Patent Document

TECHNICAL FIELD  
       [0001]     The present invention generally relates to backlit displays, and more particularly to light emitting diode (LED) backlit displays and backlights therefore.  
       BACKGROUND  
       [0002]     Backlights are widely used in connection with transmissive displays such as liquid crystal displays (LCDs). The most common types of backlights are fluorescent lamp backlights. While they are effective they suffer from a number of disadvantages, among which are the need for comparatively high driving voltage and the complexity or difficulty of providing dimming (variable luminescence) and user alterable color (variable chrominance). Also, in applications such as avionics systems where mechanical ruggedness is essential, the comparative fragility of fluorescent backlights can be a significant problem.  
         [0003]     Accordingly, it is desirable to provide an improved backlight, backlit display and method, especially apparatus and methods that are compensated for aging and capable of varying luminescence and chrominance. In addition, it is desirable that the backlight and backlit display be simple, rugged and reliable and not require moving shutters or other such mechanical parts. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.  
       BRIEF SUMMARY  
       [0004]     An apparatus is provided for a backlit display with variable luminance and chrominance. The apparatus comprises a backlight coupled to a liquid crystal display. The backlight is formed from i groups of LEDs of different colors, one or more sensors optically coupled to the LEDs, a controller receiving signals S i  from the sensors and drivers coupled to the controller for providing drive current to the LEDs. The sensors, controller, drivers and LEDs form a closed look feedback system that regulates the backlight luminance and chrominance to match desired values set by chrominance parameters K i  and command luminance signal L C . The combined light produced by the LEDs is directed to the transmissive LCD preferably through a diffusing layer. The sensors monitor the output S i  of each group of LEDs. The controller includes first electronic circuits for receiving S i  and chrominance parameter K i  for each group of LEDs and providing K i *S i . A second electronic circuit in the controller compares K i *S i  to the commanded luminance signal L C  to obtain L C −K i *S i , which difference is used to adjust control signal D i  to the drivers for each group of LEDs to achieve L C  with the desired chrominance set by K i . By changing K i  and L C  the chrominance and luminescence of the display may be varied.  
         [0005]     A method is provided for a backlit display with variable luminance and chrominance using i groups of LEDs of different colors and one or more sensors optically coupled to the LEDs and electrically coupled through a feedback controller and current drivers to the LEDs. The method comprises, obtaining a quantity S i  proportional to the intensity of light emitted by each group of LEDs, determining K i *S i  for each group of LEDs where K i  is a chrominance adjustment parameter for each group of LEDs, comparing K i *S i  to a luminance command signal L C  where L C  determines the overall luminance, and forming current drive control signal D i =L C −K i *S i  for each group of LEDs. In the preferred embodiment, three groups of LEDS are used, one for each primary color. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0007]      FIG. 1  is a simplified schematic side view of a backlit display according to the present invention, with a near side portion removed to illustrate the interior light paths;  
         [0008]      FIG. 2  is a simplified front view of a display backlight according to a first embodiment of the present invention;  
         [0009]      FIG. 3  shows a simplified electrical schematic block diagram of a control system for the display backlight of  FIGS. 1-2 , according to a first embodiment of the present invention;  
         [0010]      FIG. 4  is a simplified electrical schematic block diagram of the controller portion of the control system of  FIG. 3 , showing further details;  
         [0011]      FIG. 5  is a 1976 u′, v′ CIE Chromaticity Diagram wherein color variations available from an LED backlight of the present invention are illustrated;  
         [0012]     FIGS.  6 A-B are simplified schematic diagrams illustrating light sensors coupled to LED backlights according to several embodiments of the present invention; and  
         [0013]     FIGS.  7 A-B are simplified flow diagrams of the method of the present invention for providing backlight of a predetermined luminescence and chrominance, according to the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description. The abbreviations “LED” and LEDs” are used for “light emitting diode(s)” singular and plural respectively, and the abbreviation “LCD” and “LCDs” are used for “liquid crystal display(s)” singular and plural respectively. The suffixes “R”, “G” and “B” are used herein with various reference numbers to identify elements, connections and signals relating to one or the other of the three primary colors red (R), green (G) and blue (B), respectively, and the suffix “W” is similarly used in connection with elements, connections and signals relating to white (W) light. The subscript or suffix “i” is used to stand for any of the colors R, G, B or W, that is, i can take on the values R, G, B, and/or W.  
