Patent Publication Number: US-2006007220-A1

Title: Light emitting device with adaptive intensity control

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to displays, and in particular to light emitting devices with adaptive intensity control.  
     BACKGROUND  
      Socially and professionally, most people rely upon video displays in one form or another for at least a portion of their work and/or recreation. Cathode ray tubes (CRTs) have largely given way to displays composed of liquid crystal devices (LCDs) or light-emitting diodes (LEDs), as either can provide a visual image without the traditional bulk and weight associated with CRTs.  
      More specifically, as there is typically no tube, an LCD or LED display may be fabricated to be quite thin and light, providing for more portable laptop computers, video displays in vehicles and airplanes, and information displays to be mounted or set in eye catching locations.  
      A typical CRT display also requires far more power to operate than does a comparably sized LED display. For example a 14″ CRT display may require 110 watts of power whereas a 14″ LED display may require 30˜40 watts or less. Such difference in power consumption is extremely important in the field of portable devices that must operate off of a battery. In addition, such power conservation and low profile aspects are raising demand for in-home and in-office products where the savings in energy may total several hundred dollars per year.  
      A CRT operates by a scanning electron beam exciting phosphorous-based materials on the back side of the screen, wherein the intensity of each pixel is commonly tied to the intensity of the electron beam. With an LED display, each pixel is an individual light emitting device capable of generating its own light. With an LCD display, each pixel is a transient light emitting device, individually adjusted to permit light to shine through the pixel. For either device, the individual nature of each LED or LCD within the display introduces the possibility that each pixel may not provide the same quantity of light. One pixel may be brighter or darker than another, a difference that may be quite apparent to the viewer.  
      The human eye is able to perceive subtle differences in light intensity. This poses a challenge to display manufacturers. If the pixels in a display vary greatly in their light emitting ability, the display will be unacceptable to users. Generally, the light intensity of the display is controlled globally—all pixels are turned up or down to collectively brighten or dim the display.  
      With respect to an LED, the effective light output—the brightness—may be controlled by either of two methods: length of time on, and intensity when on. For example, LED #1 may operate at 100%, providing a light output of X, when the LED #1 is turned on for 5 nano-seconds. LED #2 may operate at 50%, providing a light output of X, when LED #2 is turned on for 10 nano-second. Cycling at a very fast rate, a user will likely be unaware that the two LED&#39;s are operating so differently. However, if both LED #1 and #2 are side by side in a display and the control logic of the display globally addresses all pixels with the same commands for when to turn on and off, the difference will likely be quite apparent.  
      To avoid such discrepancies in performance, great care is generally applied in the fabrication of LED and LCD displays in an attempt to insure that the pixels are as uniform and consistently alike as is possible. Frequently, especially with large displays, quality control measures discard a high percentage of displays before they are fully assembled. As such, displays are generally more expensive than they otherwise might be, as the manufacturers must recoup the costs for resources, time and precise tooling for the acceptable displays as well as the unacceptable displays.  
      Time, temperature and physical environmental conditions may adversely affect some pixels within a display while not affecting others. When and if such an event occurs, the user will more than likely find that the display is unacceptable as the intensity of the pixels is no longer uniform. Even where the pixels in the display age uniformly, a user may find that he or she must increase the contrast again and again in order to view the display. Eventually, even with the contrast fully increased, the display may appear too dark to be of relevant use.  
      Hence, there is a need for a light emitting device with adaptive intensity control that overcomes one or more of the drawbacks identified above.  
     SUMMARY  
      The present disclosure advances the art and overcomes problems articulated above by providing a light emitting device with adaptive intensity control.  
      In particular and by way of example only, according to an embodiment of the present invention, this invention provides a light emitting device with adaptive intensity control, including: an active display pixel providing a light; a photodetector optically coupled to the display pixel, the photodetector providing an electrical feedback signal in response to the light; a feedback controlled intensity controller electrically coupled to the photodetector and an electrical switch coupled to the active display pixel, the feedback controlled intensity controller further receiving an electrical reference signal.  
