Technical Field

The present invention relates to electronic pens that operate by capturing images of a product surface, and in particular to a camera unit for such electronic pens.

Background Art

Electronic pens are well-known in the art. One type of electronic pen is a camera pen, which is a pen-shaped handheld implement configured to capture images of a product surface while being manipulated by a user on the product surface. The images may be processed to determine the movement of the pen on the product surface, in absolute or relative positions. In one such implementation, the product surface is provided with a machine-readable code that encodes position information on the product surface. The images may alternatively be processed for extraction of other data encoded by a machine-readable code on the product surface. In a further alternative, the images are processed for recognition of handwritten or printed text in the images.

Camera pens are known to include a miniaturized camera for capturing and producing the images of the product surface. The camera is typically arranged in the front portion of the pen. By its location at the front of the pen, the dimensions of the camera may have a significant impact on the design and shape of the pen. If the camera has a large extent in the transverse (lateral) direction, it may be difficult to produce a pen with slender or slim shape. It may also be difficult to achieve a grip portion that is comfortable to the user.

Typically, the camera includes a number of separate components, such as an image sensor, one or more lens elements, an aperture stop, a radiation filter, an LED for emitting illuminating radiation, optics for redirecting/shaping the illuminating radiation emitted from the LED, etc. In view of the small dimensions, it is generally desirable to design the camera so as to facilitate its assembly in production, while ensuring that assembly and manufacturing tolerances are kept low. To control assembly tolerances, it is known to assemble or integrate at least certain components of the camera into an optical module before installation in the pen. Such optical modules are e.g. disclosed in WO03/001358, WO2005/057471 and WO2008/118085.

The design of the camera involves a complex trade-off between allowable pen orientations, depth of field, field of view, location of field of view, illumination brightness within the field of view, sensitivity of the image sensor, etc. One factor that affects the design of the camera is that interfering light may enter the camera and impinge on the image sensor. Such interfering light may reduce the contrast of the captured images or even lead to a saturation of all or part of the image sensor.

Interfering light that originates from the surroundings of the electronic pen (ambient light) may e.g. be reduced by installing radiation filter(s) matched to the radiation emitted by the LED, e.g. as proposed in the above WO03/001358, WO2005/057471 and WO2008/118085. However, it may be equally important and more difficult to limit interfering light that originates from the illuminating radiation emitted by the LED, e.g. by the illuminating radiation being diffusely or specularly reflected at the front of the pen, e.g. in the above-mentioned radiation filter, or by the illuminating radiation being specularly reflected in the product surface.

Brief Summary

It is an objective of the invention to at least partly overcome one or more limitations of the prior art.

Another objective is to reduce the impact of the camera on the shape of the electronic pen.

A further objective is to provide a camera unit for an electronic pen that is easy to assemble.

A still further objective is to provide a simple and efficient technique of reducing the amount of interfering radiation that reaches the image sensor in a camera unit for an electronic pen.

One or more of these objectives, as well as further objectives that may appear from the description below, are at least partly achieved by a camera unit and an electronic pen according to the independent claims, embodiments thereof being defined by the dependent claims.

A first aspect of the invention is a camera unit for an electronic pen. The camera unit comprises a housing that defines a front face, an inlet opening in the front face, and a compartment which extends from the inlet opening and includes a sensor mounting portion; an image sensor arranged at the sensor mounting portion; an imaging system that defines an optical axis that extends through the inlet opening to the image sensor, wherein the imaging system comprises an imaging lens in the compartment; a sheet device that defines a radiation-transmitting aperture, said sheet device being attached to the front face of the housing across the inlet opening and with the radiation-transmitting aperture on the optical axis of the imaging system; and an irradiating device which is directly or indirectly attached to the front face.

A second aspect is an electronic pen, which comprises: a stylus; the camera unit of any preceding claim, which is arranged to generate an image of a surface in temporary engagement with the stylus; a power source, which is electrically connected to the image sensor and the irradiating device of the camera unit; and a control unit, which is configured to operate the irradiating device to emit radiation onto the surface and to operate the image sensor to acquire the image.

Still other objectives, features, aspects, as well as advantages of the present invention and its various embodiments will appear from the following detailed description, from the attached claims as well as from the drawings.

Brief Description of Drawings

The accompanying drawings are schematic and provided merely to facilitate the understanding of example embodiments of the invention.

FIG. 1 is a see-through side view of an electronic pen in contact with an interaction surface.

