FIELD OF THE DISCLOSURE

The present disclosure relates to compensating for spatially varying illuminance of an event sensing device.

BACKGROUND OF THE DISCLOSURE

Spatially varying attenuation and/or obstruction of light may undesirably cause correspondingly spatially varying illuminance of an image sensor. Such spatially varying attenuation and obstruction is an inherent feature of optical lenses. In particular, vignetting may result in a gradual reduction of illuminance towards the periphery of an image sensor as compared to a centre of the image sensor. Spatially varying illuminance may impair the operation of an event sensor used for detecting changes in brightness in a scene, inasmuch that pixels of the event sensor may exhibit mutually different responses to changes in brightness of the imaged scene. For example, peripherally located pixels of an event sensor affected by vignetting may respond more slowly to changes in scene brightness/pixel illuminance than centrally located pixels, or may even fail completely to detect such changes.

SUMMARY OF THE DISCLOSURE

An objective of the present disclosure is to compensate for spatially- varying illuminance of photosensors of an event sensing device.

A first aspect of the present disclosure provides an event sensing device having a sensing region, comprising: a plurality of photosensors arranged across the sensing region, for providing respective photosensor signals indicative of an intensity of incident light, and one or more adaption circuitries configured to determine changes in intensity of light incident on the photosensors exceeding one or more threshold levels based on the respective photosensor signals provided by the photosensors, wherein a sensitivity of the one or more adaption circuitries to the changes in intensity of light incident on the photosensors depends on a distance of the respective photosensor from a centre of the sensing region. Each photosensor may, for example, comprise a photodiode. The event sensing device is thus operable to detect light incident on the plurality of photosensors arranged across, i.e., spatially-distributed about, the sensing region, e.g., in an array, and to determine changes in the intensity of light incident on the photosensors exceeding one or more threshold levels. For example, the event sensing device may output event signals in response to determination of changes in light intensity on the photosensors exceeding the thresholds. Such changes in incident light intensity may provide useful information about a scene imaged by the event sensing device. For example, spatiotemporal changes in light intensity may be indicative of motion in an imaged scene, inasmuch that a moving object may be expected to trigger brightness changes in the spatially-distributed photosensors at different times, depending on the velocity of the object, i.e., the speed and direction of motion of the object. The speed of motion of the object may thus be quantified using known values of the inter-pixel distance and the time difference between detected events.

However, spatially varying attenuation and/or obstruction of light may undesirably cause correspondingly spatially varying illuminance of the spatially-distributed photosensors, even in the instance that an optically-flat scene is imaged. Such spatially varying attenuation may, for example, be caused by a lens interposed in front of the photosensors. In particular, vignetting may result in a gradual reduction of illuminance with increasing distance of the respective photosensor from a centre of the sensing region. As a result of vignetting, photosensors located towards a periphery of the sensing region/array may be exposed to a lower illuminance compared to photosensors located centrally in the sensing region/array, even for an optically-flat scene. Such spatially varying illuminance may impair the operation of the event sensing device, inasmuch that the photosensors may exhibit mutually different responses to changes in brightness of an imaged scene, even where the imaged scene is optically flat. For example, peripherally located photosensors of the array affected by vignetting may respond more slowly to changes in scene brightness than centrally located photosensors, or may even fail completely to detect such changes. In particular, such spatially varying illuminance of the photosensors may impair quantification of motion of an object in a scene, inasmuch that the response times of photosensors in the array to changes in scene brightness caused by a moving object may be mutually different, or photosensors of the array subjected to relatively reduced illumination resulting from vignetting may even fail completely to respond to a change in light intensity, thereby complicating measurement of a time difference between detected events. The event sensing device of the present disclosure however is configured such that it’s sensitivity to changes in intensity of light incident on the photosensors depends on a distance of the respective photosensor from a centre of the sensing region. In other words, the sensitivity of the event sensing device of the present disclosure to changes in intensity of light incident on the photosensors is not constant across all of the photosensors, rather the sensitivity differs with increasing distance of the respective photosensor from the centre of the sensing region. This spatially-varying sensitivity of the event sensing device to light incident on the photosensors may at least partially compensate for correspondingly spatially-varying illuminance of the photosensors. The detection of changes in light intensity in an imaged scene, and the information regarding the scene conveyed thereby, may thereby advantageously be improved.

The present disclosure discloses two example ways in which the sensitivity of the event sensing device to changes in intensity of incident light on the photosensors may be spatially- varied. A first way involves spatially varying the one or more threshold levels, i.e., the threshold intensity above with an adaptation circuitry provided to a photosensors triggers an event. For example, the threshold levels may be varied with distance of the respective photosensor from the centre of the sensing region. A second way involves spatially varying a bias applied to the photosensors and/or the adaptation circuitries. The bias levels influence the response of the photosensors and/or the adaptation circuitries to changes in intensity of light incident on the photosensors. By either approach, the sensitivity of the event sensing device to changes in intensity of light incident on the photosensors may be spatially varied, to thereby at least partially compensate for correspondingly spatially-varying illumination of the photosensors.

