Patent Publication Number: US-2010127627-A1

Title: Light-emitting medical devices having protections against unintended light exposure

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from U.S. patent application No. 60/915,401 filed 1 May 2007 and entitled LIGHT-EMITTING MEDICAL DEVICES HAVING PROTECTIONS AGAINST UNINTENDED LIGHT EXPOSURE. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of U.S. patent application No. 60/915,401 filed 1 May 2007 and entitled LIGHT-EMITTING MEDICAL DEVICES HAVING PROTECTIONS AGAINST UNINTENDED LIGHT EXPOSURE which is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to medical devices and in particular to medical devices that emit light for diagnostic or treatment purposes. Some specific embodiments of the invention provide apparatus and methods for protecting against unintended light exposure from near infrared spectrometry (NIRS) devices. 
     BACKGROUND 
     Various medical devices emit light. Light includes visible light and invisible light. Invisible light includes ultraviolet light and infrared light. The light may be intended for a diagnostic or therapeutic purpose. Depending upon the application, the light may comprise visible light, invisible light or some combination of visible and invisible light. 
     An example of a light-emitting medical device is a NIRS system. Near Infrared Spectroscopy (“NIRS”) is a technique which involves emitting near infrared (“NIR”) light and receiving the NIR light after it has passed through a tissue or other medium of interest. NIRS can be applied to study and monitor biochemical compounds in the body. Emitted NIR light penetrates skin and other tissues and some of it is absorbed by biochemical compounds which have an absorption spectrum in the NIR region. NIR light which is not absorbed is scattered. Each biochemical compound has a different absorption spectrum. It is possible to estimate the concentration of biochemical compounds in the tissues by measuring characteristics of NIR light that has been detected after it has passed through the tissues. 
     Light can be dangerous to the eyes. Intense invisible light is particularly dangerous because the eye of an observer can receive damaging exposure to the intense light without realizing that damage is occurring. 
     There is a need for practical light-emitting medical devices that can prevent unintended light exposure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In drawings which illustrate non-limiting embodiments of the invention, 
         FIG. 1  is a block diagram of a medical apparatus according to a generalized example embodiment of the invention. 
         FIG. 2  is a partially schematic diagram showing a light guard and mechanical interlock in apparatus according to an example embodiment of the invention. 
         FIG. 3  is a block diagram of a circuit that may be used to implement a multi-trigger light output inhibiting arrangement. 
         FIG. 4  is a block diagram showing an example circuit for monitoring both light output and electrical current. 
         FIG. 5  is a flow chart illustrating a method that is performed in some embodiments of the invention. 
         FIG. 6  is a flow chart illustrating a method that is performed in some embodiments of the invention. 
         FIG. 7  is a block diagram of a medical apparatus according to another generalized example embodiment of the invention. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     The following description describes a medical device that has three different systems for preventing unintended exposure to radiation. These include a mechanical interlock, a hardware safety shut off, and firmware routines that interact with the hardware to prevent unintended exposure. These systems may be provided individually or in any suitable combinations. The invention also provides methods for preventing unintended exposure to light. 
       FIG. 1  shows an apparatus  10  according to an example embodiment of the invention. Apparatus  10  includes a light source  12  that emits light toward tissues of a subject, a light detector  14  that receives some of the light that has passed through the subject&#39;s tissues and an analysis component  16  that analyzes the detected light to obtain information about the subject&#39;s tissues. 
     For example, light source  12  may emit infrared light, and analysis component  16  may evaluate concentrations (or changes in concentration) of one or more biological compounds in the subject&#39;s tissues by analyzing detected light. In such embodiments, apparatus  10  may employ NIRS to evaluate concentrations of one or more compounds in tissues of a subject. 
     In the illustrated embodiment, light source  12  comprises one or more light emitters  20  (e.g. light emitters  20 A,  20 B,  20 C) within a housing  22 . Light emitters  20  deliver light to one or more corresponding ports  23 . Cables  24  have couplings  25  that engage ports  23 . When a coupling  25  of a cable  24  is connected to a corresponding port  23 , a light path is established from a corresponding one or more of light emitters  20  to an optical fiber  26  within cable  24 . Optical fiber  26  extends to a patch  27  which is intended to be placed against the skin of a subject when apparatus  10  is in use. In some embodiments, light detector  14  detects light received at patch  27 . In such embodiments, light detector  14  may be on or mounted to patch  27  or may receive light that is collected at patch  27  and directed to detector  14  by way of a suitable optical fiber (e.g. a receiver cable). 
     In other embodiments, optical fiber  26  carries light to a location on or in a catheter or other instrument inserted in the subject&#39;s body for diagnosis or medical treatment or surgery. For example,  FIG. 7  shows an apparatus  10 A in which optical fiber  26  extends into catheter  28  to carry light toward a tip of catheter  28 . Light detector  14  may be located at or near a tip of catheter  28  as illustrated, or at some other location on catheter  28 . Apparatus  10 A has many features in common with apparatus  10 , and the same reference numerals are used in  FIGS. 1 and 7  to label the common features. 
     As seen in  FIGS. 1 and 7 , an interlock mechanism  30  prevents light emitters  20  from being energized to emit high intensity light in certain cases where such high-intensity light could escape to cause harm. Some example interlock mechanisms that may be applied as an interlock mechanism  30  are described below. 
     Interlock mechanism  30  may be triggered to disable light emitters  20  upon various conditions including, for example, one or more of:
         A port  23  does not have a coupling  25  properly plugged in to it.   A shield (not shown in  FIG. 1 ) is not disposed to block any light escaping from ports  23 .
 
