Patent Publication Number: US-2005140641-A1

Title: Power conservation for a display apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application claims priority, under 35 U.S.C. § 119, from Korean Patent Application No. 2003-79501 filed on Nov. 11, 2003, the content of which is herein incorporated by reference in its entirety.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The invention relates generally to a display apparatus, and more particularly to a display apparatus capable of reducing power consumption without compromising brightness.  
      2. Description of the Related Art  
      A liquid crystal display (LCD) apparatus includes an LCD panel that uses light to generate images. As the LCD panel does not generate light on its own, the LCD panel uses either the light from the environment (e.g., sunlight) or an artificial light source that is optically coupled to the LCD panel.  
      The amount of light that is supplied to the LCD apparatus affects the brightness of the LCD apparatus. The light supply includes both ambient light and light from a backlight assembly. Thus, when there is sufficient light in the environment, the LCD apparatus can achieve a desired brightness level relying just on the ambient light. However, since the amount of light in the environment is not constant, the LCD apparatus typically includes a backlight assembly to ensure that there will always be a sufficient amount of light supply regardless of time and place. With the backlight assembly, the desired brightness level of the LCD apparatus is maintained at all times.  
      Although the backlight assembly is indispensable for maintaining a constant brightness level, it has the downside of increasing power consumption. In fact, it is estimated that about 70% of an LCD apparatus&#39; total power consumption is attributed to driving the backlight assembly. Thus, for mobile electric devices such as a cellular phone, a laptop computer, a PDA, etc. that rely on batteries, the presence of a backlight assembly results in the inconvenience of having to charge the batteries more frequently.  
      This power consumption problem has been addressed by decreasing the electrical power supply to the backlight assembly. However, the decreased power supply results in the brightness level undesirably going down, which is especially problematic when there is not enough ambient light. For these reasons, display apparatus manufacturers are currently unable to satisfy both the consumers&#39; desire for low power consumption and the conflicting desire for high brightness.  
      A method of reducing the backlight assembly power consumption while maintaining a desired brightness level is desired.  
     SUMMARY OF THE INVENTION  
      The invention provides a method of reducing power consumption without compromising brightness. The invention also provides a display apparatus that conserves power while supplying the desired level of brightness.  
      According to one aspect of the invention, the brightness of a display apparatus is controlled by sensing an ambient light level, comparing the ambient light level to a reference value to obtain a difference between the ambient light level and the reference value, and adjusting an applied voltage to a light source according to the difference.  
      Another aspect of the invention is a display apparatus that includes a light source, a sensor for detecting an ambient light level, and a light source driving section for adjusting a brightness of the light source according to the ambient light level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of an LCD apparatus according to an exemplary embodiment of the present invention;  
       FIG. 2  is a plan view of the display panel shown in  FIG. 1 ;  
       FIG. 3  is a cross-sectional view of the display panel shown in  FIG. 2 ;  
       FIG. 4  is a block diagram of a display apparatus according to another exemplary embodiment of the invention;  
       FIG. 5  is a graph of transmittance and reflectance as a function of applied voltage;  
       FIG. 6  is a block diagram of the display panel driving section shown in  FIG. 4 ;  
       FIGS. 7A and 7B  are circuit diagrams showing first and second gamma circuit sections of  FIG. 6 , respectively;  
       FIG. 8  is a circuit diagram showing a resistor section for a gray-scale that is built into the data driving section shown in  FIG. 6 ;  
       FIG. 9  is a cross-sectional view of a first embodiment of an LCD apparatus incorporating the invention;  
      FIG  10  is a cross-sectional view of a second embodiment of an LCD apparatus incorporating the invention; and  
       FIG. 11  is a cross-sectional view of a third embodiment of an LCD apparatus incorporating the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiments of the invention are described herein in the context of liquid crystal display (LCD) apparatuses. However, it is to be understood that the embodiments provided herein are just preferred embodiments, and the scope of the invention is not limited to the applications or the embodiments disclosed herein. For example, the invention may be adapted to other types of apparatuses that benefit from a constant light supply.  
