Abstract:
A wrist-worn device ( 100 ) comprising a background plate ( 106 ) to be illuminated for a display ( 102 ) of the wrist-worn device ( 100 ), and a light source ( 108 A,  108 B) for illuminating the background plate ( 106 ). The wrist-worn device ( 100 ) comprises a guide chamber ( 300, 302 ) at least partly surrounding the light source for collecting light emitted by the light source ( 108 A,  108 B) and for directing it to the background plate ( 106 ).

Description:
FIELD OF THE INVENTION 
     The field of application of the invention comprises wrist-worn devices, such as heart rate monitors, watches, wrist computers, or the like. The invention particularly relates to the illumination of a display on the wrist-worn device in question. 
     BRIEF DESCRIPTION OF THE RELATED ART 
     One important, and almost indispensable, characteristic of wrist-worn, watch-like devices, such as wrist watches, heart rate monitors and wrist computers, is the possibility to use the device in low ambient light and no ambient light conditions. The heart rate monitor to be described as an example is a device used in sports and exercising which incorporates the possibility to measure the user&#39;s hear rate information during physical exercise. The structure of some known heart rate monitors comprises an electrode belt to be positioned on the chest to measure heart rate information from an electric signal caused by the heart and to transmit the heart rate information to a device carried on the wrist which then shows the heart rate on its display. In conditions of low ambient light the display can be illuminated by pressing a push-button on the wrist-worn device, whereby the display is illuminated by electroluminescence or by light emitted from a light source, such as a LED (Light Emitting Diode). In prior art LED illumination solutions one or more light sources are arranged close to the background plate to allow as much of the light energy emitted by the LED as possible to be used for illuminating the background plate. In a known solution the light emitted from the light source is directed by means of lenses. In prior art solutions, the background plate is an injection-moulded, 1 mm thick plastic plate, for example. During its manufacture, the background plate is provided with macrostructures, for example projections, for directing light to the display. In addition to macrostructures, another attempt to improve the directing of light to the display is to manufacture the background plate such that its becomes gradually thinner further away from the LED. 
     The prior art solution for illuminating the background plate of a heart rate monitor involves a significant drawback in that a major portion of the directed light energy misses the background plate, thereby causing significant light energy losses. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved method and equipment implementing the method for illuminating a background plate of a display on a wrist-worn device. This is achieved with the method to be described below, which is a method for illuminating a background plate of a display on a wrist-worn device. The method employs a light source for emitting light to a guide chamber at least partly surrounding the light source for collecting the light and for directing it to the background plate. 
     The invention further relates to a wrist-worn device comprising a background plate to be illuminated for the display of the wrist-worn device and a light source for illuminating the background plate. The wrist-worn device comprises a guide chamber at least partly surrounding the light source for collecting light emitted from the light source and for directing it to the background plate. 
     The preferred embodiments of the invention are disclosed in the dependent claims. 
     As already stated, the invention relates to a method and equipment for illuminating the background plate of a display on a wrist-worn device. In this specification, ‘wrist-worn device’ refers to watches, heart rate monitors, wrist computers, or similar devices carried on the wrist. ‘Heart rate monitor’ is here defined as a device the functions of which comprise at least a function for measuring heart rate, i.e. heartbeat. ‘Wrist computer’ in turn can be defined for example as a device the functions of which comprise at least a function for locating the user. Further, in this specification, ‘display’ refers to those parts of a device which are used for visually displaying the information produced by the wrist-worn device to the user of the device. The display therefore comprises for example a background plate to be illuminated, a liquid crystal display and a plastic shield protecting the liquid crystal display and allowing it to be viewed. The wrist-worn device of the invention comprises a guide chamber for collecting light and for directing it from the light source to the background plate. In the solution of the invention, the guide chamber surrounds the light source at least partly, thus allowing light to be optimally collected from the light source. In this context, the term ‘surrounding’ means that the light source is at least partly inside the space defined by the guide chamber. The method of the invention differs for example from prior art solutions implemented by means of a lens, where the lens is used for directing light energy, but, since there is no collecting function, the loss of energy is significant. The solution of the invention is not restricted to the number of light sources or guide chambers the wrist-worn device comprises. 