A mechanical interlock  31  may be provided to invoke interlock mechanism  30  if one or more of these conditions exist.
       

     A continuous wave (CW) safety mechanism  32  determines whether the light being output by light emitters  20  is pulsed or continuous wave. If the light being output is continuous wave (or, in some embodiments, if the light being output has a time-averaged intensity in excess of a threshold value) then light emitters  20  are automatically disabled. Some example mechanisms that may be applied as CW safety mechanism  32  are described below. 
     A subject detection system  34  determines whether or not patch  27  is against the skin of a subject. If patch  27  is not against the skin of a subject then there is a possibility that light being emitted at patch  27  could enter someone&#39;s eye and cause eye damage. Subject detection system  34  may detect one or more of:
         Signals at light detector  14  that are indicative of stray light being received by light detector  14 ;   Light detector  14  is not receiving light emitted by optical fiber  26 .       

       FIG. 2  shows a mechanical interlock  31  according to an example embodiment of the invention. Port(s)  23  are provided in a front panel  40  of housing  22 . Mechanical interlock  31  comprises a guard  42  that is movable between a “closed” position in which guard  42  blocks direct viewing of port(s)  23  and an “open” position in which port(s)  23  are exposed so that couplings  25  can be coupled to or disconnected from port(s)  23 . Guard  42  is opaque (or at least opaque enough to attenuate to a safe level) the light from light emitter(s)  20 . 
     When guard  42  is in its closed position, a slot or other opening  43  is provided to allow cables  24  to pass out from behind guard  42 . The slot or other opening is not in line with ports  23  such that any light that emerges from port(s)  23  cannot shine straight through the slot or other opening. 
     When guard  42  is in its open position, light emitter(s)  20  are inhibited from emitting light. In the illustrated embodiment, a power switch  44  is located such that it blocks guard  42  from being moved to its open position when power switch  44  is in an ON position (as shown in solid outline in  FIG. 2 ), permitting light emitter(s)  20  to be energized. When guard  42  is in its open position it blocks access to power switch  44  such that power switch  44  must be in an OFF position (as shown in dashed outline in  FIG. 2 ) and cannot be turned to the ON position. 
     In other embodiments, power switch  44  is located such that when it is in the ON position and guard  42  is in the closed position, movement of guard  42  toward the open position causes guard  42  to engage with and move power switch  44  from the ON position to the OFF position. This ensures that power to light emitters  20  is shut off as soon as an operator moves guard  42  from its closed position to expose ports  23 . 
     Guard  42  may comprise:
         a sliding cover which slides between the open and closed positions (as shown in  FIG. 2 );   a pivoting cover which pivots between the open and closed positions;   a separate cover that may be attached to cover port(s)  23 ; or   a combination of two or more of the above.       

