb'.

Subsequently, the second light source 2b is switched off to favour activation of a third light source 2c in such a way as to illuminate the object "O" from a further direction and in such a way as to acquire, also in this case, an image of the object "O" during the lighting state. The process for switching on and switching off the light sources 2a, 2b, 2c is repeated for each of the light sources 2a, 2b, 2c present in the flat scanner 100.

Once all the light sources 2a, 2b, 2c have been switched on, and once the image sensor lb, lb' has acquired at each switching on an image representing the object "O", the object "O" or the acquisition device 1 is moved in such a way as to allow a gradual illumination and an acquisition of a further portion of the object "O".

In a possible embodiment, the scanner 100 comprises a movement system (not illustrated) configured for moving the acquisition device 1 whilst the scanning surface y is stationary. In this situation, the acquisition device 1 is moved according to separate positions in such a way as to acquire an image of the object "O" for each separate position.

In a further possible embodiment, the scanner 100 comprises a movement system (not illustrated) configured for moving the scanning surface

relative to the acquisition device 1.

Preferably, the movement system moves the scanning surface y along horizontal Cartesian directions.

Even more preferably, the movement system moves the object "O" in a plurality of separate positions in such a way that the acquisition device 1, and in particular the sensor lb, lb', acquire an image for each separate position.

Even more preferably, the images are contiguous and at least partly overlapping. Alternatively, the movement system is able to simultaneously move the scanning surface y and the acquisition device 1.

In other words, once, for each activation of the light sources 2a, 2b, 2c, the images relative to a portion of the object "O” are acquired, the movement system is such that a new portion of the object "O" can be scanned. In this situation, the light sources 2a, 2b, 2c are again selectively activated in such a way as to acquire, also in this case, an image relating to a further portion of the object "O" during each activation of the sources.

The operations for actuating the movement system, for activating the light sources 2a, 2b, 2c and acquisition of the images are repeated in succession until all the portions of the object "O" have been scanned.

In the preferred embodiment, the flat scanner 100 also comprises a processing unit configured for acquiring and processing data relative to successive scans of the object "O".

Preferably, the processing unit is further configured to apply a correction of the flat-field type to the images acquired using the acquisition device 1. As mentioned above, it is in effect possible that in the flat scanner 100 the luminous intensity of the beams of light emitted by each light source 2a, 2b, 2c is non-uniform on the scanning area 3 and the images representing the object "O" therefore have defects due to the lighting intensity. These defects are normally limited and can be easily corrected and compensated for with the flat-field correction.

Advantageously, the application of the flat-field type correction makes it possible to eliminate the non-uniformity of the intensity of the various light sources via software and therefore without the need to solve the problem by adopting complex solutions for arranging and positioning the light sources 2a, 2b, 2c as, on the other hand, occurred in the prior art.

In the preferred embodiment, the processing and control unit is also configured for selectively activating the light sources 2a, 2b, 2c one at a time, activating the acquisition device 1 for acquiring an image for each activation of the individual light sources 2a, 2b, 2c and for processing the images acquired defining an image representing the object to be scanned "O" positioned on the scanning surface x. With reference to the embodiments shown in the accompanying drawings, Figure 2 shows a diagram of a scanner 100 according to the invention. More specifically, the scanner 100 comprises four light sources 2a, 2b, 2c, 2d. The light sources 2a, 2b, 2c, 2d are schematically represented as points but they may be diffused light sources located at a distance from the scanning surface x such as to be able to be considered substantially collimated or light sources 2a, 2b, 2c, 2d which are able to emit collimated light.

The light sources 2a, 2b, 2c, 2d have tilt angles 6 which differ from each other by a value greater than or equal to 180°/n.

More specifically, in this embodiment, the light sources 2a, 2b, 2c, 2d have, respectively, tilt angles 6 equal to 115°, 55°, 120° and 70°.

In this embodiment, the distribution of the light sources 2a, 2b, 2c, 2d according to the above-mentioned tilt angles 6 makes it possible both to obtain an excellent scanning quality according to the Photometric Stereo technique and, at the same time, to maintain a certain construction flexibility of the scanner 100, for example it becomes possible to insert between the light sources 2a and 2b any additional device. In particular, as shown, a particularly bulky device can also be introduced between the sources 2a and 2b thanks to the large tilt angle 6 (equal to 120°). In the case of a distribution of the light sources 2a, 2b, 2c, 2d according ideal tilt angles 6, therefore equal to 90° (360°/n), this introduction might not be possible.

In use, the first light source 2a is activated and the light rays of the collimated beam illuminate the scanning area 3 with the same angle of incidence. These rays are then reflected towards the telecentric optical system la and, therefore, towards the sensor which acquires the image relative to the portion of the object "O" lying in the scanning area 3. The process for activating and acquiring the image is repeated for the second, third and fourth light sources 2b, 2c, 2d. At the end of these operations, the movement system is actuated in such a way that a further portion of the object "O" lies in the scanning area 3 and is alternately illuminated by the light sources 2a, 2b, 2c, 2d. The steps of moving, illuminating and acquiring the images continue until the entire object "O" has been scanned.