FIG. 2 is a section view of a prior-art camera unit as installed in the pen of FIG. 1.

FIGS 3 A-3C are section views of camera units according to different

embodiments of the invention.

FIGS 4A-4B are section views of a front portion of camera units according to further embodiments of the invention.

FIGS 5A-5C are perspective, front and section views of a detailed implementation of an inventive camera unit.

FIGS 6A-6C are different views of a sheet device combined with an illuminating LED for installation in the camera unit of FIGS 5A-5C.

FIG. 7 is a plan view of a pattern of electrical conductors for the sheet device in FIGS 5A-5C.

Detailed Description of Example Embodiments

Embodiments of the present invention will now be described more fully hereinafter with reference to the accompanying schematic drawings, in which some, but not all, embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure may satisfy applicable legal requirements. Like numbers refer to like elements throughout.

Also, it will be understood that, where possible, any of the advantages, features, functions, devices, and/or operational aspects of any of the embodiments of the present invention described and/or contemplated herein may be included in any of the other embodiments of the present invention described and/or contemplated herein, and/or vice versa. In addition, where possible, any terms expressed in the singular form herein are meant to also include the plural form and/or vice versa, unless explicitly stated otherwise. As used herein, "at least one" shall mean "one or more" and these phrases are intended to be interchangeable. Accordingly, the terms "a" and/or "an" shall mean "at least one" or "one or more," even though the phrase "one or more" or "at least one" is also used herein. As used herein, except where the context requires otherwise owing to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, that is, to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Embodiments of the invention are directed to a novel configuration of a camera for an electronic pen. Such a camera includes, at least, a combination of an imaging system, an image sensor and an irradiating device.

As used herein, an imaging system is an optical system for transmitting a two- dimensional distribution of radiation from an object plane to an image plane. The imaging system includes imaging optics and defines an optical axis. The optical axis is an imaginary line within the imaging system around which there is rotational symmetry. The imaging optics comprises an imaging lens, which is a transmissive optical device that affects the focus of beams of radiation through refraction and projects the object plane onto the image plane. The imaging lens may be a single lens element ("simple lens"), or a group of lens elements which may be spaced from each other or combined in to a compound lens. The imaging lens is made of transparent material, such a plastics or glass. The imaging optics further comprises an aperture stop, which is an element included in the imaging system to limit the angle of acceptance of the imaging system and thereby limit the optical energy that is admitted through the imaging system from the object plane to the image plane. The aperture stop determines the resolution, radiation transmission efficiency and the depth of field of the imaging system.

An image sensor may be arranged in or spaced from the image plane of the imaging system to electronically record an image. As used herein, an image sensor is a two-dimensional electro-optical device capable of producing a two-dimensional representation of an irradiance pattern. The image sensor may be sensitive to visible and/or invisible radiation. Such invisible radiation may comprise radiation in the ultraviolet (UV) and/or infrared (TR). The image sensor may be analog or digital.

Commonly used digital image sensors include solid-state detectors such as CCD detectors, CID detectors and active pixel sensors (APS), including CMOS detectors.

As used herein, an irradiating device comprises one or more radiation-generating elements, such as a laser diode, a filament lamp, a discharge lamp, a light-emitting diode (LED), etc, which may be encased in a housing or package. The irradiating device may also include a beam shaping element that defines the solid angle of the emitted radiation and the distribution of irradiance within the solid angle. The irradiating device may emit visible and/or invisible radiation, e.g. UV or IR radiation. An LED device may include any type of LED, including but not limited to inorganic LEDs, organic LEDs, high brightness LEDs and vertically-stacked LEDs. The irradiating device is also denoted "radiation source" herein.

The irradiating device may be configured for mounting on a substrate and may be of through-hole type or surface mount type. The substrate may be a PCB (Printed Circuit Board), also known as PWB (Printed Wire Board), or a flexible circuit that is bendable or pliable. A PCB is a rigid substrate configured to mechanically support and electrically connect electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. A flexible circuit typically comprises a thin insulating film of pliable material, e.g.