     In a wrist-worn device of a preferred embodiment of the invention the background plate to be illuminated and the one or more guiding chambers form a single, uniform piece, which is advantageous from the point of view of the manufacturing process. Consequently, in one preferred embodiment, the cross-section of the guide chamber is substantially equal in thickness with the background plate. However, the invention is not restricted to a guide chamber and background plate made of one and the same piece, but it is possible that the guide chamber is manufactured separately and then attached to the background plate. In another preferred embodiment, the guide chamber and the background plate are interconnected, i.e. they are joined together. This allows to minimize any losses of light energy caused during the transmission of light. In another preferred embodiment the guide chamber and the background plate are made of a film-like material, as distinct from pieces manufactured by injection moulding, for example. Thin, film-like material is significantly easier to form into the shape required by the guide chamber than pieces made by injection moulding. 
     In a preferred embodiment, the guide chamber comprises a guide surface on its inner surface, the guide surface being in the area between a first end and second end that the guide chamber comprises. The guide surface is preferably circular. In a preferred embodiment, the guide surface is conical, the diameter of the cross-section of the first end thus being greater than that of the cross-section of the second end. The conical guide chamber is preferably open at its first end and closed at its second end. The guide surface between the first end and the second end preferably defines an open inner space, i.e. the guide means of this embodiment is not a closed plastic cone, for example. The light source is preferably arranged to be at least partly inside the guide means, the numerical aperture of the light source being directed towards the second, i.e. closed end of the cone. Preferably all the light emitted to the numerical aperture by the light source is collected inside the area encircled by the first end of the guide chamber. 
     According to a second preferred embodiment the guide chamber is shaped like a roll, the diameters of the cross-sections of the first end and the second end of the guide chamber thus being substantially equal. The light source is preferably arranged inside the guide chamber, on the inner surface side thereof, and directed with regard to the roll-shaped guide chamber such that as much of the light emitted by the light source as possible is directed to the wall of the guide chamber, i.e. to the guide surface. This is achieved for example by placing the light source obliquely with respect to the roll-shaped guide chamber. It is apparent that the above-described conical and roll-shaped forms of the guide chamber only serve as examples, the invention not being restricted to them. The inventive idea of the present application also comprises other guide chamber shapes. 
     The guide chamber of a preferred embodiment comprises at least one guide means for directing light from the guide chamber to the background plate. The guide means is for example a mirror, prism, air bubble, or a similar means causing light to be reflected or refracted. The guide chamber preferably comprises a plural number of guide means meant to make light propagate to the background plate as a uniform flow of light. The guide members are placed either on the guide surface or inside the guide chamber material. In a preferred embodiment, the flow of light directed using the guiding means propagates inside the background material. In another preferred embodiment the flow of light propagates along the surface of the background plate, the guiding chamber and the background plate being provided with a gap between them to allow the light to propagate towards the background plate. The background plate preferably comprises diffractive elements for reflecting the light received from the guide chamber or for directing it substantially perpendicularly with regard to the background plate towards the shield on the wrist-worn device&#39;s display. 
     The wrist-worn device preferably employs a LED as a light source, although other light sources producing optical power can also be used. The guide chamber and the background plate are made of at least partly light conducting material, such as plastic or glass. The background plate preferably conducts light well enough to allow at least some of the light produced by the light source to reach the far side of the background plate when seen from the light source. 
     The method of the invention defines the steps to be taken to illuminate the background plate of the display of the above-described wrist-worn device of the invention by using a light source and a guide chamber. 
     An advantage of the invention is that, compared with prior art solutions, it allows the light energy emitted by the light source to be more efficiently used for illuminating the background plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following the invention will be described in greater detail with reference to the accompanying drawings, in which 
     FIG. 1 shows an embodiment of a heart rate monitor; 
     FIG. 2 shows a prior art solution; 
     FIG. 3A shows a background plate of an embodiment of the invention; 
     FIG. 3B shows a background plate of an embodiment of the invention; 
     FIG. 4A shows a conical guide chamber of an embodiment of the invention; 
     FIG. 4B shows a roll-shaped guide chamber of an embodiment of the invention; 
     FIG. 5 shows a preferred embodiment of a method of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following the invention will be described in connection with preferred embodiments and with reference to accompanying FIGS. 1 to  5 . FIG. 1 shows a wrist-worn device, i.e. a heart rate monitor  100 , the display of which is illuminated using a prior art solution for display illumination. The heart rate monitor  100  of FIG. 1 is carried on the wrist where the heart rate monitor  100  is attached to with a wrist band  112 . The heart rate monitor  100  measures the heart rate either from a blood vessel in the wrist, or it receives magnetic pulses, for example, of the heart rate measured by an electrode belt on the chest. From the point of view of the present invention, it is not relevant to describe in detail how the heart rate is measured or transmitted to the heart rate monitor  100 , i.e. to the wrist-worn receiver. 