      In an alternative embodiment, this invention provides a light emitting display with adaptive intensity control, including: a plurality of adaptive display pixels, each including: an active display pixel providing a light; a photodetector paired with and optically coupled to the active display pixel, the photodetector providing an electrical feedback signal in response to the light; a feedback controlled intensity controller electrically coupled to the photodetector and a switch coupled to the active display pixel, the feedback controlled intensity controller further receiving an electrical reference signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram of a light emitting device with adaptive intensity control according to one embodiment;  
       FIG. 2  is a conceptual electrical diagram of a light emitting device with adaptive intensity control according to an embodiment;  
       FIG. 3  is a conceptual electrical diagram of a light emitting device with adaptive intensity control according to yet another embodiment;  
       FIG. 4  is a partial side view of an embodiment of a light emitting device with adaptive intensity control;  
       FIG. 5  is a block diagram of a light emitting device with adaptive intensity control according to an alternative embodiment;  
       FIG. 6  is a chart illustrating the operation of the embodiment in  FIG. 2 ; and  
       FIG. 7  is a chart illustrating the operation of the embodiment in  FIG. 3 . 
    
    
     DETAILED DESCRIPTION  
      Before proceeding with the detailed description, it is to be appreciated that the present teaching is by way of example, not limitation. The concepts described herein are not limited to use or application with a specific type of light emitting device. Thus, although the instrumentalities described herein are for the convenience of explanation, shown and described with respect to exemplary embodiments, it will be appreciated that the principals herein may be equally applied in other types of light emitting devices.  
      Referring now to the drawings, and more specifically to  FIG. 1  and  FIG. 5  there is shown a portion of a block diagram for a light emitting device with adaptive intensity control (LEDAIC)  100 , according to one embodiment. More specifically, LEDAIC  100  includes an active display pixel  102 , a photodetector  104  and a feedback controlled intensity controller  106 .  
      The active display pixel  102  provides light  108 , represented as arrows in  FIGS. 1 and 5 . More specifically, active display pixel  102  may either be a light generating pixel such as a light emitting diode  130 , shown in  FIG. 1 , or a light permitting pixel such as a liquid crystal diode  132 , shown in  FIG. 5 .  
      With respect to  FIG. 1 , photodetector  104  is optically coupled to active display pixel  102 , as indicated by large arrow  122  extending from active display pixel  102  to photodetector  104 . In other words, photodetector  104  receives light  108  directly from active display pixel  102 . In at least one embodiment photodetector  104  may be physically coupled to active display pixel  102 . The photodetector  104  provides an electrical feedback signal  110  in the form of a voltage feedback in response to the received light  108 .  
      The feedback controlled intensity controller  106  is electrically coupled to photodetector  104  and to an electrical switch  112 . The electrical switch  112  is coupled to active display pixel  102  and electrically connects active display pixel  102  to a power source  114 . A reference electrical signal  116 , such as a reference voltage, is also provided to feedback controlled intensity controller  106 . This reference electrical signal  116  is used to set the intensity of LEDAIC  100 . In at least one embodiment, this reference electrical signal  116  is pre-defined. Under appropriate circumstance, this reference electrical signal  116  may be user adjustable.  
      In at least one embodiment, a light restricting device  118 , such as an aperture, may be placed between active display pixel  102  and photodetector  104 . Employing a light restricting device  118  may be desired in certain embodiments wherein it is desirable to have photodetector  104  exposed to less than the full intensity of light  108  provided by active display pixel  102 .  
      To further assist with the direct control of intensity of the active display pixel  102 , photodetector  104  is shielded from external light, i.e., light not generated by active display pixel  102 , or provided by active display pixel  102 . Such shielding may be provided by design and placement of the photodetector  104  with respect to the active display pixel  102 , and/or by providing a physical structure that serves as a external light shield, such as shielding  120 , shown in  FIG. 1 .  
      Shielding  120  serves to shield components from external light, and is represented as a dotted line in  FIG. 1 . As an active display pixel  102  may be substantially larger than photodetector  104  and feedback controlled intensity controller  106 , shielding  120  may shield all relevant components or simply the pixel  102  and photodetector  104 .  
       FIG. 2  provides a conceptual electrical schematic of LEDAIC  100  including an active display pixel  102 , a photodetector  104  and a feedback controlled intensity controller  106 . To assist with this discussion, specific elements of this schematic have been set apart by dotted boxes, specifically a display pixel  200 , a photodetector  104 , and a feedback controlled control circuit  204 . V_d is the VDD supply source for photodetector  104  and the feedback controlled control circuit  204   
      In this conceptual electrical schematic, active display pixel  102  is depicted as light emitting diode (LED)  210 . The LED  210  is electrically coupled to power source  212  by conductive line  208  running through switch S 2 , illustrated as switch  216 . When switch S 2   216  is closed, power is provided to LED  210  and light  108 , shown as arrows in  FIG. 2 , is provided.  