polymer material, having conductive circuit patterns affixed thereto and typically supplied with a thin overlay or coating, e.g. of polymer material, to protect the conductor circuit patterns. Flexible circuits are also known as flex film, flex circuits, flexible printed circuits (FPCs), flex print and flexi-circuits. As used herein, flexible circuits also include flexible foil circuits and flexible flat cables (FFCs). The flexible circuits may be formed by a variety of processes, including etching metal foil cladding (normally of copper) from polymer bases, plating metal, printing of conductive ink or metal (e.g. silver), and laminating thin metal strips between polymer layers. Flexible circuits may be of different types, including single-layer, double-sided and multi-layer. A single-layer flexible circuit comprises a flexible insulating film (base layer) provided with a single conductive layer of electrically conductive material, arranged in a trace pattern. A double-sided flexible circuit has two conductive patterned layers, one on each side of the base layer within the circuit. A multi-layer flexible circuit typically combines several single-layer or double-sided circuits which are interconnected. There are additionally various variants of flexible circuits that allow for more or less bending and stability, for instance rigid and semi-rigid flexible circuits.

FIG. 1 is a schematic side view of an electronic pen 1. The pen 1 has a casing 2 that defines a pen-shaped contour. To facilitate the following discussion, certain internal components of the pen 1 are also depicted in FIG. 1. The pen 1 includes a stylus 3 for pointing to or engaging with a product surface or interaction surface S. The stylus 3 may or may not be configured to mark the surface S. The pen 1 further comprises a camera or camera unit 4 which is arranged in the front portion of the pen 1 to illuminate the surface S and capture images of the surface S via a through-hole or a transparent window in the pen casing 2. The pen 1 further comprises a control unit 6 for controlling the operation of the pen 1, including the camera 4. The control unit 6 may, e.g., selectively activate an irradiating device to illuminate the surface S, and selectively activate an image sensor to electronically capture an image of the surface S. Depending on implementation, the image may represent illuminating radiation that is scattered by the surface S and/or fluorescence that is emitted by the surface S when illuminated. For the purpose of the following discussion, however, it is assumed that the camera 4 is at least sensitive to illuminating radiation scattered by the surface S. The control unit 6 and the camera 4 are connected to receive electrical power from a power source 7 in the pen, e.g. a battery.

The images may be processed, inside or outside of the pen 1, for data extraction. In one example, relative or absolute positions are derived from the images to represent a movement of the pen 1 on the surface S. In another example, data is extracted from a machine-readable code in the image, such as a bar code or matrix code. In yet another example, handwritten or printed text is identified in the images and processed for character recognition. For position determination, the surface S may or may not be specially formatted. In the former case, the surface S may be provided with a coding pattern which codes absolute positions on the surface S, see e.g. US6663008. In the latter case, the pen 1 may capture images of the whole surface S, so that positions may be determined with respect to a corner and an edge of the surface S, see e.g.

WO2004/077107. Alternatively, an electronic trace of the pen's movement may be determined by correlating a series of partially overlapping images, see e.g. US6985643. Still further, positions may be derived by analyzing coherent radiation reflected off the surface S, see e.g. US6452683.

It is to be understood that the pen 1 may include further components than those shown in FIG. 1, such as one or more signal processors for processing the images generated by the camera 4, electronic memory for storing data generated by the signal processors and/or processing instructions for the signal processor(s) or the control unit, a pen-down detector for detecting when the stylus 3 engages the surface S, one or more signal interfaces for transmitting signals, by wire or wirelessly, to and/or from the pen 1, one or more buttons or other mechanical controls, one or more displays, one or more loudspeakers, one or more microphones, etc. Electronic pens are known in the art and the pen 1 in FIG. 1 will not be described in further detail herein.

It should be noted that the orientation of the pen 1 may vary while it is operated on the surface S. The orientation of the pen 1 is commonly defined in relation to a pen axis PA, which may be given by the longitudinal axis of the stylus 3. In FIG. 1, the orientation is represented by an inclination angle Θ, which is the tilt angle between the pen axis PA and the normal of the surface S, and a skew angle φ, which is the rotation angle of the pen around the pen axis PA. In addition, the pen may be rotated around the normal of the surface S by a rotation angle a (not shown). The varying orientation of the pen puts specific demands on the camera 4. Normally, the camera 4 is designed to accommodate a predefined range of pen orientations, e.g. given by ranges of the above- mentioned inclination angle Θ, skew angle φ and rotation angle a.

