Abstract:
An exemplary light sensor ( 20 ) includes a supporting base, a light-sensing unit ( 21 ) provided at at least one first location of the supporting base where ambient light is received, and a compensating unit ( 22 ) provided at a second location of the supporting base shielded from ambient light, the compensating unit having a structure that is the same as the light-sensing portion. The light-sensing portion includes at least one amorphous silicon thin film transistor (TFT) ( 210 ) configured for sensing light, and the compensating unit is configured for providing a reference value current for the light-sensing unit. A display device using the light sensor is also provided.

Description:
FIELD OF THE INVENTION 
   The present invention relates to light sensors, and more particularly to a light sensor having a light-sensing unit and a compensating unit, and a display device using the light sensor. 
   GENERAL BACKGROUND 
   Because LCD devices have the advantages of portability, low power consumption, and low radiation, they have been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCD devices are considered by some to have the potential to completely replace CRT (cathode ray tube) monitors and televisions. 
   Brightness is an important parameter to evaluate the performance of a display of an LCD device. A user may adjust the brightness of the display device according to the prevailing operational environment in which he/she is viewing the display device. 
   Referring to  FIG. 9 , a conventional LCD device  1  includes a liquid crystal panel  11 , and a backlight module  12  disposed under the liquid crystal panel  11  for illuminating the liquid crystal panel  11 . 
   Also referring to  FIG. 10 , the backlight module  12  includes a light source  121 , a brightness detector  122 , and a control circuit  123 . The brightness detector  122  detects a brightness of ambient light of the LCD device  1 , generates a corresponding photocurrent, and transmits the photocurrent to the control circuit  123 . The control circuit  123  stores a plurality of reference values, the reference values having a function with photocurrent values. The control circuit  123  calculates a result according to the function of the reference values and the received photocurrent, generates a corresponding voltage signal, and adjusts the brightness of the light source  121  according to the voltage signal. Thereby, a brightness of the light beams emitted by the light source  121  corresponds with the brightness of the ambient light. 
   As detailed above, the LCD device  1  automatically adjusts the brightness of light provided by the backlight module  12  according to the brightness of the ambient light. However, the internal element parameters of the brightness detector  122  may change due to changes in the ambient environment such as changes in temperature. When this happens, the photocurrent generated by the brightness detector  122  does not necessarily accurately reflect the brightness of the ambient light. Further, the reference values stored in the control circuit  123  are changeless. Thus the control circuit  123  may not accurately adjust the brightness of the light source  121  to correspond with the brightness of the ambient light. Furthermore, the backlight module  12  also includes other components such as a plastic frame, a metal bottom plate, and various kinds of optical films. All together, the brightness detector  122 , the control circuit  123  and the other components make the backlight module  12  rather complicated and bulky. This correspondingly makes the structure of the LCD device  1  unduly complicated and bulky. 
   What is needed, therefore, is a means or mechanism which can overcome the above-described deficiencies. What is also needed is a display device employing such means or mechanism. 
   SUMMARY 
   In one aspect, a light sensor includes a supporting base, a light-sensing portion provided at at least one first location of the supporting base where ambient light is received, and a compensating unit provided at a second location of the supporting base shielded from ambient light, the compensating unit having a structure that is the same as the light-sensing portion. The light-sensing portion includes at least one amorphous silicon thin film transistor (TFT) configured for sensing light, and the compensating unit is configured for providing a reference value current for the light-sensing portion. 
   In another aspect, a display device includes a first substrate, a second substrate parallel to the first substrate, a black matrix disposed at an inner surface of the first substrate, and a light sensor disposed at an inner surface of the second substrate. The light sensor includes a light-sensing unit and a compensating unit, the compensating unit has a structure that is the same as the light-sensing unit. The black matrix includes an opening, and the opening is disposed corresponding to the light-sensing unit. The black matrix is configured for shielding the compensating unit from ambient light, the opening is configured for providing a light path for the light-sensing unit, and the compensating unit is configured for providing a reference value current for the light-sensing unit. 