     The heart rate monitor  100  of FIG. 1 further comprises a display  102  for displaying the heart rate, the training duration  104 , or some other information relating to the heart rate or the training. The display  102  is preferably implemented as a liquid crystal display. The display  102  comprises a shield made of glass or plastic, for example, for protecting the liquid crystal display against humidity and impurities. The display  102  shield is attached to a casing  114  of the heart rate monitor made of plastic or metal, for example. The heart rate monitor  100  comprises keys  110 A- 110 B for controlling the functions of the heart rate monitor  100 . Key  110 A, for example, is used for controlling the functions of the heart rate monitor  100 , for example for activating a heart rate measurement or for alternating the information shown on the display between heart rate and training duration. Key  110 B is used for controlling light sources  108 A- 108 B of the heart rate monitor  100  such that when key  110 B is pressed, the light sources  108 A- 108 B illuminate the background plate  106  of the display  102  of the heart rate monitor  100  in no ambient light conditions. Key  110 B operates such that when it is depressed, the light sources  108 A- 108 B illuminate the background plate  106  continuously, whereas when key  110 B is alternately depressed and released, the light sources  108 A- 108 B illuminate the display for 15 seconds. In FIG. 1, the light sources  108 A- 108 B are visible, for the sake of clarity, although in practice they are preferably concealed inside the casing  114  of the heart rate monitor  100  that surrounds the display  102 . The light sources  108 A- 108 B, as well as other functions of the heart rate monitor  100  that require operating energy, receive the energy they need from a battery of the heart rate monitor  100 , from sunlight, artificial light, or the like. 
     FIG. 2 illustrates in greater detail a prior art solution for illuminating the background plate  106  of the heart rate monitor  100  shown in FIG.  1  and the problems related thereto. Light source  108 A is placed on one side of the background plate  106 , a shadowed area showing a connecting area  200  of the light emitted by light source  108 A. Illumination areas  202 A and  202 B show the areas where the light energy emitted by the light source  108 A is wasted, i.e. it is not connected to the background plate  106  to be illuminated. 
     FIG. 3A shows a solution according to an embodiment of the invention for illuminating the background plate  106 . The background plate  106  comprises two guide chambers  300 - 302 : a conical guide chamber  300  on the left and a roll-shaped guide chamber  302  on the right in the Figure. The conical guide chamber  300  is formed by forming or rolling block  106 C, which is that portion of the background plate  106  drawn with a broken line, into a cone. The roll-shaped guide chamber  302  is formed by forming or rolling that portion of the background plate  106  indicated by block  106 D, which is also drawn with a broken line, into a roll-shape or cylinder. The background plate is preferably cut to comprise blocks  106 C and  106 D, from which the guide chambers  300 - 302  can then be shaped during manufacturing. It is apparent that the guide chambers  300 - 302  can be similar to or different from one another and that their number in the solution of the invention is not limited. The number of the light sources or whether all the light sources are provided with guide chambers is not relevant to the invention either. Moreover, it is apparent that the guide chambers  300 - 302  may be separate pieces, which are separately manufactured and then attached to the background plate  106  after the background plate  106  has been manufactured. In a preferred embodiment, the background plate  106  is made of thin, film-like material about 0.1-0.5 mm thick, such as light conducting plastic. The background plate  106  can be shown to comprise a back wall  106 A, which is preferably made of material that does not conduct light. The background plate  106  further comprises a front wall  106 B which is visible in the display  102  of the heart rate monitor  100  shown in FIG.  1 . The entire background plate can also be made of material that does not conduct light, such as metal, in which case the light propagates on the surface of the background plate. 
     The conical guide chamber  300  comprises a first end  300 B and a second end  300 C, the diameter of the cross-section of the first end  300 B being greater than that of the second end  300 C. In a preferred embodiment, the first end  300 B and the second end  300 C are circular. The second end  300 C is preferably closed, thus preventing light from leaving the guide chamber through the second end. According to another preferred embodiment, the second end  300 C of the conical guide chamber  300  can also be open. The first end  300 B of the conical guide chamber  300  is preferably open. Light source  108 A, such as a LED, is preferably placed inside the conical guide chamber  300  such that the numerical aperture of the LED is substantially towards the second end  300 C of the conical chamber  300 . For the sake of clarity, the Figure does not show any wire leading from a power source, such as a battery, to light source  108 A. The LED  108 A is preferably placed so that substantially all the light emitted by the LED is collected by the guide chamber  300 . The inner surface, or guide surface, of the conical guide chamber  300  is preferably provided with diffractive elements to direct light from a groove-like gap  300 A in the guide chamber  300  along the surface of the background plate. In one embodiment the gap  300 A extends along the entire distance between the first end  300 B and the second end  300 C. The light  304 A directed out of the guide chamber  300  is evenly distributed to the background plate  106 . 
