Field of the invention

The invention relates to a system which comprises a display apparatus, a detector for detecting whether an object is present in a first predetermined area near the display apparatus, and a feedback device to provide feedback if the object is within the first predetermined area. The invention further relates to a method of changing an image on a display apparatus, and a display apparatus which comprises such a detector and feedback device.

Background of the invention Children often sit directly in front of a television set. Parents are concerned that this behavior of their child has a deleterious effect on the child's eyes.

US-A-5,408,276 discloses a signal interrupt device connected externally to an electronic device such as a television set. The signal interrupt device includes a radio frequency sensor which senses when a person is within a predetermined minimum distance from the surface of the television set. The device further comprises a switch connected in series with the wire or cable providing incoming TV signals to the television set. The normally closed switch is opened when a person is sensed within the predetermined minimum distance and the incoming T. V. signals to the television are interrupted.

The sensor circuit includes a RF transmitter, a RF receiver/amplifier, and a signal intensity detector. The transmitter steadily transmits a weak radio frequency signal to obtain a field which in turn is continually received and amplified by the RF receiver/ amplifier. When an object enters into the field, the electro-magnetic waves generated by the transmitter are partially absorbed by the object, thereby creating a drop in the signal received by the receiver/amplifier. The signal intensity detector monitors the intensity of the signal received to determine a reduction in signal strength. The sensitivity threshold of the signal intensity detector can be adjusted such that a degree of signal reduction, which causes the normally closed switch to open and consequently interrupt the incoming signals to the television, is variable. Thus, if a person walks in front of the television, the absorption of waves detected is used to interrupt the signals to be displayed on the television.

In an alternatively embodiment, the transmitter is not present and the receiver/amplifier monitors FM signals in a specific broadband range to detect a drop in strength of this specific FM broadband range signal and interrupts the signals to be displayed on the television if this drop crosses a predetermined level. These FM signals are considered to be present anyhow, and need not be generated by the signal interrupt device.

Although this prior art prevents the child to view an image on the television if too near to the television, it has the drawback that the signal inputted to the television is also interrupted if a cat or dog is near the television.

Summary of the invention

It is an object of the invention to provide an apparatus and a method which has a higher discriminating ability for objects which are too near to a display apparatus.

A first aspect of the invention provides a system as claimed in claim 1. A second aspect of the invention provides a method as claimed in claim 13. A third aspect of the invention provides a display apparatus as claimed in claim 14. A fourth aspect of the invention provides the use of a tag as claimed in claim 16. Advantageous embodiments are defined in the dependent claims.

The system in accordance with the first aspect of the invention comprises a tag and a display apparatus for displaying an image. The tag is associated with a user which should be warned and/or protected when too near to the display apparatus. The tag may have any suitable form, for example, it may be a badge, a pendent, or it may be incorporated in clothing which the user wears.

A receiver receives a tag signal sent by the tag and supplies a received signal. A detector receives the received signal and supplies a detect signal which has an active status when the tag is present in a first predetermined area around the display apparatus. The detect signal has an inactive status if the tag is present in a second predetermined area. The first area is closer to the display apparatus than the second area. The detect signal has an inactive status if the tag is not present in the first predetermined area, thus, if the tag is further away from the display apparatus than the positions covered by the first predetermined area. For example, the first area comprises the positions which are closer to the display apparatus than a predetermined distance, and the second predetermined area comprises the positions which are further away from the display apparatus than the predetermined distance.

Such a detector and tag are, as such, known from WO-A-00/19235 which discloses a system for monitoring the position of one or more RFID tags. The system has a

detector incorporating circuitry for detecting changes in the range of an RFID tag from the detector, and for triggering an alarm if a detected change in range of an RFID tag exceeds a predetermined threshold, or if the RFID tag cannot be detected by the detector. Thus, the alarm is triggered if the tag is further away from the detector than a predetermined distance to prevent theft. The range may be detected by measuring the time of a returned radio signal from the tag, by measuring the strength of the returned radio signal from the tag, or by detecting changes in a periodic interval at which energy is transmitted by the tag.