      As used herein, “backlight” is light generated by the backlight assembly, as opposed to “ambient light,” which is light in the environment. The backlight assembly is usually a part of the display apparatus. The position of a backlight assembly is not limited to any particular section of the display apparatus relative to the display panel, as long as the display panel receives light from the backlight assembly. Ambient light may come from a natural source (e.g., the sun) or an artificial source (e.g., a light bulb). As used herein, a “primary light exit surface” refers to the surface of a display panel that affects image brightness most dramatically by having light exit the apparatus through that surface. The primary light exit surface is usually the surface that is closest to a user of the LCD apparatus viewing the displayed images.  
       FIG. 1  is a block diagram showing a display apparatus  1000  according to an exemplary embodiment of the invention. The display apparatus  1000  displays images by using a backlight L 1  and/or ambient light L 2 . The display apparatus  1000  includes a backlight assembly  100  for generating the backlight L 1 , a backlight driving section  200  for controlling the backlight assembly  100 , a display panel  300  for displaying images, and a display panel driving section  400  for outputting a driving signal DS for the display panel  300 .  
      The display apparatus  1000  further includes a light sensing section  500 , which senses the overall light amount, detects the amount of ambient light, and outputs an electrical signal corresponding to the amount of the ambient light L 2 . The electrical signal is herein referred to as the photocurrent (PC). Although not shown in the Figures, the light sensing section  500  includes a sensor for sensing the light and a photodetector for detecting the amount of ambient light.  
      The display apparatus  1000  includes a signal transmitting section  600  for outputting an appropriate electrical signal to the backlight assembly  100  in response to the photocurrent. The signal transmitting section  600  compares the photocurrent output from the light sensing section  500  against a predetermined reference value and determines whether to output a first sensing signal SS 1  or a second sensing signal SS 2  based on the comparison. The backlight driving section  200  adjusts the voltage V applied to the backlight assembly  100  depending on whether it receives the first sensing signal SS 1  or the second sensing signal SS 2 . The reference value is selected to correspond to a minimum ambient light level that provides a desired brightness level. Thus, if the photocurrent level indicates an ambient light level that is equal to or lower than the light level associated with the reference voltage, the backlight driving section  200  applies a voltage V to the backlight assembly  100  to turn on the backlight assembly  100 . In this case, the backlight from the backlight assembly  100  supplements the ambient light to raise the total light amount and achieve the desired brightness level. On the other hand, if the photocurrent level indicates an ambient light level that is equal to or higher than the light level associated with the reference voltage, no backlight is needed to supplement the ambient light. Thus, the backlight driving section  200  applies a voltage V to turn off the backlight assembly  100 , thereby conserving power.  
      The overall effect of the configuration is that the backlight assembly  100  is turned on when supplemental light is desired, and turned off to conserve power the rest of the time. When the ambient light level is below the desired level (i.e., the photocurrent is smaller than the reference value), the backlight driving section  200  turns on the backlight assembly  100  in response to the first sensing signal SS 1 . Otherwise, the backlight driving section  200  turns off the backlight assembly  100  in response to the second sensing signal SS 2 . Since the backlight assembly does not have to stay turned on, electrical power consumption for the backlight assembly  100  is reduced.  
      In some embodiments, the backlight driving section  200  may tune the amount of backlight L 1  according to the amount of ambient light L 2 , instead of simply turning on and turning off the backlight assembly  100 . For example, when there is a difference between the reference value and the photocurrent level, the backlight driving section  200  may increase or decrease the voltage V by an amount that corresponds to the difference. If the photocurrent value is higher than the reference value, the backlight driving section  200  may decrease the voltage V that is applied to the backlight assembly  100  by an amount that reflects the difference. Conversely, when the photocurrent is lower than the reference value, the backlight driving section  200  increases the voltage V by an amount that reflects the difference.  
       FIG. 2  is a plan view of the display panel shown in  FIG. 1 .  FIG. 3  is a cross-sectional view of the display panel shown in  FIG. 3 .  
