Abstract:
A photopic light sensor for controlling the apparent brightness to a human observer of a fluorescent backlight uses a broad spectrum photodiode and a daylight-filtered photodiode, the latter sensing predominantly the infrared region of the broad spectrum. By properly scaling and subtracting signals from these two photodiodes, an effective response of a photopic sensor is created which may be used in a feedback loop to control the brightness of a fluorescent tube in automotive applications without the expense or package constraints incident to an infrared blocking filter of a true photopic sensor.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to controllers for lamps used to illuminate liquid crystal displays (“backlights”) and the like and, in particular, to a backlight controller that accurately controls the apparent brightness of the backlight to the human eye. 
     Liquid Crystal Displays (LCD) provide a rugged and flexible display suitable for use in automotive applications. The LCD is backlit typically by a cold cathode fluorescent lamp (CCFL). Such fluorescent lamps are bright and relatively efficient and can be fabricated to provide even illumination over a large area. 
     Unfortunately, CCFL&#39;s are sensitive to temperature and vary in brightness as the passenger compartment and console warms up. One method of correcting for this variation is to make a temperature measurement at the CCFL and to vary its driving power to maintain constant illumination. The relationship between driving power, illumination and temperature is a complex function which may be implemented in lookup tables or algorithms incorporated into a microcontroller providing an output to the lamp. This approach is not entirely successful at low brightness levels and elevated temperatures where small changes in power, typically less than the quantization errors of the algorithms or tables, can result in greatly varying brightness. 
     A second approach to controlling the brightness of such lamps is to monitor their light output using an electronic light sensor and to use the measured output to control the power to the lamp by means of a feedback loop. A silicon photodiode may be used to measure the light from the lamp. Silicon photodiodes are inexpensive, rugged and available in a wide range of different package types including those suitable for surface mounting on a printed wiring board. Surface mount packages are smaller than packages that require sockets or holes in the printing wiring board to receive leads. Surface mounting lowers the cost of the photodiode. 
     Silicon photodiodes have sensitivity to a range of light frequencies that extend significantly into the infrared region invisible to the human observer. Unfortunately at cold temperatures, the CCFL&#39;s output a large amount of infrared radiation. For this reason, if an ordinary silicon photodiode is used to control a CCFL, the perceived brightness of the CCFL to a human observer will vary, being dimmer at cold temperatures, for example, when the output of infrared radiation is greater. 
     This problem may be solved by the use of an infrared filter, such as a special glass that absorbs the infrared portion of the light before it strikes the photodiode. Photodiodes with such filters, however, are relatively expensive and are not available in packages desired for use in the automotive field. 
     SUMMARY OF THE INVENTION 
     The present invention provides a photodetector that approximates the response of the human eye (the photopic curve) and which therefore may be used to control the perceived brightness to a human observer of a CCFL or other similar backlight. Instead of using a relatively expensive “infrared-filtered” silicon photodiode, the present inventor has recognized that common “daylight-filtered” silicon photodiodes (filtering out all but the infrared region) may be used in conjunction with an unfiltered silicon photodiode to produce the same result. The signal from the daylight-filtered photodiode is subtracted from the unfiltered photodiode to approximate the photopic curve. 
     In contrast to infrared-filtered photodiodes, daylight-filtered silicon photodiodes are widely available at low cost and a wide variety of packages for use in consumer electronics such as infrared remote controls where the daylight filtering prevents ambient light from interfering with the infrared control signals. For this reason, combining the two devices of a daylight-filtered and unfiltered silicon photodiode may produce a photopic detector of lower cost and smaller package size than a single infrared-filtered silicon photodiode. 
     Specifically then, the present invention provides an illumination control for a fluorescent backlight having a first light sensor producing a first electronic signal dependent on sensed light in a first and second frequency range of sensitivity where the first frequency range of sensitivity corresponds substantially to light frequencies that the human eye sees and the second range of sensitivity corresponds substantially to light frequencies that the human eye does not see. A second light sensor is also used producing a second electronic signal dependent on sensed light in a third frequency range of sensitivity substantially overlapping the second frequency range but not the first frequency range. The fluorescent backlight illuminates the first and second light sensors by an amount dependent on a drive signal from a fluorescent driver circuit, which varies the drive signal to the fluorescent bulb according to a feedback control signal input. A subtractor subtracts the second electronic signal from the first electronic signal to produce the feedback control signal. 
     Thus it is one object of the invention to provide a light sensor that effectively conforms to the photopic curve without the need for expensive filters or a limited selection of device packages. By independently sensing light that is invisible and visible to the human eye and subtracting one from the other, a feedback control signal related to the desired response curve may be obtained. 
     The first and second light sensors may be silicon photodiodes and the second light sensor may include a filter blocking light in the first range. 
     Thus it is another object of the invention to make use of commonly available silicon photodiodes having similar or identical electrical characteristics. A filter on one photodiode provides the necessary frequency discrimination. It is another object of the invention to make use of commercially available daylight-filtered photodiodes widely used in consumer electronics. 
     The fluorescent driver circuit may also receive a desired brightness signal and may also control the drive signal of the fluorescent bulb according to the desired brightness signal. 
     Thus it is another object of the invention to provide a control that may accept an independent brightness signal allowing the user to change the brightness of the fluorescent tube. 
     The fluorescent bulb may be mounted against a first face of a printed wiring board and the first and second detectors may be mounted on the first face of the printed wiring board between the fluorescent bulb and the printed wiring board to receive light directly from the back of the fluorescent bulb. Alternatively, the printed wiring board may include at least one aperture and the first and second detectors may be mounted on a second obverse face of the printed wiring board over the aperture to receive light from the fluorescent bulb through the aperture. 
     Thus it is another object of the invention to provide a rugged and low cost assembly for supporting and controlling the fluorescent bulb. 
     A scaling circuit may be used to scale at least one of the first and second electronic signals prior to receipt by the subtractor. 
     Thus it is another object of the invention to provide for corrections of errors caused by filter absorption, small differences between the electrical characteristics of the first and second light sensors, and other dependencies such as temperature dependency, through the use of a scaling circuit that may be adjusted for the particular devices. 
    