The system in accordance with the first aspect of the invention further comprises a feedback device which receives the detect signal to provide a feedback to the user if the detect signal has the active status. This feedback indicates to the user that she or he is within the first predetermined area and thus too close to the display apparatus.

It has to be noted that, although the above mentioned prior art discloses the detection of movement of an identification tag, the use of the prior art system is completely different because an alarm is triggered if the tag moves too far away from the detector. This is contrary to the present invention wherein the feedback is provided when the tag moves too close to the display to prevent harm to the user.

In an embodiment as claimed in claim 2, the feedback device provides an audible or visual feedback to the user if the detect signal has or gets the active status. The feedback may be temporarily and may be initiated by the detect signal changing from the inactive status to the active status. Alternatively, the feedback may be continuously or intermittently present during the complete period in time the detect signal has the active status.

In an embodiment as claimed in claim 3, the feedback device comprises a transducer which generates the audible or visual feedback in response to the detect signal having or getting its active status. The audible or visual feedback indicates to the user that the tag is too near to the display apparatus. The transducer, which is not incorporated in the display apparatus, may, for example, be a loudspeaker, or a light source such as a lamp or a LED.

In an embodiment as claimed in claim 4, the detector supplies the detect signal to the display apparatus. The display apparatus comprises a display device which is the feedback device. The visual feedback to the user is provided by changing the image on the display device. This has the advantage that the already present display device is used to provide the feedback on the display screen of the display device. A further advantage is that

it is very likely that the user is actually looking to the display screen, and thus will immediately see the feedback.

In an embodiment as claimed in claim 5, the feedback to the user is generated on the display device of the display apparatus. The display apparatus comprises an internal signal generator which supplies a feedback signal defining the visual feedback to the user. The display apparatus further comprises a selector which selects between the feedback signal and the normally displayed input signal of the display apparatus. The normally displayed input signal is selected to be displayed on the display device if the detect signal has the inactive status. The feedback signal is selected to be displayed on the display device if the detect signal has the active status. For example, the internal signal generator may be a teletext generator. The teletext generator may generate a teletext page comprising a message giving the feedback. Alternatively, the teletext generator may supply a subtitle providing the feedback. More in general, the feedback signal may be combined with the input signal, or the feedback signal may modulate (change) the input signal or replace the input signal. In an embodiment as claimed in claim 6, the detector supplies the detect signal to the display apparatus. The feedback means now comprise a signal generator external to the display apparatus. The selector and the display device are part of the display apparatus. The signal generator supplies the feedback signal to the display apparatus. The feedback signal may be supplied continuously when the detect signal has the active state, or only if the detect signal gets the active state. For example, both the detect signal and the feedback signal may be supplied to a SCART connector of the display apparatus. For example, the feedback signal is a CVBS signal or comprises separate RGBS signals. The selector selects between the feedback signal and the normally displayed input signal of the display apparatus. The normally displayed input signal is selected to be displayed on the display device if the detect signal has the inactive status. The external feedback signal is selected to be displayed on the display device if the detect signal has the active status. Alternatively, the feedback signal may be combined with or modulate the input signal.

In an embodiment as claimed in claim 7, the feedback means visually warns the user by changing the image to display a textual or visual warning. The textual warning may actually read that the user is too close to the display apparatus, or may warn him that the image will disappear within a predetermined period of time if the user stays too close to the display apparatus. The visual warning may, for example, be a flashing image, or an image which is immediately or slowly darkened or discolored or otherwise impaired. The amount of impairing of the image may be inverse proportional to the distance between the tag and the

display apparatus. For example, the contrast of the picture may be decreased the shorter the distance between the tag and the display apparatus becomes.

In an embodiment as claimed in claim 8, the feedback means comprises a switching device which immediately switches off the image when the detect signal gets the active status. Thus, the image is switched off immediately when is detected that the user is within the first area which is considered too close to the display apparatus. The image may reappear when the user is outside the first area and thus sufficiently far away from the display apparatus.