The one or more adaptation circuitries are employed for placing a respective one or more of the photosensors in a desired operating state, and for detecting/determining when a change in intensity of light incident on the respective photosensors exceeding one or more threshold levels has occurred. In examples, each of the photosensors may be provided with a respective adaptation circuity. In other examples, two or more of the photosensors may share all or part of a common adaptation circuitry.

In an implementation, the sensitivity of the one or more adaption circuitries to changes in intensity of light incident on the photosensors increases with a distance of the respective photosensor from the centre of the sensing region. In other words, the event sensing device may be configured to be increasingly more sensitive to changes in intensity of light incident on the photosensors with increasing distance of the respective photosensors from the centre of the sensing region. The event sensing device may thereby at least partially compensate for the corresponding reduction in illuminance of photosensors with increasing distance of the photosensor from the centre of the sensing region caused by vignetting. As a result, the detection of changes in light intensity in an imaged scene, and the information regarding the scene conveyed thereby, may thereby advantageously be improved. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured to apply mutually different threshold levels for determining changes in intensity of light incident on the photosensors depending on the distance of the respective photosensor from the centre of the sensing region.

In other words, in examples the spatially-varying sensitivity of the event sensing device is achieved by the adaptation circuitries utilising mutually different threshold levels, i.e., different levels above/below which a change in intensity of light is determined. The mutually different threshold levels thereby vary the response of the adaptation circuitries to changes in intensity of light incident on the photosensors. In particular, the different threshold levels have the result that the adaptation circuitries will determine, i.e., detect, changes in intensity of light incident on the photosensors, with differing sensitivities. A relatively low threshold level may have the result that even a relatively low change in intensity of light incident on one or more of the photosensors is detected, such that the respective photosensors may be relatively highly sensitive to changes in intensity of incident light. A relatively higher threshold level may have the result that only relatively higher changes in intensity of light incident on one or more of the photosensors is detected, such that the respective photosensors may be relatively lowly sensitive to changes in intensity of incident light. For example, the adaptation circuitries may use one or more first threshold levels for detecting changes in intensity of light incident on photosensors located centrally on the sensing region, and one or more further threshold levels, which are relatively lower for detecting changes in intensity of light incident on photosensors located towards a periphery of the sensing region. The different threshold levels may be achieved by varying threshold voltages/currents applied to comparator circuits of the adaptation circuitries. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved. In an implementation, the one or more adaption circuitries are configured to apply one or more threshold levels for determining changes in intensity of light incident on one or more of the photosensors located closer to the centre of the sensing region that are relatively higher than one or more threshold levels for determining changes in intensity of light incident on one or more other of the photosensors located relatively further from the centre of the sensing region.

In other words, the adaptation circuitries may use relatively higher threshold levels for determining changes in intensity of light incident on centrally located photosensors than for peripherally located photosensors. This has the effect of de-sensitising the event sensing device to changes in intensity of light incident on the centrally located photosensors, or expressed conversely, increasing the sensitivity of the event sensing device to changes in intensity of light incident on the peripherally located photosensors. The event sensing device may thereby compensate for the decreasing illuminance of the photosensors with increasing distance from the centre of the sensing region caused by vignetting. In the context, the reference to ‘higher’ threshold levels refers to threshold levels that are farther from zero, whether negative or positive. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured to apply mutually different bias signals for determining changes in intensity of light incident on the photosensors depending on a distance of the respective photosensor from the centre of the sensing region.

In other words, in examples the spatially-varying sensitivity of the event sensing device is achieved by the adaptation circuitries utilising mutually different bias levels. The bias levels set the operating points of the adaptation circuitries and/or the photosensors, and thus define the respective responses of the event sensing device to changes in intensity of light incident on the photosensors. In particular, the bias levels may influence the speed with which the photoreceptor output responds to changes in illumination. Higher biases may thus cause a delayed response compared to lower biases. This means that the adaptation circuitries can include a delay that depends on illuminance level. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured to apply one or more bias signals for determining changes in intensity of light incident on one or more of the photosensors located closer to the centre of the sensing region that are relatively lower than one or more bias signals for determining changes in intensity of light incident on one or more other of the photosensors located relatively further from the centre of the sensing region.

In other words, the adaptation circuitries may use relatively lower bias signals, e.g., signals having a lower current or voltage, for determining changes in intensity of light incident on centrally located photosensors than for peripherally located photosensors. This has the effect of de-sensitising the event sensing device to changes in intensity of light incident on the centrally located photosensors, or expressed conversely, increasing the sensitivity of the event sensing device to changes in intensity of light incident on the peripherally located photosensors. The event sensing device may thereby compensate for the decreasing illuminance of the photosensors with increasing distance from the centre of the sensing region caused by vignetting. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured to set the bias signals applied to the photosensors as a function of a photocurrent generated by the respective photosensor.