As seen in FIG. 1, the optical axis OA of the camera 4 is offset from the pen axis PA. This means that the distance from the camera 4 to the surface S may vary significantly with the orientation of the pen 1. The camera 4 may therefore need to be designed with a relatively large depth of field, i.e. a small numerical aperture, which limits the amount of radiation that will be transmitted by the camera 4. Thus, the camera 4 for an electronic pen may need to be designed with a high sensitivity to incoming radiation. This may make the electronic pen particularly sensitive to interfering radiation that enters the camera system.

FIG. 2 is a schematic section view of a prior art camera 4 of the type disclosed in above-mentioned WO2005/057471. The camera 4 in FIG. 2 will serve as a reference for presenting embodiments of the invention and their merits. The camera 4 is located at the front of the pen 1 (cf. FIG. 1), with part of the casing 2 being located in the vicinity of the camera 4 but outside its field of view. In the illustrated example, the camera 4 is located behind a transparent window 8. The window 8 may be installed to shield the interior of the pen from ingress of dust, moisture, etc. The window 8 may also be a radiation filter for suppression of ambient light, e.g. by the filter only transmitting radiation in a confined wavelength range that matches the wavelength range of the radiation that is projected by the camera 4 onto the surface S. The camera 4 comprises a housing 9 that defines an internal compartment or radiation channel 10, which extends from a front face 11 of the housing 9 and is defined by side walls 9'. The front face 11 defines an inlet opening or hole 13 that admits radiation into the camera 4. A rear portion 12 of the housing 9 defines a mounting portion 14 for receiving an image sensor 15. A lens 16 is mounted or integrated in the radiation channel 10 to define an imaging system of the camera 4. The imaging system defines an optical axis OA that extends through the radiation channel 10 to the image sensor 15, which is engaged with the mounting portion 14 and is operable to electronically capture images of objects, such as the surface S, that are located within the depth of field of the camera 4. The depth of field is defined by an aperture stop 17, which is a small opening in a lateral wall 18 that projects from the side walls 9' of the radiation channel 10. Thereby, the lateral wall 18 may also be seen to separate the radiation channel 10 into a front compartment and an inner compartment, where the inner compartment extends between the lateral wall 18 and the rear portion 12. A radiation source 19 is attached to the outer surface of the housing 9 and is operable to project distributed (diverging) radiation onto the surface S. The distributed radiation is emitted by the radiation source 19 over a predefined solid angle which is matched so as to properly illuminate the field of view of the camera 4 on the surface S for the predefined range of pen orientations.

FIG. 2 also illustrates two rays that are emitted by the radiation source 19 and give rise to interfering radiation in the camera 4. It may be noted that all refractions of radiation in transmissive objects (e.g. window 8 and lens 16) have been omitted in all figures herein. In FIG. 2, one of the rays is specularly reflected in the window 8 back towards the inlet hole 13, passes the lens 16 and the aperture stop 19 and is specularly and/or diffusely reflected towards the sensor 15 by one or more walls 9' of the radiation channel 10. The other ray hits and is diffusely reflected (scattered) by the pen casing 2, as indicated by short lines. As is well-known to the skilled person, "diffuse reflection" refers to reflection from a surface such that an incident ray is reflected at many angles rather than at just one angle as in "specular reflection". One of the diffusely reflected rays passes the lens 16 and the aperture stop 17 and is specularly and/or diffusely reflected towards the sensor 15 by at least one wall 9' of the radiation channel 10. Thus, the camera 4 in FIG. 2 is likely to admit interfering radiation into the inner compartment behind the aperture stop 17. Since the inner compartment effectively forms a closed box, the interfering radiation may be repeatedly reflected inside the inner compartment until it hits the sensor 15. The interfering radiation may significantly increase the background radiation at the sensor 15 and thereby decrease the contrast in the resulting images. The interfering radiation may also cause local or complete saturation of the image sensor 15. Although not shown in FIG. 2, saturation of the image sensor 15 may also be caused by radiation which is emitted from the radiation source 19 and specularly reflected in the surface S. In fact, such specular reflection is likely to occur for a substantial number of different pen orientations when the camera 4 is installed in the pen of FIG. 1. The problem of specular reflections is particularly pronounced when the surface S is highly reflective, e.g. when the pen is operated on a display surface, on glossy paper, or glossy plastic.

FIGS 3 A-3C illustrate improvements of the camera 4 in FIG. 2. For brevity of presentation, the following description will focus on differences over the camera 4 in FIG. 2. Unless explicitly stated otherwise, it can be assumed the description of the camera 4 in FIG. 2 is equally applicable to the camera 4 in FIGS 3A-3C.