In an embodiment as claimed in claim 9, the feedback means comprises a switching device for switching off the image a predetermined period of time after the detect signal got the active status. Now, the image is switched off after a predetermined period of time when is detected that the user is within the first area which is considered too close to the display apparatus. This has the advantage that the display apparatus is not switched off if the user is only a short time within the first area, which allows the user to control the display apparatus or any other apparatus near the display apparatus without immediately being unable to see the image. This is, for example, especially relevant if the user has, for example, to change a DVD in a DVD player which is present in a rack beneath the display apparatus. Another advantage is that first a warning can be presented to the user before the image is switched off such that the user has some time to move out of the first area. In an embodiment as claimed in claim 10, the tag is an active radio frequency identity tag, and the detector comprises a transceiver for wireless transmission of signals to said tag and for wireless reception of signals from said tag. The construction and operation of such a tag and transceiver are, as such, known from WO-A-00/19235.

In an embodiment as claimed in claim 11, the tag is a passive radio frequency identity tag, and the detector comprises a receiver for wireless reception of signals from said tag. The detector now need not comprise a transmitter. The construction and operation of such a tag and transceiver are, as such, also known from WO-A-00/19235.

Such an identity tag as defined in the embodiments as claimed in claim 10 or 11, has a unique code which identifies it. This has the advantage that the user is uniquely identifiable, making it possible to tailor the system to the user. For example, if the user is a young child, the distance between the child and the display apparatus at which a feedback or protective action has to be taken may be larger than for an older child. Or, the time during which the young child is allowed to be near the display apparatus is shorter than for an older child. Also the warning can be tailored to the ID of the tag. For example, if the tag of a young

child is detected to be too near to the display apparatus, the image may be, for example, smoothly dimmed, flashed, or colored reddish, while when the tag of an older child is detected to be too near, this may cause a text warning to be displayed on the display screen. In an embodiment of the display apparatus as defined in claim 15, the display apparatus is a so called mirror display. Such a mirror display comprises a polarizing mirror arranged between the user and the display device of the display apparatus. The polarizing mirror has a first plane directed to the user for reflecting light of a first polarization direction. The polarizing mirror transmits light having a second polarization direction different than the first polarization direction. The display device emits light with the second polarization direction. A switchable optical element is arranged in-between the display device and the polarizing mirror. This optical element may change the polarization direction of the light passing said optical element. Dependent on the change of the direction of the polarization of the light impinging on the optical element and the polarization of the light leaving the optical element, the mirror display may act as a display which shows the image on the display device, or as a mirror which reflects the ambient incident light to the user. The construction and operation of such a mirror display is elucidated in detail in WO-A-03079318.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

Brief description of the drawings

In the drawings:

Fig. 1 shows schematically a system in accordance with an embodiment of the invention,

Fig. 2 shows a block diagram of a receiver for receiving a tag signal and a display apparatus for displaying a feedback on the display screen of the display apparatus,

Fig. 3 shows a block diagram of a receiver for receiving a tag signal and a transducer for providing the feedback,

Fig. 4 shows a block diagram of a receiver for receiving a tag signal and a display apparatus for displaying a feedback on the display screen of the display apparatus, and

Figs. 5 A and 5B show schematically a construction of a mirror display to elucidate its operation.

Detailed description

Fig. 1 shows schematically a system in accordance with an embodiment of the invention. The person 22 wears a tag 21 which wirelessly transmits a tag signal 27. A display apparatus 4 comprises a transceiver 31, and a display device 43 with a display screen. The transceiver 31 is connected to an antenna 30 to transmit a transceiver signal 26. This transceiver signal 26 is received by the tag 21. The tag 21 may transmit the tag signal 27 in response to the received transceiver signal 26, or the tag 21 may autonomously transmit the tag signal 27, for example at intermittent instants. The tag 21 may comprise a battery, or may retrieve its energy from the received transceiver signal 26. Alternatively, the transceiver 31 is only a receiver and does not transmit a signal. The transceiver 31 receives the tag signal 27 and determines whether the tag

