WO2013030561A1

WO2013030561A1 - Pixel circuit

Info

Publication number: WO2013030561A1
Application number: PCT/GB2012/052104
Authority: WO
Inventors: Bhaskar CHOUBEY
Original assignee: The University Court Of The University Of Glasgow
Priority date: 2011-08-26
Filing date: 2012-08-28
Publication date: 2013-03-07
Also published as: GB201114898D0

Abstract

A circuit for detecting light incident on the circuit and providing an electrical representation of that light (generally known as a pixel) is provided, the circuit Including; a photo-sensitive device having a photosensor input and a photosensor output; a signal generator generating a reference potential that varies with time according to a predetermined function; a first device arranged to set a potential at a first node, said first node being connected to said photosensor output; and a second device arranged to compare the potential at said first node to said reference potential to determine a capture moment from the comparison and to output the value of said reference potential at said capture moment, wherein first device and said second device are devices which are arranged to conduct current between two of their nodes when the potential at a third node is higher than that at one of those nodes. Preferably the first and second devices are nMOS transistors. The pixel can provide a device which is able to capture illumination across a wide range of intensities, which is formed of just one type of device and which provides a large area for light capture (has a high fill factor). An Image sensor incorporating a plurality of such circuits and a camera or other recording device have such an Image sensor are also provided.

Description

PIXEL CIRCUIT

Field of the Invention

The present invention relates to a circuit which converts light into electrical voltage. Such circuits are often referred to as "pixel circuits", or just "pixels". The invention is particularly, but not exclusively, concerned with a pixel circuit which uses N OS type transistors. The invention also relates to image sensors for electronic imaging device having such pixel circuits and imaging devices have such imaging sensors.

Background of the Invention

Electronic imaging devices such as digital cameras (both still and video) are generally based around an image sensor having an array of light-sensitive detectors. Each detector is arranged to detect the light falling on a part of the image sensor and its output is combined with the outputs of the other detectors in the image sensor to produce a complete picture. As such, each light-sensitive detector relates to a picture element (which is normally referred to by the shorthand "pixel") and so is referred to as a pixel detector or pixel circuit. Light falling on each pixel detector generates an output signal corresponding to the amount of light (which may be the amount of light of a particular colour) falling on each of those detectors. The output signals from the pixel detectors are digitised and stored in an electronic file that contains the information which makes up the complete image.

Many known electronic imaging devices use a CCD sensor, which measures light arriving at the sensor by integrating the photocurrent of a number of pixel detectors over a set period to obtain a measurement of the charge that has passed through the detector. Other devices use CMOS sensors and integrate the photocurrent from those sensors.

Integration of the photocurrent is known to suffer from problems where the scene to be captured has a large dynamic range as the pixels described above have a linear output in a relatively low dynamic range.

For example, in nature, the amount of illumination varies from around 0.05 Lux On a moonlit night to around 100,000 Lux in bright daylight which is a range of over 6 orders of magnitude (also referred to as "decades"). Cameras which are currently available are generally only able to capture around 2-3 decades of light and this can lead to saturation in the bright parts of a captured image.

Techniques have been proposed to enhance the dynamic range of a pixel, including using multiple sampling at different integration times or logarithmic image capture. However, these techniques tend to be limited by the large time required to capture the image or inherent temporal noise present in these electronic circuits.

It is therefore an object of the present invention to provide a device which is able to capture illumination across a wide range of intensities, preferably at least 6 decades of intensity.

Furthermore, equipment to display images, including paper, printers, computer screens and projectors are also limited in their abilities and display only 2-3 decades of intensity. Hence even if a device is provided which is able to capture a wide dynamic range (or which is able to artificially produce an image having a wide dynamic range), it has to be subsequently manipulated (which may require complex mathematical techniques) to fit the dynamic range of the display equipment. This process is referred to as tone mapping.

It is therefore an object of the present invention to provide a device for recording an image which is able to reduce the processing requirements for tone mapping by recording the image in a form which reduces or even removes the need for tone mapping prior to display of the image.

