Abstract:
A color balanced solid-state backlight provides feedback control of each color using a single photodetector by imposing a modulation pattern on the solid-state lamps revealing individual colors to the photodetector. The photodetector signal provides feedback controlling color balance over a small range of instantaneous brightness less than larger range of average brightness of the display to provide for accurate color balance throughout a large range of average brightnesses.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   BACKGROUND OF THE INVENTION 
   The present invention relates to backlights for instruments such as those using liquid crystal displays and, in particular, to a backlight suitable for avionics and providing a wide range of brightness in a color-balanced white output formed from the combination of light from multiple colored sources. 
   Graphic displays, such as those employing a liquid crystal display (“LCD”) screen provide a field of pixel elements each of which may be independently controlled to block or pass light, for example, from an underlying backlight. 
   A common backlight for use with an LCD screen provides a transparent panel edge-lit or backlit by one or more fluorescent tubes. In the edge-lit design, a reflective rear surface of the panel directs the edge illumination towards an LCD screen positioned against a front surface of the panel. The reflective rear surface of the panel may be gradated to produce an even field illumination behind the LCD compensating for an inherent falloff of brightness with distance of the fluorescent tube. 
   Fluorescent tubes provide a relatively high efficiency light source providing a broad color spectrum output suitable for backlighting color LCD screens in which pixels associated with red, green, and blue light components must be evenly illuminated for good color rendition. 
   When backlit LCD screens are used in avionics applications, a wide range of illumination output is desirable to allow the avionics display to be easily readable, both in bright sunlight and in levels of very low light and over a wide range of ambient temperatures. In low light situations, too much illumination can interfere with dark adaptation and night vision goggles or similar equipment. 
   Fluorescent tubes have a number of disadvantages in avionics applications including: the need for a high voltage power supply, a fragility of the glass tube, a tendency to fail unexpectedly, low efficiency at low ambient temperatures, and a limited ability to change brightness level. For these reasons, it is known to use light-emitting diodes (“LEDs”) as a replacement for fluorescent tubes, particularly in avionics and other demanding applications. In order to provide a multi-spectral output needed for color LCD screens, such LED backlights provide clusters of red, blue, and green LEDs. Preferably, each color of LED may be separately controlled in brightness. When these different colors of LEDs are energized together with the correct relative brightness, they produce a light that appears substantially white to the human eye. 
   The relative brightness of each of the LEDs must normally be adjusted electronically to obtain the correct color balance to provide white light. Maintaining this color balance as the backlight is varied in brightness, can be difficult because of different and often non-linear relationships between light output and current for each of the different colors of LEDs. That is, over a given range, a uniform change in current provided to the LEDs for each color will tend to cause a color shifting of the backlight. The function relating brightness to current can change with the temperature and age of the LED further complicating attempts to maintain color balance over a wide range of illumination. 
   SUMMARY OF THE INVENTION 
   The present invention provides a color-balanced LED backlight that maintains color balance over a wide range of illumination by means of a set of feedback loops, one for each color. Sensing the light output for each feedback loop requires only a single photodetector which distinguishes among colors by a “measurement modulation” of the LEDs during a first period of time, to reveal each color in isolation. For example, during this first period of time, the LED&#39;s of only one color will be energized at a time. Brightnesses of each color determined during the measurement modulation are held and used after the measurement modulation to control the LEDs when the LEDs are energized simultaneously during a second period of time. 
   This brief measurement modulation period eliminates the need for color filters on multiple photodetectors that may age or degrade, or the need to balance the signals from multiple photodetectors, or correct for variations in those signals caused by age and temperature of different photodetectors. The feedback control of the LEDs may be combined with open loop pulse width modulation of the LEDs to permit an extremely wide range of illumination while retaining precise color balance enforced by the much narrower range of feedback color control. A narrower range of feedback allows use of a photodetector that has a narrower range but greater precision. 
   Specifically then, the present invention provides a backlight having a set of groups of solid state lamps, the lamps of each group providing a different color of light. A photodetector is positioned to receive light from all the groups to produce a measurement signal, and a modulator communicating with each group modulates the brightness of light from each group during a first period when the groups are jointly energized to provide a multi-spectral backlight of predetermined color and brightness, and a second period wherein the groups are independently excited to provide measurement signals revealing relative brightness of each color. 
