Abstract:
An opto-isolator with a correction circuit is disclosed. The correction circuit is configured to make adjustments for degradation of the light source of the opto-isolator. The correction circuit may comprise a photodetector for detecting degradation of the light source of the opto-isolator. When the light source degrades below a predetermined level, the correction circuit may be configured to make adjustments.

Description:
BACKGROUND 
     A galvanic isolator provides a means for moving a signal from one electrical circuit to another electrical circuit in a control system when the two electrical circuits must otherwise be electrically isolated from one another. Usually the two electrical circuits operate at different voltages, and thus, must be electrically isolated. For example, consider an application in which a 5V battery powered controller board is configured to control a motor circuit operating at 240V. In this example, it is essential to electrically isolate the 240V motor circuit from the 5V controller circuit, while permitting the 5V controller circuit to send or receive signals from the 240V motor circuit. In this type of application, an isolator may be used to provide voltage and noise isolation, while permitting the information exchange between the two circuit systems. Opto-isolator, also known as optocoupler, is one of the most commonly used galvanic isolators. 
     Generally, an opto-isolator comprises an optical emitter and an optical receiver. Over time, degradation may occur and optical signals emitted from the optical emitter may become weak, and eventually the optical emitter may fail to function. For many control systems, failure of such optical parts may be vulnerable and may be dangerous as the control systems may be relying on the optical signals to control fast moving motors or machinery parts. It may be desirable to take preventive steps to replace the parts before complete failure, or to address the degradation to prolong the lifetime of the opto-isolators. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments by way of examples, not by way of limitation, are illustrated in the drawings. Throughout the description and drawings, similar reference numbers may be used to identify similar elements. The drawings are for illustrative purpose to assist understanding and may not be drawn per actual scale. 
         FIG. 1  illustrates a block diagram of an opto-isolator having correction circuitry; 
         FIG. 2  illustrates a schematic diagram of another opto-isolator embodiment; 
         FIG. 3  illustrates a schematic diagram of another opto-isolator embodiment; 
         FIG. 4  illustrates a schematic diagram of another opto-isolator embodiment; 
         FIG. 5  illustrates a cutaway side view of a packaged opto-isolator. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a block diagram of an opto-isolator  100  for electrical isolation of an electrical input signal from an electrical output signal. Opto-isolator  100  may comprise a signal input terminal  112  configured to receive the electrical input signal. An optical emitter  114  may be coupled to the signal input terminal  112  and configured to generate emitted light in response to the electrical input signal. 
     A first optical receiver  116  may be arranged proximate to the optical emitter  114  to receive a first portion of the emitted light. In particular, a first photodetector (not shown) of the first optical receiver  116  may be arranged proximate to the optical emitter  114  to receive the first portion of the emitted light. The first optical receiver  116  may be configured to generate the electrical output signal in response to the first portion of the emitted light. A signal output terminal  118  may be coupled to the first optical receiver  116  to receive the electrical output signal. 
     Opto-isolator  100  may further comprise a second optical receiver  120  (and/or a second set of optical receivers  120 ) arranged proximate to the optical emitter  114  to receive a second portion of the emitted light. In particular, a second photodetector (not shown) of the second optical receiver  120  may be arranged proximate to the optical emitter  114  to receive the second portion of the emitted light. In the figures, the first and second portions of emitted light are representatively illustrated by first and second dashed line arrows. The second optical receiver  120  (and/or the second set of optical receivers  120 ) may be configured to generate one or more light output electrical signals in response to the second portion of the emitted light. 
     The terms “light” and “optical” as used herein may be visible and/or may be invisible. All possible variations of electromagnetic waves should be taken into consideration when a specific type of light or radiation or optical emitter or optical received is discussed, unless explicitly expressed otherwise. For example, ultra-violet, infrared and other invisible radiation should be included when considering the terms “light” or “optical” as used herein, even though light may often be used in the art to refer to radiation that is visible to the human eye. 
     Opto-isolator  100  may further comprise correction circuitry  130  configured to generate one or more correction signals. Correction circuitry  130  may be configured to generate one or more correction signals, when a light source of the optical emitter  114  may degrade over time, for example, when light emitted by the light source may degrade below a predetermined level. Correction circuitry  130  may comprise a drive correction signal generator  132  to provide for adjusting drive of the optical emitter. Alternatively or additionally correction circuitry  130  may comprise a threshold correction signal generator  134  to provide for adjusting a threshold of the first optical receiver  116 . 