WO 2007/051964 discloses an image sensor in which each pixel detector includes a photosensor that is arranged to detect incident light and provide a signal that represents a time integral of the detected light intensity. A signal generator device is connected and arranged to generate a reference signal that varies according to a predetermined function. A comparator has a first input connected to receive the photosensor signal and a second input connected to receive the reference signal and an output for providing a capture signal. The comparator is arranged to generate a capture signal at a time determined by comparing the photosensor signal and the reference signal. A read out device is arranged to capture a read out signal upon receiving the capture signal. The read out signal represents a logarithm of the integrated detected light intensity.

The sensor disclosed in this document uses two different kinds of transistors (NMOS and P OS) in the same pixel, thereby requiring several additional processing steps compared to a pixel with the same kind of transistors throughout. This results in higher manufacturing costs. Furthermore, the use of P OS transistors in the pixel disclosed in this document means that a large area is required to electrically isolate these transistors from the substrate of other transistors in the pixel, and therefore a significant area of the pixel does not capture light. This reduces the "fill factor" (the ratio of the area in the pixel which captures the light compared to the total area of the pixel) and therefore the sensitivity of the pixel.

It is a further object of the present invention to provide a circuit for detecting and measuring incident light which is formed of just one type of device. It is a further object of the present invention to provide a circuit for detecting and measuring incident light which provides a large area for light capture (has a high fill factor).

Existing pixels (including those in WO 2007/051964) directly measure the incident light on the photo-sensitive element of the pixel. Typically this is a potential resulting from the integration of the charge accumulated on the photosensitive device.

Furthermore, as indicated above, existing image sensors used in electronic imaging devices rely on integration of the incident light on a capacitor for a known integration time and the recording of the voltage stored after that time. The pixels are then reset and a new integration cycle can be started. However, in order to ensure that each pixel is integrated for the same amount of time, rows of pixels are reset and readout in succession across the device. In a simplified example, first the first horizontal row of pixels in the sensor is read and reset, then the second horizontal row of pixels is read and reset and so on. This approach is known as "local shutter". However, this process introduces a time lag in the reading of various rows and is a problematic feature of such sensors, particularly when recording images of moving objects where it will introduce defects into the image.

It is therefore an object of the present invention to provide an image sensor in which all of the pixels can be reset globally (a "global shutter") and a method of carrying out such shuttering.

Summary of the Invention

At its broadest, a first aspect of the present invention provides circuit or pixel circuit which measures the output of a photo-sensitive device indirectly as a result of an independent process. Accordingly, a first aspect of the present invention preferably provides a circuit for detecting light incident on the circuit and providing an electrical representation of that light, the circuit including: a photo-sensitive device having a photosensor input and a photosensor output; a signal generator generating a reference potential that varies with time according to a predetermined function; a first device arranged to set a potential at a first node, said first node being connected to said photosensor output; and a second device arranged to compare the potential at said first node to said reference potential, to determine a capture moment from the comparison and to output the value of said reference potential at said capture moment, wherein first device and said second device are devices which are arranged to conduct current between two of their nodes when the potential at a third node is higher than that at one of those nodes.

Preferably the first and second device are n OS type transistors. However, transistors having similar behaviour may also be used.

This circuit can produce a unique response to a wide dynamic range of input illumination. In addition, this circuit can produce any monotonically changing relationship between the input light and output voltage by choice of the predetermined function for the reference potential.

By outputting the value of the reference potential at the capture moment (rather than, for example, the value of the voltage at the photosensor output), the circuit makes an indirect measurement of the light incident on the photo-sensitive device. This means that the circuit can output a potential which is under control of the circuit, but the setting of that potential can be determined by the light incident on the photo-sensitive device. nMOS type transistors are generally much smaller than p OS or other kinds of transistors for a given performance. This means that the electronic part of the pixel can be relatively small thereby allowing for a greater space to be allocated to the photodiodes. By providing large photodiodes, the light capturing area in the pixel can be increased, which can consequently increase the sensitivity of the pixel. Hence the circuit can provide for a high fill factor (as described above) and so less incident light is "wasted" by not being captured by the pixel, and is also easy to manufacture in any standard fabrication process of transistors.