         [0015]      FIG. 1  is a simplified side view of backlit display  10  according to the present invention, with the near side portion removed to illustrate interior construction and light or light rays  12 ,  14 . Display  10  comprises backlight  16 , diffuser  18  and LCD  20  emitting patterned light signal  22  toward observer  25  according to whatever character or graphic has been sent to LCD  20 . Diffuser  18  is located between backlight  16  and LCD  20 . As is well understood in the art, LCD  20  receives electrical signals that locally alter the polarization of light passing through the LCD so that light is either emitted, for example as patterned image  22 , or not. Backlight  16  desirably contains multiple LEDs  24  that emit light or light rays  12  toward diffuser  18 . LEDs  24  preferably comprise red LEDs  24 R, green LEDs  24 G, blue LEDs  24 B and (optionally) white LEDs  24 W. Diffuser  18  scatters light rays  12  received from LEDs  24  into multiple light rays  14  that are the optical sum of the various colored (or white) light rays received from LEDs  24 . Light  14  impinges upon rear face  19  of LCD  20 . LCD  20  selectively transmits portions of light  14  to form character or graphic light pattern  22  according to the drive signals that it has received from its associated character generator (not shown). As will be subsequently explained in more detail, by varying the light output from different color LEDs  24  (e.g.,  24 R,  24 G,  24 B and/or (optionally  24 W) light  14 ,  22  which is the sum of light  12  emitted by backlight  16  can be varied in color (chrominance) and intensity (luminence). While LEDs  24  are illustrated herein as including red, green, blue and (optionally) white LEDs  24 R,  24 G,  24 B and  24 W respectively, this is merely for convenience of explanation and not intended to be limiting. Any combination of LED colors capable of collectively producing the desired color for display image  22  may be used. Display  10  also desirably includes one or more light detectors  30 ,  30 ′ mounted on or in optical proximity to backlight  16  so as to receive light emitted from one or more LEDs  24 . Inward facing surfaces  13 ,  15 ,  17  of display  10  are desirably reflective and/or reflective-diffusive rather than absorptive so as to enhance forward propagation of light  12 ,  14  toward backside  19  of LCD  20 .  
         [0016]      FIG. 2  is a simplified plan view of backlight  16  according to a preferred embodiment of the present invention, looking toward interior surface  17 . Backlight  16  is composed of N×M matrix  36  of LEDs  24 . In the example of  FIG. 2 , N=10 and M=15, but this is merely for convenience of explanation and not intended to be limiting. Any number of LEDs  24  can be used according to the resolution and size of the LCD display and desired backlight uniformity. In the example of  FIG. 2 , LEDs  24  comprise approximately (N×M)/3 red LEDs  24 R, (N×M)/3 green LEDs  24 G and (N×M)/3 blue LEDs  24 B randomly distributed in N×M matrix  36 , but this is not essential. Some white LEDs may also be included, as for example white LED  24 W. Any arrangement of LEDs  24  that provides the desired degree of light and color uniformity may be used. For example and not intended to be limiting, matrix  36  can be square, triangular, concentric circles, spiral, and so forth. Light sensors  30  are conveniently disposed over corresponding LEDs located, for example and not intended to be limiting, in the corners of the N×M LED array, but this is not essential. In the preferred embodiment, each light sensor  30  is conveniently associated with LED  24  of a particular color. For example, light sensor  30 R conveniently overlies one of red LEDs  24 R, light sensor  30 G conveniently overlies one of green LEDs  24 G, light sensor  30 B,conveniently overlies one of blue LEDs  24 B and optional light sensor  30 W conveniently overlies one of (optional) white LEDs  24 W. Thus, sensor  30 R provides signal S R  proportional to overall red light emission, sensor  30 G provides signal S G  proportional to overall green light emission, sensor  30 B provides signal S B  proportional to overall blue light emission and sensor  30 W provides signal S W  proportional to the intensity of light emitted by white LEDs  24 W. As noted earlier, inward facing surface  17  of backlight  16  as well as inward facing surfaces  13  and  15  shown in  FIG. 1  are desirably reflective or reflective-diffusive, that is, not significantly absorptive. This helps to maximize the amount of light reaching diffuser  18 . Light reflected or back-scattered from diffuser  18  is desirably redirected back toward diffuser  18 . Optional light sensor  30 ′ located within optical cavities  21  or  23  or both of display  10  can also be used. Sensor  30 ′ detects a light signal related to the combined output of all LEDs  24  of backlight  16 . Sensors  30 ,  30 ′ are described in greater detail in connection with FIGS.  6 A-B.  