     In one embodiment, the drive correction signal generator  132  may be coupled with the second optical receiver  120  (and/or one or more members of the second set of optical receivers  120 ) for receiving one or more of the light output electrical signals therefrom. As the optical emitter  114  may degrade over time, for example, when the light source of the optical emitter  114  may degrade below a predetermined level, one or more of the light output electrical signals may likewise decline below one or more predetermined signal levels. In response to the one or more of the light output electrical signals of the second optical receiver  120  (and/or of one or more members of the second set of optical receivers  120 ), the drive correction signal generator  132  may generate one or more drive correction signals. The optical emitter  114  may be coupled to the drive correction signal generator  132  for adjusting drive of the optical emitter  114  in response to the one or more drive correction signals. 
     In one embodiment, the threshold correction signal generator  134  may be coupled with the second optical receiver  120  (and/or another one or more members of the second set of optical receivers  120 ) for receiving another one or more of the light output electrical signals therefrom. As the optical emitter  114  may degrade over time, for example, when the light source of the optical emitter  114  may degrade below a predetermined level, the another one or more of the light output electrical signals may likewise decline below another one or more predetermined signal levels. In response to the another one or more of the light output electrical signals of the second optical receiver  120  (and/or of another one or more members of the second set of optical receivers  120 ), the threshold correction signal generator  134  may generate one or more threshold correction signals. The first optical receiver  116  may be coupled to the threshold correction signal generator  134  for adjusting the threshold of the first optical receiver  116  in response to the one or more threshold correction signals. 
       FIG. 2  illustrates a schematic diagram of another opto-isolator embodiment  200  for electrical isolation of an electrical input signal from an electrical output signal. Opto-isolator  200  may comprise a signal input terminal  212  configured to receive the electrical input signal. Optical emitter  214  may be coupled to the signal input terminal  212  and configured to generate emitted light in response to the electrical input signal. Optical emitter  214  may comprise a photoemitter  252  (e.g. a Light Emitting Diode  252 ), which may be coupled with the signal input terminal  212  via a drive circuit  254  and input logic  258 . 
     First optical receiver  216  may comprise first photodetector  242 , which may be arranged proximate to the optical emitter  214  to receive a first portion of the emitted light. The first optical receiver  216  may be configured to generate the electrical output signal in response to the first portion of the emitted light. Signal output terminal  218  may be coupled to the first optical receiver  216  to receive the electrical output signal. 
     Second optical receiver  220  may comprise second photodetector  228 , which may be arranged proximate to the optical emitter  214  to receive a second portion of the emitted light. In  FIG. 2 , the first and second portions of emitted light are representatively illustrated by first and second dashed line arrows. Second photodetector  228  may be configured to generate a light output electrical signal in response to the second portion of the emitted light. 
     As the optical emitter  214  may degrade over time, for example, when light emitted by the photoemitter  252  may degrade below a predetermined level, the light output electrical signal generated by second photodetector  228  in response thereto may likewise decline below a predetermined signal level. The light output electrical signal from second photodetector  228  may be coupled with a first input of a first comparator  226  via first transimpedance amplifier  227 . First comparator  226  may monitor when the light output electrical signal may decline below the predetermined signal level by comparing the light output electrical signal to a first reference signal. The first reference signal may be set so that the first comparator  226  may generate an activating transition of a first comparison signal, when the light output electrical signal may decline below the predetermined signal level. 
     The second optical receiver  220  may comprise a first reference signal generator  224  configured to generate the first reference signal. Accordingly, it should be understood that the first comparator  226  may be coupled with the first reference signal generator  224  and the second photodetector  228 , and may be configured to generate a first comparison signal in response to the first reference signal and the light output electrical signal. 
     A correction signal generator, e.g. a threshold correction signal generator  234 , may be coupled with the first comparator  226 , and configured to generate a correction signal, e.g. a threshold correction signal, in response the first comparison signal, as generated by the first comparator  226 . Therefore, in accordance with the foregoing discussion, it should be understood that the threshold correction signal generator  234  may generate the threshold correction signal, in response to the light output electrical signal of the second optical receiver  220 . The threshold correction signal generator  234  may comprise a decoder  235 , which may be configured to decode the first comparison signal, so as to generate the threshold correction signal. 