By setting the bias signals applied to the photosensors as a function of the respective photocurrent, the response of the adaptation circuits to changes in intensity of incident light may be set as a function of the specific illuminance of the photosensor, thereby resulting in adaptation of the event sensing device to different light levels. In particular, this technique may reduce a power consumption of the adaptation circuitries and maintain a constant resonance of the photosensor irrespective of lighting level.

In an implementation, the one or more adaption circuitries are configured to set the bias signals applied to the photosensors as a function of a sum of photocurrents generated by the respective photosensor and one or more other photosensors located adjacent to the respective photosensor.

In other words, the bias signals for each photosensor may be set as a function of a sum of photocurrents generated by the respective photosensor and one or more other local photosensors. This may achieve the adaptation of the event sensing device to different light levels, whilst advantageously minimising the effect of a spurious or otherwise unreliable photocurrent generated by a photosensor. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured such that differences in sensitivity to changes in intensity of light incident on the photosensors depends on an intensity of light incident on the photosensors.

In other words, the sensitivities of the adaptation circuitries may be set in dependence on an illuminance of one or more of the photosensors. This may advantageously allow the response of the adaptation circuitries, and in particular, the relatively sensitivities of the photosensors to take account of the lighting conditions. For example, in high-level lighting conditions it may be acceptable to de-sensitise adaptation circuitries provided to centrally-located photosensors relatively highly, to thereby apply relatively high compensation for vignetting, as the lighting conditions may be sufficiently high to still result in reliable detection of changes in intensity of light. In contrast, in relatively low-level lighting conditions, de-sensitising of the adaptation circuitries may result in an unacceptably high risk of significant changes in intensity of light not being determined/detected. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured such that a difference in sensitivity to changes in intensity of light incident on the photosensors is positively correlated to an intensity of light incident on the photosensors.

In other words, the compensation for spatially- varying illuminance of the photosensors may be used more fully when imaging in high lighting level conditions. This may advantageously minimise the effect of vignetting, whilst still ensuring that changes in intensity of incident light are determined reliably. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

In an implementation, the one or more adaption circuitries are configured such that the application of a relatively higher threshold level for determining changes in intensity of light incident on the one or more photosensors located closer to the centre of the sensing region is dependent on an intensity of light incident on the photosensors. In other words, the technique for varying the sensitivity of the adaptation circuitries by varying the threshold levels may be utilised in dependence on a level of illuminance of the photosensors.

This is because the threshold levels employed influence the degree of noise in detections. For example, in relatively low-level lighting conditions, relatively low threshold levels may be appropriate in order to reliably determine significant changes in intensity of incident light. However, usage of such relatively low threshold levels may disadvantageously result in an increased level of spurious detections, i.e., increased noise. Thus, in relatively low-level lighting conditions, it may be desirable to vary the sensitivity of the adaptation circuitries only relatively lowly, or not at all, by the technique of setting spatially varying threshold levels, and instead vary the sensitivities using an alternative technique, e.g., using spatially-varying bias signals. Thereby noise in the detections may advantageously be reduced. Thus, as a result of this feature, the determination of changes in intensity of incident light may be improved.

A second aspect of the present disclosure provides a method for determining changes in light incident on an event sensing device having a sensing region, the method comprising determining changes in intensity of light incident on a plurality of photosensors located across the sensing region exceeding one or more threshold levels based on respective photosensor signals provided by the photosensors, wherein the determining changes in intensity of light comprises determining changes in intensity of light incident on the photosensors with a sensitivity that depends on a distance of the respective photosensor from a centre of the sensing region.

In an implementation, the sensitivity to changes in intensity of light incident on the photosensors increases with a distance of the respective photosensor from the centre of the sensing region.

In an implementation, the determining changes in intensity of light comprises determining changes in intensity of light incident on the photosensors exceeding mutually different threshold levels which depend on the distance of the respective photosensor from the centre of the sensing region. In an implementation, the determining changes in intensity of light comprises determining changes in intensity of light incident on one or more of the photosensors located closer to the centre of the sensing region that are relatively higher than one or more threshold levels for sensing changes in intensity of light incident on one or more threshold levels for determining changes in intensity of light incident on one or more other of the photosensors located relatively further from the centre of the sensing region

In an implementation, the determining changes in intensity of light comprises sensing changes in intensity of light incident on the photosensors by applying mutually different bias signals depending on a distance of the respective photosensor from the centre of the sensing region.

In an implementation, the determining changes in intensity of light comprises determining changes in intensity of light incident on the photosensors by applying mutually different bias signals depending on a distance of the respective photosensor from the centre of the sensing region.