      Referring to  FIGS. 2 and 3 , the display panel  300  includes a first member  310 , a second member  320  positioned in a plane that is substantially parallel to the first member  310 , and a liquid crystal layer  330  disposed between the first and second members  310  and  320 . The display panel  300  may be divided into a display area DA for displaying the image and a peripheral area PA adjacent to the display area DA.  
      A plurality of pixels are formed in a matrix configuration in the display area DA. The first member  310  includes a gate line GL, a data line DL that is substantially perpendicular to the gate line GL, a thin film transistor (TFT)  311  that is connected to the gate lines GL and data lines DL, a transparent electrode  312  connected to the TFT  311  and a reflective electrode  313  coupled to the transparent electrode  312 . As shown, the reflective electrode  313  may be formed on the transparent electrode  312 . The TFT  311  includes a gate electrode  311   a  that is connected to the gate line GL, a source electrode  311   b  that is connected to the data line DL, and a drin electrode  311   c  that is connected to the transparent electrode  312  and the reflective electrode  313 .  
      The first member  310  further includes a storage electrode  315 , which is located to be covered by the transparent and reflective electrodes  312  and  313 . An insulating layer is disposed over the storage electrodes  315  and transparent electrode  312  so that the insulating layer covers the storage electrode  315 . The storage electrode  315  receives a common voltage.  
      The second member  320  includes a color filter  321 , which imparts red, green, and blue (RGB) colors to the pixels, and a common electrode  322 . The common electrode  322  is coupled to the color filter  321  and preferably borders the liquid crystal layer  330 .  
      Hereinafter, an area of the display panel  300  where the reflective electrode  313  is formed is referred to as a “reflective area” (RA) and an area on which the reflective electrode  313  is not formed and the transparent electrode  312  is formed is referred to as a “transmissive area” (TA). The display panel  300  may operate in a transmissive mode and/or in a reflective mode. In the transmissive mode, the display panel  300  displays the image by letting the backlight L 1  pass through the transmissive area TA (refer to  FIG. 1 ). In the reflective mode, the display panel  300  displays the image by reflecting the ambient light L 2  in the reflective area RA.  
      The display panel driving section  400 , which includes a gate driving section  410  and a data driving section  420 , is formed in the peripheral area PA. The gate driving section  410  feeds a gate driving voltage to the gate line GL in response to various control signals from external devices (not shown). Similarly, the data driving section  420  feeds a data voltage to the data line DL.  
      When the backlight assembly  100  is turned on due to the amount of ambient light L 2  being below a desired level, the display panel  300  operates in the transmissive mode using the backlight L 1  from the backlight assembly  100 . When the backlight assembly  100  is turned off, however, the display panel  300  operates in the reflective mode using primarily the ambient light L 2 .  
      When the display panel  300  operates in the transmissive mode using the backlight L 1 , the transmissive voltage is applied to the transparent and reflective electrodes  312  and  313  through the TFT  311 . The display panel  300  displays images in the transmissive area TA using the backlight L 1 . When the amount of ambient light L 2  is below a desired level, the display panel  300  operates in the transmissive mode so that the display panel  300  does not display images in the reflective area RA.  
      When the display panel  300  operates in the reflective mode using the ambient light L 2 , the reflective voltage is applied to the transparent and reflective electrodes  312  and  313  through the TFT  311 . The display panel  300  displays images in the reflective area RA using the ambient light L 2 . When the backlight assembly is turned off, the display panel  300  operates in the reflective mode so that the display panel  300  does not display images in the transmissive area TA.  
      The display panel  300  may operate in the transmissive mode using the backlight L 1  or the reflective mode using the ambient light L 2 , although the transparent electrode  312  is connected to the reflective electrode  313 .  
      The transmissive and reflective voltages will be described below in reference to  FIG. 5 .  
      The above exemplary embodiment was illustrated in the context of a transflective-type display panel  300 , which has both the transmissive and reflective areas. However, as will be described below in reference to  FIG. 10  and  FIG. 11 , the invention is not limited to a display apparatus using a transflective-type display panel.  