         [0017]     FIGS.  3  shows simplified electrical schematic block diagram  40  of feedback control system  42  for LEDs  24  of backlight  16  according to a first embodiment of the present invention utilizing, for example, red, green and blue LEDs. As noted above, the suffixes “R”, “G”, “B” indicate elements, signals and connection related respectively to red, green, and blue emitters. Control system  42  comprises sensors  30 , controller  48  and LED drivers  44 . Driver  44 R is coupled to output  47 R of controller  48  via lead  45 R and supplies a controlled current to series-coupled red LEDs  24 R in response to control signals received from controller  48 . Driver  44 G is coupled to output  47 G of controller  48  via lead  45 G and supplies a controlled current to series-coupled green LEDs  24 G in response to control signals received from controller  48 . Driver  44 B is coupled to output  47 B of controller  48  via lead  45 B and supplies a controlled current to series coupled blue LEDs  24 B in response to control signals received from controller  48 . Sensors  30   i  are conveniently positioned within housings  32   i  so that sensor  30 R receives portion  12 R′ of red light  12 R from red LEDs  24 R, sensor  30 G receives portion  12 G′ of green light  12 G from green LEDs  24 G, and sensor  30 B receives portion  12 B′ of blue light  12 B from LEDs  24 B. Sensor  30 R is coupled via lead  31 R to input  33 R of controller  48  and provides to controller  48  a measure of red light output  12 R from red LEDs  24 R of backlight  16 . Sensor  30 G is coupled via lead  31 G to input  33 G of controller  48  and provides to controller  48  a measure of green light output  12 G from green LEDs  24 G of backlight  16 . Sensor  30 B is coupled via lead  31 B to input  33 B of controller  48  and provides to controller  48  a measure of blue light output  12 B from blue LEDs  24 B of backlight  16 . Controller  48  also has input  56  for setting the overall luminance level of backlight  16  and chrominance control inputs  58  for setting the color mix. Inputs  58  desirably include one or more separate inputs  58 R,  58 G,  58 B for varying the amount of one or more of the colors making up light  12 .  
         [0018]     The following equations describe the LED drive function provided by system  42 : 
 
 D   R   =L   C   −K   R   *S   R ,   [1]
 
 D   G   =L   C   −K   G   *S   G ,   [2]
 
 D   B   =L   C   −K   B   *S   B ,   ]3]
 
 or more generally 
 
 D   i   =L   C   −K   i   *S   i ,   [4]
 
 where i=R, B, G (or other colors) and where 
        D R =Red LED drive control signal,     D G =Green LED drive control signal,     D B =Blue LED drive control signal,     L C =Luminescence command signal,     K R =Red LED color calibration coefficient,     K G =Green LED color calibration coefficient,     K B =Blue LED color calibration coefficient,     S R =Red color sensor output,     S G =Green color sensor output, and     S B =Blue color sensor output. 
 
 These equations are implemented by system  42  of  FIG. 3 , as explained in more detail in connection with  FIG. 4 . 