21 is in a first area Al near to the display apparatus 4. The first area Al may comprises positions which have a smaller distance to the display apparatus 4 than a predetermined distance Dl. The second area A2 is an area which covers positions which are further away from the display apparatus 4 than the positions within the first area Al. In the embodiment shown in Fig. 1 , the positions within the second area A2 have a distance with respect to the display apparatus 4 which is larger that the predetermined distance Dl. Usually, actually, the distance between the tag 21 and the antenna 30 is determined, but this distance may be corrected such that in fact the distance to the screen of the display device 43 is used. The first area Al may be selected to deviate from a sphere, and in fact may occupy an arbitrary volume, what counts is that the positions in the first area Al are shorter to the display apparatus 4 than the positions in the second area A2. Thus, if the tag 21 is present in the first area Al it is considered too near to the display apparatus 4 and an action will be performed to warn and/or protect the user 22 to which the tag 21 is associated.

The distance can be measured with different methods such as disclosed in WO-A-00/19235. In the example shown in Fig. 1, both a child 22 wearing the tag 21 and a cat 23 not wearing any tag are within the first area Al . It will be detected that the tag 21 which is associated to the child 22 is too close to the display apparatus 4 and an action is taken to provide a feedback to the child or to protect the child 22 from being too close. However, the cat 23 will not be detected because it does not wear a tag and thus will not cause a falsely taken action.

Although the transceiver 31 is shown to be present in the display apparatus 4, the transceiver 31 may also be mechanically separated from the display apparatus 4 such that it can be added as an option. The transceiver 31 may be replaced by a receiver only.

The action may be a feedback to the child 22 that it is too close to the screen of the display device 43. Such a feedback preferably is an audible or visual feedback, which is provided by a feedback providing device 33. The audible feedback may be generated by a feedback providing device 33 comprising a transducer in the form of a separate loudspeaker (see Fig. 3) or may be provided via the loudspeaker(s) (not shown) of the display apparatus 4. The visual feedback may be generated by a feedback providing device 33 comprising a transducer in the form of a separate light source or display, by a light indicator of the display apparatus 4, or on the display screen (not shown) of the display device 43. Usually, the light indicator of the display apparatus comprises a LED. The separate elements used to provide the feedback are especially relevant if the protection system is optionally and externally added to the display apparatus 4. The protection system comprises the transceiver 31, a detector 32 to detect whether the distance between the tag 31 and the display apparatus 4 is too small, and the feedback providing device 33 as shown in Fig. 3.

The feedback may only be a warning without changing the operation of the display apparatus 4. Alternatively, the feedback may visually warn the user by changing the image displayed on the display device 43. The image may be switched off immediately or a predetermined period of time after it is detected that a tag 21 enters the first area Al. The image may be changed to display a textual or visual warning. The textual warning may actually read that the user 22 is too close to the display apparatus 4, or may warn the user 22 that the image will disappear within a predetermined period of time if the user 22 stays too close to the display apparatus 4. The visual warning may, for example, be a flashing image, or an image which is immediately or gradually darkened or discolored or otherwise impaired. If several users 22 have to be protected, all the users 22 may wear a same type tag 21 and the protection system acts always in the same manner if a tag 21 enters the first area Al . Alternatively, to different users 22 different tags 21 may be associated, each with a unique identification. Now, the protection/warning system is able to act differently on different tags 21. For example, the predetermined area Al may be larger for a young child than for an older child. Or, a period in time during which the image is displayed after is detected that the tag 21 entered the first area Al is shorter for a young child 22 than for an older child 22. Also the type of warning may depend on the tag ID detected: a textual warning is presented only to users 22 who are able to read, other warnings are presented to users 22 which are unable to read.

It would even be possible to keep track of the total amount of time a tag 21 with a particular ID has been too near to the display apparatus 4. The action to be taken may

depend on this accumulated time. For example, it may be displayed how many hours during a particular period of time (for example a day, a week, a month) the particular tag 21 has been too close to the display apparatus 4, or how much time still is left during the particular period of time that the particular tag 21 is still allowed to be too near. The feedback and/or protective action may be provided if the time left drops below a predefined limit.