       FIG. 4  is a block diagram showing a display apparatus according to another exemplary embodiment of the present invention. Like the embodiment of  FIG. 1 , this embodiment adjusts the backlight assembly according to the amount of ambient light available. This embodiment, however, also adjusts the gray data voltage and the common voltage of the display panel  300  according to the amount of ambient light. The gray data voltage and the common voltage are adjusted differently depending on whether the ambient light level is sufficient for the apparatus to operate in a primarily reflective mode or insufficient such that the apparatus operates in a primarily transmissive mode.  
      Unlike the display apparatus  1000  of  FIG. 1 , the display apparatus  1100  includes a mode converting section  700 . As in the display apparatus  1000 , the signal transmitting section  600  outputs a first or second sensing signal SS 1 /SS 2 . Unlike in the display apparatus  1000 , however, the signal transmitting section  600  also outputs a third sensing signal SS 3  and a fourth sensing signal SS 4  to the mode converting section  700 . The mode converting section  700  receives a third sensing signal SS 3  and a fourth sensing signal SS 4  from the signal transmitting section  600  and outputs either a first mode selecting signal FMS or a second mode selecting signal SMS, depending on the signal that is received. The mode selecting signals FMS, SMS determine the operational mode of the display panel  300 . The display panel driving section  400  receives the mode selecting signals FMS or SMS and outputs a first driving signal FDS and a second driving signal SDS in response to the first and second mode selecting signals FMS and SMS, respectively. The display panel  300  displays images according to the driving signal FDS/SDS that is received.  
      The operational modes of the display panel  300  are the transmissive mode and the reflective mode. In the transmissive mode, the primary light source is the backlight assembly  100 . Images are displayed in a transmissive area TA (see  FIG. 3 ) by using the backlight L 1  that passes through the display panel  300 . The signal transmitting section  600  outputs the third sensing signal SS 3  when the photocurrent is smaller than the reference value, for example when the level of ambient light L 2  is low. In response to the third sensing signal SS 3 , the mode converting section  700  outputs the first mode selecting signal FMS to select the transmissive mode.  
      In the reflective mode, the primary light source is ambient light and images are displayed in a reflective area RA (refer to  FIG. 3 ) by using the ambient light. The signal transmitting section  600  outputs the fourth sensing signal SS 4  when the photocurrent is greater than the reference value, for example when there is a lot of ambient light. In response to the fourth sensing signal SS 4 , the mode converting section  700  outputs the second mode selecting signal SMS to select the reflective mode. The display panel driving section  400 , which receives the signals output by the mode converting section  700 , operates the display panel  300  in the transmissive mode or reflective mode depending on whether the received signal is the first mode selecting signal FMS or second mode selecting signal SMS.  
       FIG. 5  is a graph of transmittance (TG) as a function of the transmissive voltage that is applied to the transparent electrode  312  (see  FIG. 3 ) through the TFT  311 . The graph also shows the reflectance (RG) when the reflective voltage is applied to the reflective electrode  313  through the TFT  311 .  
      As  FIG. 5  shows, when a voltage of about 4.2 volts is applied to the liquid crystal layer  330  (see  FIG. 3 ) in the transmissive area TA, the display apparatus  1000  has a maximum transmittance of about 40%. When a voltage of about 2.6 volts is applied to the liquid crystal layer  330  in the reflective area RA (see  FIG. 3 ), the display apparatus  1000  has a maximum reflectance of about 38%. As illustrated, the applied voltage for achieving the maximum transmittance is different from the applied voltage for achieving the maximum reflectance. Thus, different voltages may be applied to the TFT  311  in the transmissive mode, and the reflective voltage may be applied to the TFT  311  in the reflective mode. In one embodiment, the transmissive voltage is about 4.2V and the reflective voltage is about 2.6V. By applying different voltages to the transmissive area TA and the reflective area RA, the display apparatus  1000 / 1100  operates at maximum transmittance and maximum reflectance.  
