Image sensor and image capturing apparatus

1. An image sensor comprising a plurality of pixels, each pixel including:

an avalanche photodiode that is driven in Geiger mode and outputs an output voltage that varies in response to a photon entering;

a quenching resistor;

at least one comparator that compares the output voltage with a first threshold voltage or a second threshold voltage different from the first threshold voltage and outputs a single signal each time the output voltage varies so as to exceed either of the first and second threshold voltages; and

at least one counter that counts a number of signals output by the comparator upon comparing the output voltage with the first and second threshold voltages and outputs a count value, for each of the first and second threshold voltages.

2. The image sensor according to claim 1 ,

wherein the comparator includes:

a first comparator that compares the output voltage with the first threshold voltage and outputs a single signal each time the output voltage varies so as to exceed the first threshold voltage; and

a second comparator that compares the output voltage with the second threshold voltage and outputs a single signal each time the output voltage varies so as to exceed the second threshold voltage, and

the counter includes:

a first counter that counts a number of signals output from the first comparator and outputs a first count value; and

a second counter that counts a number of signals output from the second comparator and outputs a second count value.

3. The image sensor according to claim 2 ,

wherein a difference between the output voltage and the first threshold voltage when a photon is not entering is lower than a difference between the output voltage and the second threshold voltage when a photon is not entering.

4. The image sensor according to claim 1 ,

wherein

each of the plurality of pixels further includes a switch that switches between the first threshold voltage and the second threshold voltage and supplies the voltage to the comparator; and

the counter counts a number of signals output from the comparator when comparing the output voltage with the first threshold voltage and outputs a first count value, and counts a number of signals output from the comparator when comparing the output voltage with the second threshold voltage and outputs a second count value.

5. The image sensor according to claim 4 ,

wherein a difference between the output voltage and the first threshold voltage when a photon is not entering is lower than a difference between the output voltage and the second threshold voltage when a photon is not entering.

6. The image sensor according to claim 1 ,

wherein the plurality of pixels are all the pixels in the image sensor.

7. The image sensor according to claim 1 , further comprising:

a determinator that determines that a pixel for which a difference between the count values obtained for the first and second threshold voltages is greater than or equal to a predetermined threshold is a saturated pixel.

8. The image sensor according to claim 7 ,

wherein a predetermined count value is output to the exterior of the image sensor from the pixel determined to be a saturated pixel.

9. An image sensor comprising:

a pixel array including a plurality of first pixels and a plurality of second pixels, each pixel having an avalanche photodiode that is driven in Geiger mode and outputs an output voltage that varies in response to a photon entering,

wherein each of the first pixels includes:

a first quenching resistor;

a first generator that generates a first threshold voltage;

a first comparator that compares the output voltage with the first threshold voltage and outputs a single signal each time the output voltage varies so as to exceed the first threshold voltage; and

a first counter that counts a number of signals output from the first comparator and outputs a first count value, and

each of the second pixels includes:

a second quenching resistor;

a second generator that generates a second threshold voltage different from the first threshold voltage;

a second comparator that compares the output voltage with the second threshold voltage and outputs a single signal each time the output voltage varies so as to exceed the second threshold voltage; and

a second counter that counts a number of signals output from the second comparator and outputs a second count value.

10. The image sensor according to claim 9 ,

wherein a difference between the output voltage and the first threshold voltage when a photon is not entering is lower than a difference between the output voltage and the second threshold voltage when a photon is not entering.

11. The image sensor according to claim 9 , further comprising:

a determinator that determines that a pixel for which a difference between the count values obtained for the first and second threshold voltages is greater than or equal to a predetermined threshold is a saturated pixel.

12. The image sensor according to claim 11 ,

wherein a predetermined count value is output to the exterior of the image sensor from the pixel determined to be a saturated pixel.

13. An image capturing apparatus comprising:

the image sensor comprising a plurality of pixels, each pixel including:

an avalanche photodiode that is driven in Geiger mode and outputs an output voltage that varies in response to a photon entering;

a quenching resistor;

at least one comparator that compares the output voltage with a first threshold voltage or a second threshold voltage different from the first threshold voltage and outputs a single signal each time the output voltage varies so as to exceed either of the first and second threshold voltages; and

at least one counter that counts a number of signals output by the comparator upon comparing the output voltage with the first and second threshold voltages and outputs a count value, for each of the first and second threshold voltages; and

a determinator that determines that a pixel for which a difference between the count values obtained for the first and second threshold voltages output from the image sensor is greater than or equal to a predetermined threshold is a saturated pixel.

14. An image capturing apparatus comprising:

the image sensor comprising:

a pixel array including a plurality of first pixels and a plurality of second pixels, each pixel having an avalanche photodiode that is driven in Geiger mode and outputs an output voltage that varies in response to a photon entering,

a first quenching resistor;

wherein each of the first pixels includes:

a first generator that generates a first threshold voltage;

a first comparator that compares the output voltage with the first threshold voltage and outputs a single signal each time the output voltage varies so as to exceed the first threshold voltage; and

a first counter that counts a number of signals output from the first comparator and outputs a first count value, and

each of the second pixels includes:

a second quenching resistor;

a second generator that generates a second threshold voltage different from the first threshold voltage;

a second comparator that compares the output voltage with the second threshold voltage and outputs a single signal each time the output voltage varies so as to exceed the second threshold voltage; and

a second counter that counts a number of signals output from the second comparator and outputs a second count value; and

a determinator that determines that a pixel for which a difference between the count values obtained for the respective threshold voltages output from the image sensor is greater than or equal to a predetermined threshold is a saturated pixel.