In an implementation, the determining changes in intensity of light comprises determining changes in intensity of light incident on the photosensors by applying one or more bias signals for determining changes in intensity of light incident on one or more of the photosensors located closer to the centre of the sensing region that are relatively lower than one or more bias signals for determining changes in intensity of light incident on one or more other of the photosensors located relatively further from the centre of the sensing region.

A third aspect of the present disclosure provides a computer program comprising machine- readable instructions which, when executed by a computer, cause the computer to carry out the method of any implementation of the second aspect of the present disclosure.

A fourth aspect of the present disclosure provides a computer-readable data carrier having the computer program of any implementation of the third aspect of the present disclosure stored thereon.

The foregoing and other objectives are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the Figures. These and other aspects of the invention will be apparent from the embodiment s) described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 schematically shows an example of an event sensing device embodying an aspect of the disclosure, comprising a photosensor array and a change detector system;

Figure 2 schematically shows an example of the change detector system identified with reference to Figure 1, comprising one or more adaptation circuitries;

Figure 3 schematically shows an example of the photosensor array identified with reference to Figure 1;

Figure 4 schematically shows an example of the adaptation circuitries identified with reference to Figure 2;

Figure 5 shows example processes involved in a method for sensing changes in intensity of light using the event sensing device, which includes a process of configuring the event sensing device to compensate for spatially-varying illuminance of the photosensor array; and

Figure 6 shows example processes involved in the compensating for spatially-varying illuminance of the photosensor array.

DETAILED DESCRIPTION OF THE DISCLOSURE

Referring firstly to Figure 1, in examples, an event sensing device 101 embodying an example of an aspect of the present disclosure comprises a plurality of photosensors forming a photosensor array 102, a change detector system 103, input/output interface 104, and communication link 105. Photosensor array 102 comprises a two-dimensional array of plural discrete photosensors, each photosensor being responsive to incident light to convert light energy into electrical charge to thereby generate an electrical signal proportional to an intensity of incident light. In examples, the photosensors are photodiodes. Photosensor array 102 will be described in further detail with particular reference to Figure 3.

Change detector system 103 comprises circuitry functional to detect changes in intensity of light incident on photosensors in the array 102 based on the respective electrical signals output by the photosensors. In particular, change detector system 103 is functional to detect changes in intensity of light incident on the photosensors in the array exceeding threshold intensity change levels. In examples, change detector system 103 comprises plural discrete detection circuitry, each circuit being functional to detect changes in intensity of light incident on a respective one of the photosensors in the array. Change detector system 103 is configured to output an event signal when a change in intensity of light incident on a photosensor in the array 102 exceeds a threshold, the event signal defining the x, y location of the triggered photosensor in the array, the time t, and the 1-bit polarity of the change (i.e., brightness increase (“ON”) or decrease (“OFF”). Thus, the output of the change detector system 103 is a variable data-rate sequence of digital “events” or “spikes”, with each event representing a change of brightness of predefined magnitude at a photosensor at a particular time. Change detector system 103 will be described in further detail with particular reference to Figure 2.

Input/output interface 104 is provided for communication of the event sensing device 101 with external systems. For example, input/output interface 104 may allow coupling of the event sensing device 101 to a system supporting a human-machine interface device, to allow control of the event sensing device 101 by an operator.

The components 102 to 104 of the event sensing device 101 are in communication via communication link 105. In examples, communication link 105 may comprise a system bus. In other examples, communication link 105 may comprise a network.

Thus, as will be described in further detail herein, in examples, event sensing device 101 is operable to detect changes in the intensity of light incident on photosensors in the array 102, and to output respective event signals in response to changes in light intensity on the photosensors exceeding threshold values. Such changes in incident light intensity may provide useful information about an imaged scene. For example, spatiotemporal changes in light intensity may be indicative of motion in an imaged scene. Where an object in the imaged scene is moving, the moving object may be expected to trigger brightness changes in the photosensors of the array 102 at different times, depending on the velocity of the object, i.e., the speed and direction of motion of the object. The speed of motion of the object may thus be quantified using known values of the inter-pixel distance and the time difference between detected events. Motion, and a quantity/magnitude, of the motion may thus be detected directly from events, or via an intensity change edge map generated from detected events.

However, spatially varying attenuation and/or obstruction of light may undesirably cause correspondingly spatially varying illuminance of the photosensors in the array 102, even in the instance that an optically-flat scene is imaged. Such spatially varying attenuation may, for example, be caused by a lens interposed in front of the photosensor array 102. In particular, vignetting may result in a gradual reduction of illuminance towards the periphery of the photosensor array 102 compared to a centre of the array 102. Spatially varying illuminance may impair the operation of the event sensing device 101, inasmuch that photosensors of the array 102 may exhibit mutually different responses to changes in brightness of an imaged scene, even where the imaged scene is optically flat. For example, peripherally located photosensors of the array 102 affected by vignetting may respond more slowly to changes in scene brightness than centrally located pixels, or may even fail completely to detect such changes. Such spatially varying illuminance of photosensors in the array 102 may thus, for example, impair quantification of motion of an object in a scene, inasmuch that the response times of photosensors in the array to changes in scene brightness caused by a moving object may be mutually different, or photosensors of the array subjected to relatively reduced illumination may even fail completely to respond to a change in light intensity, thereby complicating measurement of a time difference between detected events. An objective of the present disclosure therefore is to compensate at least partially for spatially varying illuminance of photosensors in the array 102 of the event sensing device 101 when using the event sensing device 101 to sense changes in light intensity in an imaged scene. The detection of changes in light intensity in an imaged scene, and the information regarding the scene conveyed thereby, may thereby be improved.