   In still another aspect, a display device includes a first substrate, a second substrate parallel to the first substrate, a black matrix disposed at an inner surface of the first substrate, a color filter disposed alternately with the black matrix at the inner surface of the first substrate, and a light sensor disposed at an inner surface of the second substrate. The light sensor includes a first light-sensing unit, a second light-sensing unit, and a compensating unit, the compensating unit has a structure that is the same as each of the first and second light-sensing units. The black matrix includes an opening, and the opening is disposed corresponding to the first light-sensing unit. The black matrix is configured for shielding the compensating unit from ambient light, the opening is configured for providing a light path for the first light-sensing unit, the color filter is configured for providing a light path for the second light-sensing unit, and the compensating unit is configured for providing a reference value current for the light-sensing unit. 
   Other novel features and advantages will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. In the drawings, all the views are schematic. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side cross-sectional view of part of a non-display area of a display device according to a first embodiment of the present invention, the display device including a light sensor. 
       FIG. 2  is a circuit diagram of the light sensor of  FIG. 1 . 
       FIG. 3  is a circuit diagram of a light sensor of a display device according to a second embodiment of the present invention. 
       FIG. 4  is a circuit diagram of a light sensor of a display device according to a third embodiment of the present invention. 
       FIG. 5  is a side cross-sectional view of part of a non-display area of a display device according to a fourth embodiment of the present invention, the display device including a light sensor. 
       FIG. 6  is a circuit diagram of the light sensor of  FIG. 5 . 
       FIG. 7  is a circuit diagram of a light sensor of a display device according to a fifth embodiment of the present invention. 
       FIG. 8  is a circuit diagram of a light sensor of a display device according to a sixth embodiment of the present invention. 
       FIG. 9  is an exploded, side cross-sectional view of a conventional LCD device, the LCD device including a backlight module. 
       FIG. 10  is a block diagram of components of the backlight module of  FIG. 9 . 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference will now be made to the drawings to describe various embodiments of the present invention in detail. 
     FIG. 1  is a side cross-sectional view of part of a non-display area of a display device  2  according to a first embodiment of the present invention. The display device  2  may for example be an LCD device. The display device  2  includes a light sensor  20 , a black matrix  25 , a first substrate  23 , and a second substrate  24 . 
   The first substrate  23  is parallel to the second substrate  24 . The light sensor  20  is disposed on an inner surface of the first substrate  23 . The light sensor  20  includes a light-sensing unit  21  and a compensating unit  22 , and the compensating unit  22  has a structure that is the same as the light-sensing unit  21 . The light-sensing unit  21  is used for sensing light, and the compensating unit  22  is used for providing a reference value current for the light-sensing unit  21 . The light-sensing unit  21  includes an amorphous silicon thin film transistor (TFT)  210 , and the compensating unit  22  includes an amorphous silicon TFT  220 . The amorphous silicon TFTs  210  and  220  have the same structure, and are made from a gate electrode layer  201 , an insulating layer  202 , an amorphous silicon layer  203 , an N-doped amorphous silicon layer  204 , and a source electrode and drain electrode semiconductor layer  205  formed on the inner surface of the first substrate  23  from bottom to top in that order. The amorphous silicon TFTs  210  and  220  may be formed in a same process with the TFT array of a display area (not shown) of the display device  1 . The black matrix  25  is disposed on an inner surface of the second substrate  24 . The black matrix  25  includes an opening  250  corresponding to the light-sensing unit  21 . The black matrix  25  is used to shield the compensating unit  22  from ambient light, and the opening  250  is used to provide a light path for the light-sensing unit  21 . 