     The first optical receiver  216  may be coupled to the threshold correction signal generator  234  for adjusting a threshold of the first optical receiver  216  in response to the threshold correction signal. For example, the threshold of the first optical receiver  216  may be lowered in response to the threshold correction signal. This lowered threshold of the first optical receiver  216  may provide for continuing detection of degraded light, even when light emitted by the photoemitter  252  may degrade below the predetermined level. 
     In addition to the first photodetector  242 , the first optical receiver  216  may further comprise an adjustable reference signal generator  236  having an input coupled with the threshold correction signal generator  234  and configured to generate an adjustable reference signal that adjusts in response to the threshold correction signal. The adjustable reference signal generator  236  may comprise a selector  238  having an input coupled with the threshold correction signal generator  234  for variably selecting from among a plurality reference voltages generated by a plural voltage reference generator  237 , so as to provide the adjustable reference signal in response to the threshold correction signal. 
     A second comparator  246  may have a first input coupled with the first photodetector  242  via a second transimpedance amplifier  244 . The second comparator  246  may have a second input coupled with the adjustable reference signal generator  236 . The second comparator  246  may be configured to generate a second comparison signal in response to the first photodetector signal and the adjustable reference signal. The first optical receiver  216  may further comprise output logic  248  coupled between signal output terminal  218  and an output of the second comparator  246 . The output logic  248  may be configured to generate the electrical output signal of the opto-isolator  200  in response to the second comparison signal. 
     As shown in  FIG. 2 , opto-isolator  200  may further comprise an electrical insulator  222 . Electrical insulator  222  may be interposed between the optical emitter  214  and the first and second optical receivers  216 ,  220 . Electrical insulator  222  may be configured to electrically isolate the first and second optical receivers  216 ,  220  from the optical emitter  214 . The electrical insulator  222  may be substantially transparent to light emitted by the optical emitter  214 . Accordingly, respective first and second photodetectors  242 ,  228  of each of the first and second optical receivers  216 ,  220  may receive through the electrical insulator  222  each of the first and second portions of light emitted by the optical emitter  214 . 
     The photoemitter  252  of the optical emitter  214  may comprise a first photoemitter die. The first photodetector  242  of the first optical receiver  216  may comprise a first photodetector die. The second photodetector  228  of the second optical receiver  220  may comprise a second photodetector die. The first photoemitter die and the first and second photodetector dies may be packaged together. 
     Alternatively, the first and second photodetectors  242 ,  228  may be integrated onto a single monolithic substrate. To provide for even greater integration, the first and second optical receivers  216 ,  220  may be integrated onto a single monolithic substrate. 
       FIG. 3  illustrates a schematic diagram of another opto-isolator embodiment  300  for electrical isolation of an electrical input signal from an electrical output signal. Opto-isolator  300  may comprise a signal input terminal  312  configured to receive the electrical input signal. Optical emitter  314  may be coupled to the signal input terminal  312  and configured to generate emitted light in response to the electrical input signal. Optical emitter  314  may comprise a photoemitter  352  (e.g. a Light Emitting Diode  352 ), which may be coupled with the signal input terminal  312  via an adjustable drive circuitry  354  and input logic  358 . 
     First optical receiver  316  may comprise first photodetector  342 , which may be arranged proximate to the optical emitter  314  to receive a first portion of the emitted light. The first optical receiver  316  may be configured to generate the electrical output signal in response to the first portion of the emitted light. Signal output terminal  318  may be coupled to the first optical receiver  316  to receive the electrical output signal. 
     Second optical receiver  320  may comprise second photodetector  328 , which may be arranged proximate to the optical emitter  314  to receive a second portion of the emitted light. In  FIG. 3 , the first and second portions of emitted light are representatively illustrated by first and second dashed line arrows. Second photodetector  328  may be configured to generate a light output electrical signal in response to the second portion of the emitted light. 