In examples, event sensing device 101 has a unitary structure, in which the components 102 to 104 are co-located in a single unit. For examples, the event sensing device 101 could comprise a hand-held unit, in which two or more of the components 102 to 104 are located inside a single casing. In such examples, in which two or more of the components 102 to 104 are co-located, communication link 105 may comprise a system bus for communicating the co-located components.

In other examples, one or more of the components 102 to 104 of event sensing device 101 may be located remotely of one or more other of the components. For example, in other examples, photosensor array 102 and change detector system 103 may be located in mutually different locations, and may communicate via communication link 105, which could, in examples, be a network implemented, for example, by a wide area network (WAN) such as the Internet, a local area network (LAN), a metropolitan area network (MAN), and/or a personal area network (PAN), etc. Such a network could be implemented using wired technology such as Ethernet, Data Over Cable Service Interface Specification (DOCSIS), synchronous optical networking (SONET), and/or synchronous digital hierarchy (SOH), etc.) and/or wireless technology e.g., Institute of Electrical and Electronics (IEEE) 802.11 (Wi-Fi), IEEE 802.15 (WiMAX), Bluetooth, ZigBee, near-field communication (NFC), and/or Long-Term Evolution (LTE), etc.). The network may include at least one device for communicating data in the network. For example, the network may include computing devices, routers, switches, gateways, access points, and/or modems.

Referring next to Figure 2, in examples, change detector system 103 comprises one or more adaptation circuitries 201, processor 202, computer storage/memory 203, input/output interface 204 and communication link 205. The change detector system 103 is configured to run a computer program stored in storage 203 for sensing changes in intensity of light incident on photosensors in the array 102.

The one or more adaptation circuitries 201 are functional to detect changes in intensity of light incident on photosensors in the array 102 exceeding one or more threshold levels, based on respective photosensor signals output by the photosensors, and to output event signals when a change in light intensity sensed by a photosensor exceeds a threshold. More particularly, the photosensor circuitries 201 are functional to supply bias signals to photosensors in the array to place the photosensors in a desired operative state, receive photosensor signals from photosensors in the array, compare the received photosensor signals to threshold values, and generate the event signals when a change in the magnitude of the received photosensor signals exceeds the threshold. Photosensor circuitries 201 will be described in further detail with reference to Figure 4.

Processor 202 is configured for execution of instructions of a computer program for detecting changes in intensity of light incident on photosensors in the array 102.

Storage/memory 203 is configured for non-volatile storage of computer programs for execution by the processor 202. In the embodiment, the computer program for detecting changes in intensity of light incident on photosensors in the array is stored in storage/memory 202. Storage/memory 203 is further configured as read/write memory for storage of operational data associated with computer programs executed by the processor 202.

Input/output interface 204 is provided for communicating change detector system 103 with communication link 105. The components 201 to 204 of the change detector system 103 are in communication via communication link 205.

Referring next particularly to Figure 3, in examples, photosensor array 102 comprises a plurality of photosensors, such as photosensor 301, located on a substrate 302 to form a two- dimensional array defining a sensing region of the event sensing device 101, i.e., a region of the event sensing device 101 that is sensitive to incident light. In examples, each of the photosensors 301 is substantially like, and comprises a photodiode responsive to incident light to convert light energy into electrical charge, to thereby generate an electrical signal proportional to an intensity of incident light. For example, each photodiode may generate a voltage or current proportional to an intensity of incident light.

The array of photosensors located on the substrate 302 form a generally circular sensing region, having a centre 303 and a periphery 304, wherein the photosensors are arranged across the sensing region. A first plurality of the photosensors of the array 302 are located centrally on the substrate 302, each photosensor of the first plurality being a first radial distance from the centre 302 of the sensing region, to thereby form a central region 305 of the photosensor array 302. A second plurality of the photosensors of the array 102 are located on an annular middle region of the substrate 302, each photosensor of the second plurality being a second radial distance from the centre 303 of the sensing region, to thereby form a middle region 306 of the photosensor array 102. And a third plurality of the photosensors of the array 102 are located about a periphery of the substrate 302, each photosensor of the third plurality being a third radial distance from the centre 303 of the sensing region, to thereby form a peripheral region 307 of the photosensor array 102.