The camera 4 in FIG. 3 A implements at least two new concepts for cameras in electronic pens. A first concept is that the radiation source 19 is directly or indirectly attached to the front face 11 of the housing 9 outside of the field of view of the imaging system. As used herein, the "front face" spans the combination of the inlet opening 13 and the coherent surface portion of the housing 9 that surrounds the inlet opening 13 as seen in a direction towards the inlet opening 13 along the optical axis OA. The first concept makes it possible to decrease the lateral extent of the camera 4, by reducing or even eliminating the space needed on the lateral side of the housing for accommodating the radiation source 19. In this context, "lateral" is a direction transverse to the optical axis OA of the camera 4. Looking at FIG. 1, it is understood that the lateral extent of the camera 4 may limit the freedom of designing the pen casing 2, since the pen casing 2 needs to accommodate the camera 4 at the front of the pen 1. The front of the pen 1 will determine the thickness of the pen, at least if the pen 1 should have a slender design. By decreasing the lateral extent of the camera 4, the freedom of design is increased. This is not only an aesthetic issue, but may also increase the ability to design a grip portion at the front of the pen that is comfortable to the user and possibly more ergonomic. The first concept also enables simple manufacture of the camera 4, since the radiation source 19 may be attached with its bottom surface towards the front face 11, e.g. by

conventional pick-and-place equipment.

A second concept embodied in FIG. 3 A is that a separate sheet device 20 is applied to the front face 11 to cover the inlet opening 13 of the housing 9. The sheet device 20 defines an aperture 21 for transmitting radiation, at least in the wavelength range of the radiation source 19. The aperture 21 is arranged on the optical axis OA to form a shield to interfering radiation that otherwise would have entered the camera 4. The aperture 21 effectively defines the front opening of the camera 4. The aperture 21 may but need not be centered on the optical axis OA. However, the aperture 21 is preferably arranged so as not to restrict the field of view of the imaging system. In the embodiment of FIG. 3 A, the aperture 21 is dimensioned and located to block all of the interfering radiation caused by specular reflection in the window 8 and diffuse reflection in the pen casing 2. Dashed lines indicate that the blocked rays would have entered the inner compartment in the absence of the sheet device 20.

In the embodiments shown herein, the sheet device 20 is a thin, substantially flat element with a thickness that is much smaller than its two-dimensional extent. Other configurations of the sheet device 20 are conceivable. The sheet device 20 is manufactured separately from the housing 9 and may thus define the aperture 21 with high precision. It should be understood that, by contrast, the housing 9 is typically a cheap and simple component, often made of plastic, manufactured with relatively low precision. Furthermore, the sheet device 20 may have a thickness that is much smaller than the wall thickness of the housing 9. This also means that the peripheral surface (cf. 2Γ in FIGS 4A-4B) around the aperture 21 may have a small extent. The peripheral surface has actually been found to contribute significantly to the interfering radiation inside the camera 4, by reflecting unwanted radiation into the camera 4. Thus, for a given size of the aperture 21, the interfering radiation in the camera 4 is reduced by thinning the material that defines the aperture 21. It is currently believed that the material thickness at the perimeter of the aperture 21 should be no more than approximately 2 mm, and preferably much smaller. In a currently preferred

embodiment, the perimeter thickness is limited to approximately 0.5 mm, 0.4 mm or 0.3 mm. In other embodiments, the perimeter thickness is limited to 0.2 mm or 0.1 mm.

It should be noted that second concept combines with the first concept to improve the performance of the camera 4. As the radiation source 19 is moved closer to the optical axis OA, by the first concept and compared to FIG. 2, the radiation reflected in the window 8 impinges on the inlet opening 13 at more shallow angles with respect to the optical axis OA. In the absence of the sheet device 20, the placement of the radiation source 19 on the front face 11 may actually increase the amount of interfering radiation inside the camera 4. Thus, the combination of the first and second concepts makes it possible to provide a compact camera with good suppression of interfering radiation.

However, the first and second concepts need not be implemented in combination. It is conceivable that only one of the first and second concepts is implemented in a camera for installation an electronic pen, to attain the respective technical advantages discussed in the foregoing.

The radiation source 19 is preferably spaced from the aperture 21, so that there is a distance between the perimeter of the radiation source 19 and the perimeter of the aperture 21, as seen in plan view towards the sheet device 20. This mitigates the risk for unwanted radiation being reflected into the camera 4 by a surface portion on the radiation source 19.