   Thus it is an object of at least one embodiment of the invention to provide for measurement of the light from each color group without the need for isolating filters or multiple photodetectors associated with each color. By using modulation of the light sources to isolate the colors, a single photodetector may be used simplifying the design and preventing the need to calibrate or compensate among multiple detectors and further eliminating the cost and expense of filters and their possible degradation with time and temperature. 
   The first period may be greater than nine times longer than the second period. 
   Thus it is an object of at least one embodiment of the invention to provide a modulation that reveals the light output for each separate color group and yet does not significantly affect the total output of the backlight, for example, if each color were energized for one-third of the total time. 
   During the second period, the lamps of each group may be sequentially energized while lamps of the remaining groups are not energized. 
   Thus it is an object of at least one embodiment of the invention to provide for an extremely simple measurement of the light output of each lamp group. 
   Alternatively, multiple groups of lamps may be energized simultaneously during the second period. 
   Thus it is an object of at least one embodiment of the invention to provide an alternative embodiment in which isolated intensities for the color groups may be algebraically extracted. 
   The invention may include a sample circuit sampling the measurement signal at a subset of time of sequential illumination of each lamp during the second period. 
   Thus it is an object of at least one embodiment of the invention to minimize the length of the second period by short modulation pulses while eliminating artifacts measurement signal rise and fall times. 
   The invention may include feedback circuitry controlling the modulator according to the relative intensities of the colors determined during the second period to provide a predetermined color. 
   Thus it is an object of at least one embodiment of the invention to provide for ongoing color correction of the backlight. 
   Feedback circuitry may provide separate feedback loops for each group. 
   Thus it is an object of at least one embodiment of the invention to allow for color correction that accommodates variations in characteristics of LEDs of different colors. 
   The circuit may include a memory circuit, for example, a sample and hold, storing the relative intensities of the groups for use during the first period. 
   Thus it is an object of at least one embodiment of the invention to separate the time of measurement of color balance from the time of illumination to prevent interference in the color measurement from changes in the total brightness of the backlight. 
   The system may include a controller providing the modulator with a joint modulation signal for controlling brightness and color-specific modulation signals for controlling color. 
   Thus it is an object of at least one embodiment of the invention to provide independent control of color balance over a wide range of brightness. 
   The modulator may provide a duty cycle modulation of the lamps according to the first signal and a current control of the lamps according to a second signal. 
   It is thus another object of at least one embodiment of the invention to require only limited feedback range in color control (determined by the pulse heights) over a much wider range of brightness control (determined by the pulse heights and widths). 
   The controller may employ a duty cycle control of the lamps during a first range of brightness and current control of the lamps during a second range of brightness less bright than the first range of brightness. 
   It is another object of at least one embodiment of the invention to preserve a measurement modulation period by limiting duty cycle modulation for low levels of brightness. 
   These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view of an LCD screen and backlight of the present invention employing an LED matrix and a controller receiving a brightness signal; 
       FIG. 2  is a fragmentary, plan/schematic view of the LED matrix showing positioning of red, green, and blue light emitting diodes with respect to an integrated photodetector; 
       FIG. 3  is a block diagram of the controller of  FIG. 1  showing a processor in the controller such as provides first analog modulation signals for red, green and blue current control and second binary red green and blue modulation signals, and showing local feedback loops responding to only the analog modulation signals; 
       FIG. 4  is a chart showing the two control regimes implemented by the processor of  FIG. 3  providing current control for low light outputs and duty cycle modulation for high light outputs; 
       FIG. 5  is a timing diagram showing the activation of the red, green and blue LEDs during a measurement modulation period and showing a composite received signal from the photodetector with an enlarged inset showing a sample point for one color of the received signal from the photodetector; 
       FIG. 6  is a flowchart showing operation of the processor in implementing the regimes of  FIG. 4 ; 
       FIG. 7  is a set of timing diagrams providing alterative measurement modulation methods per the present invention; 
       FIG. 8  is a perspective view of an alternative backlight embodiment using an edge-lit panel also suitable for the present invention; and 
       FIG. 9  is a side view of the panel of  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to  FIG. 1 , an avionics display  10  may, for example, include a transmissive liquid crystal display (“LCD”)  12  attached by a cable  14  to avionics electronics  16 . Avionics electronics  16  may, for example, provide signals to the avionics display  10  producing graphic representations of indicator gauges and the like based on data  17  received from sensors in the aircraft. 