     The example of FIG. 3A shows the roll-shaped guide chamber  302  made by rolling block  106 D to the front surface  106 B of the background plate. In one embodiment the ends of the guide chamber  302  are open, but they can naturally be also covered with a reflecting material, for example. Light source  108 B is preferably placed obliquely with respect to the guide chamber  302 , whereby the light emitted by light source  108 B is reflected or refracted from the guide means on the inner surface of the guide chamber  302 , and, in the end, a uniform flow of light  304 B is directed along the background plate material  106 . Although some of the light may be wasted through the ends of the guide chamber  302  and possibly through the guide surface between the ends, the described solutions of the invention produce a significant improvement to the prior art solutions. 
     FIG. 3B shows an embodiment of the illumination solution. The flow of light  304 A emitted by the light source  108 A is directed on the background plate  106  as a uniform flow of light in the area between the between the back surface  106 A and the front surface  106 B, or along the front surface  106 B. In an embodiment of the invention, the back surface  106  of the background plate  106  is provided with diffractive elements reflecting light beams  310  from the back surface of the background plate  106  perpendicularly towards the front surface. The light beams  310  continue through the front surface  106 B, the user of the heart rate monitor, for example, perceiving this as illumination of the background plate  106 . 
     In the following the invention is described with reference to FIGS. 4A and 4B. FIG. 4A shows light source  108 A located in connection with the conical guide chamber  300 . In a preferred embodiment of the invention, the light source is placed such that substantially all the light emitted by light source  108 A to its numerical aperture collects into the guide chamber  300 , i.e. particularly to its inner surface, or guide surface  406 A, facing light source  108 . The numerical aperture of light source  108 A can be defined as the angle between outer light beams  404 A and  404 B emitted by light source  108 . The guide means  300  further comprises one or more guide members, such as a mirror  400  and prism  402  shown in FIG.  4 A. The function of the guide members is to direct the light  304 A towards the background plate. FIG. 4B shows light source  108 B and the roll-shaped guide chamber  302 . Light source  108 B is arranged obliquely with respect to the guide chamber  302 , whereby as much of the light emitted by light source  108 B to the numerical aperture defined by the beams  404 A- 404 B as possible meets the guide surface  406  of the guide chamber  302 . Further, the light  304 B is directed towards the background plate by reflecting or refracting it with the guide members  400 - 402 . As already stated in connection with FIG. 4A, the guide members can be implemented in a prior art manner using for example a mirror, prism, air bubble, or the like. The guide member  400 - 402  can also be made of a different material than the guide surface  406 B, whereby the light is strongly refracted for example from the guide surface  406 B which allows it to be directed towards the background plate. It is to be noted that for the sake of clarity, the above FIGS. 4A-4B do not show reflection or refraction patterns for all the light beams. It is also apparent that the light beams can reflect in the guide chamber ( 300 - 302 ) a number of times before they are directed out of the guide chamber ( 300 - 302 ) towards the background plate. 
     In the following, the method of the invention for illuminating a background plate will be described with reference to a working example and to FIG.  5 . In method step  502  light is emitted using a LED of the heart rate monitor. The LED is placed with respect to a guide chamber guiding light such that substantially all the light emitted by the LED into its numerical aperture is collected, in method step  504 , by a conical or roll-shaped guide chamber. The thickness of the guide chamber is substantially equal to that of the background plate. Substantially equal thickness means that the thickness of the guide chamber and that of the background plate differ by 0.2 mm or less. The guide chamber and the background plate are preferably made of the same piece, such as a thin plastic film, whereby they are defined to be of equal thickness within the error limits applied in the manufacturing process. In method step  504  the light is further directed towards the background plate. This is carried out using for example diffractive elements, such as mirrors or prisms, or by varying the refraction coefficient of the guide chamber material by means of material changes or air bubbles. Light is directed from the guide chamber to the background plate either inside the background plate material or along the surface of the background plate. In one implementation of an embodiment, if the light is directed along the surface of the background plate, there is left a gap between the guide chamber and the background plate to allow the light to propagate. In method step  506 , the background plate receives the substantially unidirectional and evenly distributed light for reflection to the display of the heart rate monitor. Reflection taking place on the background plate is carried out similarly as the guiding of light that takes place in the guide chamber. 
     Although the invention is described above with reference to examples according to the accompanying drawings, it is apparent that the invention is not restricted to them, but may vary in many ways within the inventive idea disclosed in the claims.