       FIG. 6  is a block diagram of a display panel driving section  400  shown in  FIG. 1 . In addition to the gate driving section  410  and the data driving section  420  shown in  FIG. 2 , the display panel driving section  400  includes a first gamma circuit section  430 , a second gamma circuit section  440 , a first common voltage generating section  450 , and a second common voltage generating section  460 .  
       FIGS. 7A and 7B  are circuit diagrams of the first and second gamma circuit sections  430 , 440  shown in  FIG. 6 .  
      As shown in  FIG. 7A , the first gamma circuit section  430  includes eight resistors, RT 1  to RT 8 , for the transmissive mode connected to each other in series. The eight resistors RT 1  to RT 8  have resistances suitable for optimizing the transmittance of the transmissive mode as shown in  FIG. 5 .  
      Upon receiving the first mode selecting signal FMS from the mode converting section  700 , the first gamma circuit section  430  outputs the electrical potentials of the eight connection nodes as gamma voltages TGM 1  to TGM 8  for the transmissive mode. The gamma voltages TGM 1  to TGM 8  are provided to a gray-scale resistor section  421  (see  FIG. 8  below), which outputs a gray-scale voltage VT for the transmissive mode that corresponds to the received gamma voltages TGM 1  to TGM 8 .  
      As shown in  FIG. 7B , the second gamma circuit section  440  includes eight resistors RR 1  to RR 8  for the reflective mode that are connected to each other in series. The eight resistors RR 1  to RR 8  have resistances suitable for optimizing the reflectance of the display apparatus  1100  shown in  FIG. 5 . The resistances of resistors RR 1  to RR 8  may be different from the resistances of resistors RT 1  to RT 8 .  
       FIG. 8  is a circuit diagram showing a gray-scale resistor section  421  for a gray-scale that is built into the data driving section  420  of  FIG. 6 . The gray-scale resistor section  421  includes a plurality of resistors connected to each other in series. The number of resistors is a function of the number of gray scales. For example, when the display apparatus  1000  displays the image in 256 (2 8 ) gray scales, the gray-scale resistor section  421  includes 256 units of gray-scale resistors connected to each other.  
      The gray-scale resistor section  421  includes a first terminal to which a first electrical potential (e.g., VDD) is applied and a second terminal to which a second electrical potential (e.g., ground voltage GND) is applied. The gray-scale resistor section  421  shows 256 gray-scale resistors, each of which has a connection node represented by 1 st  to 256 th  gray-scale voltages VG 0  to VG 255 . Each connection node for the gray-scale resistors has a different electrical potential from the other connection nodes.  
      The second gamma circuit section  440  outputs the electrical potentials of the connection nodes associated with the resistors RR 1  to RR 8 . These electrical potentials are gamma voltages for the reflective mode, RGM 1  to RGM 8 , that are generated upon receiving the second mode selecting signal SMS from the mode converting section  700 . The gamma voltages RGM 1  to RGM 8  are provided to the gray-scale resistor section  421 . In response to the gamma voltages, the gray-scale resistor section  421  outputs a reflective mode gray-scale voltage VR that corresponds to the received gamma voltage.  
      As shown in  FIG. 6 , the first common voltage generating section  450  receives a power voltage Vp from an external source (not shown). The power voltage Vp is constant. If the display panel driving section  400  receives the first mode selecting signal FMS from the mode converting section  700 , the first common voltage generating section  450  converts the power voltage Vp to a common voltage VT com  and outputs the common voltage VT com . Similarly, if the second common voltage generating section  460  receives the second mode selecting signal SMS from the mode converting section  700 , it converts the power voltage Vp to a common voltage for the reflective mode (VR com ) and outputs VR com . The first and second voltage generating sections  450 ,  460  receive the power voltage Vp constantlybut convert it to VT com  or VR com  in response to the signals FMS/SMS.  
      The gate driving section  410  outputs a gate driving voltage Vg in response to a control signal CS. The pixels that receive the gate driving voltage Vg receive signals through their data lines DL.  