   The LCD screen  12  provides a plurality of electronically controllable pixels for each of three colors: red, green and blue, to provide for a color display when backlit by a multi-spectral and preferably white or nearly white light. 
   Positioned behind the LCD screen  12  may be a backlight  15  comprised of a diffuser  18  and an LED array  20 . The diffuser  18  positioned between the LED array  20  and the LCD screen  12  serves to spread the light from many point source LEDs in the LED array  20 . The diffuser  18 , may, for example, also include a lens or holographic screen that collimates or directs the light toward a preferential viewing angle. 
   Referring also to  FIG. 2 , the LED array  20  holds a set of multi-LED units  22  arranged, for example, on a regular grid over a mirrored planar surface commensurate with the area of the LCD screen  12 . Upstanding mirrored side walls  24  around the grid of multi-LED units  22  provide an enclosure open toward the diffuser  18  that serves to spread light from the multi-LED units  22  uniformly within the enclosure to provide a more even field of illumination. 
   Each of multi-LED units  22  may include red, green, and blue LEDs  26 ,  28  and  30 , respectively. Matching colors of the red, green and blue LEDs  26 ,  28  and  30  are grouped together and wired commonly, either in series or preferably in parallel to be controllable as independent groups of a single color. Thus, for example, red LEDs  26  of each of the multi-LED units  22  are wired to a red control line  32  (providing two conductors for power and a return) to be controlled as a group. Similarly, green LEDs  28  of each of the multi-LED units  22  are connected to be controlled by green control line  34 , and blue LEDs  30  of each of the multi-LED units  22  are connected to be controlled by blue control line  36 , each to be controllable as a group independently of the other groups. Each of the control lines  32 ,  34  and  36  are received by a controller  38  that also receives a brightness signal  40  and providing electrical signals on control lines  32 ,  34  and  36  to control the brightness and color of the backlight  15  formed of diffuser  18  and LED array  20 . 
   Referring to  FIG. 2 , a photodetector  42 , for example, a photodiode, may be positioned within the reflective chamber formed by upstanding reflective and preferably mirrored sidewalls  24  to receive light  44  from multiple ones of the multi-LED units  22 . The photodetector  42  is attached to lead lines  46  to provide a measurement signal indicating the brightness of the light within the enclosure as contributed from many ones of the multi-LED units  22 . The photodetector  42  is generally multi-spectral sensitive to each different color of light from LEDs  26 ,  28  and  30  to provide the electrical signal proportional thereto. 
   Referring to  FIG. 3 , the controller  38  may generally employ a processor  48  being in the preferred embodiment, a micro controller executing a stored program but also possibly being discrete circuitry or a programmable gate array. The processor  48  receives the brightness signal  40  and provides for two distinct sets of modulation signals. The first set is red, green and blue binary control signals  50 ,  51  and  53  providing, during a first period, a binary signal having a varying on time proportional to a desired brightness of the backlight  15 , and during a second period a measurement modulation to be described. The second set of modulation signals is red, green, and blue analog control signals  52 ,  54  and  56  providing an analog or continuous signal indicating a desired relative brightness of each of the LEDs  26 ,  28  and  30 . 
   Generally, as will be described, the processor sets the initial relative values of the analog red, green, and blue analog control signals  52 ,  54  and  56  according to a desired color balance stored in memory  58 , in the processor  48  or hardwired into its circuitry through potentiometers and the like. When the brightness signal has a high value, indicating the backlight  15  should have a high light output, the values of the analog red, green, and blue analog control signals  52 ,  54  and  56  remain essentially constant and brightness is varied by changing the on-time of the red, green and blue binary control signals  50 ,  51  and  53 . For low light levels, the red, green, and blue analog control signals  52 ,  54  and  56  are changed by equal percentage adjustments to provide for extremely low light control. 