It should be noted that the second concept also may serve to mitigate the above- described problem that radiation originating from specular reflection in the surface S enters the camera 4, by providing the camera 4 with a front opening given by a well- defined aperture 21 that has a thin peripheral surface 2 . Thus, the second concept may limit the range of pen orientations that may result in saturation of the image sensor 15.

In the particular example of FIG. 3 A, the radiation source 19 is arranged and fixed to the sheet device 20, which is in turn attached to the front face 11. This arrangement may further facilitate assembly of the camera 4. For example, the radiation source 19 may be attached to the sheet device 20 in precise relation to the aperture 21 in a preparatory assembly step, whereupon the resulting combination is attached to the front face 11 in a subsequent assembly step. It should be understood that the embodiment in FIG. 3 A is merely given as an example. For example, the housing 9 and the compartment 10 may have any extent and shape, the optical axis OA need not be straight but may be redirected inside the compartment 10, and the image sensor 15 need not be mounted at the rear portion 12 of the housing 9 but may e.g. be located in a sidewall 9' of the housing 9. Further, the lens 16 may instead be located behind the aperture step 18.

FIG. 3B shows a variant of the camera 4 in FIG. 3 A. Compared to FIG. 3 A, the radiation source 19 is placed closer to the aperture 21, which has approximately the same size and location as in FIG. 3 A. As indicated, a small portion of the radiation reflected by window 8 enters the inner compartment of the camera 4, via the aperture stop 17. To prevent this radiation from reaching the image sensor 15, a baffle 22 inserted in or integrated with the housing 9 so as form a radiation barrier in the compartment 10. The baffle 22 forms a lateral wall that defines an opening 23 for transmitting the image of the surface S. The baffle 22 is thus placed and dimensioned to block at least part of interfering radiation that enters the camera 6 via the aperture 21 and the aperture stop 17. As an alternative or supplement to the baffle 22, the inner surface of the housing 9 may be provided with one or more controlling surfaces that direct specular reflections way from the image sensor 15. In FIG. 3B, the controlling surfaces define a recess 24 configured to capture and attenuate impinging radiation. As a further alternative or supplement, the inner surfaces of the housing 9 may be roughed or textured to reduce specular reflections. As a further alternative or supplement, the inner surfaces of the housing 9 may be coated by or formed in a material that absorbs the interfering radiation. These measures for suppressing interfering radiation may be used in any combination in all embodiments disclosed herein.

FIG. 3C shows a further variant of the camera 4 in FIG. 3 A. In this variant, the aperture 21 in the sheet device 20 is dimensioned to form the aperture stop 17 of the imaging system. This may further reduce the amount of interfering radiation in the camera 4, in particular interfering radiation caused by specular reflection in the surface S. To further limit interfering radiation, a baffle 22 is arranged in the compartment 10. In this example, the baffle 22 is located directly behind the lens 16. The baffle 22 may thereby also serve to locate the lens 16, e.g. if the lens 16 is a separate component that is inserted via the inlet opening 13 into contact with the baffle 22. It is to be understood that the lateral wall 18 in the embodiments of FIGS 3A-3B may serve a corresponding function of locating the lens 16 along the radiation channel 10.

FIGS 4A-4B are section views of the front portion of the camera 4 according to further embodiments of the invention. In the illustrated embodiments, the housing 9 comprises a lateral front wall or support surface 9" that extends from the side walls 9' towards the optical axis OA to define the inlet opening 13. Thus, compared to the embodiments in FIGS 3A-3C the inlet opening 13 is smaller. The lateral wall 9" and the inlet opening 13 define the front face 11 of the housing 9. Like in the foregoing embodiments, a sheet device 20 is attached to the front face 11, e.g. by an adhesive, glue or the like (not shown). The aperture 21 is smaller than and arranged to overlap the inlet opening 13, so that the aperture 21 defines the front opening of the camera 4. As discussed above, the peripheral surface 2Γ of the aperture 21 may have a small thickness, typically much smaller that the material thickness of the lateral wall 9' (as shown), and thereby reduce the risk of unwanted radiation being reflected into the camera 4 by the peripheral surface 2 . In this context, it should be emphasized that the sheet device 20 need not have a uniform thickness throughout its extent, but should be thin at least around the aperture 21.