    
     The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration a preferred embodiment of the invention. Such embodiment does not necessary represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective, exploded view of an automotive control console showing an LCD positioned behind a bezel of the control console and in front of a fluorescent lamp, the latter of which is supported by a printed wiring board holding control circuitry of the present invention; 
     FIG. 2 is a simplified block diagram of the control circuitry and fluorescent lamp of FIG. 1; 
     FIG. 3 is a simplified representation of the photopic curve plotting perceived brightness against light frequency and indicating sensitivity of the human to different frequencies of light; 
     FIG. 4 is a figure similar to that of FIG. 3 showing the sensitivity of a commercial silicon photodiode with and without daylight-filtering and showing an effective sensitivity produced by subtracting daylight-filtered and unfiltered photodiode signals from each other per the present invention to realize a curve similar to that of FIG. 3; 
     FIG. 5 is a detailed schematic of a circuit to implement the blocks of FIG. 2; 
     FIG. 6 is a cross-section through the assembly of FIG. 1 taken along lines  6 — 6  showing a first method of mounting the photodetectors to a printed circuit board adjacent to the fluorescent display; and 
     FIG. 7 is a figure similar to that of FIG. 6 showing a second method of mounting the photodetectors to the printed wiring board. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, an automotive console  10  includes a bezel  12  supporting user controls  14  and a display opening  16 . Positioned behind the display opening  16  is a Liquid Crystal Display (“LCD”)  18  followed by a fluorescent backlight  20 . The fluorescent backlight  20  surrounds a light pipe  21  to provide a large area, even illumination commensurate with the area of the LCD  18 . The backlight  20  provides light passing through the LCD  18  so as to make figures displayed on the LCD  18  visible through the opening  16  to a driver or passenger for all lighting conditions ranging from fill sunlight to conditions of low ambient light. 
     A circuit card  22  may be positioned behind the backlight  20  to support control electronics of the present invention as well as the necessary control electronics for the LCD  18 . 
     Referring now also to FIG. 2, the backlight  20  may connected to a driver  21  typically including an inverter converting a source of direct current from the automotive battery (not shown) to an alternating current (“AC”) drive waveform whose power may be controlled by one or more inputs  24  to the driver  21 . Typically the inputs  24  include a brightness input  26  providing means for a user or automatic control to vary the brightness of the backlight  20  and optionally other control inputs  28  such as may adjust the fluorescent backlight RMS current level  23  according to other conditions such as ambient illumination and/or bulb temperature. A feedback control signal  30  is also received by the driver  21  per the present invention as will be described below. Driver  21  may use Pulse Width Modulation (PWM) techniques to control the brightness of the fluorescent backlight  20 . 
     Referring now to FIGS. 2,  3  and  4 , a first and second silicon photodiode  32  and  34  are positioned on the circuit card  22  so as to receive light  36  from the fluorescent backlight  20 . The photodiodes  32  and  34  are preferably silicon photodiodes operated in a photo-amperic mode as will be described below. The photodiodes  32  and  34  are preferably electrically similar, however, photodiode  34  is daylight-filtered by the incorporation of a filter layer  38  between the photodiode  34  and the fluorescent backlight  20  such as admits substantially only infrared light  40  but blocks visible light  42 . Such daylight-filtered photodiodes  34  are used as receptors for infrared communications in a wide variety of consumer electronics. In these applications, daylight sensitivity is not desired because daylight or other household illumination tends to saturate the photodiode blinding it to the weaker infrared signal that it was intended to receive. 
     As shown in FIG. 4, the response for the silicon photodiode  32  without the filter is a broad spectrum indicated by curve  44  extending substantially from 400 nanometers to 1,100 nanometers in frequency range of sensitivity. In contrast as shown in FIG. 3, the photopic curve  46  indicating the sensitivity of the human eye extends approximately from 380 nanometers to 750 nanometers, the range beyond 750 nanometers being generally the infrared region. 
     The filter layer  38  when used with the photodiode  34  creates a response that is largely sensitive to the infrared region only as indicated by curve  48  extending from approximately 750 nanometers to 1,100 nanometers. 
     Referring now to FIGS. 2 and 4, a signal  50  from the photodiode  34  is received by a scaling circuit  52 , whose purpose will be described, which provides multiplication of that signal by a fixed scaling factor  54 . The scaled signal  50 ′ is then received by a summing circuit  56  and subtracted from a signal  58  from the photodiode  32 . As shown in FIG. 4, this subtraction yields the feedback control signal  30  approximating the photopic curve  46 . This feedback control signal  30  is provided to the driver  21  and serves to increase the drive signal  23  with decreases in the feedback control signal  30 . 