   Referring still to  FIG. 3 , each of the red, green, and blue analog control signals  52 ,  54  and  56  provides a command input to a corresponding summing junction  60 ,  62  and  64 , the summing junctions implementing separate feedback loops for each color and producing error signals when red, green, and blue analog control signals  52 ,  54  and  56  are compared to sampled feedback signals  66 ,  68  and  70 . The sampled feedback signals  66 ,  68  and  70  are received from corresponding sample-and-hold circuits  72 ,  74  and  76 , respectively, which in turn receive the output of the photodetector  42  to sample its light output signal as will be described below. 
   The error signal from the summing junction  60 ,  62  and  64  is received by gating current amplifiers  78 ,  80  and  82  which also receive the red, green and blue duty cycle binary control signals  50 ,  51  and  53 , the latter which gate the gating current amplifiers  78 ,  80  and  82  to block or pass the brightness signal to control lines  32 ,  34  and  36  ultimately to the groups of LEDs  26 ,  28  and  30 . 
   Generally, the feedback loops formed as described above serve to provide a regulated output for the groups of LEDs  26 ,  28  and  30  that is indifferent to aging, temperature effects, and nonlinearities intrinsic to the LEDs  26 ,  28  and  30 . Note that the sampled feedback signals  66 ,  68  and  70  from the photodetector  42  are used only in the local feedback loops and are not provided to the processor  48  or used by the processor  48  to modify the binary control signals  50 ,  51  and  53  or the analog red, green, and blue analog control signals  52 ,  54  and  56 . This is true even though the brightness of a given group of LEDs  26 ,  28  and  30  will be dependent, both on the red, green and blue duty cycle binary control signals  50 ,  51  and  53  and the error voltage from the summing junctions  60 ,  62  and  64  as possibly amplified by a constant amount by gating current amplifiers  78 ,  80  and  82 . 
   Referring now to  FIG. 4 , the brightness of the backlight  15  may vary over a range of 20,000:1, in a preferred embodiment, from approximately 0.01 foot-lamberts to 200 foot-lamberts. The processor  48  provides for this range of operation by using one of two modulation regimes  81  and  83  depending on the brightness signal  40 . The boundary between modulation regimes  81  and  83  can be varied but in a preferred embodiment, for range of 0.01 to 0.2 foot-lamberts, variations in brightness are obtained in the first low-light regime  81  by uniformly scaling the amplitude  84  of the red, green, and blue analog control signals  52 ,  54  and  56  (holding a constant pulse width  86 , e.g. zero). Thus, different values of the red, green, and blue analog control signals  52 ,  54  and  56 , as set for a desired color balance, are multiplied by a common scaling factor. Nonlinearities that differ among the LEDs  26 ,  28  and  30  and that may cause a slight shifting of color balance in this low-light regime  81  are controlled by feedback. 
   When the brightness signal  40  commands a brightness above 0.2 foot-lamberts, in the second bright-light regime  83 , the red, green, and blue analog control signals  52 ,  54  and  56  are held constant in amplitude  84  and the red, green and blue duty cycle binary control signals  50 ,  51  and  53  are used to vary the pulse widths  86  in duty cycle, pulse width, or pulse density-type modulation. 
   Referring now to  FIGS. 3 and 5 , in order to provide for independent feedback loops for each of the groups of LEDs  26 ,  28  and  30 , the signal on lines  46  from photodetector  42  must be processed to provide separate measurements of the brightness of each group of LEDs  26 ,  28  and  30 . Thus, feedback control of the group of red LEDs  26  requires a measurement of red light isolated from green and blue light, and similarly the feedback control of the groups of green LEDs  28  requires a measurement of green light isolated from red and blue light, and feedback control of the groups of blue LEDs  30  requires a measurement of blue light isolated from green and red light. 
   In the preferred embodiment, this decomposition of the measurement signal from the photodetector  42  into separate color measurements is done by using the red, green and blue duty cycle binary control signals  50 ,  51  and  53  to provide a separate brightness modulation period  90  and a measurement modulation period  92 . During brightness modulation period  90 , each of the binary control signals  50 ,  51  and  53  provide identical duty cycle modulation of the group of LEDs  26 ,  28  and  30  varying an on-time proportion in proportion to the brightness signal  40  to control the average illumination of the backlight  15 . 