A tag 21 with a particular ID may be also be attached to an animal, and if such a special tag 21 is detected to be too near to the display apparatus 4, the feedback should be selected to confront the animal with a nuisance such that it avoids to be to near to the display apparatus 4. But preferably, the image should not change such that humans who are looking at the image shown are not disturbed.

Fig. 2 shows a block diagram of a receiver for receiving a tag signal and a display apparatus for displaying a feedback on the display screen of the display apparatus. The receiver 3 for wirelessly receiving the tag signal 27 comprises a transceiver 31 coupled to an antenna 30 to wirelessly transmit the transceiver signal 26 and to receive the tag signal 27 transmitted by the tag 21. Preferably, the wireless link is a radio frequent link. However, the link may be an optical or infrared link. The transceiver 31 supplies the received signal RS to the detector 32. The detector 32 supplies a detect signal DS which has the active status if the tag 21 is present in the area Al nearer than the distance Dl to the display apparatus 4. The detect signal DS has the inactive status if the tag 21 is not present in the area Al and thus is further away from the display apparatus 4 than the predetermined distance Dl . The active status of the detect signal DS does only imply that it must have any particular level or particular code word which is understood by circuits which use the detect signal DS that the tag 21 is considered to be too near to the display apparatus 4.

The display apparatus 4 comprises an input circuit 41 which receives a signal normally to be displayed on the display device 43 of the display apparatus 4. In Fig. 2, by way of example, the input circuit 41 is a tuner coupled to an antenna 40. Alternatively, the input circuit 41 may receive a CVBS, SVHS, RGB or other baseband signal or a high frequent signal from an external or internal apparatus. The external apparatus may be, for example, a DVD player or a settop box. The internal apparatus may be, for example, a teletext decoder or a DVD player. The output signal IS of the input circuit 41 is supplied as an input signal IS to a selector 42. A signal generator 44 generates the feedback signal WS which is also supplied to the selector 42. The selector 42 further receives the detect signal DS to select either the input signal IS or the feedback signal WS to be supplied as the selected signal DRS to the display device 43. The input signal IS is supplied to the display device 43

if the detect signal DS has the inactive status, the feedback signal WS is supplied to the display device 43 if the detect signal DS has the active status. The feedback FB to the user 22 is provided by the image displayed on the display device 43 of the display apparatus 4.

In one embodiment, the receiver 3 is mechanically separated from the display apparatus 4. Now, the receiver 3 only has to supply the detect signal DS to the display apparatus 4. The display apparatus 4 has to comprise the selector 42 and the signal generator 44. In a preferred embodiment, the signal generator 44 is a teletext decoder which anyhow is available in the display apparatus 4. Also the selector 42 is anyhow already present in the display apparatus 4 to switch between an input video signal IS and the teletext signal WS. Only minimal adaptations are required in the display apparatus 4 to make it suitable for cooperation with the receiver 3. The teletext decoder 44 has to know that it has to display a special teletext page or a subtitle if the detect signal DS has the active status. Therefore, the detect signal DS may be fed to the teletext decoder 44 or to a control unit (not shown) which is present in the display apparatus 4 and controls the operation of the display apparatus 4. The control unit usually comprises a control microprocessor which also controls the teletext decoder 44 and the selector 42. The selector 42 may also be directly controlled by the detect signal DS. A special input may have to be provided on the display apparatus to receive the detect signal DS. In a preferred embodiment, the receiver 3 is integrated into the display apparatus 4, and no special connection is required to receive the detect signal DS. Fig. 3 shows a block diagram of a receiver for receiving a tag signal and a transducer for providing the feedback. The transceiver 31 is coupled to the antenna 30 to receive the tag signal 27 from the tag 21 and supplies the received signal RS. The detector 32 receives the received signal RS and supplies the detect signal DS to a feedback device 33. The feedback device 33 may comprise a transducer 33 to provide an audible or visual feedback FB to the user. This system does not need any connection to the display apparatus 4 and does not require any special adaptation of the display apparatus 4. However, this system is only able to provide a warning and cannot change the image displayed on the display device 43. Although, not shown, it is possible to add a unit which in response to the detect signal DS, immediately or after a predetermined period of time, disconnects an input signal from the display apparatus 4 such that the image disappears. Still, this system is completely autonomous and can be simply added to an existing display apparatus 4.