     Power switch  44  may incorporate a lever, a push button, a toggle, a rocker switch or any suitable mechanism which switches power on and off, and engages with guard  42  to prevent unintended exposure of light emerging from ports  23 . 
     A circuit that inhibits operation of light emitters  20  may also, or in the alternative, be operated in response to micro-switches or other switches which assume a state such that power is shut off or the inhibition signal is present when guard  42  is its open position (and/or guard  42  is not in its closed position). 
       FIG. 3  shows a circuit  50  that may be used to implement a multi-trigger light output inhibiting arrangement in apparatus according to embodiments of the invention. Circuit  50  comprises a data processor  52  such as a programmable controller, a digital signal processor (DSP), a microprocessor, or the like. Data processor  52  controls light emitters  20  by way of a control interface  54 . Light emitters  20  emit light only when they are enabled by control interface  54 . 
     In the illustrated embodiment, data processor  52  receives an input from a light detector  55  that detects light emitted by a light emitter  20 . In the illustrated embodiment, there are three light emitters  20 A,  20 B and  20 C (collectively light emitters  20 ) and three corresponding light detectors  55 A,  55 B, and  55 C. When they are energized, light emitters  20 A,  20 B and  20 C emit light at different wavelengths. Some of the light is detected by light detectors  55 A,  55 B and  55 C. Light emitters  20  may be lasers, and are typically solid-state lasers such as laser diodes. 
     Outputs from light detectors  55  are sampled by one or more analog-to-digital converters (ADCs)  56  to yield digital signals that indicate the amount of light emitted by each light emitter  20 . The digital signals are provided to data processor  52 . In some embodiments, one or more ADCs  56  are integrated with data processor  52 . In other embodiments, ADCs are provided separately or light detectors  55  are of a type that provides a digital output. 
     Having light detectors  55  that monitor the light emitted by emitters  20  before the light passes through tissues of a subject is optional. Some embodiments lack such light detectors. In such embodiments information regarding the intensity of light emitted by light emitters  20  can be obtained from the intensity of light detected by light detector  14 . 
     The signals from light detectors  55  as well as any other signals may be subjected to suitable amplification, filtering, combinations thereof, or other suitable signal conditioning steps either in the digital or analog domain before those signals are processed by data processor  52 . The extent to which such signal conditioning is desirable or necessary in any particular embodiment is a matter of design choice. 
     Data processor  52  also receives digitized signals from light detector  14 . Light detector  14  is intended to detect light that has passed through tissues of a subject S. If light detector  14  receives light collected at a location on patch  27  then light detector  14  may also detect stray light if patch  27  is not properly against the skin of subject S. 
     Data processor  52  also receives signals from one or more switches or circuits  56  that detect whether couplings  25  are properly engaged with ports  23  and signals from one or more switches or circuits  58  that detect whether guard  42  is in its closed position. 
     Data processor  52  also receives signals from one or more user controls  59  (which may comprise switches, inputs made by way of a graphical or other computer interface, or the like) which indicate whether a user, such as a physician or medical technician desires to operate light emitters  20 . 
     Data processor  52  executes instructions  60  in a program store  62  (which may be, but is not necessarily integrated with data processor  52 ). Instructions  60  cause data processor  52  to generate a signal that permits and/or causes interface  54  to operate light emitters  20  to emit light when a set of one or more criteria is satisfied. 
     The set of criteria may, for example, permit light emitters  20  to emit light only if the following conditions are all met:
         User controls  59  indicates that a user wishes to operate the apparatus in a mode that requires light emitters  20  to be operational; AND   Switches or circuits  58  indicate that guard  42  is in its closed position; AND   Switches or circuits  56  indicate that couplings  25  are properly connected to ports  23 ; AND   The signal output from light detector  14  does not contain more than a threshold amount of noise (which could indicate exposure to ambient light); AND   The signal output from light detector  14  correlates with the operation of light emitters  20  (i.e. light detector  14  is detecting signals when one or more of light emitters  20  is emitting light and is not detecting significant amounts of light at other times); AND   The signal outputs from light detectors  55  indicates that light emitters  20  are each operating in a desired time sequence (For example, if light emitters  20  are intended to operate in a pulsed mode, this condition may require one or more of: the outputs of light detectors  55  have a corresponding pulsed waveform; at least a certain fraction of samples of the outputs of light detectors  55  is less than a threshold; or the like); AND   The signal outputs from light detectors  55  do not exceed threshold values.
 
In the above, AND represents the logical AND operation.
       

     It is not mandatory that data processor  52  check all of these conditions. Instructions  60  may cause data processor  52  to check:
         one or more of the above conditions; or   any combination of one or more of the above conditions with one or more other conditions.
 