     FIG. 2  is a circuit diagram of the light sensor  20 . The amorphous silicon TFTs  210  and  220  respectively include a gate electrode (not labeled), a drain electrode (not labeled), and a source electrode (not labeled). A gate voltage V g1  is supplied to the gate electrodes of the amorphous silicon TFTs  210  and  220 , a fixed voltage V 21  is supplied to the drain electrode of the amorphous silicon TFT  210 , and a fixed voltage V 22  is supplied to the drain electrode of the amorphous silicon TFT  220 . The source electrodes of the amorphous silicon TFTs  210  and  220  are grounded. 
   When the amorphous silicon TFT  210  is not irradiated by ambient light, a drain current I 21  flows through the drain electrode of the amorphous silicon TFT  210 , and a drain current I 22  flows through the drain electrode of the amorphous silicon TFT  220 . When the amorphous silicon TFT  210  is irradiated by ambient light, the drain current I 21  increases, and the amount of the increase depends on the quantity of light received by the amorphous silicon TFT  210 . However, because the black matrix  25  shields the compensating unit  22  from ambient light, the amorphous silicon TFT  220  is not irradiated by the ambient light, and the drain current I 22  of the amorphous silicon TFT  220  is not changed. An external control circuit (not shown) measures and compares the drain currents I 21  and I 22 , calculates a result according to a function of the drain currents I 21  and I 22 , generates a corresponding control signal, and adjusts the brightness of a light source (not shown) of the display device  2  according to the control signal. Thereby, a brightness of the light beams emitted by the light source corresponds with the brightness of the ambient light. 
   Because the light-sensing unit  21  and the compensating unit  22  have the same structure, the amorphous silicon TFTs  210  and  220  are made of same material. Even if the light-sensing unit  21  is influenced by ambient environment conditions (e.g. change in temperature) that change the internal element parameters of the light-sensing unit  21 , the internal element parameters of the compensating unit  22  undergo the same change. Therefore, the compensating unit  22  provides the drain current I 22  as the reference value to the external control circuit, and the external control circuit can accurately adjust the brightness of the light source to correspond with the brightness of the ambient light. Furthermore, the light sensor  20  is formed on the first substrate  23  in a same process with the TFT array of the display area of the display device  1 . This simplifies the overall process of fabricating the display device  2 , and lowers costs. 
   The light-sensing unit  21  and the compensating unit  22  can further include more amorphous silicon TFTs, respectively. Nevertheless, the number of amorphous silicon TFTs of the light-sensing unit  21  is equal to that of the compensating unit  22 . The gate voltage V g1  is supplied to all the gate electrodes of the amorphous silicon TFTs, the fixed voltage V 21  is supplied to all the drain electrodes of the amorphous silicon TFTs of the light-sensing unit  21 , and the fixed voltage V 22  is supplied to all the drain electrodes of the amorphous silicon TFTs of the compensating unit  22 . The source electrodes of all the amorphous silicon TFTs are grounded. All the amorphous silicon TFTs of the light-sensing unit  21  are used for receiving the ambient light. A sum of the drain currents of all the amorphous silicon TFTs of the compensating unit  22  is used as the reference value. The reference value and a sum of the drain currents of all the amorphous silicon TFTs of the light-sensing unit  21  are provided to the external control circuit. In a typical embodiment, the two sum currents are large enough to be measured conveniently by the external control circuit without the need for an amplifying circuit. 
     FIG. 3  is a circuit diagram of a light sensor  30  of a display device according to a second embodiment of the present invention. The light sensor  30  includes a light-sensing unit  31  and a compensating unit  32 , and the compensating unit  32  has a structure that is the same as the light-sensing unit  31 . The light-sensing unit  31  is used for sensing light, and the compensating unit  32  is used for providing a reference value current for the light-sensing unit  31 . The light-sensing unit  31  includes two amorphous silicon TFTs  311 ,  312 , and a resistor  313  that functions as a voltage-dividing element. The compensating unit  32  includes two amorphous silicon TFTs  321 ,  322 , and a resistor  323  that functions as a voltage-dividing element. 