       
 
         [0029]      FIG. 4  shows simplified electrical schematic block diagram  60  providing further details of controller  48  of system  42  of  FIG. 3 , according to a preferred embodiment. Controller  48  conveniently comprises three channels, one for each primary color; channel  62 R controls red LEDs  24 R, channel  62 G controls green LEDs  24 G and channel  62 B controls blue LEDs  24 B. The three channels are substantially identical and will be discussed together without use of suffixes “R”, “G” and “B”, which will be understood as applied to the individual channels. Controller  48  is a feedback controller, that is, it receives signals  35  (e.g., S i ) from sensors  30  and, (a) optionally adjusts signals  35  according to chromaticity reference signals  59  (e.g., K i ) on inputs  58  to produce chrominance adjusted feedback signals  65  (e.g., K i *S i ), and (b) compares chrominance adjusted feedback signals  65  (e.g., K i *S i ) to reference or luminance commanded signal  57  (e.g., L C ) to produce LED driver control signals  70  (e.g., D i ). Signals  70  are sent to drivers  44  thereby causing LEDs  24  to produce the color mix (chromaticity) and brightness (luminance) that will reduce the difference between chrominance adjusted feedback signals  65  and luminance signal  70  to be substantially zero. Persons of skill in the art will understand that a small offset is always present in such a differential feedback system. For convenience of explanation it is neglected here.  
         [0030]     Each channels  62  has first variable gain amplifier or level shifter  64  and second differential amplifier  66 , wherein amplifiers  64 ,  66  are series coupled between control input  33  and output  47  leading to driver  44 . First input  33  of amplifier  64  receives feedback signal  35  (e.g., Si) from corresponding photo-sensor  30 . Second input  58  of amplifier  64  receives optional chromaticity adjustment signal  59 . In a preferred embodiment signal  59  conveniently adjusts the gain of amplifier or level shifter  64 , that is, has the effect of multiplying the signal S i  received from sensor  30  by an adjustable constant K i  that may be different for each group of LEDs (each value of “i”). Thus, output signal  65  from amplifier  64  is K i *S i . Channels  62 R,  62 G,  62 B generate intermediate feedback signals  65 R,  65 G,  65 B given by K R *S R , K G *S G , K B *S B  respectively, where subscripts R, G, B identify the individual colors being handled in the present example and where K R , K G  and K B  are determined by the value of chrominance adjustment signals  59 R,  59 G,  59 B at inputs  58 R,  58 G,  58 B respectively. Individual chromaticity adjustment signals  59 R,  59 G,  59 B going to channels  62 R,  62 G,  62 B respectively, are optional and may be the same or different for each channel  62 R,  62 G,  62 B, or may be supplied to only one channel or to only two channels or to all three channels, depending upon the range of colors desired for light  12 . Signals  59  allow the color provided by backlight  16  to be varied to meet the needs of the system designer or user. This is explained more fully in connection with  FIG. 5 .  
         [0031]     Output signal  65  of first amplifier  64  is fed to first input  67  of second amplifier  66 . Second amplifier  66  has second input  69  that receives luminance command (L C ) signal  57  from external input  56  of controller  48 . In the preferred embodiment, L C  signal  57  is common to all three channels  62 R,  62 G.  62 B, but this is not essential and not intended to be limiting. Command luminance signal (CLS)  57  allows the designer or user to set the overall light output (luminance) of backlight  16  by varying the overall drive levels provided by controller  48  to drivers  44  and thence to LEDs  24 . In the preferred embodiment, changing signal  57  causes more or less current to flow through all LEDs  24 . In general, light output from an LED tracks the current through the device so that increasing the current substantially uniformly through all LEDs causes a change in luminance without a significant change in color. If LEDs of different colors have different current-luminance responses, this can be taken into account either in controller  48  or drivers  44  or both, so that signal  57  can control overall luminance without a significant change in color. Second amplifier  66  is desirably a difference amplifier that causes output  70  to increase (or decrease) until resulting adjusted feedback signal  65  (i.e., K i *S i ) appearing at input  67  of amplifier  66  substantially equals L C  signal  57  at input  69  of amplifier  66 . It will be appreciated based on the description herein, that the present invention can compensate for aging effects, so as to maintain the predetermined luminance and chrominance. This is a particular feature of the present invention.  