If one or more of the conditions that data processor  52  checks is not satisfied then data processor  52  inhibits interface  54  so that light emitters  20  cannot operate.
       

     Some embodiments of the invention provide a circuit that independently verifies that the light being delivered by light emitters  20  meets certain emission criteria and inhibits the operation of light emitters  20  otherwise.  FIG. 3  shows a monitoring circuit  70  that has this function. Monitoring circuit  70  may monitor light emitters  20  to ensure one or more of the following:
         light emitters  20  are emitting pulsed light (as opposed to continuous wave light);   the outputs of light emitters  20  do not exceed a threshold; or the like.
 
Monitoring circuit may comprise one or more light detectors (such as light detectors  55 ) that detect light emitted by light emitters  20  and/or may monitor the electrical supply (current and/or voltage) delivered to light emitters  20 .
       

       FIG. 4  shows an example monitoring circuit  70 A which monitors both light output and electrical current supplied to a laser diode  72 . Circuit  70 A has been simplified for purposes of illustration. Conventional elements such as power supplies and the like have been omitted for clarity. In any particular embodiment additional signal conditioning circuitry may be necessary or desirable to achieve good results. Such additional circuitry is known to electrical engineers and others skilled in the field and is not shown in  FIG. 4  to avoid obscuring the invention. 
     In circuit  70 A, a fraction of the light emitted by laser diode  72  is intercepted by a light sensor detector  74  that generates an output signal proportional to the intensity of the detected light. This output signal is provided to a comparator  75  that compares the signal to a threshold voltage. The electrical current driving laser diode  72  is monitored by measuring a voltage drop across a series-connected resistor  76 . A signal indicating the voltage drop is passed through a low-pass filter  77 . The output from low-pass filter  77  is monitored by a comparator  78 . Outputs from comparators  75  and  78  are combined at OR gate  79  to provide an inhibition signal at the output of OR gate  79 . 
     Circuit  70 A may provide circuit elements for detecting CW operation instead of or in addition to a low-pass filter  77 . For example, an integrator configured to integrate the voltage drop signal over an integral number of periods of the driving signal for laser diode  72  or a timer configured to time pulses in the driving current for laser diode  72  could be used as alternative means for detecting CW operation of laser diode  72 . 
     In other embodiments, CW operation may be monitored by sampling the signal detected by light detector  14  or light detectors  55  at a frequency greater than the light pulse frequency. If light emitters  20  were emitting pulses of light, it is expected that some of the sampled signals would indicate that there is no light output (e.g. these sampled signals would be below a threshold value). The absence of any sampled signals below a threshold value may be an indication of CW operation of light emitters  20 . 
     The inhibition signal is applied to inhibit light emitters  20  from operating if either the peak light output from a light emitter  20  exceeds a threshold or if a light emitter  20  is operating in a continuous wave mode (or is delivering significantly longer-than-intended pulses). The inhibition signal preferably controls a switch or relay that is independent of the state of interface  54  such that inhibition circuit  70  can shut off light emitters  20  even if interface  54  fails. In the alternative, or in addition, circuit  70  may deliver the inhibition signal to interface  54 . 
     In preferred embodiments, light-emitting apparatus has multiple redundant systems for preventing damaging exposure to light including two or more of, and preferably all three of:
         A mechanical interlock that prevents operation of a switch that must be switched on to supply power to at least the part of the apparatus that powers light emitters  20 ;   An electronic circuit  70  that monitors at least electrical current being supplied to light emitters  20  and shuts off the current supplied to light emitters  20  if the electronic circuit detects that one or more of light emitters  20  is operating in a continuous wave mode (or in a mode that does not match a pattern being monitored for by the circuit); and,   A data processor that monitors one or more inputs and inhibits the operation of light emitters  20  by way of an interface  54  if any of the conditions fails to be satisfied.       