Fig. 4 shows a block diagram of a receiver for receiving a tag signal and a display apparatus for displaying a feedback on the display screen of the display apparatus.

In this embodiment, the communication between the tag 21 and receiver 31 is performed with electro-magnetic waves via the antennas 20 and 30, respectively. The receiver 31 transmits a receiver signal 26 into the air, and receives the tag signal 27 from the tag 21. Any suitable communication protocol may be used. In a preferred embodiment, different tags 21 may be used which are allowed to send their identification ID when asked for by the receiver 31.

The detector 32 receives the received signal RS from the transceiver 31 and supplies the detect signal DS to both a signal generator 35 and a selector 45. The signal generator 35 supplies the feedback signal WS to the selector 45. The selector 45 further receives an input signal IS and supplies a selected signal SES. The processing circuit 46 processes the selected signal SES to obtain a drive signal DRS suitable to drive the display device 43. If the detect signal DS has the inactive status, the input signal IS is displayed by the display device 43. If the detect signal DS has the active status, the feedback signal WS is displayed by the display device 43, resulting in the visual feedback FB. All the blocks 31, 32, 35, 45, 46, 43 may be incorporated into the display apparatus 4. Alternatively, the blocks 31, 32, 35 may be mechanically separated from the display apparatus 4 which then comprises the blocks 45, 46, 43. In a preferred embodiment, the detect signal and the feedback signal WS are inputted to the display apparatus 4 via a SCART input. Such a SCART input has inputs for both receiving a video signal and a switch signal. The switch signal is used in the display apparatus 4 to switch to display the video signal supplied to the SCART input. Thus, a standard display apparatus 4 with a standard SCART connector can be used.

It has to be noted that if tags 21 are used with different ID's, the receiver 31 may retrieve the ID and indicate to the detector 32 what the ID of the tag 21 is. The detector 32 may use the ID to adapt the detection. For example, for different ID's the detect signal DS is made active for different distances between the tag 21 and the display apparatus 4. If the feedback signal WS should depend on the tag-ID, the tag-ID has also to be forwarded to the signal generator 35. If the signal generator 35 is present in the display apparatus 4, the tag-ID should be transferred to the display apparatus 4. Figs. 5A and 5B show schematically a construction of a mirror display to elucidate its operation. The display apparatus 4 now comprises a polarizing mirror 2 arranged between the user 22 and the display device 43 of the display apparatus 4. The ambient light 16 incidents on the polarizing mirror 2. The ambient light 16 may be considered to comprise two perpendicular arranged polarized light components 16'and 16". The polarizing mirror 2

has a plane directed to the user 22 to reflect impinging ambient light of a first polarization direction 16'. The polarizing mirror 2 is transmissive for light having a second polarization direction 16" different than the first polarization direction 16'. The display device 43 emits light 15 which has the polarization direction 15'. For example, the display device 43 is an LCD (Liquid Crystal Display) which emits light with the polarization direction 15'.

Alternatively, if the display device 43 does not emit polarized light, a polarizing mirror 13 is added in front of the display device 43. This polarizing mirror 13 transmits the polarization direction 15 'of the light 15 of the display 11.

A switchable optical element 12 is arranged in-between the display device 43, or the polarizing mirror 13, and the polarizing mirror 2 to selectively rotate a polarization direction of light passing the optical element 12 over at least two predetermined different angles dependent on a status of the optical element 12. For example, the optical element is a half lambda plate realized with known LCD technique, and the different states are obtained by applying different voltages across the LCD material. Possible embodiments of the construction of mirror display are disclosed in WO-A-03079318.

