Imaging array with enhanced event detection

1. An imaging device comprising an array of pixels in which, for each pixel, there is provided:

a sensor which is sensitive to a variable quantity to be imaged and for outputting a signal representative of the variation in said quantity during a monitoring period;

a comparator having a first input and a second input;

a connection for connecting the output of said sensor to said first input in such a way that said comparator continuously monitors said output signal; and

a connection for applying a reference voltage to said second input;

said comparator being such as to change its output state at the point in time at which the amplitude of said output signal crosses said reference voltage level,

wherein each pixel further comprises a hit flag generator which is operable to generate a flag signal indicative of whether, during the monitoring period, the output state of said comparator changes, and readout circuitry which is operable to read the contents of a memory unit during a read period, following the monitoring period, only if the hit flag generator indicates that the output state of the comparator changed during the monitoring period.

2. An imaging device as claimed in claim 1 wherein each pixel further comprises a memory unit operable to store a value representative of the said point in time at which the amplitude of said output signal crosses said reference voltage level.

3. An imaging device as claimed in claim 2 further comprising a counter unit which generates a steadily increasing or decreasing value during the monitoring period, which value is written to the memory unit at said point in time to thereby represent the timing of said point in time.

4. An imaging device as claimed in claim 3 further including a reset unit for resetting said counter unit at the beginning of the monitoring period so that the value stored in the memory unit is representative of the timing of said point in time with respect to the beginning of the monitoring period.

5. An imaging device as claimed in claim 4 wherein said counter unit is a Gray code generator for generating a steadily increasing digital value, and wherein said memory unit comprises a register for storing the digital value reached at said point in time.

6. An imaging device as claimed in claim 1 wherein said hit flag generator receives input from the output of said comparator such that said flag signal indicates a “hit” if the output state of the comparator changes during the monitoring period, but not otherwise.

7. An imaging device as claimed in claim 1 , wherein said hit flag generator receives input from the output of said memory unit and wherein the flag signal is set to indicate a “hit” only if the memory unit contains a value other than a reset value.

8. An imaging device as claimed in claim 1 wherein said hit flag generator is set so as to indicate a “hit” only if the output state of the comparator changes within a certain range of times from the beginning of the monitoring period.

9. An imaging device as claimed in claim 1 wherein each pixel further comprises a memory unit for storing a unique value identifying the pixel within each column of the pixel array.

10. An imaging device as claimed in claim 1 wherein the reference voltage is a d.c. voltage which remains unchanged over time.

11. An imaging device as claimed in claim 1 wherein the reference voltage changes over the monitoring period in such a direction as to bring forward said point in time at which the amplitude of the sensor output signal crosses the reference voltage level.

12. An imaging device as claimed in claim 11 wherein said reference voltage changes over the monitoring period according to a law which tends towards making a quantity to be imaged having a particular value in intensity space have the same value as its corresponding value in time space.

13. An imaging device comprising an array of pixels in which, for each pixel, there is provided:

a sensor which is sensitive to a variable quantity to be imaged and for outputting a signal representative of the variation in said quantity during a monitoring period;

a comparator having a first input and a second input;

a connection for connecting the output of said sensor to said first input in such a way that said comparator continuously monitors said output signal; and

a connection for applying a reference voltage to said second input;

said comparator being such as to change its output state at the point in time at which the amplitude of said output signal crosses said reference voltage level,

wherein each pixel comprises a plurality of memory units, wherein the output of the comparator is connected to a multi-way switch which is operable to connect the output of the comparator to one of said memory units at a time, and wherein a reset unit is provided for resetting the output of said sensor after a change of state of the comparator output so that a further change of the output state of the comparator within the monitoring period can be detected, said multi-way switch being controlled in such a way as to store timing information relating to each change of the output state of the comparator within the monitoring period in a separate one of said memory units.