Preferably the photosensitive device is a photodiode. In other embodiments, the

photosensitive device may be a photogate, a buried photodiode, a lateral bipolar

phototransistor or a vertical phototransistor. The photodiode could also be a vertically implanted photodiode made of materials such as amorphous silicon, nanotubes or other synthetic or organic chemicals.

Preferably the circuit further includes a third device arranged to store said outputted value from said second device until it is read-out from the circuit. In a particular arrangement, this third device is a source follower-switch pair of transistors, and those transistors are preferably nMOS type transistors as well.

As the same type of transistor is preferably used for all of the switches, they can be manufactured at the same time in the same process. This results in a high yield in fabrication and can reduce costs.

The reference function is preferably constantly increasing over time. In certain embodiments the reference function may be chosen so as to result in a relationship between the voltage output of the circuit and the incident light on the photo-sensitive device which is one of: a logarithmic function, a power law function, or a linear function. In preferred arrangements it is possible to select or change the reference function applied to the circuit.

In a particular embodiment, the relationship between the voltage output and the photocurrent generated by the photo-sensitive device is V = αΙ_Ρ0 ^β, wherein l_P0 is the current produced in the photo-sensitive device due to light falling on it.

The above relationship is chosen to provide an output which is in the form of Steven's power law. Steven's power law is used to model the physiological function of the response of the human eye to incident light. Accordingly, by providing a circuit which has the above relationship, the performance of the circuit can closely follow the performance of the human eye with respect to different levels of illumination.

In this embodiment, the reference function can be in the form V(t) = a - bf in which the parameters a, 6 and c are defined as follows:

3 ⁼ V reset ~ ½,M4

where V_Kse, is the potential set at said first node by said first switch, V_t,M4 is the threshold voltage of the second device, which is the difference in potential between said potential at said first node and said reference potential which causes said second device to determine said capture moment, and Cpo is the capacitance of the photo-sensitive device.

In another embodiment, the reference function is defined by:

where ^is the reference function, V_fesef is the potential set at said first node by said first switch, V_t.M4 is the threshold voltage of the second device, which is the difference in potential between said potential at said first node and said reference potential which causes said second device to determine said capture moment, C_PD is the capacitance of the photosensitive device, O is an offset and S is the slope of the logarithmic function desired and l_r is a reference photocurrent, so as to produce a logarithmic response.

A logarithmic response allows light intensity to be accurately recorded over a wide dynamic range (e.g. 6 or more decades).

In certain embodiments the time taken for the reference potential to cover the entire voltage range from zero to the maximum potential of said circuit can be adjusted. Alternatively or additionally the reference potential is arranged so that it does not start increasing until a predetermined amount of time has elapsed from the time at which said potential at said first node has started to be changed as a result of light falling on said photo-sensitive device.

Both of the above features, either individually or in combination, allow the circuit to "zoom" on a particular range of intensities. By delaying the start of the change in the reference function, it is possible to cause the circuit to effectively ignore higher intensities as these will never be captured by the circuit. It is also possible to choose the intensity at which the circuit starts to potentially result in a capture. By causing the reference function to span the available range of potentials in a shorter time, a greater resolution of intensities in that range can be achieved. By combining these functions, the circuit can be adjusted to only record intensities within a selected range, but to do so at higher resolution than if the entire dynamic range of intensities were being captured.

The above features are preferably used in conjunction with the feature of the reference function being selectable or adjustable so that the user of a device containing one or more of the circuits of this aspect can choose to "zoom" in this fashion. In a preferred arrangement, said first device is a reset switch which is arranged to set the potential at said first node to a high potential and then to isolate said first node from the source of said potential, and said photo-sensitive device is arranged to discharge said potential at said first node as light falls on it. In particular, in such a circuit, said capture moment may occur when said potential at said first node is a predetermined amount below said reference potential. This may be the threshold voltage of the first device.

In one embodiment, the reference potential varies in such a way so as to produce a compressed image which is suitable for display on various media without further adjustment. This allows the device to reduce the processing requirements for tone mapping by recording the image in a form which reduces or even removes the need for tone mapping prior to display of the image.

The circuit of the first aspect may include some, all or none of the above described optional and preferred features in any combination.

A second aspect of the present invention provides an image sensor including a plurality of circuits according to the above first aspect, including some, all or none of the optional and preferred features of that aspect, and a control unit for reading out the output of those circuits.