      In at least one embodiment, photodetector  104  is a CMOS active pixel sensor  220 , also referred to as an APS. A typical CMOS active pixel sensor  220  is understood and appreciated to consist of a photosensitive diode, biased by a power supply, a capacitor functioning as an integrator, a transistor switch to discharge and set the initial conditions on the capacitor, and a transistor that acts as a source follower. As further described below and illustrated in  FIG. 4 , when light  108  is incident upon active pixel sensor  220 , active pixel sensor  220  will provide an electrical output, such as for example V_i.  
      The feedback controlled intensity controller  106  is composed of several components, namely, in at least one embodiment, an integration capacitor  230  electrically coupled to both CMOS active pixel sensor  220  and a comparator  222 . A reference signal, such as V_ref, is provided to comparator  222 . This reference signal V_ref is used to externally set the intensity of LEDAIC  100 . A reset switch S 1 , illustrated as switch  224 , is also provided to discharge the integration capacitor  230  and reset control circuit  204 . A Bias signal is also provided as an external control signal that is used to set the sensitivity of comparator  222 .  
      The advantageous autonomous control of LEDAIC  100  is achieved as follows. Light  108  emitted by LED  210  is received by CMOS active pixel sensor  220  and converted from a light sensitive photo current to a voltage, V_i by integrating the photo current over a display time interval. This V_i is then compared to a reference voltage V_ref, by comparator  222 . V_ref is an analog signal provided by an external control circuit (not shown) to control light  108  emitted from display pixel  102  to a predetermined amount. When the amount of emitted light  108  generates a V_i equal to V_ref, comparator  222  turns off LED  210  by opening switch S 2   216 . The opening of switch S 2   216  is accomplished by sending signal V_b through conductive line  218 .  
      Stated another way, the feedback controlled intensity controller  106  is operable to open electrical switch S 2   216  when electrical feedback signal V_i is equal to or greater than the electrical reference signal, V_ref. The feedback controlled intensity controller  106  is further operable to close electrical switch S 2   216  when the electrical feedback signal V_i is less than the electrical reference signal V_ref.  
      Moreover, the rate at which integration capacitor  230  is charged is fully dependent upon the intensity of light  108  provided by display pixel  200  to photodetector  104 . In other words, LEDAIC  100  is converting the intensity of light  108  into a duration of time. The amount of light  108  perceived by a user observing a LEDAIC  100  is dependent upon both the intensity of light  108  and the duration of the light  108 . A high current through LED  210  for a short duration or a low current through LED  210  for a long duration can yield the same user-perceived intensity of light  108 .  
       FIG. 6  provides a set of graphs illustrating the lifecycle of feedback signal V_i as it is related to switches S 1   224  and S 2   216  as well as signals V_b shown in  FIG. 2 . As shown, at time value 0, V_i is substantially zero. Switch S 1   224  is activated to reset the LEDAIC  100  and correspondingly switch S 2   216  is turned off. At time value X, Switch S 1   224  is turned off and switch S 2   216  is turned on. As a result, power is supplied to LED  210 , which in turn provides light  108  that is incident upon photodetector  104  (such as active pixel sensor  220 ). As a result of this incident light, the voltage on integration capacitor  230  is ramped up to V_i. When V_i =V_ref, comparator  222  opens switch S 2   216  with a pulse. More specifically, V_a is an internal signal of comparator  222 . The lower transistors of the comparator  222  form a current mirror load circuit which develops a large voltage swing on node V_b depending on the relative value of the gate voltages V_i and V_ref on the source followers connected to the current mirror load circuit.  
      Typically, in operation, the light emissive device such as LED  210 , is cycled repeatedly, and/or connected to a refresh circuit. In addition, the period of the cycle is generally so fast that LED  210  is perceived as a substantially steady light source and not a blinking one.  
       FIG. 3  illustrates an alternative embodiment for LEDAIC  100  further providing a logical gate, such as for example, a logic NOR gate  300 . The logic NOR gate  300  is coupled to switch S 2   216 . The logic NOR gate  300  is controlled by feedback signal V_b from comparator  222  on NOR terminal B, and by a control signal V_reset. V_reset is also provided and coupled to switch S 1   224  and logic NOR gate  300 . The logic NOR gate  300  is provided to further improve both performance and design complexity such as, for example when a plurality of LEDAIC  100  devices are used in a large display.  