14. An imaging device comprising an array of pixels in which, for each pixel, there is provided:

a sensor which is sensitive to a variable quantity to be imaged and for outputting a signal representative of the variation in said quantity during a monitoring period;

a comparator having a first input and a second input;

a connection for connecting the output of said sensor to said first input in such a way that said comparator continuously monitors said output signal; and

a connection for applying a reference voltage to said second input:

said comparator being such as to change its output state at the point in time at which the amplitude of said output signal crosses said reference voltage level,

wherein each pixel further comprises amplitude measurement circuitry for measuring the amplitude of the output signal from the sensor at the point in time at which the amplitude of the output signal crosses said reference voltage level, and a memory unit for storing a value representative of the amplitude of the output signal from the sensor, and

wherein each pixel further includes a switching circuitry means associated with said comparator for causing said comparator to operate in one of two modes:

a) a “timing” mode in which said reference voltage level is constant and said second input to the comparator is connected to receive said reference voltage level, and said output is connected to said memory unit; or

b) an “amplitude” mode in which said second input to the comparator is connected to receive a ramp voltage to cause the comparator to operate as a ramp ADC, and said output is connected to said memory unit

and a control unit for switching the comparator to “timing” mode at the beginning of the monitoring period, and switching to “amplitude” mode in the event that the comparator output changes state, thus enabling the analogue value of the amplitude to be stored as a digital value, by means of the ADC action of the comparator, in said memory unit.

15. An imaging device as claimed in claim 14 wherein the control unit is such as to switch the comparator to “amplitude” mode immediately upon a change of state of the comparator output so that the amplitude value stored in the memory unit is representative of the variation in said quantity from the beginning of the monitoring period to the point in time at which the amplitude of said output signal crosses said ramp voltage.

16. An imaging device as claimed in claim 14 wherein the control unit is such as to switch the comparator to “amplitude” mode at the end of the monitoring period, only provided that, during the monitoring period, the output of the comparator has changed state, so that the amplitude value stored in the memory unit is representative of the variation in said quantity over the whole monitoring period.

17. An imaging device comprising an array of pixels in which, for each pixel, there is provided:

a sensor which is sensitive to a variable quantity to be imaged and for outputting a signal representative of the variation in said quantity during a monitoring period;

a comparator having a first input and a second input;

a connection for connecting the output of said sensor to said first input in such a way that said comparator continuously monitors said output signal; and

a connection for applying a reference voltage to said second input;

said comparator being such as to change its output state at the point in time at which the amplitude of said output signal crosses said reference voltage level,

wherein each pixel further comprises amplitude measurement circuitry for measuring the amplitude of the output signal from the sensor at the point in time at which the amplitude of the output signal crosses said reference voltage level, and a memory unit for storing a value representative of the amplitude of the output signal from the sensor, and

wherein each pixel includes a further comparator whose output is connected to said memory unit, whose first input is connected to receive the output signal of said sensor and whose second input is connected to receive a ramp voltage so as to cause the further comparator to operate as a ramp ADC to enable the analogue value of the amplitude of the sensor output signal to be stored as a digital value in said memory unit.

18. An imaging device as claimed in claim 17 wherein each pixel further includes switching circuitry associated with said comparators for causing each comparator to operate in one of two modes:

a) a “timing” mode in which said reference voltage level is constant and the second input to the comparator is connected to receive said reference voltage level, or

b) an “amplitude” mode in which said second input to the comparator is connected to receive said ramp voltage to cause the comparator to operate as a ramp ADC

and a control unit for switching said comparator to “timing” mode and said further comparator to “amplitude” mode at the beginning of a first monitoring period, and for switching said comparator to “amplitude” mode and said further comparator to “timing” mode at the beginning of the next following monitoring period, and so on in a repeating sequence.

19. An imaging device as claimed in claim 18 , wherein each pixel further includes further switching circuitry connected to the outputs of said comparators and operable to switch each comparator output to a respective one of a pair of memory units according to whether the comparator is in “timing” mode or “amplitude” mode.