Preferably the control unit is arranged to read out the output of each of the circuits and to reset all of said circuits at the same time. By resetting all of the circuits at the same time, it is possible to provide a "global shutter" effect whereby the whole of the image sensor is recording incident light from the same point in time.

The ability to provide such a global shutter results in particular from the ability of the circuits to record an output of the circuit at a particular point during a capture cycle, and to hold that output without change until it is read out by the control unit. A global shutter is a particularly advantageous feature as it reduces or preferably eliminates any lag between the time at which an image is captured by each circuit in the image sensor. The reduction or elimination of such lag is particularly advantageous in respect of the capture of images of moving objects. The global shutter can be applied in conjunction with any form of reference function, including functions which result in a relationship between the incident light and the output potential which is in a linear form similar to existing pixels.

In certain arrangements, the signal generator for each of said plurality of circuits is the same device. Therefore the image sensor need not duplicate the signal generator for each pixel and therefore keep the space associated with the image sensor as small as possible.

However, the image sensor may have a plurality of signal generators, or a single signal generator which is able to produce different forms of reference function. This can allow the control unit to change the reference function applied to at least one of said circuits. The different reference signal may be applied to all of said circuits, or only to selected ones of said circuit. As such, in a particular image capture at least two of said circuits may have different reference functions which operate over the same recording time.

The image sensor preferably further includes an analogue to digital converter for converting the output of said circuits to digital form.

A third aspect of the present invention provides a camera having an image sensor according to the above second aspect including some, all or none of the optional or preferred features of that aspect. The camera of this aspect may include additional components, such as lenses, memory devices, screens and buttons or other components which permit user interaction with the camera and any combination of such components.

A fourth aspect of the present invention provides a method of recording a level of light incident on a photo-sensor in a manner which measures the light intensity indirectly as a consequence of an independent process.

By measuring the incident light indirectly, the method can output a potential which is a known value, but the selection of that potential can be determined by the light incident on the device.

In particular, the fourth aspect of the present invention may provide a method of recording a level of light incident on a device including the steps of: varying a reference signal according to a predetermined function from a designated start time; and recording a value of said reference signal at a point in time after said start time determined by the amount of light incident on said device since said start time. The predetermined function of said reference signal may be any of the functions described in relation to the first aspect above.

The method of this aspect preferably, but not necessarily, involves using a circuit according to the above first aspect, including some, all or none of the optional or preferred features of that aspect.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows a circuit forming a pixel according to an embodiment of the present invention;

Figure 2 shows a graph of various potentials in the circuit of Figure 1 and is used to illustrate its operation;

Figure 3 shows the measured response from a circuit according to an embodiment of the present invention;

Figures 4a and 4b show graphs of various potentials in a circuit according to an embodiment of the present invention and are used to illustrate the "zoom" feature; and

Figure 5 shows graphs of various potentials in a circuit according to an embodiment of the present invention and is used to illustrate operation as a linear pixel with a global shutter.

Detailed Description

Figure 1 shows a circuit which is a pixel according to an embodiment of the present invention. The circuit has 4 nMOS type transistors M1-M4, and one photodiode, PD.

Although the transistors of this embodiment are nMOS type, any kind of transistor having functioning similar to that of an nMOS transistor can be used.

Whilst the pixel of the present embodiment has circuitry built from nMOS type transistors in a standard planer CMOS process, the same circuit can also be built by transistors having similar behaviour, that is, the transistor conducts current between two nodes (switches on), when the potential on a third node is higher than one f the nodes. For example the pixel could be made from vertical or multi-gate transistors (ike FinFets, trigate transistors and Junction field effect transistors (JFETs).

Furthermore, whilst the photo-sensitive device of this embodiment is a photodiode PD, in other embodiments, the photosensitive device may be a photogate, a buried photodiode, a lateral bipolar phototransistor or a vertical phototransistor. The photodiode may be a vertically implanted photodiode made of materials such as amorphous silicon, nanotubes or other synthetic or organic chemicals.