In all embodiments, the aperture 21 may be formed as a window of radiation- transparent material, e.g. plastic or glass. The window may be an integral portion of the sheet device 20 or may be a separate element attached to the sheet device 20 to cover a through-hole. Such a window aperture 21 may shield the camera 4 against ingress of dust and moisture. It is also conceivable to implement the window of a material that absorbs or blocks ambient radiation. Such a window aperture 21 may replace or supplement the window 8 (FIGS 3 A-3C).

Alternatively, the aperture 21 may be a through-hole in the sheet device 20. Such an aperture 21 may be manufactured with high precision using standard techniques and may transmit the image without distortion or scattering of radiation.

In all embodiments, the sheet device 20 is configured, outside of the aperture 21 and at least in the region overlapping the inlet opening 13, to block (and possibly absorb) incoming radiation in the wavelength range of the radiation source 19. The sheet device 20 may be provided with a dedicated layer for blocking such incoming radiation. The layer may be internal or provided on the either side of the sheet device 20, or on both sides.

The radiation source 19 is attached to the sheet device 20 by fixation means 25, e.g. adhesive tape, glue or solder. Although not shown in FIG. 4A, it is conceivable that the lateral wall 9" is inclined towards the optical axis OA, at least in a region beneath the radiation source 19, e.g. to achieve a precise main direction of the cone of radiation emitted by the radiation source 19.

In the embodiment of FIG. 4A, the sheet device 20 further comprises two electrical conductors 27 which at least extend to a dedicated mounting area for the radiation source 19. The conductors 27 are intended to be electrically connected to the power source of the pen (cf. 7 in FIG. 1). The sheet device 20 is preferably configured such that the radiation source 19 is electrically connected to the conductors 27 by its mere attachment to the sheet device 20, where the radiation source 19 is a surface- mount device (SMD). In the example of FIG. 4A, the conductors 27 are formed as an internal structure of the sheet device 20, and the source 19 is contacted by conductive pins or tabs on its bottom surface being brought in contact with the conductors 27, which may be exposed at the mounting area to facilitate contacting. In a variant (not shown), the conductors 27 are instead located on the outside of the sheet device 20, e.g. on the side facing away from the front face 11, which may simplify contacting of the radiation source 19 to the conductors 27. It is understood that the provision of electrical conductors on or in the sheet device 20, and in particular if the radiation source 19 is an SMD, facilitates manufacture of the camera 4. In one embodiment, the sheet device 20 is a flexible circuit. Flexible circuits may be manufactured with any type of conductor pattern using standard techniques and are commercially available in thicknesses down to approximately 0.1 mm, or even less. The flexible circuit may be of any type, including single-layer, double-sided and multi-layer. In another embodiment, the sheet device 20 is a printed circuit board (PCB), which typically has a thickness in the approximate range of 0.5-2 mm, although smaller thicknesses may be available in customized production.

In the embodiment of FIG. 4B, the radiation source is directly attached to the front face 11, in this example the lateral wall 9", and the sheet device 20 is arranged to overlie the inlet opening 13 to define the front opening of the camera 4. The radiation source 19 is electrically connected to the conductors 27 of the sheet device 20 by a pair of wires 27'. If the sheet device 20 is configured without conductors 27, the wires 27' may be otherwise connected to the power source of the pen.

FIGS 5-7 illustrate a specific implementation of a camera for an electronic pen.

The implementation is based on the principles and embodiments described in the foregoing, and the following presentation will therefore be brief. FIGS 5A-5C illustrate the camera 4 in a perspective view (FIG. 5 A), a front view (FIG. 5B) and a section view (FIG. 5C) taken along line C-C in FIG. 5B. FIGS 6A-6C illustrate a combination of a sheet device 20 and a radiation source 19 for installation in the camera 4, in a perspective view (FIG. 6A), an elevated side view (FIG. 6B) and a plan view (FIG. 6C). FIG. 7 illustrates a trace pattern of conductors 27 in the sheet device 20, in plan view.

The camera 4 is principally similar to the embodiments in FIG. 3B and FIG. 4A but differs in details further explained below. The housing 9 is tray shaped and forms the closed compartment 10 after engagement to a PCB 28, which is configured to receive electrical power from the power source in the pen. The image sensor 15 is mechanically and electrically connected to the PCB 28 and is located at a sensor mounting portion 14 of the housing 9 when the PCB 28 is engaged with the housing 9. A mirror 29 is arranged in the housing 9 to re-direct the image-forming radiation and thus the optical axis OA towards the image sensor 15. Although not shown in detail, the side walls 9' inside the compartment 10 define controlling surfaces that are arranged to direct specular reflections away from the image sensor 16. The housing 9 defines a baffle 22 that projects into the compartment 10 to block interfering radiation. A separate element 22' is press-fit into the tray-shaped housing 9 to define a further baffle.