Figure 3 shows a flat scanner 100 having an acquisition device 1 comprising a matrix sensor lb', that is to say, a sensor defining a scanning area 3 in which are acquired images relative to a portion of the object "O" lying in the scanning area 3, and a telecentric optical system la in such a way as to facilitate the joining together of the images acquired by the matrix sensor lb'. Since the flat scanner 100 is of the xy type, the dimensions of the scanning area 3 are contained and it has been chosen to insert three diffused light sources 2a, 2b, 2c in the flat scanner 100 located at a sufficiently large distance from the scanning area 3 in such a way that the beam of light emitted by each can be considered collimated on the scanning area 3. In other words, the light sources 2a, 2b, 2c are spaced relative to the scanning area 3 in such a way as to guarantee a sufficient parallelism of the rays of light and therefore a sufficient uniformity of angles of incidence of the light beam in the scanning area 3. More specifically, the three light sources 2a, 2b, 2c each have a slant angle 9 equal to 55°. In this situation, the flat scanner 100 is optimised for Lambert surfaces. The light sources 2a, 2b, 2c also have optimum tilt angles 6 for implementing the Photometric Stereo technique, that is to say, the angles differ from each other by 120° (3607n with n equal to three). In this embodiment, unlike the embodiment shown in Figure 2, there are only three light sources 2a, 2b, 2c and are distributed according to optimum tilt angles 6. In this situation, the scanning quality obtained by means of the scanner 100 is less than that obtained with the scanner of Figure 2 in which there were four luminous sources 2a, 2b, 2c, 2d even though with tilt angles 6 different from the optimum ones.

In fact, after numerous verifications, as mentioned above, the Applicant has noticed that since the greater the number of light sources 2a, 2b, 2c in the Photometric Stereo technique (each having a direction of illumination different from the others), the more precise is the scanning, a large number of light sources 2a, 2b, 2c, 2d even though having tilt angles 6 which are different from each other by a value greater than or equal to 1807n (and thus a non-optimum value) still allow very good scanning results to be obtained (very close to the results obtained with optimum tilt angles 6 equal to 3607n) and greater than a scanner 100 with fewer light sources 2a, 2b, 2c but positioned according to optimum tilt angles 6. Advantageously, the distribution of the light sources 2a, 2b, 2c, 2d according to the tilt angles 6, claimed in the invention, makes it possible to obtain a good scanning quality and at the same time allows greater flexibility of arrangement and/or addition of the light sources 2a, 2b, 2c.

Figure 4 shows a flat scanner 100 of the xy type comprising four light sources 2a, 2b, 2c, 2d each made with a matrix of LEDs equipped with lenses designed to concentrate and collimate the light in the scanning area 3 to guarantee an optimum parallelism of the rays of light and therefore an optimum uniformity of angles of incidence of light in the scanning area 3. In this embodiment, the light sources 2a, 2b, 2c, 2d all have a slant angle 9 equal to 55° whilst they have tilt angles 6 such as to have a difference of values equal to or greater than 1807n with n equal to four (for example 120°, 55°, 115°, 70°). In that embodiment, the flat scanner 100 comprises a sensor of images of the matrix type lb' and a traditional optical system la wherein the lens is located at a distance from the scanning surface v such that it acts as a telecentric lens, thus guaranteeing a good joining of the images acquired. In order to reduce the overall dimensions of the scanner 100 relative to those shown in Figure 3 or 4, wherein the light sources 2a, 2b, 2c or the acquisition device 1 were located at a large distance from the scanning area 3, in order to guarantee a collimation of the light or to act as a telecentric lens, both light sources 2a, 2b, 2c, 2d equipped with collimation elements and an optical system la equipped with a telecentric lens have been introduced in the flat scanner 100 of Figure 5.

More specifically, four light sources 2a, 2b, 2c, 2d each made with a matrix of LEDs equipped with lenses designed to concentrate and collimate the light in the scanning area 3 have been introduced in the scanner of Figure 5 to guarantee an optimum parallelism of the rays of light and therefore an optimum uniformity of angles of incidence of light in the scanning area 3.

The embodiment of Figure 6 shows, on the other hand, a flat scanner 100 wherein the acquisition device 1 comprises a linear sensor lb, that is to say, a sensor which is able to acquire images relative to a strip of the object "O" lying along a scanning line 3, and a telecentric optical system la in such a way as to facilitate the joining of the images acquired by the linear sensor lb. The flat scanner 100 also comprises four light sources 2a, 2b, 2c, 2d each made with a matrix of LEDs equipped with lenses designed to concentrate and collimate the light in the scanning line 3.

More specifically, in this embodiment, the light sources have optimum tilt angles 6 or in any case such as to have a difference between them equal to or greater than 1807n where n is the number of light sources 2a, 2b, 2c, 2d. With regard to the slant angles 9, on the other hand, two of the light sources 2a, 2b have a slant angle 9 equal to 60° whilst the remaining two have a slant angle 9 equal to 45°.