Node N111 is connected to a high voltage, which in the preferred embodiment is the power supply to the circuit. Through node N105 a pulse reset signal is applied to the circuit. Node N100 is connected to a low potential, which in a preferred embodiment is the ground potential.

Devices M2 and M3 form a source follower-switch pair which can be used to selectively connect to the circuit in a large array of such pixels, for example in a image sensor according to a further embodiment of the present invention. In such arrangements, the output nodes N101 and N102 are connected to other circuits as required to readout the output of the pixel circuit. In alternative embodiments the data from the pixel may be read out through alternative switches and amplifiers (or a series of amplifiers). For example there could be a further amplifier (or amplifiers) connected to a collection of pixels (e.g. a "column" in an image sensor). Such amplifiers and switches are well known in the art and will not be described further here.

Through node 104, a monotonieally increasing signal (i.e. one that is constantly changing and ramping up) is fed into the circuit. This monotonieally increasing signal is dependant on the kind of input-output relationship desired between the incident light on the photodiode and the output voltage.

In essence, as long as the potential at the gate of device M4 is higher than its source potential and hence device M4 is in the "on" configuration, the voltage at node N104 is transferred to node N2. The gate capacitance at node N2 stores this constantly changing voltage.

The operation of this circuit will be explained in more detail with reference to Figure 2. First at time ti, the reset signal at node N105 feeding the gate of the device 1 is pulled high, switching the device 1 on. This also ensures the high voltage at the node N1 1 is transferred to node N1 connected to the photodiode PD.

After a small interval, at time to, the reset 105 is pulled low thereby disconnecting the photodiode PD from high voltage at node N 01.

With light falling on photodiode PD, electric charges are generated inside the diode, which start to discharge the initial potential at node N1.

Simultaneously, the voltage at node N104 is swept up according to the output pattern desired which will be discussed in more detail below. Assuming that an illumination 11 causes the potential at N1 (and therefore at the gate of device 4) to reduce as shown by the dotted line in Figure 2, at time f1, the potential at the gate of the device M4 is lower than the potential at its source (N104) and hence, the voltage applied at node N104 is transferred to node N2. In fact the potential at the gate of the device must be sufficiently lower than the potential at the gate (N1) to switch the device 4 off. The nature of transistors such as M4 is that this switching does not occur at exact equality of potentials.

For all illuminations of the photodiode PD, the potential of node N1 decreases and that of node N 04 increases from time (0. As a result, there comes a time when the gate potential of device 4 becomes lower than its source potential. At this point, the device switches off and the potential at node N2 remains stuck at the cutting off potential.

The cutoff potential is determined by the photocurrent in the diode PD and the kind of sweep applied at node 104. One example of the voltage applied at node 04 is shown in Figure 2.

Finally, after the completion of integration period, the voltage at node N2 is read out by switching on the device M3 using node N103. This makes the potential at N2 available at node N 02.

After reading out the integrated voltage, the reset signal at node N 05 feeding the gate of the device 1 is pulled high again, thereby switching the device M1 on and hence removing all charge at node N1. This restarts the process from the first step above. As shown in Figure 2, the output voltage is V1 for high input light 11. Lower light levels 12 and 13 produce higher voltages V2 and V3 respectively.

The use of different potentials as applied to the node N104 to elicit different responses according to various embodiments of the present invention will now be described.

The voltage at node N1 falls linearly according to the following equation:

V_/iii - V_KSet— t. Ipo/Cpo ( 1 ) where V,_eset is the voltage at node N1 at time W when it is reset, f is the time elapsed, W is the current produced in the photodiode due to light falling on it and C_P0 is the diode capacitance. Hence the amount of voltage accumulated over the integration time is ί./«/0_ΡΟ·

If the output transduction function required from the image sensor is given an arbitrary yet monotonically decreasing function f defined as:

At the time when device M4 switches off, these two voltages should be equal, i.e.

V_feSe_t - ti_P! Cp_D = f(lp_D) (3)

The known properties of device 4 are such that it will switch off when the difference between its gate voltage (node N1) and its source voltage (node 104) is less than the threshold voltage of the device (V,_iM4).

