Photodetector calibration of an optical measurement system

1. A wearable system for use by a user comprising:

a head-mountable component configured to be attached to a head of the user, the head-mountable component comprising an array of photodetectors configured to detect photons from a light pulse after the light pulse reflects off a target within the head; and

a computing device comprising a processor configured to:

measure a noise level of a photodetector included in the array of photodetectors, the noise level comprising a dark count rate that measures a dark count divided by a time period;

determine that the noise level meets a predetermined threshold comprising a dark count rate threshold; and

prevent, based on the determining that the noise level meets the predetermined threshold, an output of the photodetector from being used in generating a histogram based on a temporal distribution of photons detected by the array of photodetectors, the preventing comprising switching the output to a monitoring circuit that monitors a characteristic of the optical measurement system separate from the photodetector.

2. The wearable system of claim 1 , further comprising:

a plurality of counting circuits each corresponding to a different photodetector of the array of photodetectors, the plurality of counting circuits including a counting circuit for the photodetector;

wherein the measuring of the dark count rate of the photodetector comprises directing the counting circuit to count output signals from the photodetector during a predetermined period of time while a light source is not emitting the light.

3. The wearable system of claim 2 , wherein the processor is further configured to:

measure a dark count rate for each photodetector of the array of photodetectors using the plurality of counting circuits during the predetermined period of time; and

prevent an output of each photodetector included in the array of photodetectors that has a dark count rate above the predetermined threshold from being used in generating the histogram.

4. The wearable system of claim 3 , further comprising:

a one-time programmable memory configured to store data representative of each photodetector having a dark count rate that is above the predetermined threshold;

wherein the processor is further configured to periodically measure the dark count rate for each photodetector not represented in the one-time programmable memory as having the dark count rate that is above the predetermined threshold.

5. The wearable system of claim 1 , wherein the measuring of the noise level of the photodetector comprises measuring a signal-to-noise ratio (SNR) of the photodetector.

6. The wearable system of claim 5 , further comprising a plurality of counting circuits each corresponding to a different photodetector of the array of photodetectors, the plurality of counting circuits including a counting circuit for the photodetector;

wherein the measuring of the SNR of the photodetector comprises directing the counting circuit to:

count output signals from the photodetector for a first predetermined period of time while a light source is not emitting light, and

count output signals from the photodetector for a second predetermined period of time while the light source is emitting a uniform light.

7. The wearable system of claim 6 , wherein the determining that the noise level meets the predetermined threshold comprises determining that the SNR is below the predetermined threshold.

8. The wearable system of claim 6 , wherein the processor is further configured to:

measure an SNR for each photodetector of the array of photodetectors using the plurality of counting circuits during the first and second predetermined periods of time; and

prevent an output of each photodetector included in the array of photodetectors that has an SNR below the predetermined threshold from being used in generating the histogram.

9. The wearable system of claim 1 , wherein the preventing of the output of the photodetector from being used in generating the histogram comprises disabling the photodetector.

10. The wearable system of claim 1 , wherein the monitoring circuit is configured to determine a temperature associated with the optical measurement system based on the noise level of the photodetector.

11. The wearable system of claim 1 , wherein the monitoring circuit is configured to determine a peak location of a light pulse emitted by a light source.

12. The wearable system of claim 1 , wherein the monitoring circuit is configured to determine an excess bias voltage of the photodetector.

13. The wearable system of claim 1 , wherein the computing device is housed in the head-mountable component.

14. The wearable system of claim 1 , wherein each photodetector of the array of photodetectors comprises:

a single photon avalanche diode (SPAD); and

a fast gating circuit configured to arm and disarm the SPAD.

15. A method comprising:

measuring, by a computing device comprising a processor, a noise level of a photodetector included in an array of photodetectors, the noise level comprising a dark count rate that measures a dark count divided by a time period;

determining, by the processor, that the noise level meets a predetermined threshold comprising a dark count rate threshold; and

preventing, by the processor, based on the determining that the noise level meets the predetermined threshold, an output of the photodetector from being used in generating a histogram based on a temporal distribution of photons detected by the array of photodetectors the preventing comprising switching the output to a monitoring circuit that monitors a characteristic of the optical measurement system separate from the photodetector.