   A fixed voltage V 31  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  311 , a fixed voltage V 32  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  312 , a fixed voltage V 33  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  321 , and a fixed voltage V 34  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  322 . A gate voltage V g2  is supplied to gate electrodes (not labeled) of the amorphous silicon TFTs  311 ,  321 . A source electrode (not labeled) of the amorphous silicon TFT  311  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  312 , and is grounded via the resistor  313 . A source electrode (not labeled) of the amorphous silicon TFT  321  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  322 , and is grounded via the resistor  323 . Source electrodes (not labeled) of the amorphous silicon TFTs  312 ,  322  are grounded. 
   The amorphous silicon TFT  311  is used for receiving ambient light. The resistors  313 ,  323  are used for dividing the fixed voltages V 31 , V 33  respectively, and supplying gate voltages to the amorphous silicon TFTs  312 ,  322  respectively. When the amorphous silicon TFT  311  is irradiated by the ambient light, the internal impedance of the amorphous silicon TFT  311  decreases, a drain current I 31  of the amorphous silicon TFT  311  increases, the gate voltage of the amorphous silicon TFT  312  increases, and a drain current I 32  of the amorphous silicon. TFT  312  increases. Due to the amplification effects of the amorphous silicon TFTs  312 ,  322 , the drain current I 32  of the amorphous silicon TFT  312  is larger than the drain current I 31  of the amorphous silicon TFT  311 , and a drain current I 34  of the amorphous silicon TFT  322  is larger than a drain current I 33  of the amorphous silicon TFT  321 . Therefore an external control circuit (not shown) can accurately adjust the brightness of a light source of the display device to correspond with the brightness of the ambient light via measuring and comparing the drain currents I 32  and I 34 , without the need for an amplifying circuit. 
     FIG. 4  is a circuit diagram of a light sensor  40  of a display device according to a third embodiment of the present invention. The light sensor  40  includes a light-sensing unit  41  and a compensating unit  42 , and the compensating unit  42  has a structure that is the same as the light-sensing unit  41 . The light-sensing unit  41  is used for sensing light, and the compensating unit  42  is used for providing a reference value current for the light-sensing unit  41 . The light-sensing unit  41  includes three amorphous silicon TFTs  411 ,  412 , and  413 . The compensating unit  42  includes three amorphous silicon TFTs  421 ,  422 , and  423 . 
   A fixed voltage V 41  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  411 , a fixed voltage V 42  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  412 , a fixed voltage V 43  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  421 , and a fixed voltage V 44  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  422 . A gate voltage V g3  is supplied to gate electrodes (not labeled) of the amorphous silicon TFTs  411 ,  413 ,  421 , and  423 . A source electrode (not labeled) of the amorphous silicon TFT  411  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  412  and a drain electrode (not labeled) of the amorphous silicon TFT  413 . A source electrode (not labeled) of the amorphous silicon TFT  421  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  422  and a drain electrode (not labeled) of the amorphous silicon TFT  423 . Source electrodes (not labeled) of the amorphous silicon TFTs  412 ,  413 ,  422 , and  423  are grounded. 
   The amorphous silicon TFT  411  is used for receiving ambient light. The amorphous silicon TFTs  413 ,  423  function as voltage-dividing elements. That is, the amorphous silicon TFTs  413 ,  423  are used for dividing the fixed voltages V 41 , V 43  respectively, and supplying gate voltages to the amorphous silicon TFTs  412 ,  422  respectively. When the amorphous silicon TFT  411  is irradiated by the ambient light, the internal impedance of the amorphous silicon TFT  411  decreases, a drain current I 41  of the amorphous silicon TFT  411  increases, the gate voltage of the amorphous silicon TFT  412  increases, and a drain current I 42  of the amorphous silicon TFT  412  increases. Due to the amplification effects of the amorphous silicon TFTs  412 ,  422 , the drain current I 42  of the amorphous silicon TFT  412  is larger than the drain current I 41  of the amorphous silicon TFT  411 , and a drain current I 44  as the reference of the amorphous silicon TFT  422  is larger than a drain current I 43  of the amorphous silicon TFT  421 . Therefore, an external control circuit (not shown) can accurately adjust the brightness of a light source of the display device to correspond with the brightness of the ambient light via measuring and comparing the drain currents I 42  and I 44 , without the need for an amplifying circuit. 