Therefore the equation for the source voltage at N 104 must satisfy the simultaneous equations W) = V reset ~ I lpc Cp_D - V,_iM4 (4) and

Wf) = « (5)

These equations can be solved to eliminate l_PD and obtain an expression for the sweep function required at node N104. In one embodiment of the present invention, the transduction function f{l_PD) is the Steven's power law which may be used to obtain a wide dynamic response and to model the physiological function of the human eye.

The voltage output of a sensor using Steven's law would be related to the photocurrent in the following way:

V= F - al_PD ^p (6) where F is an offset which, in the preferred embodiment, is V,_eSet - ½.

By inserting this equation in equations (4) and (5), a parametric form of the sweep function to be applied at node N104 is

V_Nm{t) = a - bf (7) where the parameters a, b and c are:

c = W- 1)

In a further embodiment of the present invention, the pixel is adapted to produce a logarithmic response. To do this, the reference voltage applied at node 104 needs to produce a logarithmic output. This function can take the form of a function previously described in H.Y. Cheng, B. Choubey and S. Collins, "A High Dynamic Range Integrating Pixel with an Adaptive Logarithmic Response", in IEEE Photonics Technology Letters, Volume 19, Issue 15, Aug 1 , 2007, pages 1169-1171.

In particular the sweep function to be applied at node 104 to produce a logarithmic response may be defined by:

where O is an offset and S is the slope of the logarithmic function desired and l_r is a reference photocurrent, leading to the following form of the output function: y = o-sio_$(i_PD /i_r) (10) The measured response from a circuit according to an embodiment of the present invention in which the above function is applied at node 104 is shown in Figure 3 and shows a logarithmic response over a wide dynamic range.

In a further embodiment of the present invention, the pixel can be arranged so as to deliberately reduce the dynamic range of the pixel in order to "zoom in" on a certain range of intensities.

To do so, the sweep voltage applied at node N104 may be modified to cover the entire voltage range within a smaller amount of time. Furthermore, the sweep voltage can be time shifted (delayed) to ensure it only covers a specific region of intensity. For example, the sweep voltage at node N104 shown in Figure 4a is delayed and has been designed so as to cover only specific regions of intensities.

This configuration of the sweep voltage ensures that the output only spans a desired region of intensities, as shown schematically in Figure 4b. The 'normal' curve in Figure 4b shows the general response of a pixel, whereas the 'zoomed' curve shows the response function of the pixel has been limited to provide greater resolution in a certain region of intensities.

A further embodiment of the present invention provides an image sensor having a plurality of pixel circuits according to the present invention. The pixel circuits are generally arranged in a two-dimensional array, although other configurations (such as a one-dimensional array for capturing a single line image). In addition to the pixel circuits, the image sensor includes circuits for reading out the image data captured by the pixel circuits. For example, in a two-dimensional array, the image sensor includes row and column scanners to individually access the information stored by each pixel.

The image sensor also includes an analogue-to-digital converter for converting the analogue voltage outputs of the pixel circuits into a digital format for storage in a computer-readable format.

In certain embodiments, the image sensor may have one or more analogue-to-digital converters for every column. In one particular embodiment, an anatogue-to-digital converter may be provided for each pixel in order to increase the speed of operation. Similarly, a further embodiment of the present invention provides a pixel as described above with an analogue-to-digital converter.

Image sensors having a plurality of pixel circuits according to the present invention can also operate a global shutter such that the data can be read from the pixels simultaneously and the pixels reset at the same time as each other.

In the pixel circuits described in the embodiments above, the output of the pixel is held at the point at which the device M4 switches off. As the sweep voltage applied at node N104 covers the full possible output range of voltages at node N1 (which are determined by the output of the photodiode PD), the device M4 will always switch off during a particular integration frame or image capture. The voltage at node N2 will be held at the readout voltage until it is read out.

Therefore, it is not necessary to read out and reset the pixels on a row by row basis as recorded voltage is not changing during the time waiting for read-out to take place.

Accordingly, once the entire image data has been recorded, it can be read-out, e.g. on a row- by-row basis, but it is not necessary to reset the pixels until read-out from all pixels in the image sensor has happened, at which point all pixels can be reset at the same time, thereby providing a global shutter.