The invention achieves the above-mentioned aims, eliminating the drawbacks of the prior art.

More specifically, the positioning of the light sources 2a, 2b, 2c makes the flat scanner 100 versatile, optimised for the implementation of the Photometric Stereo technique and able to be used both with linear optical sensors lb and matrix scanners lb'. Inother words, the flat scanner 100 overcomes the drawbacks relating to the positioning of the light sources 2a, 2b, 2c guaranteeing a uniformity in the angles of incidence of the light beams and a reduced non-uniformity in the intensity of the beams of light in the scanning area, which is easily compensated by "flatfield” correction.

The positioning of the light sources 2a, 2b, 2c claimed makes it possible to obtain a scanner 100 wherein the introduction of one or more additional light sources, their arrangement and their positioning are precise but at the same time simple in such a way as to greatly simplify the design of a scanner 100 of this type and at the same time leave large margins of construction flexibility.

The invention provides a general solution to the problem of how to place and add to the light sources in a scanner 100 designed to implement the Photometric Stereo technique. More specifically, the invention introduces a formulation which makes it possible to construct a scanner 100 with any number of light sources defining in a sufficiently precise manner the angles (tilt 6 and slant 9) of the light sources 2a, 2b, 2c and the relative constraints for arrangement and positioning.

Claims

1. A flat scanner (100) of the xy type configured to implement the Photometric Stereo technique, said flat scanner (100) comprising:

- a scanning surface (T) on which an object (O) to be scanned can be positioned;

- an acquisition device (1) positioned along a vertical optical axis (A) and perpendicular to said scanning surface (T), said acquisition device (1) being configured for acquiring at least one image of said object to be scanned;

- at least three light sources (2a, 2b, 2c) which can be selectively activated to emit a beam of light collimated on said scanning surface (T), each light source (2a, 2b, 2c) defining a tilt angle (6) between a projection of said light source (2a, 2b, 2c) on said scanning surface (T) and a reference axis (B) lying on the scanning surface (T) and a slant angle (9) between the light source (2a, 2b, 2c) and said optical axis (A), said light sources (2a, 2b, 2c) being angularly distributed around said optical axis (A) in such a way that each light source (2a, 2b, 2c) has a slant angle (9) of between 20° and 80° and a tilt angle (6) in such a way as to differ from the tilt angles (6) of the other light sources (2a, 2b, 2c) by a value greater than or equal to 180°/n where n corresponds to the number of light sources (2a, 2b, 2c).

2. The scanner (100) according to claim 1, wherein said acquisition device (1) comprises a telecentric optical system (la).

3. The scanner (100) according to claim 1 or 2, wherein said acquisition device (1) comprises a sensor for linear images (lb) or a sensor for matrix images (lb').

4. The scanner (100) according to any one of the preceding claims, wherein said at least three light sources (2a, 2b, 2c) have the same slant angle (9).

5. The scanner (100) according to any one of claims 1 to 3, wherein said at least three light sources (2a, 2b, 2c) have different slant angles (9) to each other.

6. The scanner (100) according to any one of the preceding claims, comprising a movement system configured to move the scanning surface (T) relative to the acquisition device (1).

7. The scanner (100) according to the preceding claim, wherein said movement system moves the scanning surface (T) along horizontal Cartesian directions.

8. The scanner (100) according to claim 6 or 7, wherein said movement system moves into a plurality of separate positions said object (O) and wherein the acquisition device (1) acquires an image for each separate position.

9. The scanner (100) according to claim 8, wherein said images are contiguous and at least partly overlapping.

10. The scanner (100) according to any one of claims 1 to 5, comprising a movement system configured to move said acquisition device (1) relative to the scanning surface (T).

11. The scanner (100) according to claim 10, wherein said movement system moves the acquisition device (1) in a plurality of separate positions in such a way that the acquisition device (1) acquires an image of the object (O) for each separate position, preferably said images being contiguous and at least partly overlapping.

12. The scanner (100) according to any one of the preceding claims, also comprising a processing unit configured for acquiring and processing data relative to successive scans of said object (O).

13. The scanner (100) according to claim 12, wherein said processing unit is further configured for applying a correction of the flat-field type to said images acquired using the acquisition device (1).

14. The scanner (100) according to any one of the preceding claims, comprising at least one auxiliary light source configured to illuminate said scanning surface (T).

15. The scanner (100) according to any one of the preceding claims, wherein said at least three light sources (2a, 2b, 2c) can be selected from: arrays of LED lights; matrices of LED lights; telecentric lights.

16. The scanner (100) according to any one of the preceding claims, wherein the at least three light sources (2a, 2b, 2c) comprise respective mechanical deflectors configured for directing the respective beam of collimated light on the scanning surface (T).

17. The scanner (100) according to any one of the preceding claims, wherein the control unit is configured for:

- selectively activating the light sources (2a, 2b, 2c) one at a time;

- activating the acquisition device (1) for acquiring an image for each activation of the individual light sources (2a, 2b, 2c);

- processing the images acquired defining an image representing an object to be scanned (O) positioned on said scanning surface (T).