       FIG. 5  is a flow chart for a decision-making method  80  that may be implemented in a processor or logic circuits of a light-emitting apparatus. In block  82 , method  80  samples a signal detected at light detector  14  at spaced apart times. Where light output by light emitters  20  is pulsed, the times are spaced more closely together than the duration of light pulses so that light pulses will not be missed. For example, light emitters  20  are controlled to emit light in pulses having durations of 3 to 5 microseconds (for example about 4 microseconds) in some embodiments. In such embodiments, the output of light detector  14  may be sampled periodically with an interval between samples that is shorter than the pulse length (for example less than 3 microseconds). 
     In block  83 , method  80  compares the sampled signal value to a threshold (the threshold is selected to be indicative of a signal level that could correspond to a valid detected pulse). Where the sampled signal value is less than the threshold, method  80  branches to block  84 . 
     In block  84  method  80  determines whether or not a light pulse is expected to be detected at detector  14  (i.e. whether an emitter  20  should have been emitting light at the time of taking the sample). This may be determined by receiving a signal from emitter  20  indicative of whether emitter  20  is operating to emit light (e.g. the signal may indicate that power supply to emitter  20  is switched ON, etc.). In the event of a NO decision at block  84  then method  80  returns to block  82  as indicated at block  84 A. If the apparatus is in auto-recovery mode (as described below) then the apparatus is returned to its normal operating mode in block  84 B. 
     In the event of a YES decision at block  84  (indicating that a pulse was not detected but ought to have been detected) method  80  proceeds to block  85  which inhibits operation of light emitters  20  and block  86  which generates a message (such as a display, warning light, sound, etc.) indicating to users what has occurred. In block  88  method  80  waits for further instructions from a user (for example, method  80  may wait while the user checks the application of patch  27  and then resets the apparatus). 
     If block  83  determines that the sampled signal has a value exceeding the threshold then method  80  proceeds to block  89 . It is a design choice whether method  80  branches to block  85  or  89  when the sampled value is equal to the threshold. In block  89  method  80  determines whether or not a light pulse is expected to be detected at detector  14 . 
     In the event of a YES decision at block  89  then method  80  returns to block  82  as indicated at block  94 . In the event of a NO decision at block  89  (indicating that light emitters  20  could be emitting light when they are not intended to be on) method  80  proceeds to block  91  which inhibits operation of light emitters  20  and block  92  which generates a message (such as a display, warning light, sound, etc.) indicating to users what has occurred. Method  80  then proceeds to block  93 . In block  93 , method  80  may wait for further user input (such as at block  88 ) to resume operation of light emitters  20  or to perform some other action. 
     In some embodiments, block  93  may invoke an auto-recovery mode (or auto-discovery mode). In such embodiments, while auto-recovery mode is invoked, operation of light emitters  20  is inhibited and the signal at light detector  14  is monitored for a pattern that indicates that light detector  14  is shielded from ambient light (e.g. that patch  27  is properly in place on a subject in the case that light detector  14  is on patch  27  or senses light collected at patch  27 ). The pattern may include observed characteristics or trends (e.g. above normal variations, or signals indicative of ambient light noise) in the sampled signal values over time. 
     In some embodiments, for each value of the sampled signal detected at light detector  14 , a flag may be set corresponding to certain conditions (e.g. the sampled signal value is higher or lower than a threshold, and emitted light is or is not expected at the time of sampling). In some embodiments, a first flag may be set if the sampled signal value is lower than a threshold and a light pulse is expected; a second flag may be set if the sampled signal value is lower than a threshold, a light pulse is not expected and the device is operating in auto-recovery mode; or a third flag may be set if the sampled signal value is above a threshold and no light pulse is expected. A series of bits (or a bit string) may be generated to maintain a record of flags which are set for each sampled signal value. Patterns in the series of bits may be used to identify the existence of a condition. For example, if the first flag is set for numerous sampled signal values over a time period (indicating that a light pulse is expected but the detected light is below a threshold), this may be a pattern indicative of broken or disconnected optical fibers, malfunctioning light emitters or failure of the light detector to receive light or transmit a signal. Similarly, if the third flag is set for numerous sampled signal values over a time period (indicating that no light pulse is expected but the detected light is above a threshold), this may be a pattern indicative of light leaking to the light detector, a loose or detached patch or light detector not being shielded from ambient light. 
     Instead of generating a series of bits to record the flags which are set, as described above, other values, such as the sampled sensor signal values obtained by A/D sampling of the signal detected at light detector  14 , may be recorded. Patterns in the recorded sensor signal values may then be identified and compared with predetermined pattern conditions. 