         [0032]      FIG. 5  shows 1976 u′, v′ CIE Chromaticity Diagram  80  illustrating the color variations available from LED backlight  16  according to the present invention. Such Chromaticity Diagrams are well known in the art and are described, for example by G. J. and D. G. Chamberlin in Color:  Its Measurement, Computation and Application,  Heyden and Sons Press Ltd, 1980, pages 60 ff. The human visible color spectrum is contained within outline  88 . Region  81  is the approximate locus of primary red (R), region  82  the approximate locus of primary green and region  83  the approximate locus of primary blue. White regions  84  is at about u′˜0.22 and v′˜0.48. Intermediate colors have other u′, v′ values. Arrows  85 ,  86 ,  87  illustrate respectively the effect on color of varying chrominance parameters K R , K G , and K B  in equations [1] through [3]. Thus, by varying K R , K G , and K B , most colors within Chromaticity Diagram  80  can be obtained. This is a particular feature of the present invention.  
         [0033]     FIGS.  6 A-B are simplified schematic diagrams  90 ,  91  illustrating light sensors  30 ,  30 ′ coupled to LED backlight  16  to measure light emission therefrom according to different embodiments of the present invention. Referring now to  FIG. 6A , diagram  90  illustrates the arrangement utilized in  FIG. 3 , where individual sensor  30   i  is mounted within housing  32   i  coupled to and desirably enclosing one of LEDs  24   i,  where i identifies the LEDs of the same color. Sensor  30   i  receives light  12   i   40  from the single LED within housing  32   i,  which is proportional to light  12   i  emitted by series connected same color LEDs  24   i.  This arrangement is simple and rugged and provides chrominance signal S i  for each color i=R, G, B, W, etc. A disadvantage is that one LED for each color is used to obtain chrominance signal S i  and therefore does not contribute to light output  12  of backlight  16  directed toward LCD  20 . Where a large number of LEDs  24  are used in backlight  16 , this is not a significant penalty.  
         [0034]     Schematic diagram  91  of  FIG. 6B  illustrates an alternate arrangement whereby sensor  30 ′ is provided in optical cavity  21  or  23  or both of display  10  (see  FIG. 1 ) and receives a portion of light  12  emitted from substantially all of LEDs  24  rather than from just one or two or three, etc. Sensor  30 ′ comprises housing  92  with opening  93  oriented so as to receive light  12  from backlight  16  if placed in cavity  21  or receive light  14  if placed in cavity  23 . For purposes of explanation and not intended to be limiting, it is assumed in the discussion that follows that sensor  30 ′ is located in cavity  23  and receives light  14 . Located within housing  92  are three sensors  94  having thereon color filters  95 . Thus, red color filter  95 R overlies sensor  94 R so that sensor  94 R provides on output  31 R, signal S R  proportional to the red content of light  14 . Similarly, green color filter  95 G overlies sensor  94 G and blue filter  95 B overlies sensor  94 B so that signals S G  and S B  appear on output leads  31 G,  31 B associated with sensors  94 G,  94 B respectively. Advantages of the arrangement of  FIG. 6B  are that all of LEDs  24  contribute to light output  12 ,  14  and only one triple-sensor pickup is needed. A potential disadvantage is that if stray light from outside display  10  is coupled into cavity  21  or  23  it may be detected by sensor  30 ′ thereby potentially causing a measuring error. However, the stray light must be significant compared to the light being emitted by backlight  16  for this to be troublesome. Therefore, the arrangement of either  FIGS. 6A  or  6 B is useful.  
