Lighting device having an interim operable state

1. A lighting device comprising:

an emitter configured to emit light;

a detector configured to generate a detector signal in response to a luminous flux of the light emitted by the emitter; and

a control circuit configured to:

adjust a drive current conducted through the emitter in response to the detector signal and a measured value of a first operational characteristic when operating in a normal state;

determine if the measured value of the first operational characteristic is outside of a range when operating in the normal state;

operate in an interim operable state if the measured value of the first operational characteristic is outside of the range; and

adjust the drive current conducted through the emitter in response to the detector signal and a measured value of a second operational characteristic when operating in the interim operable state.

2. The lighting device of claim 1 , wherein the control circuit is configured to determine if the measured value of the first operational characteristic is outside of the range when operating in the interim operable state, and operate in an inoperable state if the measured value of the first operational characteristic is outside of the range when operating in the interim operable state.

3. The lighting device of claim 2 , wherein the control circuit is configured to return to the normal state if the measured value of the first operational characteristic is within the range when operating in the interim operable state.

4. The lighting device of claim 3 , wherein, when operating in the interim operable state, the control circuit is configured to increase a counter if the measured value of the first operational characteristic is outside of the range and decrease the counter if the measured value of the first operational characteristic is within the range, the control circuit configured to operate in the inoperable state if the counter exceeds a failure threshold, and return to the normal state if the counter drops to zero.

5. The lighting device of claim 3 , wherein the control circuit is configured to operate in the inoperable state in response to continuing to detect measured values of the first operational characteristic that are outside of the range during a first time period when operating in the interim operable state; and

wherein the control circuit is configured to return to the normal state in response to detecting no measured values of the first operational characteristic that are outside of the range during a second time period when operating in the interim operable state.

6. The lighting device of claim 2 , wherein the control circuit is configured to turn off the emitter when operating in the inoperable state.

7. The lighting device of claim 1 , wherein the first operational characteristic comprises a forward voltage of the emitter, and the second operational characteristic comprises a forward voltage of the detector.

8. The lighting device of claim 7 , wherein the control circuit is configured to adjust the drive current conducted through the emitter in response to the detector signal and the measured value of the forward voltage of the emitter when operating in the normal state.

9. The lighting device of claim 8 , wherein the control circuit is configured to adjust the drive current conducted through the emitter in response to the detector signal and the measured value of the forward voltage of the detector when operating in the interim operable state.

10. The lighting device of claim 9 , wherein the control circuit is configured to not adjust the drive current in response to a measured value of the forward voltage of the emitter when operating in the interim operable state.

11. The lighting device of claim 1 , wherein the control circuit is configured to determine if the measured value of the first operational characteristic is outside of the range by determining if a difference between a measured value of the first operational characteristic and a measured value of the second operational characteristic is greater than a threshold.

12. A method for controlling a lighting device having at least one emitter and at least one detector, the method comprising:

measuring a luminous flux of light emitted by the at least one emitter using the at least one detector;

adjusting a drive current for the at least one emitter in response to a measured value of the luminous flux and a measured value of a forward voltage of the at least one emitter when operating in a normal state;

determining if the measured value of the forward voltage of the at least one emitter is outside of a range when operating in the normal state;

operating in an interim operable state if the measured value of the forward voltage of the at least one emitter is outside of the range; and

adjusting the drive current for the at least one emitter in response to a measured value of the luminous flux and a measured value of a forward voltage of the at least one detector when operating in the interim operable state.

13. The method of claim 12 , further comprising:

determining if a measured value of the forward voltage of the at least one emitter is outside of the range when operating in the interim operable state; and

operating in an inoperable state if the measured value of the forward voltage of the at least one emitter is outside of the range when operating in the interim operable state.

14. The method of claim 13 , further comprising:

returning to the normal state if the measured value of the forward voltage of the at least one emitter is within the range when operating in the interim operable state.

15. The method of claim 14 , further comprising:

increasing a counter if the measured value of the forward voltage of the at least one emitter is outside of the range when operating in the interim operable state; and

decreasing the counter if the measured value of the forward voltage of the at least one emitter is within the range when operating in the interim operable state;

wherein operating in an inoperable state further comprises operating in the inoperable state if the counter exceeds a failure threshold; and

wherein returning to the normal state further comprises returning to the normal state if the counters drops to zero.

16. The method of claim 14 , further comprising:

continuing to detect measured values of the forward voltage of the at least one emitter that are outside of the range during a time period when operating in the interim operable state;

wherein operating in an inoperable state comprises operating in the inoperable state in response to continuing to detect measured values of the forward voltage of the at least one emitter that are outside of the range during the time period.

17. The method of claim 14 , further comprising:

detecting no measured values of the forward voltage of the at least one emitter that are outside of the range during a time period when operating in the interim operable state;

wherein returning to the normal state comprises returning to the normal state in response to detecting no measured values of the forward voltage of the at least one emitter that are outside of the range during the time period.

18. The method of claim 13 , further comprising:

turning off the at least one emitter when operating in the inoperable state.

19. The method of claim 13 , further comprising:

exiting the inoperable state to enter the normal state in response to receiving a special command.

20. The method of claim 12 , wherein determining if the measured value of the forward voltage of the at least one emitter is outside of a range further comprises determining if a difference between the measured value of the forward voltage of the at least one emitter and a measured value of the forward voltage of the at least one detector is greater than a threshold.