The radiation source 19 is attached to the sheet device 20 such that the perimeter of the radiation source 19 is spaced from the perimeter of the aperture 21. The sheet device 20 is in turn arranged to overlie the front face 11 of the housing 9. An adhesive tape 25' is applied to the housing material on the front face 11 to attach the sheet device 20 thereto. The sheet device 20 is precisely positioned onto the front face 11 by fitting guiding holes 3 OA, 30B, 30C in the sheet device 20 onto corresponding guiding pins 31 A, 3 IB, 31C that project from the front face 11. At least one of the guiding pins 31 A, 3 IB, 31C may also serve to fix the sheet device 20 onto the front face 11 by heat staking, in which the respective guiding pin is deformed by heating and application of a pressing force, as is known in the art. As seen in the front view of FIG. 5B, the radiation source 19 is arranged to partly overlap the inlet opening 13 (indicated by dashed lines), and thereby also the imaging lens 16 (which has similar lateral dimensions as the inlet opening 13, see FIG. 5C).

The sheet device 20 comprises a cover portion 20A, adapted to engage the front face 11 of the housing 9, and a flap portion 20B, adapted to be brought into engagement with the PCB 28. The sheet device 20 is a flexible circuit comprising a flexible film that is patterned with electrically conductive traces to define a pair of conductors 27. The conductive traces are located in an internal layer and are thus not visible in FIGS 5-6. FIG. 7 illustrates the layout of this inner layer of the sheet device 20. The conductors 27 extend from two contact elements 27A, located on the flap portion 20B, to a respective contact pad 27B on the cover portion 20A. The contact elements 27A brought into contact with corresponding contact elements (not shown) on the PCB 28 when the flap portion 20A is bent into engagement with the PCB 29, as indicated in FIG. 5A. It is understood that the image sensor 15 and the radiation source 19 receive electrical power via the PCB 28, subject to control by the pen's control unit (cf. 6 in FIG. 1).

The sheet device 20 further comprises a stiffener 20', which is a rigid or semirigid plate attached to the flexible film behind the cover portion 20A. The stiffener 20' may e.g. be made of plastics, glass, glass fiber or metal. The stiffener 20' is provided to increase the rigidity of the cover portion 20A, in particular in the mounting area for radiation source 19. The stiffener 20' may improve the durability of the bond between the radiation source 19 and the sheet device 20. The stiffener 20' may define an opening that is larger than the opening in the flexible film, such that the aperture 21 is defined by the latter. This will minimize the thickness of the peripheral surfaces around the aperture 21 (cf. 2Γ in FIGS 4A-4B). In another embodiment, the stiffener 2 may define an opening that is smaller than the opening in the flexible film, such that the aperture 21 is defined by the stiffener 2 .

Although not clearly seen in FIGS 5-7, the sheet device 20 is provided with at least one layer of metal 32 (or another conductive material) that spans a major part of the cover portion 20A and is arranged in thermal contact with a heat transfer portion of the radiation source 19, e.g. a part of its bottom surface. The layer 32 thereby forms a heat dissipation element for leading off thermal energy from the radiation source 19. The promoted heat dissipation may increase the durability of the bond between the radiation source 19 and the sheet device 20, as well as increase the life of the radiation source 19. In FIG. 6B, the layer 32 is provided on the front surface of the sheet device 20, but it may alternatively be provided on the rear surface towards the front face 11, or be an internal layer of the sheet device 20. The layer 32 may be further configured to block radiation, including the radiation emitted by the radiation source 19. This may be advantageous, e.g., if the bulk of the sheet device 20 is made of radiation-transmissive material, since the layer 32 may be provided on or in the sheet device 20 to surround the aperture 21. In such an embodiment, the layer 32 extends from the perimeter of the aperture 21 to the radiation source 19. The layer 32 may also serve the additional function of providing a ground plane in the sheet device 32. Such a layer 32 may, e.g., serve to shield electronic components of the camera 4, such as the image sensor 15, against electromagnetic interference (EMI).