Spatially varying attenuation and/or obstruction of incident light may result in a correspondingly spatially varying illuminance of the photosensors in the array 102. For example, vignetting may result from reduced illuminance of the photosensors in the peripheral region 307 of the array 102 than for photosensors in the middle or central regions 306, 305 respectively, even in the instance that an optically-flat scene is imaged. In particular, the illuminance of the photosensor array 102 may be expected to reduce gradually from the centre 303 towards the periphery 304 due to vignetting. In examples therefore, the event sensing device 101 is configured such that the change detector system 103 is less sensitive to changes in intensity of light incident on the photosensors in the central region 305 of the array 102 than for photosensors in the middle region 306, and less sensitive still to changes in intensity of light incident on the photosensors in the middle region 306 than for photosensors in the peripheral region 307. In other words, in examples, the event sensing device 101 is configured to be increasingly more sensitive to changes in intensity of light incident on photosensors of the array as a function of a distance of the respective photosensors from the centre 305 of the array 102. In the example, the event sensing device 101 is therefore more sensitive to changes in intensity of light incident on photosensors located in the peripheral region 307 of the photosensor array 102 than to light incident on photosensors located in the middle and central regions 306 and 305 respectively of the photosensor array 102. This increased sensitivity of the event sensing device to light incident on photosensors of the array 102 located further from the centre of the array, and in particular to photosensors located in the peripheral region 307 of the array 102 may at least partially compensate for the expected reduction in illuminance of the peripheral region 307 of the array compared to the central region 305 resulting from vignetting.

In the example depicted in Figure 3, the photosensor array is shown as comprising relatively few photosensors 301, namely, four photosensors in the central region 305, and eight photosensors in each of the middle region 306 and the peripheral region 307. In other examples, the photosensor array 102 may comprise more or fewer photosensors. For example, in examples the photosensor array may comprise several million photosensors spread across the central, middle and peripheral regions, 305, 306 and 307 respectively of the photosensor array. Referring next particularly to Figure 4, in examples, the change detector system 103 comprises a plurality of discrete adaptation circuitries 201, each photosensor 301 in the array 102 being provided with a respective adaptation circuitry 201 for controlling the operation of the respective photosensor for detecting changes in intensity of incident light on the photosensor, and for generating event signals in response to a change in intensity of light incident on the respective photosensor exceeding a threshold change value.

Thus, in the example depicted in Figure 4, each of the photosensors of the array 102 is provided with a respective adaptation circuitry, such as adaptation circuity 201 provided to photosensor 301. In other examples, two or more of the photosensors of the photosensor array 102 may share all or part of an adaptation circuitry such as adaptation circuitry 201.

In examples, adaptation circuitry 201 comprises six functional stages 401 to 406, namely, a photoreceptor stage 401, a source follower stage 402, a differentiator amplifier stage 403, a comparator-based event signal generator stage 404, a bias generator 405, and a threshold generator 406.

The photoreceptor stage 401 couples to the respective photosensor, e.g., photosensor 301, and stabilises the voltage across the photosensor to create a voltage signal which is proportional to the log of the light intensity (the photosensor signal). The photoreceptor stage comprises a saturated NMOS transistor Ma, for supplying a bias current I _Ph,Sum to the photosensor 301. The bias current I _Ph,sum sets the operating point of the photosensor. The bias current I _Ph,sum is supplied by the bias generator 405. The bias current I _Ph,sum may, for example, be set by the bias generator 405 globally, i.e., as a function of a sum of photocurrents generated by all of the photosensors of the array 102. Alternatively, the bias current may be set locally, as a function of a photocurrent generated by the respective photosensor, or as a function of a sum of photocurrents generated by the respective photosensor and one or more other photosensors located adjacent the respective photosensor in the array 102.

The gate of the NMOS transistor Ma is connected to the output of an inverting amplifier (M _pr , M _cas , M _n ) whose input is connected to the photosensor 301, where M _pr is biased by a bias voltage V _b,Pr . This bias controls the amplifier in the photoreceptor stage, and limits the speed with which the output of the photoreceptor stage can respond to changes in intensity of light incident on the photosensor 301. An instantaneous change in illumination causes a change in the light-related signal which takes a finite time to readjust. The magnitude of the bias voltage Vb, _P r thus influences the speed with which a pixel can respond to changes in light (the “bandwidth”). If the pixel bandwidth is high then it will detect faster oscillations of illumination; however it will also respond to higher frequency electronic noise, therefore producing more noise events (especially in low lighting conditions). The photosensor 301 may thus produce a current proportional to an intensity of incident light, in response to which a voltage, V _P r _, is generated by the amplifier.

The photoreceptor stage output V _pr is buffered by the source follower 402, a type of amplifier, to pass the signal from the photoreceptor stage 401 through to the differencing amplifier 403 whilst reducing coupling from the differencing amplifier 403 back to the photoreceptor stage 401. The source follower stage 402 thus isolates the photoreceptor stage 401, and the photosensor 301, from the rapid transients in the differencing circuit 403.