     As the optical emitter  314  may degrade over time, for example, when light emitted by the photoemitter  352  may degrade below a predetermined level, the light output electrical signal generated by second photodetector  328  in response thereto may likewise decline below a predetermined signal level. The light output electrical signal from second photodetector  328  may be coupled with a first input of a first comparator  326  via first transimpedance amplifier  327 . First comparator  326  may monitor when the light output electrical signal may decline below the predetermined signal level by comparing the light output electrical signal to a first reference signal. The first reference signal may be set so that the first comparator  326  may generate an activating transition of a first comparison signal, when the light output electrical signal may decline below the predetermined signal level. 
     The second optical receiver  320  may comprise a first reference signal generator  324  configured to generate the first reference signal. Accordingly, it should be understood that the first comparator  326  may be coupled with the first reference signal generator  324  and the second photodetector  328 , and may be configured to generate a first comparison signal in response to the first reference signal and the light output electrical signal. 
     A correction signal generator, e.g. a drive correction signal generator  334 , may be coupled with the first comparator  326 , and configured to generate a correction signal, e.g. a drive correction signal, in response the first comparison signal, as generated by the first comparator  326 . Therefore, in accordance with the foregoing discussion, it should be understood that the drive correction signal generator  334  may generate the drive correction signal, in response to the light output electrical signal of the second optical receiver  320 . In turn, the adjustable drive circuitry  354  may adjust or may raise or may increase drive of the photoemitter  352  in response to the drive correction signal generated by the drive correction signal generator. 
     In other words, the adjustable drive circuitry  354  of the optical emitter  314  may be coupled to the drive correction signal generator  334  for adjusting drive of the photoemitter  352  in response to the threshold correction signal. For example, drive of the photoemitter  352  may be raised or may be increased in response to the drive correction signal. This raised or increased drive of the photoemitter  352  may provide for remedying degraded light of the photoemitter  352  by raising or increasing light output of the photoemitter  352 , when light emitted by the photoemitter  352  may have degraded below the predetermined level. 
     In addition to the first photodetector  342 , the first optical receiver  316  may further comprise a second reference signal generator  345 , which may be configured to generate a second reference signal. A second comparator  346  may have a first input coupled with the first photodetector  342  via a second transimpedance amplifier  344 . The second comparator  346  may have a second input coupled with the second reference signal generator  336 . The second comparator  346  may be configured to generate a second comparison signal in response to the first photodetector signal and the second reference signal. The first optical receiver  316  may further comprise output logic  348  coupled between signal output terminal  318  and an output of the second comparator  346 . The output logic  348  may be configured to generate the electrical output signal of the opto-isolator  300  in response to the second comparison signal. 
     As shown in  FIG. 3 , opto-isolator  300  may further comprise an electrical insulator  322 . Electrical insulator  322  may be interposed between the optical emitter  314  and the first optical receiver  316 . Electrical insulator  322  may be configured to electrically isolate the first optical receiver  316  from the optical emitter  314 . In contrast, the second optical receiver  320  may be electrically coupled with the optical emitter  314 . The electrical insulator  322  may be configured so as to be limited to electrically isolating the first optical receiver  316  from the optical emitter  314 , and so as to avoid electrically isolating the second optical receiver  320  from the optical emitter  314 . 
     The electrical insulator  322  may be substantially transparent to light emitted by the optical emitter  314 . Accordingly, as shown in  FIG. 3 , first photodetector  342  of the first optical receiver  316  may receive through the electrical insulator  322  the first portion of light emitted by the optical emitter  314 . 
       FIG. 4  illustrates a schematic diagram of another opto-isolator embodiment  400  for electrical isolation of an electrical input signal from an electrical output signal. Opto-isolator  400  may comprise a signal input terminal  412  configured to receive the electrical input signal. Optical emitter  414  may be coupled to the signal input terminal  412  and configured to generate emitted light in response to the electrical input signal. Optical emitter  414  may comprise first and second photoemitters  452 A,  452 B (e.g. first and second Light Emitting Diodes  452 A,  452 B), which may be coupled with the signal input terminal  412  via input logic  458  and an adjustable drive circuit  454  and emitter selection logic  455 . 
     First optical receiver  416  may comprise first photodetector  442 , which may be arranged proximate to the optical emitter  414  to receive a first portion of the emitted light. The first optical receiver  416  may be configured to generate the electrical output signal in response to the first portion of the emitted light. Signal output terminal  418  may be coupled to the first optical receiver  416  to receive the electrical output signal. 