     To the extent that the sensitivity of the photodiodes  34  and  32  are identical and the sensitivity of the photodiode  34  precisely indicates the amount of invisible infrared light  40  received from the backlight  20 , the range of the photopic curve is well approximated. Because the shape of the curves  44  and  48  outside of the spectrum emitted by the backlight  20  is immaterial, neither curve must be precisely related to the other provided they overlap in the appropriate infrared region. 
     The signal from photodiode  34  may be somewhat less than photodiodes  32  as a result of the absorption of the filter layer  38  in the visible spectrum or as a result of different degrees of temperature sensitivity caused by different manufacturers and variations in the manufacturing process. The scaling of the scaling circuit  52  by the scaling factor  54  is used to null-out such differences. 
     Referring now to FIG. 5, circuitry for implementation of the blocks of FIG. 2 may include a first and second operational amplifier  60  and  62 , the former associated with photodiode  34  and the latter associated with photodiode  32 . The photodiodes  34  and  32  in each case are connected directly across the inverting and non-inverting inputs of the respective operational amplifier  60  and  62  with their cathodes facing the inverting input. 
     Feedback resistors R 1  for operational amplifier  62 , and R 2  for operational amplifier  60  extending from the output of the respective operational amplifiers to its inverting input in normal inverting mode and provide by their ratio, a relative scaling between signals  50  and  58  produced by those operational amplifiers represented by scaling circuit  52  of FIG. 2 described above. 
     The non-inverting inputs of the operational amplifiers  60  and  62  may be connected to a divider circuit  64  providing the necessary offset for operation of the operational amplifier with a single-sided automotive supply. 
     Outputs  58  and  50  from operational amplifier  62  and  60  are received by operational amplifier  66  operating as a summing (subtraction) circuit  56  described in FIG. 2 according to methods well known in the art to produce the feedback control signal  30 . 
     Succeeding operational amplifier  68  provides integration of the feedback control signal  30  of the summing operational amplifier  66  (which is received at the inverting input of operational amplifier  68 ). Operational amplifier  68  further receives, at its non-inverting input, input signals  24  such as may provide the commanded brightness of the backlight  20  independent of the feedback control signal  30 . Thus, the amplifier  68  controls the inverter drive level such that the feedback light level signal is equal to the commanded or desired light level signal  24 . By operating amplifier  68  in a high gain integrator configuration, the DC loop gain is extremely high thus yielding negligible error between the feedback brightness  30  and the commanded brightness  24 . The output of the operational amplifier  68  connects to an inverter  73 , which together with operational amplifier  68 , forms the driver  21  of FIG.  2 . The power supplied by the inverter  73  to the backlight  20  is a function of the output of the operational amplifier  68  either through a modification of voltage, current or duty cycle of drive signal  23 . 
     Referring now to FIGS. 2 and 6, the backlight  20  emits light both from its front surface toward the LCD  18 , and through its rear surface toward a front face of the circuit card  22 . Apertures  72  may be cut through the circuit card  22  and a rear face  74  of the circuit card, obverse to the front face adjacent to the fluorescent backlight  20 , may support the photodiodes  32  and  34  in surface mount packages attached to wiring  76  on the rear face  74  of the circuit card  22 . An insulating spacer  78  may be placed on the front side of the circuit card  22  between the backlight  20  and the circuit card  22  to provide insulation and support for circuit card  22  and may on its face, toward the fluorescent backlight  20 , include a metallic shield  80  and reflective light surface  82 . The metallic layer  80  needed to “strike” the lamp also provides electrical shielding from the noise generated by the fluorescent backlight  20  and the reflective layer  82  and the strike ground return  80  and increases the efficiency of the tube by reflecting rearward exiting light toward the LCD  18 . Each of insulating spacer  78 , metallic shield  80  and reflective light surface  82  have apertures  81  aligned with and corresponding to apertures  72 . 
     In an alternative embodiment shown in FIG. 7, the aperture  72  may be dispensed with and the photodiodes  32  and  34  may be mounted on the front face of the circuit card  22  toward the fluorescent backlight  20  in the apertures  81  in the insulating spacer  78 , metal layer  80  and reflective light surface  82 . 
     In either case as so assembled, the fluorescent backlight  20  and circuit card  22  provide a rugged integrated assembly. 
     The above description has been that of a preferred embodiment of the present invention, it will occur to those that practice the art that many modifications may be made without departing from the spirit and scope of the invention. In order to apprise the public of the various embodiments that may fall within the scope of the invention, the following claims are made. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 10 
                 automotive console 
               