The source follower stage 402is biased by bias signal Vbs f, which dictates the speed at which this amplifier works. If this bias is set sufficiently high then it should have no effect on performance. However, if this bias is low then it can limit the bandwidth of the pixel in much the same way as the Pr bias can. The source follower thus transfers the Voltage V _pr , to an input terminal of the capacitor Cl of the differencing amplifier circuit 403 to drive the capacitive input of the differencing circuit 403.

The differencing amplifier 403 removes a direct current (DC) component of the Voltage V _pr , using the capacitor Cl and a capacitor C2, amplifies the Voltage V _pr , based on a ratio between the capacitors Cl and C2, and outputs a Voltage Vdiff. The differencing amplifier 403 is balanced with a reset switch that shorts its input and output together, resulting in a reset voltage level. Resetting of the differencing amplifier 403 may thus be performed between each instance of receiving a photosensor signal from the photosensor 301.

The current comparator stage 404 detects changes in the light-related signal and produces digital “ON” and “OFF” signals, indicative of a change in the photosensor signal exceeding a threshold change, and hence indicative of a change in intensity of light incident on the photosensor 301 exceeding a threshold intensity. An “ON” signal indicates an intensity of light on the photosensor increased by a threshold intensity value. An “OFF” signal indicates an intensity of light on the photosensor decreased by a threshold intensity value. The comparators (MON _P , MON _p , MOFF _II , MOFF _p ) compare the output of the differencing amplifier stage 403 against thresholds that are offset from the reset voltage to detect increasing and decreasing changes. If the input of a comparator overcomes its threshold the ON or OFF event is generated. _More particularly, the current comparator stage 404 generates a current Ii based on the Voltage V _diff , provided to a gate of the P-type transistor MO _Np . Also, the current comparator generates a current IO _N based on an ON threshold bias voltage provided from the threshold generator 406 to a gate of the N-type transistor MO _NP . When a value of the current Ii is greater than a value of he current IO _N , the adaptation circuitry 201 outputs an ON event signal. When the value of the current Ii is less than or equal to the value of the current IO _N , the adaptation circuitry 201 does not output the ON event signal. In addition, the current comparator 404 generates a currenth based on the voltage V _diff provided to a gate of another P-type transistor MO _FFp . Also, the current comparator generates a current IO _FF based on an OFF threshold voltage provided from the threshold generator 406 to a gate of another N-type transistor MO _FFII . When a value of the currenth is greater than a value of the current IO _FF , the adaptation circuitry 201 outputs an OFF_event signal. When the value of the currenth is less than or equal to the value of the current IO _FF , the adaptation circuitry does not output the OFF event signal.

When an event in which an intensity of light increases is sensed by the adaptation circuity 201, the adaptation circuitry may output an ON event signal. For example, when an amount of an increase in the intensity of the light is greater than a first threshold variation, the sensing element 110 may output an ON event signal. When an event in which an intensity of light decreases is sensed by the adaptation circuitry 201, the adaptation circuitry may output an OFF event signal. For example, when an amount of a decrease in the intensity of the light is greater than a second threshold variation, the sensing element 110 may output an OFF event signal.

The first threshold variation may be set based on the ON threshold voltage applied to a gate of the transistor MO _NP . When a value of the ON threshold voltage signal increases, the first threshold variation may increase. Accordingly, to output the ON event signal, an intensity of light incident on the photosensor 301 may need to further increase.

Similarly, the second threshold variation may be set based on the OFF threshold voltage signal applied to a gate of the transitor MO _FFII . When a value of the OFF threshold voltage 122 increases, the second threshold variation may increase. Accordingly, to output the OFF event signal, an intensity of light incident on the photosensor 301 may need to further decrease.

Accordingly, by controlling the ON threshold voltage and the OFF threshold voltage generated by the threshold generator 406, a sensitivity of the adaptation circuitries 201 to generate an event signal may be adjusted.

The bias generator 405 is functional to generate one or more of the bias signals I _P h,sum, Vb,pr, Vb,sf for biasing the adaptation circuitry 201 in a way as previously described, to thereby adjust the response of the adaptation circuitry 201 to a photosensor signal output by the respective photosensor 301.

The threshold generator 406 is functional to generate one or more threshold signals for application to the transistors MON _P , MOFF _II of the current comparator stage 404, to thereby set the threshold variations defining the magnitude of a change in illumination of the photosensor 301 that results in generation of ON or OFF event signals.

Referring next to Figure 5, in examples, event sensing device 101 is configured to perform a procedure for determining changes in intensity of light incident on photosensors in the array 102 comprising four stages. In examples, the procedure may be controlled by change detector system 103, in accordance with a computer program stored in storage 203.

