Photodetector calibration of an optical measurement system
An illustrative optical measurement system includes a light source configured to emit light directed at a target, an array of photodetectors configured to detect photons of the light after the light is scattered by the target, and a processing unit 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.
1. An optical measurement system comprising:
a light source configured to emit light directed at a target;
an array of photodetectors configured to detect photons of the light after the light is scattered by the target; and
a processing unit 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 optical measurement 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 the light source is not emitting the light.
3. The optical measurement system of claim 2 , wherein the processing unit 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 optical measurement 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 processing unit 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 optical measurement 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 optical measurement 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 the 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 optical measurement 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 optical measurement system of claim 6 , wherein the processing unit 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 optical measurement system of claim 8 , wherein the preventing the 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 comprises switching the output of each photodetector included in the array of photodetectors that has an SNR below the predetermined threshold to the monitoring circuit.
10. The optical measurement 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 optical measurement system of claim 1 , wherein the monitoring circuit is configured to determine a peak location of a light pulse emitted by the light source.
12. The optical measurement system of claim 1 , wherein the monitoring circuit is configured to determine an excess bias voltage of the photodetector.
13. The optical measurement 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.
14. The optical measurement system of claim 1 , wherein the array of photodetectors is included in a wearable device configured to be worn by a user.
15. The optical measurement system of claim 14 , wherein the wearable device includes a head-mountable component configured to be worn on a head of a user.
16. A method comprising:
measuring a noise level of a photodetector of an optical measurement system, the noise level comprising a dark count rate that measures a dark count divided by a time period;
determining that the noise level meets a predetermined threshold comprising a dark count rate threshold; and
preventing, based on the determining that the noise level meets the predetermined threshold, an output of the photodetector from being used to determine a histogram based on a temporal distribution of photons detected by an 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.
17. The method of claim 16 , further comprising:
measuring a dark count rate for each photodetector of the array of photodetectors using a plurality of counting circuits during the predetermined period of time; and
preventing 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.
18. The method of claim 17 , further comprising:
measuring, periodically, the dark count rate for each photodetector not represented in a one-time programmable memory as having the dark count rate that is above the predetermined threshold, the one-time programmable memory configured to store data representative of each photodetector having a dark count rate that is above the predetermined threshold.
19. The method of claim 17 , wherein the preventing the 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 comprises switching the output of each photodetector included in the array of photodetectors that has a dark count rate above the predetermined threshold to the monitoring circuit.
20. The method of claim 16 , wherein the monitoring circuit is configured to determine a temperature associated with the optical measurement system based on the noise level of the photodetector.