   In contrast during measurement modulation period  92 , no duty cycle modulation is provided, but in sequence, light from all of the groups of LEDs  26 ,  28  and  30 , but one, are suppressed. Thus, during measurement modulation period  92 , first, the group of red LEDs  26  only is activated for a short pulse  94  using binary control signal  50 . Next, a short pulse  96  of binary control signal  51  activates only the green LEDs  28 , and then a pulse  98  of binary control signal  53  activates only the blue LEDs  30 . 
   The photodetector  42  thus provides three corresponding pulses  94 ′,  96 ′ and  98 ′ during measurement modulation period  92 , each pulse  94 ′,  96 ′ and  98 ′ being proportional in height to the light output of a single group and thus a single color of LEDs  26 ,  28  and  30 , respectively. The processor  48  provides capture signals (not shown) to sample-and-hold circuits  72 ,  74  and  76 , respectively, to sample each of the pulses  94 ,  96  and  98  to provide the sampled feedback signals  66 ,  68  and  70 , respectively. The sampling occurs during sample intervals  100  centered within the pulse&#39;s  94 ′,  96 ′ and  98 ′ so as to eliminate the effect of rise time and decay time on the measurement. 
   Referring now to  FIGS. 4 and 5 , because the signals to the LEDs  26 ,  28  and  30  on control lines  32 ,  34  and  36  vary in amplitude only during the low-light regime  81  and not during the bright-light regime  83 , the dynamic range in brightness that must be accommodated by photodetector  42  is substantially limited. In this example, the photodetector  42  must only accommodate a 20 to 1 rather than 20,000 to 1 variation in instantaneous light output. This allows for an extremely precise relative brightness control of each of the groups of LEDs  26 ,  28  and  30  ensuring stable color control. Whereas, brightness variation in the backlight  15  on the order of 10 to 20 percent may be readily accommodated for total multi-spectral brightness, such a variation among each of the color components would result in undesirable color shifting. Accordingly, eliminating feedback control of the total dynamic range of brightness of 20,000 to 1 provides for improved color accuracy. The approach relaxes the requirements of the photodetector  42 , allowing standard photodetectors  42  to be used with minor colors sensitivity variation being accommodated with calibration factors stored in memory  58  as described above. 
   Referring now to  FIG. 6 , the processor  48  operates to accept brightness signal  40  as indicated by process block  101 . The values of analog red, green, and blue analog control signals  52 ,  54  and  56  are set to provide the desired color balance as indicated by process block  102  as may be precomputed or preset at the factory to a constant value or, in an alternative embodiment, varied according to the brightness signal  40  to preserve a desired color balance. 
   At decision block  104 , the processor  48  determines whether the brightness signal  40  is above or below the threshold level between control low-light regime  81  and bright-light regime  83  shown in  FIG. 4 . If a low light condition does not exist, then bright-light regime  83  is indicated, and as represented by process block  106 , a duty cycle is calculated on an open loop basis to create the desired brightness of the backlight  15 . Because the duty cycle modulation of bright-light regime  83  operates the LEDs  26 ,  28  and  30  at essentially constant current levels, non-linearities in the relationship between brightness and current may be largely ignored while providing this open loop control. Further, as indicated by process block  108 , the relative brightness of each of the groups of LEDs  26 ,  28  and  30  during on times of the duty cycle is held fixed according to the ratios established at process block  102  as maintained by the feedback loops. 
   If at decision block  104 , the low light regime  81  is indicated by the brightness signal  40 , then the program branches to process block  110  to provide a scaling of the values for analog red, green, and blue analog control signals  52 ,  54  and  56  (from the values previously set per process block  102 ) reducing the command brightness values by equal percentages while preserving the offsets and thus the ratios between the brightness values represented by analog red, green, and blue analog control signals  52 ,  54  and  56 . At this time, brightness modulation periods  90  may provide for a small or zero on-time of the LEDs  26 ,  28  and  30  and illumination provided by simply the sampling values of pulses  94 ,  96  and  98  shown in  FIG. 5 . In this case, the measurement modulation period  92  also provides for brightness modulation by current control. 