At stage 501, the change detector system 103 initiates a light intensity detection procedure. For example, the change detector system 103 may initiate the procedure in response to receiving a prompt from a human-machine interface, or other input system, coupled to input/output interface 104.

At stage 502, the change detector system 103 performs a procedure for compensating for spatially- varying illuminance of photosensors in the array 102. In particular, as will be described in further detail with reference to Figure 6, stage 502 may involve configuring respective sensitivities of the one or more adaptation circuitries 201 of the change detector system 103 to changes in intensity of light incident on a respective one or more of the photosensors of the array, e.g., photosensor 301. The respective sensitivities of the adaptation circuities 201 may thereby be spatially- varied to compensate for correspondingly spatially- varying illuminance of the photosensors of the array 102. In examples, the illuminance variation compensation procedure may be implemented based on a location of the respective photosensor in the array 102. For example, the sensitivity of the one or more adaptation circuitries 201 to changes in intensity of light incident on the photosensors in the array 102 could be set as a function of a distance of the respective photosensor from the centre 303 of the array 102. This may thereby wholly or partially compensate for spatially- varying illuminance of the photosensor array 102 resulting from vignetting. For example, the sensitivity of the one or more adaptation circuitries 201 to changes in intensity of light incident on a respective one or more of the photosensors of the array 201 may be set to be greater for photosensors located a relatively greater distance from the centre of the array 102, e.g., for photosensors in the peripheral region 307 of the array, than for photosensors located relatively closer to the centre 303 of the array, e.g., for photosensors located in the centre region 305 of the array 102.

At stage 503, the event sensing device 101 images a scene using the photosensor array 102 at different time-points, and uses the adaptation circuitries 201 to detect changes in scene brightness between the time-points. Such changes in scene brightness could, for example, be interpreted to infer motion of objects in the imaged scene. Detection of such changes in scene brightness, e.g., detection of motion of objects in the scene, may provide useful information about the scene. The one or more adaptation circuitries 201 may subsequently generate “ON” or “OFF” event signals in the event of detection of a change in intensity of light incident on a respective one or more of the photosensors of the array 102 exceeding a respective threshold value. Stage 503 may, for example, involve the adaptation circuity storing such “ON” or “OFF” event signals in storage/memory 203.

At stage 504, the change detector system 103 may output “ON” or “OFF” event signals generated by the adaptation circuitries 201 at stage 503. For example, the change detector system 103 may output the “ON” or “OFF” event signals to an external computing device coupled to input/output interface 104 of event sensing device 101.

Referring next particularly to Figure 6, in examples, the method of stage 502 for compensating for variation in illuminance of the photosensor array comprises three stages. In examples, the method of stage 502 is implemented by the processor 202 of change detector system 103, in accordance with instructions of the computer program stored in storage 203. At stage 601, the computer program stored in storage 203 causes the processor 202 to control the bias generators 405 of each of the one or more adaptation circuitries 201 to determine bias signals levels, e.g., bias signal levels for application to one or more of the photoreceptor stage 401, the source follower stage 402, and the differencing amplifier stage 403. The determined bias signals may thus cause the adaptation circuitries 201 to detect changes in intensity of light incident on a respective one or more of the photosensors with relative sensitivities to at least partially compensate for spatially-varying illuminance of the photosensor array. Thus, by the process of stage 601, respective responses of the adaptation circuitries 201 to changes in intensity of light incident on a respective one or more of the photosensors of the array 102 may be varied.

At stage 602, the computer program stored in storage 203 causes the processor 202 to control the threshold generators 406 of each of the one or more adaptation circuitries 201 to determine threshold signals level for application to the current comparator stage 404, to set respective threshold variation levels at which “ON” or “OFF” event signals are generated by the respective current comparator stage 404. The determined threshold signals may thus cause the adaptation circuitries to detect changes in intensity of light incident on a respective one or more of the photosensors exceeding mutually different threshold change values to at least partially compensate for spatially- varying illuminance of the photosensor array. Thus, by the process of stage 602, respective responses of the adaptation circuitries 201 to changes in intensity of light incident on a respective one or more of the photosensors of the array 102 may be further varied.

The processes of either or both of stages 601 and 602 may be performed in dependence on an intensity of light incident on the photosensors, to thereby vary the absolute and relative sensitivities of the adaptation circuits in dependence on the level of illumination of the photosensors. For example, the process of stage 602 for determining threshold levels may take into account the level of illumination of one or more of the photosensors of the array 102.

At stage 603, the computer program stored in storage 203 causes the processor 202 to set the respective sensitivities of the adaptation circuitries 201 to changes in intensity of light incident on a respective one or more of the photosensors of the array 102 by application of the respective bias and or threshold signals generated at stages 601 and 602 respectively. Thus, by the process of stage 603, respective responses of the adaptation circuitries 201 to changes in intensity of light incident on a respective one or more of the photosensors of the array 102 occurring during later stage 503 may be defined.

Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. _In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.