         [0035]     FIGS.  7 A-B are simplified flow diagrams of method  100 ,  100 ′ of the present invention for providing backlight of a predetermined luminance and chrominance.  FIG. 7A  shows method  100  and  FIG. 7B  shows method  100 ′ which differ in detail. The same reference numbers are used for analogous steps in both method  100  and  100 ′. Method  100 ,  100 ′ begins with START  102  that desirably occurs on system power-up, that is, when display  10  is energized, or at least backlight  16  is energized. In step  104 , sensors  30 ,  30 ′ are interrogated to provide chromaticity outputs S i , where i corresponds to one or the other of the LED colors (e.g., R, B, G, W, etc.). Signals S i  are fed, for example, to inputs  33   i  of controller  48 . In step  106  of method  100 , the K i  values needed to obtain the desired backlight chrominance are set, as for example, via inputs  58   i  of controller  48  (e.g., see  FIG. 4 ). In step  108  the resulting values K i *S i  are compared with luminance command signal L C , and in step  110  LED drive control signals D i  supplied to LED drivers  44   i  (see  FIG. 4 ) are adjusted so that K i *S i  and L C  are approximately equal (abbreviated as L C −K i *S i ˜0 in FIGS.  7 A-B). Once substantial equality has been achieved so that backlight  16  is emitting the correct color and luminance, method  100  proceeds to END  112 . However, persons of skill in the art will understand based on the description herein that method  100 ,  100 ′ may continually loop back to START  102  as shown by path  113  as long as power is applied to display  10  and/or backlight  16  so as to maintain light emission therefrom at the desired intensity and color and respond to LED aging or any adjustments that may, from time to time, be made by the user. The method of  FIG. 7A  is conveniently implemented using an analog type controller such as is illustrated in  FIGS. 3-4 .  
         [0036]      FIG. 7B  differs from  FIG. 7A  in how steps  106  and  110  are carried out. Method  100 ′ of  FIG. 7B  is convenient for digital control of display  10  and backlight  16  wherein queries may be performed to determine whether the adjustable parameters L C  and K i  are currently at the desired values or not. For example, step  106  of method  100  is replaced in method  100 ′ by query  106 - 1  wherein it is determined whether or not the current values of K i  correspond to the backlight color desired by the user. If the outcome of query  106 - 1  is YES (TRUE) then the current value of K i  is used in step  108  where K i *S i  is compared to L C . If the outcome of query  106 - 1  is NO (FALSE) then method  100 ′ proceeds to step  106 - 2  where K i  is adjusted to the correct value for the color desired by the user, and this modified value of K i  is used in step  108 . Similarly, in method  100 ′ step  110  is divided into sub-steps  110 - 1 ,  110 - 2  and  110 - 3 . In step  110 - 1  the LED drive is adjusted up or down depending upon the sign of the difference obtained in COMPARE step  108 . In subsequent step  110 - 2 , sensors  30 ,  30 ′ are re-interrogated to obtain the resulting new value of Si. This value of S i  is used in query  110 - 3  to determined whether or not the condition L C −K i *S i ˜0 is now satisfied. If the outcome of query  110 - 3  is NO (FALSE) then method  100 ′ loops back as shown by path  111  until the outcome of query  110 - 3  is YES (TRUE), whereupon method  100 ′ proceeds to END  112  or loops back to START  102  as shown by optional path  113  as previously discussed. Persons of skill in the art will understand based on the description herein how to implement method  100 ,  100 ′ 
         [0037]     While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. For example, and not intended to be limiting, while  FIGS. 3 and 4  are illustrated for three colors R, G, B persons of skill in the art will understand based on the description herein that white LEDs  24 W may also be used and, for example, additional white channel  62 W provided in controller  48  responsive to sensors  30 W coupled to white LEDs  24 W. Such white channel may respond to luminance signal  57  and/or to an independent luminance signal  57 ′ coupled only to white channel  62 W, depending upon the needs of the user. While the present invention has been described in terms of using primary color LEDs, e.g., red (R), green (G) and blue (B) and, optionally, white (W), the present invention is not limited merely to LEDs of those colors. Any color LEDs can be used that are capable when their light output is combined of achieving the desired color for the display. Thus, R, G, B and W LEDs are merely preferred examples and not limitations of the present invention. Further, while three so-called primary colored groups of LEDs are used in the preferred embodiment this is not essential. Less than three groups of different colored LEDs may be used and still achieve user variable luminance although with more limited user variable chrominance, provided that the available chrominance range can achieve the desired color for the display. For example, if a particular display requires only blends of red and greed colors, there is no need to include blue LEDs, or if the display only requires white and red blends, there is no need to include green and blue LEDs since red and while LEDs are sufficient. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.

Technology Category: 3