     Second optical receiver  420  may comprise second photodetector  428 , which may be arranged proximate to the optical emitter  414  to receive a second portion of the emitted light. In  FIG. 4 , the first and second portions of emitted light are representatively illustrated by first and second dashed line arrows. Second photodetector  428  may be configured to generate a light output electrical signal in response to the second portion of the emitted light. 
     As the optical emitter  414  may degrade over time, for example, when light emitted by the first photoemitter  452 A may degrade below a predetermined level, the light output electrical signal generated by second photodetector  428  in response thereto may likewise decline below a predetermined signal level. The light output electrical signal from second photodetector  428  may be coupled with a first input of a first comparator  426  via first transimpedance amplifier  427 . First comparator  426  may monitor when the light output electrical signal may decline below the predetermined signal level by comparing the light output electrical signal to a first reference signal. The first reference signal may be set so that the first comparator  426  may generate an activating transition of a first comparison signal, when the light output electrical signal may decline below the predetermined signal level. 
     The second optical receiver  420  may comprise a first reference signal generator  424  configured to generate the first reference signal. Accordingly, it should be understood that the first comparator  426  may be coupled with the first reference signal generator  424  and the second photodetector  428 , and may be configured to generate a first comparison signal in response to the first reference signal and the light output electrical signal. 
     A correction signal generator, e.g. a drive correction signal generator  434 , may be coupled with the first comparator  426 , and configured to generate a correction signal, e.g. a drive correction signal, in response the first comparison signal, as generated by the first comparator  426 . Therefore, in accordance with the foregoing discussion, it should be understood that the drive correction signal generator  434  may generate the drive correction signal, in response to the light output electrical signal of the second optical receiver  420 . In turn, emitter selection logic  455  may selectively activate second photoemitter  452 B in addition to, or in place of, activation of first photoemitter  452 A, when the first photoemitter  452 A may have become degraded. Further, the adjustable drive circuitry  454  may adjust or may raise or may increase drive of the photoemitter  452  in response to the drive correction signal generated by the drive correction signal generator. 
     In other words, emitter selection logic  455  of the optical emitter  414  may be coupled to the drive correction signal generator  434  for selectively activating second photoemitter  452 B in addition to, or in place of, activation of first photoemitter  452 A, when the first photoemitter  452 A may have become degraded. Further, the adjustable drive circuitry  454  of the optical emitter  414  may be coupled to the drive correction signal generator  434  for adjusting drive of one or both of the photoemitters  452 A,  452 B in response to the threshold correction signal. For example, drive of the one or both of the photoemitters  452 A,  452 B may be raised or may be increased in response to the drive correction signal. This raised or increased drive of the first photoemitter  452 A may provide for remedying degraded light of the first photoemitter  452 A by raising or increasing light output of the photoemitter  452 A, when light emitted by the first photoemitter  452 A may have degraded below the predetermined level. Further, the raised or increased drive of the adjustable drive circuit may provide for remedying degraded light of the first photoemitter  452 A by providing drive for the second photoemitter  452 B, when light emitted by the first photoemitter  452 A may have degraded below the predetermined level. 
     In addition to the first photodetector  442 , the first optical receiver  416  may further comprise a second reference signal generator  445 , which may be configured to generate a second reference signal. A second comparator  446  may have a first input coupled with the first photodetector  442  via a second transimpedance amplifier  444 . The second comparator  446  may have a second input coupled with the second reference signal generator  436 . The second comparator  446  may be configured to generate a second comparison signal in response to the first photodetector signal and the second reference signal. The first optical receiver  416  may further comprise output logic  448  coupled between signal output terminal  418  and an output of the second comparator  446 . The output logic  448  may be configured to generate the electrical output signal of the opto-isolator  400  in response to the second comparison signal. 
     As shown in  FIG. 4 , opto-isolator  400  may further comprise an electrical insulator  422 . Electrical insulator  422  may be interposed between the optical emitter  414  and the first optical receiver  416 . Electrical insulator  422  may be configured to electrically isolate the first optical receiver  416  from the optical emitter  414 . In contrast, the second optical receiver  420  may be electrically coupled with the optical emitter  414 . The electrical insulator  422  may be configured so as to be limited to electrically isolating the first optical receiver  416  from the optical emitter  414 , and so as to avoid electrically isolating the second optical receiver  420  from the optical emitter  414 . 