The ability of global shuttering can equally be applied to pixels which have a linear output similar to that found in known pixels. A further embodiment of the present invention provides an image sensor having such pixels. As set out above, the output of the pixels can be chosen by the choice of the reference potential which is supplied to the node N104. In the pixels of the image sensor of this embodiment, a linear function is supplied to the node N104 in accordance with the region of intensities required. This results in a linear relationship between the input light intensity and the output potential as shown in Figure 5.

However, as described above, the device M4 will always switch off at some point during the integration frame or image capture with the potential at that point being held at node N2 until it is read-out. Thus the global shutter feature described above can be used in conjunction with the linear pixels as well.

Image sensors may be constructed with electronic scanning circuits in every row and column to enable pixel by pixel readout, These can also be used for selective access to each pixel. An image sensor which is a two-dimensional array of pixels may also have an inverting amplifier at the end of each column. Such an amplifier may be used to ensure that the monotonically decreasing response of the pixel is converted into a monotonically increasing output. Wide dynamic images in general cannot be displayed on standard screens owing to their limited dynamic range. In order to do so, the image's brightness range is compressed through a process known as tone mapping. Essentially, tone mapping involves application of a chosen mathematical operator to the response of every pixel.

Several mathematical operators have been defined which perform such tone mapping. For example, a power law function similar to the Steven's law response can be used to obtain a tone mapped image (Tumblin, J., and Rushmeier, H. 1993 "Tone reproduction for computer generated images" IEEE Computer Graphics and Applications Vol. 13, No. 6, pp. 42-48.)

Another function which has been used is a logarithmic operator, with its slope varied for different intensity regions (Drago, F., Myszkowski, K., Annen, T., and Chiba, N. 2003 "Adaptive logarithmic mapping for displaying high contrast scenes" Proc. of

EUROGRAPHICS 2003, P. Brunei and D. W. Fellner, Eds., vol. 22 of Computer Graphics Forum, 419- 426.)

As described above, both of these functions, as well as any other monotonically decreasing function, can be reproduced by a pixel. Therefore, the pixel can be used to produce a tone mapped image ready for direct display on a screen. Furthermore, the monotonically increasing function fed to the pixel can be generated using a look-up table, thereby generating responses for which no simple mathematical form exists. This helps in reproducing even complex tone mapping operators.

Accordingly, a further embodiment of the present invention provides pixel circuits which are specifically adapted for producing images which are suitable for display on computer screens and projectors, or for printing where the display or printer is only capable of displaying a limited range of intensity, typically 2-3 decades.

The use of such pixels means that the standard process of post-processing an image using a mathematical operator to make it suitable for display or printing on such equipment (known as tone mapping) is no longer necessary, reducing the time and processing power required to produce an image in the appropriate form.

In order to produce a tone mapped image, the reference potential applied to node N104 is changed to meet the functionality of the required output function and thereby apply the tone mapping effects at the time of recording the image, reducing the need for post-recording processing of the image.

As indicated above, the most common tone mapping operators include logarithmic, sigmoid or power law functions. A common power law function is 1/3 power.

For power law tone mapping, equations (7) and (8) above can be used to generate a tone- mapped image. For a 1/3 power law tone mapping function, the parameter β is set to 1/3.

For sigmoid tone mapping, the appropriate response function can be achieved by utilizing equations (4) and (5) with the appropriate sigmoid function.

For logarithmic tone mapping, the function described in Cheng et al. (reference above) can be used.

The tone mapped image produced by the image sensor can then be directly displayed on a screen.

A further embodiment of the present invention provides an image sensor having a plurality of pixels which are able to acquire images and which use a plurality of different reference functions taking the same overall integration time. This allows the image sensor to zoom in on different regions of intensities in different parts of the image sensor.

Further embodiments of the present invention provide cameras having pixels or image sensors of the above embodiments. The cameras of these embodiments include additional components, such as lenses, memory devices, screens and buttons or other components which permit user interaction with the camera, as known on existing cameras,

In general an image sensor in a camera

While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

All references referred to above are hereby incorporated by reference;

Claims

1. A circuit for detecting light incident on the circuit and providing an electrical representation of that light, the circuit including:

a photo-sensitive device having a photosensor input and a photosensor output;

a signal generator generating a reference potential that varies with time according to a predetermined function;

a first device arranged to set a potential at a first node, said first node being connected to said photosensor output; and

a second device arranged to compare the potential at said first node to said reference potential, to determine a capture moment from the comparison and to output the value of said reference potential at said capture moment,

wherein first device and said second device are devices which are arranged to conduct current between two of their nodes when the potential at a third node is higher than that at one of those nodes.