The display 11 emits linear polarized light 15 of a first polarization direction 15', either directly or after the light 15 emitted passes the polarizing mirror 13. If the polarizing mirror 13 is not required, the display 11 is the screen of display device 43. If the polarizing mirror 13 is required, the display device 43 comprises the combination of the display 11 and the polarizing mirror 13. The switchable half- lambda plate 12 is provided between the polarizing mirror 13 and the linear polarizing mirror 2. The polarization directions of the mirrors 2 and 13 have preferably an angle of 90 degrees.

The mirror 2 reflects incident light 16 with the polarization direction 16' and transmits light with the other polarization direction indicated by 16". If the switchable plate 12 is in the "off or "inactive" state (see Fig. 5A), the light with polarization direction 16" transmitted by the mirror 2 passes the plate 12 and is reflected on the linear polarizing mirror 13. After reflection, the reflected light 18 with the polarization direction 16" passes the plate 12 and the linear polarization mirror 2 again (see arrows 18). The light 15' of the display 11 has a polarization direction such that it passes the polarizing mirror 13 and the plate 12, but is blocked by the polarizing mirror 2. As a result, substantially all ambient light 16 is reflected and the image on the display screen 11, when present, is not visible to the user. Consequently, the display screen of the display apparatus 4 has a mirror like appearance.

If the switchable plate 12 is in the "on" or "active" state (see Fig. 5B), the polarization direction 15' of the display light 15 is rotated to the other polarization direction 15" such that this light will be transmitted through the polarizing mirror 2 and the image, when present, is visible to the user 22 (see arrows 15). Now, the polarization direction 16" of the incident light 16 passes the switchable plate 12 and is rotated in polarization direction such that the light 16' is forwarded to the polarizing mirror 13. Light with the polarization direction 16" does not pass the polarizing mirror 13 and cannot be reflected by the display 11. The polarization direction indicated by 16' is reflected by the linear polarizing mirror 13 and then passes the switchable half- lambda plate 12 and the linear polarizing mirror 2, see arrow 17.

The system comprising the tag 21 and the display apparatus 4 can now be arranged to switch the mirror display to the mirror mode if it is detected that the tag 21 is too near to the display apparatus 4. Preferably, the display apparatus 4 is switched to the mirror mode only if the tag 21 is longer than a predetermined period of time too near to the display apparatus 4. The user 22 need not take any action to be able to use the mirror display as a mirror. As soon as he is near to the display apparatus 4, or after the predetermined period of time, the display apparatus 4 gets the mirror function. This mirror function is usually only relevant if the user is near the display apparatus 4.

Preferably, the display device 43 is switched off such that the user 22, if too near to the display apparatus 4, is protected from being too near to a display apparatus 4 which shows an image. But, it is not essential that the image displayed on or by the display screen of the display device 43 is removed, it will not be visible anyhow.

Without the use of the tag 21, if the user wants to watch the image, the switchable plate 12 has to be activated into the on-mode, for example by switching on the mirror display either by remote control or by pressing the ON-switch of the display apparatus 4. If the user wants to use the mirror iunction of the mirror display he has to switch off the display apparatus 4. The use of the tag 21 automates the mirror function, even if the display apparatus 4 is on, the mirror mode is attained if the user is sufficiently long near to the display apparatus 4. However, this advantage would also be reached if the display apparatus 4 is able to detect the presence of user 22 within an area near to the display apparatus 4 without the use of a tag 21. Thus, any detection system which would be able to detect when a user 22 is within a predetermined area near the display apparatus 4 could be used to switch the status of the switchable plate 12.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

For example, instead of switching off the display, it may be changed in any way such that the user is made aware of the fact that she or he is too close to the display screen. For example only, if the distance between the tag and the display apparatus decreases, the color or the complete picture may fade away, the dimensions of the image may shrink, an increasing part of the picture may be covered by a warning or an not interesting portion such as a colored block, a warning may be displayed with increasing intensity, or an irritating noise or sound may be generated with an increasing level, or an audible warning may be generated which occurs more often or with higher volume.

The display apparatus may, for example be a television apparatus, a computer monitor or any other apparatus which displays an image.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.