     The electrical insulator  422  may be substantially transparent to light emitted by the optical emitter  414 . Accordingly, as shown in  FIG. 4 , first photodetector  442  of the first optical receiver  416  may receive through the electrical insulator  422  the first portion of light emitted by the optical emitter  414 . 
       FIG. 5  illustrates a cutaway side view of a packaged opto-isolator  500 . The opto-isolator package  500  may comprise a plurality of leads  531 , a die attach pad  532 , a photoemitter die  533 , a die  534  of a plurality of photodetectors (e.g. photodetector die  534 ), an optional correction circuit die  535 , an electrical isolation layer  536 , an encapsulant  538 , and an optional opaque encapsulant  539 . The die  534  of the plurality of photodetectors (e.g. photodetector die  534 ) may be integrated onto a single monolithic substrate  534 . A portion of one of the leads  531  may be extended to define the die attach pad  532  configured to accommodate the die  534  of the plurality of photodetectors (e.g. photodetector die  534 ). The die  534  of the plurality of photodetectors (e.g. photodetector die  534 ) may be larger than the photoemitter die  533 , as well as the optional correction circuit die  535 . Therefore, the die  534  of the plurality of photodetectors (e.g. photodetector die  534 ) may be configured to receive the photoemitter die  533  and the optional correction circuit die  535  as shown in  FIG. 5 . 
     For the purpose of electrically isolating the die  534  of the plurality of photodetectors (e.g. photodetector die  534 ) from the photoemitter die  512 , the electrical isolation layer  536  may be disposed on the die  534  of the plurality of photodetectors (e.g. photodetector die  534 ), and subsequently the photoemitter die  533  may be disposed on the electrical isolation layer  536 . Alternatively, the photoemitter die  533  and the correction circuit die  535  may be disposed on another one of the plurality of leads  531 . In another embodiment, the die  535  may be disposed on the electrical isolation layer  536 . However, stacking up the dies  533 ,  534 ,  535  as shown in  FIG. 5  may reduce space needed for the opto-isolator package  500 . The electrical connections between the dies  533 ,  534  and leads  531  may be established through wire bonds  537 . 
     One of the leads coupled to the photoemitter die  533  may provide a signal input terminal and may be configured to receive the electrical input signal. One of the leads coupled to the die  534  of the plurality of photodetectors (e.g. photodetector die  534 ) may provide a signal output terminal and may be configured to output the electrical output signal. 
     The photodetector die attach pad  532  may be arranged proximate to the photoemitter die  533 . The photodetector die  533  may be configured to receive the emitted light. The photodetector die  533  may be configured to generate the electrical output signal in response to emitted light. The second photodetector may be configured to generate a light output electrical signal in response to the emitted light. The embodiment shown in  FIG. 5  may be a lead frame package, but in another embodiment, the package may comprise a printed circuit board with the plurality of leads  531  being the conductive traces of the printed circuit board. 
     The encapsulant  538  may be substantially transparent to the light emitted by the photoemitter die  533 . The encapsulant  538  may not be transparent to the human eye as the light may include invisible light such as infra-red light as explained earlier. The encapsulant  538  may be silicone, epoxy or any other similar material suitable to encapsulate the photoemitter die  533 , the die  534  of the plurality of photodetectors, and the optional correction circuit die  535 . The optional opaque encapsulant  539  may be opaque to the light emitted from the photoemitter die  533 . In some applications, the light emitted by the photoemitter die  533  may be blocked within the opto-isolator package  500  because the light may become noise to other electronic components (not shown) or to human eyes. For these applications, the encapsulant  538  may be required to be covered by the opaque encapsulant  539 . 
     Different aspects, embodiments or implementations may, but need not, yield one or more of the following advantages. For example, the correction circuitry may extend the service life of the opto-isolator, when the light source of the optical emitter degrades below a predetermined level. Using the threshold correction signal generator may provide for adjusting the threshold of the first optical receiver. Using the drive correction signal generator may provide for adjusting drive of the optical emitter. 
     Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. It is to be understood that the illustration and description shall not be interpreted narrowly. For example, the light sources shown may comprise a Light Emitting Diode (LED), but alternatively or additionally may comprise a die with an integrated LED and circuitry or a light source using future technologies. The scope of the invention is to be defined by the claims appended hereto and their equivalents.