     FIG. 5  is a side cross-sectional view of part of a non-display area of a display device  5  according to a fourth embodiment of the present invention. The display device  5  is similar to the display device  1 , and may for example be an LCD device. However, the display device  5  further includes a color filter  56 . The color filter  56  and a black matrix  55  are disposed alternately across the inner side of a second substrate (not labeled) of the display device  5 . A light sensor  50  of the display device  5  includes a first light-sensing unit  51 , a second light-sensing unit  57 , and a compensating unit  52 . The compensating unit  52 , the first light-sensing unit  51 , and the second light-sensing unit  57  have the same structure. The first and second light-sensing units  51 ,  57  are used for sensing different lights. The compensating unit  52  is used for providing a reference value current for the first and second light-sensing units  51 ,  57 . The black matrix  55  is used to shield the compensating unit  52  from ambient light, an opening  550  of the black matrix  55  is used to provide a light path for the first light-sensing unit  51 , and the color filter  56  is used to provide a light path for the second light-sensing unit  57 . The first light-sensing unit  51  includes an amorphous silicon TFT  510 , the second light-sensing unit  57  includes an amorphous silicon TFT  570 , and the compensating unit  52  includes an amorphous silicon TFT  520 . The amorphous silicon TFT  510  is disposed corresponding to the opening  550 , and is used for receiving ambient light through the opening  550 . The amorphous silicon TFT  570  is disposed corresponding to the color filter  56 , and is used for receiving ambient light through the color filter  56 . 
     FIG. 6  is a circuit diagram of the light sensor  50 . A gate voltage V g4  is supplied to gate electrodes (not labeled) of the amorphous silicon TFTs  510 ,  570 , and  520 . A fixed voltage V 51  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  510 , a fixed voltage V 52  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  520 , and a fixed voltage V 57  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  570 . Source electrodes (not labeled) of the amorphous silicon TFTs  510 ,  570 , and  520  are grounded. 
   An external control circuit (not shown) measures a drain current I 51  of the amorphous silicon TFT  510 , a drain current I 57  of the amorphous silicon TFT  570 , and a drain current I 52  of the amorphous silicon TFT  520  as the reference value, respectively. Then the external control circuit calculates two results according to a function of the drain currents I 51  and I 52  and a function of the drain currents I 57  and I 52 , respectively, compares the two results, generates a corresponding control signal, and adjusts the brightness of a light source (not shown) of the display device  5  according to the control signal. Thereby, a brightness of the light beams emitted by the light source corresponds with the brightness of the ambient light. 
     FIG. 7  is a circuit diagram of a light sensor  60  of a display device according to a fifth embodiment of the present invention. The light sensor  60  includes a first light-sensing unit  61 , a second light-sensing unit  67 , and a compensating unit  62 . The compensating unit  62 , the first light-sensing unit  61 , and the second light-sensing unit  67  have the same structure. The first and second light-sensing units  61 ,  67  are used for sensing ambient light at two selected different locations on the display device. The compensating unit  62  is used for providing a reference value current for the first and second light-sensing units  61 ,  67 . The first light-sensing unit  61  includes two amorphous silicon TFTs  611 ,  612 , and a resistor  613  that functions as a voltage-dividing element. The second light-sensing unit  67  includes two amorphous silicon TFTs  671 ,  672 , and a resistor  673  that functions as a voltage-dividing element. The compensating unit  62  includes two amorphous silicon TFTs  621 ,  622 , and a resistor  623  that functions as a voltage-dividing element. 