      The control signal for switch S 2  is adaptively generated from both an external signal that initiates the display cycle (V_reset), and an internal feedback signal (V_b). Specifically, design efficiency is improved by integrating a low transistor count, logic NOR gate  300  into LEDAIC  100  and generating a control signal for display pixel  102  from an external control signal V_reset, and an internal feedback signal V_b.  
      This method of control advantageously simplifies and improves the adaptive intensity control of display pixel  102  individually, and the plurality of LEDAIC  100  devices in a display. This improvement is achieved by turning on all display pixels  102  in a selected group (the entire display or a specific sub-group) and causing individual LEDAIC  100  devices to turn off when an amount of emitted light is equivelant to a threshold specified by an analog voltage (V_ref) externally supplied to each LEDAIC  100 .  
      The operational characteristics of LEDAIC  100  (specifically the condition of switch S 2  as open or closed), as the signals provided to logic NOR gate  300  terminals A and B determine the signal provided to NOR terminal C controlling switch S 2 , are shown in the following table.  
                                       A   B   C                  0   0   1       0   1   0       1   0   0       1   1   0                  
 
      The logic NOR gate  300  is an effective control element that combines the integrator reset switch S 1  with the control signal V_reset to turn on display pixel  102  and initiate the intensity control circuit  204 . When V_reset is high, S 1  is on and the voltage on capacitor  230  is held at ground. The output of logic NOR gate  300  is held low so that switch S 2  is off and display pixel  102  is off. The output of comparator  222 , specifically V_b, is also held low (V_i&lt;V_ref). When V_reset is switched low, switch S 1  is opened and the output of logic NOR gate  300  will go high, and turn on switch S 2 , thus causing display pixel  102  to emit or pass light  108 . This relationship for this condition is V_i&lt;V_ref causing V_b to be made low.  
      Light  108  from display pixel  102  passing through light restricting device  118  causes a photo current to ramp up the voltage in the integration capacitor for a display time interval until V_i&gt;V_ref. When V_i&gt;V_ref, comparator  222  switches so that V_b goes from a low potential to a potential greater than the switch threshold of logic NOR gate  300 . This switch causes the output of logic NOR gate  300  to go from high to low. When the output of logic NOR gate  300  switches from high to low, switch S 2  turns off and display pixel  102  is turned off completing the display cycle.  
      Similar to  FIG. 6 ,  FIG. 7  provides a set of graphs illustrating the lifecycle of feedback signal V_i as it is related to the signals V_reset, V_G 2  and V_b shown in  FIG. 3 . Specifically,  FIG. 7  illustrates V_ref as it may be applied to a dark pixel {V_ref(d)} and a lighter pixel {V_ref(l)}. As the chart demonstrates, less charge is needed from the photo diode for a darker pixel than for a lighter pixel.  
      For a given value of a gray scale, or brightness value for a color, it will take the low intensity LEDAIC  100 , i.e., a “Cold” pixel, a longer time for integration capacitor  230  to develop a charge equal to the supplied V_ref than a high intensity LEDAIC  100 , i.e., a “Hot” pixel. With respect to  FIG. 7 , Hl=Hot(light pixel), Hd =Hot(dark pixel), Cl=Cold(light pixel) and Cd=Cold(dark pixel). Specifically,  FIG. 7  demonstrates how a Hot and Cold pixel will control switch S 2   216  gate potential V_G 2 . The illustrated time sequences are as follows: 
          thd=active display time for a Hot pixel and a darker display     thl=active display time for a Hot pixel and a light display     tcd=active display time for a Cold pixel and a darker display     tcl=active display time for a Cold pixel and a light display        

      In a typical visual display, thousands of pixels are provided, working in concert to present visual information to the user. Typically, the resolution of the display is provided with direct reference to the number of pixels provided, for example, common resolutions include 640×480, 800×600, 1024×768 and 1600×1200. A higher resolution display can usually operate in a backward compatible mode to display lower resolution images.  
      With a 14″ display screen, a 1600×1200 pixel resolution yields approximately 20,000 pixels per square inch. Though this number may appear large, contemporary submicron-technology manufacturing processes permit the fabrication of diode structures, such as photosensitive diodes, measured on a nano-meter scale. More specifically, whereas a single horizontal inch may generally include approximately 142 display pixels, a single horizontal inch may easily include several thousand photosensitive diodes.  