      As described above, the display apparatus  1100  switches on/off the backlight assembly  100  based on the amount of the ambient light L 2 . In response to this switching of the backlight assembly  100 , the display apparatus  1100  adjusts the operating mode of the display. When the amount of ambient light L 2  is lower than the reference value, the backlight assembly  100  is turned on and the display panel  300  operates primarily in the transmissive mode. On the other hand, when the amount of ambient light L 2  is higher than the reference value, the backlight assembly  100  is switched off and the display panel  300  operates primarily in the reflective mode.  
       FIGS. 9, 10 , and  11  are cross-sectional views of display apparatuses  1100 ,  1200 , and  1300 , which are variations of the display apparatus  1000 . In each of the embodiments, the primary light exit surface is the surface through which light is shown to leave the apparatus, as indicated by arrows.  
      The embodiment of  FIG. 9  employs the display panel  300  shown in  FIG. 3 . The display panel  300  has a primary light exit surface  300   a.  The display apparatus  1100  includes a backlight assembly  100  for generating the backlight L 1  and the display panel  300 . The backlight assembly  100  and the display panel  300  are coupled such that the display panel  300  is able to use the backlight L 1  to display images. The backlight assembly  100  includes a lamp  110  for generating the backlight L 1  and a light guiding plate  120  for guiding the backlight L 1  to the display panel  300 .  
      The “lamp  110 ,” which is also referred to herein as the “light source,” may be implemented with one or more of any well-known light source such as LED, fluorescent, phosphorescent, or incandescent light source. The light guiding plate  120  has a planar shape. The light guiding plate receives the backlight L 1  through a side surface and guides the received light to the display panel  100 . A reflecting plate  140  is disposed near the light guiding plate  120  to reflect any light that leaks from the light guiding plate  120  back toward the display panel  300 . One or more optical sheets  130  are positioned between the light guiding plate  120  and the display panel  300  to enhance the brightness of the light coming from the light guiding plate  120 . The optical sheets  130  also improve the viewing angle of the display apparatus  1100 .  
      As described above in reference to  FIG. 3 , the display panel  300  includes a first member  310 , a second member  320 , and a liquid crystal layer (not shown) disposed between the first and the second members  310  and  320 . As shown in  FIG. 3 , the first member  310  is divided into a reflective area RA and a transmissive area TA. The display panel  300  may operate in a transmissive mode or in a reflective mode, depending on whether the primary light source is backlight L 1  or ambient light L 2 . In the transmissive mode, the display panel  300  displays images by using primarily the backlight L 1  from the backlight assembly  100 . In the reflective mode, the display panel  300  displays images through the reflective area RA by using the ambient light L 2 . In embodiments that allow both transmissive and reflective modes to operate simultaneously, the primary light source may be the backlight assembly  100  any ambient light may be reflected to contribute to the brightness, or vice versa.  
      The display apparatus  1100  switches the backlight assembly  100  on or off based on the amount of the ambient light L 2 . Further, the display panel  300  switches between the transmissive mode and the reflective mode depending on whether the backlight assembly  100  is on or off. By adjusting the state of the backlight assembly  100 , the overall power consumption of the display apparatus  1100  is reduced compared to the conventional embodiments where the backlight assembly  100  has a constant state. Since the state of the backlight assembly  100  depends on the amount of ambient light L 2  that is available, this power conservation is achieved without compromising the brightness of the display apparatus  1100 .  
       FIG. 10  shows an LCD apparatus  1200  that includes the backlight assembly  100 , a transmissive display panel  301 , and a reflective/transmissive film  350  for transmitting the backlight L 1  and reflecting the ambient light L 2 . The transmissive display panel  301  has a primary light exit surface  301   a.    
      Like the display panel  300 , the display panel  301  includes a first member  310 , a second member  320 , and a liquid crystal layer (not shown) disposed between the first and second members  310  and  320 . However, unlike the transflective display panel  300 , the transmissive display panel  301  has a transparent electrode but no reflective electrode. Instead of the reflective electrode, the LCD apparatus  1200  includes the reflective/transmissive film  350 . The reflective/transmissive film  350  is disposed between the display panel  301  and the backlight assembly  100  to transmit the backlight L 1  coming from the backlight assembly  100  and reflect the ambient light L 2 . The reflective/transmissive film  350  is well known and commercially available. For example, Dual Brightness Enhancement Film (DBEF) made by 3M may be used as the reflective/transmissive film  350 .  