   A fixed voltage V 61  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  611 , and a fixed voltage V 62  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  612 . A fixed voltage V 63  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  621 , and a fixed voltage V 64  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  622 . A fixed voltage V 65  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  671 , and a fixed voltage V 66  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  672 . A gate voltage V g5  is supplied to gate electrodes (not labeled) of the amorphous silicon TFTs  611 ,  621 , and  671 . A source electrode (not labeled) of the amorphous silicon TFT  611  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  612 , and is grounded via the resistor  613 . A source electrode (not labeled) of the amorphous silicon TFT  621  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  622 , and is grounded via the resistor  623 . A source electrode (not labeled) of the amorphous silicon TFT  671  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  672 , and is grounded via the resistor  673 . Source electrodes (not labeled) of the amorphous silicon TFTs  612 ,  622 , and  672  are grounded. 
   An external control circuit (not shown) measures a drain current I 62  of the amorphous silicon TFT  612 , a drain current I 66  of the amorphous silicon TFT  672 , and a drain current I 64  of the amorphous silicon TFT  622  as the reference value without an amplifying circuit, respectively. Then the external control circuit calculates two results according to a function of the drain currents I 62  and I 64  and to a function of the drain currents I 66  and I 64 , respectively, compares the two results, generates a corresponding control signal, and adjusts the brightness of a light source (not shown) of a display device according to the control signal. Thereby, a brightness of the light beams emitted by the light source corresponds with the brightness of the ambient light. 
     FIG. 8  is a circuit diagram of a light sensor  70  of a display device according to a fifth embodiment of the present invention. The light sensor  70  includes a first light-sensing unit  71 , a second light-sensing unit  77 , and a compensating unit  72 . The compensating unit  72 , the first light-sensing unit  71 , and the second light-sensing unit  77  have the same structure. The first and second light-sensing units  71 ,  77  are used for sensing ambient light at two selected different locations on the display device. The compensating unit  72  is used for providing a reference value for the first and second light-sensing units  71 ,  77 . The first light-sensing unit  71  includes three amorphous silicon TFTs  711 ,  712 , and  713 . The second light-sensing unit  77  includes three amorphous silicon TFTs  771 ,  772 , and  773 . The compensating unit  72  includes three amorphous silicon TFTs  721 ,  722 , and  723 . 
   A fixed voltage V 71  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  711 , and a fixed voltage V 72  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  712 . A fixed voltage V 73  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  721 , and a fixed voltage V 74  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  722 . A fixed voltage V 75  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  771 , and a fixed voltage V 76  is supplied to a drain electrode (not labeled) of the amorphous silicon TFT  772 . A gate voltage V g6  is supplied to gate electrodes (not labeled) of the amorphous silicon TFTs  711 ,  713 ,  721 ,  723 ,  771 , and  773 . A source electrode (not labeled) of the amorphous silicon TFT  711  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  712  and a drain electrode (not labeled) of the amorphous silicon TFT  713 . A source electrode (not labeled) of the amorphous silicon TFT  721  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  722  and a drain electrode (not labeled) of the amorphous silicon TFT  723 . A source electrode (not labeled) of the amorphous silicon TFT  771  is connected to a gate electrode (not labeled) of the amorphous silicon TFT  772  and a drain electrode (not labeled) of the amorphous silicon TFT  773 . Source electrodes (not labeled) of the amorphous silicon TFTs  712 ,  713 ,  722 ,  723 ,  772 , and  773  are grounded. 
   An external control circuit (not shown) measures a drain current I 72  of the amorphous silicon TFT  712 , a drain current I 76  of the amorphous silicon TFT  772 , and a drain current I 74  of the amorphous silicon TFT  722  as the reference value without an amplifying circuit, respectively. Then the external control circuit calculates two results according to a function of the drain currents I 72  and I 74  and to a function of the drain currents I 76  and I 74 , respectively, compares the two results, generates a corresponding control signal, and adjusts the brightness of a light source (not shown) of the display device according to the control signal. Thereby, a brightness of the light beams emitted by the light source corresponds with the brightness of the ambient light. 
   It is to be understood, however, that even though numerous characteristics and advantages of preferred and exemplary embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only; and that changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.