       FIG. 6  is a flow chart for a decision-making method  100  that may be implemented in a processor or logic circuits of a light-emitting apparatus. Method  100  may be performed together with method  80  but may also be performed independently of method  80 . 
     In block  102 , method  100  samples a signal detected at light detector  14  at spaced apart times. In block  103 , method  100  compares the sampled signal value to a threshold (the threshold is selected to be indicative of a signal level that could correspond to a valid detected pulse). Where the sampled signal value is less than the threshold, method  100  branches to block  104 . Where the sampled signal value is greater than the threshold, method  100  branches to block  106 . 
     Blocks  104  and  106  each determine whether or not a pulse is expected to be detected at detector  14 . In a case where method  80  and  100  are being performed together, blocks  82 ,  83 ,  84  and  89  may be shared between methods  80  and  100  and may provide blocks  102 ,  103 ,  104  and  106  of method  100 . 
     In the event of a NO result in block  104  (i.e. no pulse is expected at detector  14 ), method  100  proceeds to block  108  which determines whether the auto-recovery mode has been invoked. If not, method  100  returns to block  102 . If auto-recovery mode has been invoked then method  100  proceeds to block  109  which sets a bit of a bit string (FLAG 2 ) and then proceeds to block  110 . 
     In block  110 , bit string FLAG 2  is compared to a predetermined pattern, PATTERN 2 , that would be expected for the case that light detector  14  is shielded from ambient light and patch  27  is properly in place on a subject. If PATTERN 2  is not matched by bit string FLAG 2 , method  100  returns to block  102 . Otherwise, at block  111 , the apparatus is placed into its normal running mode (i.e. the operation of light emitters  20  is not inhibited and light emitters  20  are controlled to emit light in a desired operational mode—such as a desired pattern of pulses). Method  100  then returns to block  102 . 
     In the event of a YES result in block  104  (i.e. a pulse is expected at detector  14 ), then in block  115  a bit is set in a bit string (FLAG 1 ) and method  100  proceeds to block  116 . In block  116 , bit string FLAG 1  is compared to a predetermined pattern, PATTERN 1 , that would be expected for the case that there is a broken or disconnected optical fiber  26 , light detector  14  is not receiving light or is not transmitting a signal (e.g. broken or disconnected receiver cables), or light emitters  20  are not emitting light (e.g. malfunctioning lasers). If bit string FLAG 1  does not match PATTERN 1  (NO result in block  116 ) then method  100  returns to block  102 . Otherwise, method  100  inhibits light emitters  20  in block  118 , and generates a message (such as a display, warning light, sound, etc.) indicating to users what has occurred in block  119 . In block  120 , method  100  pauses and waits for a user input before proceeding. For example, in block  120  method  100  may wait while the user reconnects a cable that has become disconnected and then resets the apparatus. 
     In the event of a YES result in block  106  (indicating that a light pulse is expected) then method  100  returns to block  102 . Otherwise, method  100  proceeds to block  125  where a bit is set in a bit string (FLAG 3 ) and method  100  proceeds to block  126 . In block  126 , the bit string FLAG 3  is compared to a predetermined pattern, PATTERN 3 , that would be expected for the cases where there is light leaking to light detector  14 , patch  27  is loose, light detector  14  is not shielded from ambient light, or patch  27  has been removed from a subject. If block  126  returns a NO result then method  100  returns to block  102 . 
     If block  126  returns a YES result then method  100  inhibits light emitters  20  in block  128 , and generates a message (such as a display, warning light, sound, etc.) indicating to users what has occurred in block  129 . In block  130  method  100  places the apparatus in the auto-recovery mode (including inhibiting operation of light emitters  20 ). Method  100  then returns to block  102 . 
     Certain implementations of the invention comprise computer processors which execute software instructions which cause the processors to perform a method of the invention. For example, one or more processors in a light-emitting device may implement the methods of  FIGS. 5 and 6  by executing software instructions in a program memory accessible to the processors. The invention may also be provided in the form of a program product. The program product may comprise any medium which carries a set of computer-readable signals comprising instructions which, when executed by a data processor, cause the data processor to execute a method of the invention. Program products according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, physical media such as magnetic data storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, or the like. The computer-readable signals on the program product may optionally be compressed or encrypted. 
     Where a component (e.g. a software module, processor, assembly, device, circuit, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many modifications, permutations, additions and sub-combinations are possible in the practice of this invention without departing from the spirit or scope thereof. For example:
         Electronic circuit  70  may include a programmable device such as a data processor, may be provided on an application specific integrated circuit (ASIC), may comprise a suitably-configured field programmable gate array (FPGA), may be made with discrete components or a combination of integrated circuits and discrete components, or the like.
 
It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.