      When there is an insufficient amount of ambient light L 2 , the transmissive display panel  301  operates in the transmissive mode. In the transmissive mode, images are displayed with the backlight L 1  that is transmitted through the reflective/transmissive film  350 . When there is a sufficient level of ambient light L 2 , however, the display panel  301  switches to the reflective mode and the lamp  110  is turned off. Thus, the images are displayed by reflecting the ambient light L 2  with the reflective/transmissive film  350 .  
      The LCD apparatus  1200  switches the backlight assembly  100  on or off according to the amount of the ambient light L 2 . Thus, the backlight assembly  100  does not stay turned on and power is conserved. At the same time, since the backlight assembly  100  turns on to supplement the ambient light L 2  when the amount of ambient light L 2  is insufficient, the desired level of brightness can be achieved for the LCD apparatus  1200  regardless of the amount of ambient light L 2 .  
       FIG. 11  shows an LCD apparatus  1300  that includes a backlight assembly  102  for generating the backlight L 1  and a reflective display panel  302  for displaying images. The reflective display panel  302  has a primary light exit surface  302   a.  Like the display panels  300  and  301  described above, the display panel  302  may display images by using either the backlight L 1  or the ambient light L 2 . Unlike the display panels  300  and  301 , however, the reflective display panel  302  has only a reflective electrode and no transparent electrode. Thus, the display panel  302  operates in a reflective mode regardless of whether the light is the ambient light L 2  or the backlight L 1 .  
      In contrast to the LCD apparatuses  1100  and  1200 , where the backlight assembly  120  is located on the side of the display panel  300 / 301  that does not include the primary light exit surface  300   a / 301   a,  the backlight assembly  102  is positioned on the side of the display panel  302  that includes the primary light exit surface  302   a.  Although the light sensing section  500  is continuously sensing the amount of ambient light, the voltage of the backlight assembly  100  is not continuously adjusted. The backlight assembly  101  is switched on only when the amount of the ambient light L 2  falls below a predetermined level. As explained above in reference to  FIGS. 1 and 4 , the amount of ambient light L 2  dropping below a predetermined level causes the photocurrent value to become lower than a reference value. When the photocurrent value is lower than the reference value, the backlight assembly  101  is switched on. The backlight assembly  102  turning on achieves a desired brightness level for the display panel  302 . The backlight assembly  102  is turned off when the amount of ambient light L 2  is higher than the reference value.  
      When measuring the amount of ambient light L 2 , the amount of backlight L 1  emitted from the backlight assembly  102  is taken into consideration. In an embodiment where a light sensing section (not shown) that senses the amount of ambient light L 2  is built into the display panel  302 , the light sensing section receives the backlight L 1  with the ambient light L 2 . The light sensing section subtracts the amount of backlight L 1  from the total amount of light sensed by the light sensing section to determine the amount of the ambient light L 2 . The amount of backlight L 1  is predetermined.  
      In summary, the sensing section outputs the sensing signal in response to the amount of the ambient light that is available to the display panel. The backlight driving section turns on or turns off the backlight assembly that provides the backlight to the display panel in response to the sensing signal.  
      Accordingly, when the amount of ambient light is greater than a predetermined amount, the display panel displays images by using the ambient light and the backlight assembly is turned off. On the other hand, when the amount of the ambient light is less than the amount corresponding to the reference value, the display panel displays images using the backlight that is provided by the backlight assembly. Since the backlight assembly does not remain turned on, the LCD apparatus can operate with a lower power consumption.  
      Although the exemplary embodiments of the present invention have been described, it is understood that the present invention should not be limited to these exemplary embodiments but various changes and modifications can be made by one ordinary skilled in the art within the spirit and scope of the present invention as hereinafter claimed.