       FIG. 4  conceptually illustrates an actual LEDAIC  100  as an autonomous device. A plurality of LEDAICs  100  are preferably used to provide a full light emitting display with adaptive intensity control. In such a display, as described above, each photodetector  104  is optically coupled to an active display pixel  102 , the two forming a matched pair.  
      With respect to LEDAIC  100  illustrated in  FIG. 4 , stated simply, LED  210  is a simplistic type of semiconductor device. Generally speaking, a diode is created by layering two different conductive materials (such as Silicon, Aluminum, Gallium or other appropriate material) together in a specific way. In pure form, the atoms of these materials will bond perfectly, leaving no free electrons to conduct current. By doping, the addition of impurities adds additional atoms that change the balance, either adding free electrons or creating electron holes—locations where electrons can go. Doping to add electrons produces materials that are known as N-type. Doping to add holes produces materials that are P-type.  
      The LEDAIC  100  shown in  FIG. 4  includes a light emitting diode (LED)  400  having a layer of N-type material  402  coupled to a section of P-type material  404 , with electrodes  406 ,  408  attached to each section respectively. When LED  400  is at rest, with no applied charge, electrons and holes migrate and balance along junction  420  between the first and second layers  416 ,  418 , forming a depletion zone. By applying a positive current to the P-type section (P-type material  404 ) and a negative charge to the N-type section (N-type material  402 ), a charge will move across the diode.  
      The functional properties of a semiconductor, such as an LED  400 , result in part from providing electrons in different energy states separated by bands, or gaps, of no energy states. The highest occupied band is a valence band and the lowest unoccupied band is a conduction band, with a gap existing in between. As used, the terms “highest” and “lowest” refer to energy levels and not physical vertical separation. Visible light emitting diodes are made of materials providing wide gaps between the valance band and the conduction band. As an electron moves from a high band to a lower band, it releases energy in the form of photons. The size of the gap determines the frequency of the photon, and consequently, the color of the light produced.  
      As is conceptually illustrated, light emitting diode  400  is substantially larger than photodetector  104  and feedback controlled intensity controller  106 . A simplified illustration of photodetector  104  is shown as an enlargement  452 , bounded by a dotted line. As such, light emitting diode  400 , photodetector  104  and feedback controlled intensity controller  106  are all housed within a protective housing  450  of the LEDAIC  100 . Conventional semiconductor fabrication techniques permit the fabrication of light emitting diode  400 , photodetector  104  and potentially feedback controlled intensity controller  106  collectively and upon the same substrate material to be later placed within protective housing  450 .  
      As stated above, photodetector  104 , such as CMOS active pixel sensor  220 , includes a photosensitive diode  410 . More specifically, photosensitive diode  410  is a diode that provides electron hole pairs (e− h+) when light photons  412  are incident upon surface  414  of diode  410 . The photodetector  104  may be disposed below light emitting diode  400 , as shown, or adjacent to light emitting diode  400 . In addition, a light restricting device  454 , such as an aperture, may be disposed between photosensitive diode  410  and light emitting diode  400  to restrict the amount of light  108  incident upon photosensitive diode  410 . Moreover, to insure proper feedback control over light emitting diode  400 , photodetector  104  is positioned so as to receive light  108  only from its paired active light emitting diode  400 .  
      Most commonly, photosensitive diode  410  provides a built-in field for separating charged carriers, such as a PN junction, PIN junction, Schottky barrier device or other type of “electronic valve” device as known in the art. Internally, at least two layers are provided. A first layer  416  with a first electrical connectivity, such as a P-type layer, and a second layer  418  with a second electrical connectivity, such as a layer of N-type material  402 , physically coupled to the first layer  416 . The electrical connectivity of each layer  402  and  416  is determined by factors such as differences in carrier concentrations, carrier types, and or band structures. The coupled area provides an interface, also know as a junction  420 .  
      Light  108  from LED  210  is incident upon outer surface  414  of active pixel sensor  220 . Light photons  412  excite electron hole pairs, otherwise known as charged carriers. Some fraction of the generated carriers of one sign (either electrons or holes) will be swept across junction  420 .  
      Depending upon the configuration of photodetector  104 , the movement of the carriers will result in either an electric potential, such as a voltage potential, or an active current, either of which is detected by a simple control circuit  422  and provided as electrical feedback output to the feedback controlled intensity controller  106  via feedback conductor  424 . In at least one embodiment, CMOS active pixel sensor  220  provides a voltage potential in response to the incidence of light  108 .  