               
                   
                 11 
                 scaling circuits 
               
               
                   
                 12 
                 bezel 
               
               
                   
                 14 
                 buttons 
               
               
                   
                 16 
                 window 
               
               
                   
                 18 
                 LCD 
               
               
                   
                 19 
                 frequency range 
               
               
                   
                 20 
                 backlight 
               
               
                   
                 21 
                 driver 
               
               
                   
                 22 
                 circuit card 
               
               
                   
                 23 
                 drive signal 
               
               
                   
                 24 
                 inputs 
               
               
                   
                 26 
                 brightness input 
               
               
                   
                 28 
                 other control input 
               
               
                   
                 30 
                 control signal (see also #28) 
               
               
                   
                 32 
                 photodiode 
               
               
                   
                 34 
                 photodiode 
               
               
                   
                 36 
                 light 
               
               
                   
                 38 
                 filter layer 
               
               
                   
                 40 
                 infrared light 
               
               
                   
                 42 
                 visible light 
               
               
                   
                 44 
                 broad spectrum curve 
               
               
                   
                 46 
                 photopic curve 
               
               
                   
                 48 
                 line 
               
               
                   
                 50 
                 signal 
               
               
                   
                 52 
                 scaling circuit 
               
               
                   
                 54 
                 scaling factor 
               
               
                   
                 56 
                 summing circuit 
               
               
                   
                 58 
                 signal (see also #50) 
               
               
                   
                 60 
                 operational amplifier 
               
               
                   
                 62 
                 operational amplifier (see also #60) 
               
               
                   
                 64 
                 divider circuit 
               
               
                   
                 66 
                 operational amplifier (see also #62) 
               
               
                   
                 68 
                 operational amplifier (see also #66) 
               
               
                   
                 72 
                 aperture 
               
               
                   
                 73 
                 inverter 
               
               
                   
                 74 
                 rear face 
               
               
                   
                 76 
                 wiring 
               
               
                   
                 78 
                 insulating spacer 
               
               
                   
                 80 
                 metallic shield 
               
               
                   
                 81 
                 apertures (see also #74) 
               
               
                   
                 82 
                 reflective light surface