2. A circuit according to claim 1 wherein said first device and said second device are nMOS transistors.

3. A circuit according to claim 1 or claim 2 further including a third device arranged to store said outputted value from said second device until it is read-out from the circuit.

4. A circuit according to claim 3 wherein said third device is a source follower-switch pair of transistors.

5. A circuit according to any one of the preceding claims wherein the reference function is constantly increasing over time.

6. A circuit according to claim 5 wherein the reference function results in a relationship between the voltage output of the circuit and the incident light on the photo-sensitive device which is one of: a logarithmic function, a power law function, or a linear function.

7. A circuit according to claim 6 wherein the relationship between the voltage output and the photocurrent generated by the photo-sensitive device is V = αΙ_Ρο^β, wherein Ipo is the current produced in the photo-sensitive device due to light falling on it.

8. A circuit according to claim 7 wherein the reference function is in the form V(f) = a - bf in which the parameters a, b and c are defined as follows:

where V^, is the potential set at said first node by said first switch, V_tiM4 is the threshold voltage of the second device, which is the difference in potential between said potential at said first node and said reference potential which causes said second device to determine said capture moment, and C_PD is the capacitance of the photo-sensitive device.

9. A circuit according to claim 6 wherein the reference function is defined by:

where VN_m(t) is the reference function, V_Kset is the potential set at said first node by said first switch, V_{t M}4 is the threshold voltage of the second device, which is the difference in potential between said potential at said first node and said reference potential which causes said second device to determine said capture moment, Cpo is the capacitance of the photosensitive device, O is an offset and S is the slope of the logarithmic function desired and l_r is a reference photocurreht, so as to produce a logarithmic response.

10. A circuit according to any one of the preceding claims wherein the time taken for the reference potential to cover the entire voltage range from zero to the maximum potential of said circuit can be adjusted.

11. A circuit according to any one of the preceding claims wherein the reference potential does not start increasing until a predetermined amount of time has elapsed from the time at which said potential at said first node has started to be changed as a result of light falling on said photo-sensitive device.

12. A circuit according to any one of the preceding claims wherein said first device is a reset switch which is arranged to set the potential at said first node to a high potential and then to isolate said first node from the source of said potential, and said photo-sensitive device is arranged to discharge said potential at said first node as light falls on it.

13. A circuit according to claim 12 wherein said capture moment occurs when said potential at said first node is a predetermined amount below said reference potential.

14. A circuit according to any one of the preceding claims wherein the reference potential varies in such a way so as to produce a compressed image which is suitable for display on various media without further adjustment. 5. An image sensor including a plurality of circuits according to any one of the preceding claims and a control unit for reading out the output of those circuits,

16. An image sensor according to claim 15 wherein the control unit is arranged to read out the output of each of the circuits and to reset all of said circuits at the same time.

17. An image sensor according to claim 15 or claim 16 wherein said signal generator for each of said plurality of circuits is the same device.

18. An image sensor according tb any one of claims 15 to 17 wherein said control unit is able to change the reference function applied to at least one of said circuits.

19. An image sensor according to any one of claims 15 tb 18 wherein at least two of said Circuits have different reference functions which operate over the same recording time.

20. An image sensor according to any one of claims 15 to 19 further including an analogue to digital converter for converting the output of said circuits to digital form.

2 . A camera having an image sensor according to any one of claims 15 to 20.

22. A method of recording a level of light incident on a device including the steps of: varying a reference signal according to a predetermined function from a designated start time; and

recording a value of said reference signal at a point in time after said start time determined by the amount of light incident on said device since said start time.

23. A method according to claim 22 wherein the reference function is constantly increasing over time.

24. A method according to claim 23 wherein the reference function results in a relationship between the recorded value and the incident light on the device which is one of: a logarithmic function, a power law function, or a linear function.