      With respect to  FIG. 4 , it is appreciated that a plurality of LEDAICs  100  operating collectively can and will provide an advantageous light emitting display. As described above, each LEDAIC  100  is capable of autonomous operation to provide a consistent and pre-determined intensity of light output based on a provided reference signal, V_ref. As such, the operational characteristics from one LEDAIC  100  to another may vary.  
      More specifically, the fabrication tolerances may be somewhat relaxed as each LEDAIC  100  within the display will advantageously self adjust. In addition, the longevity of the display incorporating a plurality of LEDAICs  100  will likely be improved as each LEDAIC  100  can and will self adjust due to age and environmental factors, which may or may not affect the display in its entirety.  
      In addition, as may be appreciated in  FIG. 4 , in at least one embodiment, the reference signal, V_ref is provided to the feedback controlled intensity controller  106  from outside the physical structure of the LEDAIC  100 . As such, the same V_ref may be provided to the plurality of LEDAICs  100 , comprising a display. Providing the plurality of LEDAICs  100  with the same V_ref advantageously insures that all of the LEDAICs  100  are self comparing to the same reference threshold, thus further insuring the uniform intensity of light throughout the display. In at least one embodiment, the value of V_ref is predetermined. Under appropriate circumstances, such as where a user is permitted to adjust the intensity of the display, the value of V_ref may be user adjustable.  
      The above embodiments have involved the use of an active display pixel such as an LED  210 , a device which actively generates light. Substantially the same methodology and structure may be employed where LEDAIC  100  utilizes a liquid crystal display (LCD), a device actively adjusted to pass light.  
      Generally speaking, and with reference to  FIG. 5 , to create an LCD, a first and a second polarized glass plate  500 ,  502  are provided, each having microscopic groves in the surface opposite from but in line with a polarizing film. The first and second polarized glass plates  500 ,  502  are parallel to one another with the respective polarizing film of each transverse to the other. For illustrative purposes, the polarizing film and groves of first glass plate  500  run parallel to the page, such that they are represented as solid line  504 . In contrast, the polarizing film and groves of second glass plate  502  are perpendicular to the page, such that they are represented as parallel cross sections  506 .  
      Nematic liquid crystals  508  are then added between the first and second glass plates  500 ,  502 . The groves will cause the layer of molecules of liquid crystals  508  that are in contact with the grooved glass to align with the groves. As the groves of one glass are transverse to the groves of the other glass, the Nematic liquid crystal  508  will twist. In the 2-D illustration of  FIG. 5  this is represented as nematic liquid crystal  508  appearing to diminish in size as it progresses from glass plate  500  to glass plate  502 .  
      As light  108  provided by an external light source  510  strikes first glass plate  500 , it is polarized. The molecules in each layer of nematic liquid crystal  508  then guide the light  108  from layer to layer within nematic liquid crystal  508 , and in so doing, twist the light  108  to align with the groves and the polarized filter of the second glass plate  502 .  
      If an electric charge is applied to nematic liquid crystal  508 , the molecules will untwist. As nematic liquid crystal  508  straightens out, the angle of the light  108  passing through from first glass plate  500  to second glass plate  502  will also change, and the cross polarization orientation will block the passage of light  108 . By varying the degree of untwisting, the LCD utilizing nematic liquid crystal  508  can control how much of light  108  passes through, thus providing a gray scale.  
      As with the description provided above for active display pixel  102  and LED  210 , feedback signal  110  provided by photodetector  104  is compared to a reference electrical signal  116  provided by feedback controlled intensity controller  106 . Based on the evaluation of this comparison, feedback controlled intensity controller  106  opens or closes electrical switch  112 , thus causing an electric field to be applied to, or removed from, the nematic liquid crystal  508 .  
      As in the above discussion, a light restricting device  118  may be provided between photodetector  104  and LCD pixel  132 . Moreover, it is understood and appreciated that photodetector  104  is so positioned and/or shielded that it does not receive external light, i.e., light that does not come from or pass through active display pixel  102  or light emitting diode  130 .  
      Changes may be made in the above methods, systems and structures without departing from the scope hereof. It should thus be noted that the matter contained in the above description and/or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method, system and structure, which, as a matter of language, might be said to fall therebetween.