   Because a single photodetector  42  may be used in this application, balancing of light between photodetectors is not required and possible unequal aging, or temperature effects in the photodetectors are largely eliminated. Precise brightness feedback control is provided for color balance without the need for high compliance or operating range in the photodetector  42 . The modulation performed during measurement modulation period  92  eliminates the need for separate photodetectors or filters or the attachment of individual photodetectors to individual LEDs to serve as a proxy for other devices. It will be recognized, however, that the benefits of limiting the range of feedback control to improve color balance compliance, may also benefit these other techniques that employ filters or multiple photodetectors. 
   Referring now to  FIG. 7 , the invention is not limited to the modulation shown in  FIG. 5 , but may be used with other modulation schemes so long as they provide the photodetector  42  or multiple ganged photodetectors to provide an independent measurement of the light intensities of each of the groups of LEDs  26 ,  28  and  30 . Thus, as shown by the left half of the timing diagram of  FIG. 7 , the measurement modulation period  92  may be distributed among the brightness modulation periods  90  so that the two are merged with negative-going pulses serving to darken two of the colors from the groups of LEDs  26 ,  28  and  30  (for each of three combinations of the two colors) so as to unambiguously reveal the individual colors. Thus, at a first time  120 , negative-going pulses  122  and  124  may be applied to the red and green duty cycle binary control signals  50  and  51  so as to effectively provide that during time  120  only a brightness of the blue LEDs  30  is measured. Likewise, at times  126  and  128 , red and blue duty cycle binary control signals  50  and  53 , and then green and blue duty cycle modulation signals  51  and  53  may be suppressed by corresponding negative-going pulses so that time  126  reveals the brightness of green LEDs  28  and time  128  reveals the brightness of red LEDs  26 . 
   Alternatively, referring to the right side of  FIG. 7 , a single negative-going pulse for each of times  120 ,  126  and  128  may occur in each of the red, green and blue duty cycle binary control signals  50 ,  51  and  53 , staggered in time. Thus, a negative-going pulse  130  at time  120  in red binary control signal  50  provides the photodetector  42  with a reading of the combined brightness of the green LEDs  28  and blue LEDs  30 . A later negative-going pulse  132  at time  126  in signal  51  provides a reading of the combined brightness of the red LEDs  26  and blue LEDs  30 , and a later negative-going pulse  134  at time  128  provides a reading of the combined brightness of the red LEDs  26  and green LEDs  28 . A simple algebraic combination of these three values yields independent values for red, green and blue. 
   Referring again to  FIG. 1 , the LED array  20  of LEDs may alternatively employ an edge-lit light panel having a reflective rear surface or other method of producing uniform light fields using point sources well known in the art. 
   Referring now to  FIG. 8 , an alternative embodiment of a LCD backlighting system includes an edge-lit backlight system  200 . The edge-lit backlight system  200  includes first and second LED assemblies  202 ,  204  arranged opposite one another and separated by a clear light guide panel  206 . Engaged with a back  208  of the light guide panel  206  is a reflector film backing  210  configured to reflect light injected by the LED assemblies  202 ,  204  into the guide panel  206  toward a front  212  of the guide panel  206 . 
   This arrangement is further illustrated in  FIG. 9 , where the reflecting film  210  is arranged against the back  208  of the light guide panel  206 . Also arranged at the back  208  of the light guide panel  206  may be a diffusing layer  214  that may be disposed between the reflecting film  210  and the guide panel  206  to diffuse light directed from the light guide panel  206  toward the reflecting film  210  and light directed back from the reflecting film  210  toward the front  212  of the light guide panel  206 . Additionally, it is contemplated that one or more brightness enhancing and/or light directing films  216  may be arranged in front of the light guide panel  206 . Finally, an LCD panel  218  is arranged forwardly of the edge-it backlight system  200  to receive light generated by the LED assemblies  202 ,  204 . The photodetector  42  (not shown) may also be placed at one edge of the light guide panel  206  to receive light from multiple ones of the LEDs of assemblies  202 ,  204 , which may be controlled as described above. 
   It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.