Patent Description:
The image acquisition device according to the invention ensures a high quality of images in the presence of scenes with low brightness, high brightness, and with a light intensity variable in a wide range passing from very low to very high light intensity.

Electro-optical sensors are known comprising a plurality of photodetector devices adapted to detect light signals by means of a photosensitive element and to transmit them, in the form of electrical signals, to a computing unit which processes them, obtaining images. Such images are then transmitted to storage or display devices which allow a user to view such images or information derived therefrom, even over time.

These electro-optical sensors are generally based on CMOS-type (Complementary Metal Oxide Semiconductor) silicon technology, and are adapted to detect scenes with a fixed and normally limited brightness range, by means of a linear transformation of the information conveyed by the incident light into an electrical signal (linear response sensors). The light information is acquired during a time interval, called integration or exposure time. During such time, the photosensitive element, comparable to a capacitor, discharges more or less quickly depending on the amount of incident light until the time interval defined for the integration ends, a condition in which an electrical signal is output from the photosensitive element which corresponds to the incident light, or to the state corresponding to the complete discharge of the photosensitive element, a condition defined saturation, and which means that the photosensitive element is no longer able to map the light information. Typically, once the integration step is completed, next there is the reading of an entire matrix of photosensitive elements and which corresponds to the image of the scene shot.

Such sensors are normally able to offer high image quality even under highly diversified lighting conditions within the same scene, exploiting different conversion techniques and mapping the light information in a more than linear manner into a corresponding electrical signal. Such techniques, which aim to extend the range of light intensity manageable by the photosensitive element before it reaches the saturation level, are defined as high light dynamics.

Among the most common techniques, it is known that the range of lights detectable by a sensor by means of a logarithmic compression of the signal within the photosensitive element can be increased. Solutions are known in which the compression is achieved by connecting a CMOS-type transistor in diode configuration to the photosensitive junction, as described for example in <CIT>. Such state-of-the-art solutions, while significantly extending the range of light intensity manageable by the photosensitive element, produce low quality images due to the mapping of a high light range within a limited voltage range with respect to that which is typical of linear sensors combined with the inability to accurately distinguish the different voltage levels into which the light information is mapped, especially in case of very intense light. Furthermore, the known techniques are characterized by an intrinsic slowness of the photosensitive elements with respect to rapid changes in light intensity in poorly lit scenes, causing artefacts in the presence of moving objects.

A technique exploiting the emission of subsequent reset pulses is known from <CIT>. In this, different reset voltages can be applied to the pixels at different times. From a structural point of view, the above technique can be implemented by using buffered switches.

<CIT> also discloses a reset pulse technique similar to the previous one. The architecture of the system disclosed includes the presence of a block which generates signals for PRST, RST and TX to apply reset pulses.

<CIT> discloses another solution similar to the previous ones in which the propagation of the reset signal is not specified.

The technique of acquiring multiple linear images to obtain information in different brightness ranges is also known and widespread, which are then combined to obtain a single image which contains the information deriving from the different images and thus obtaining images with high light dynamics. Such a technique, as appropriate, is implemented both through the aid of software tools and directly on physical substrates, typically silicon, which integrate, in addition to the photosensitive element, also the electronics necessary to implement the control system and generate the images deriving from the combination of the acquired images.

Such techniques suffer from a high complexity resulting from the large amount of information to be processed and/or the electronics necessary to implement such solutions. Furthermore, by combining multiple images obtained in succession, it is highly likely to have image artefacts caused by the different position of objects moving over time.

Most of the limitations of these techniques have been overcome with the solution proposed in the previous patents <CIT> and <CIT> of the same applicant, which present techniques for acquiring high dynamic images based on the control of the discharge of the photosensitive element, appropriately piloting the reset signal of the photosensitive element itself and accordingly modelling the electrical signal which maps the power of the incident light.

The advantages of this technique are the high flexibility in programming the output light dynamics, both in amplitude and shape, based on the reset control signal and, consequently, the possibility of optimizing the response in terms of dynamics and resolution in the desired light range, rather than increasing the compression thereof and obtaining very high dynamic images. All the above with the acquisition of a single image, and thus avoiding artefacts, limiting processing complexity and obtaining high quality in any light condition.

The limitation of the technique presented in the aforementioned patents is the ability to obtain high dynamic images only in the case of controlled light. This is because a single reset curve is generated and distributed simultaneously to the entire matrix of photosensitive elements and it is therefore necessary to interrupt the luminous flux affecting the pixels to stop the discharge process during the reading step. It is clear that this constraint limits the scope of the above-mentioned technology to those situations where it is possible to control the lighting conditions of the scene.

The possibility is also known of controlling the exposure time of each photosensitive element individually so as to avoid saturation at the end of the light exposure period. An implementation example of this technique is disclosed in <CIT> and includes integrating into each photosensitive element a flip-flop and an AND-type port to locally control the onset instant of the integration time and exposure. However, this technique includes the acquisition of a linear signal and, consequently, even if the different photosensitive elements will work with different acquisition intervals and therefore on different dynamics, each photosensitive element will individually have linear dynamics and therefore low dynamics when compared to the aforementioned cases.

The advantage is also known in the state of the art of being able to define areas within the same image which are subject to different integration times in order to maximize the informative content within the image, locally maximizing the contrast of the image. Such techniques are generally used to solve the problem of displaying high dynamic images on <NUM>-bit monitors which only have <NUM> grey values to map information content which, to be properly expressed, would need <NUM>-<NUM>-bit monitors.

Various techniques are known in the state of the art to implement such a function. For example, a publicly available application, called HDRView, allows the user to open a high dynamic image and locally vary the exposure by selecting a certain area of the image. A similar technique is also presented in <CIT>. However, all the techniques known in the state of the art have the disadvantage of requiring operator intervention and being implemented entirely in software with the consequent need for a large computational load and a large memory to maintain all the information necessary to enable such a function.

Another technique for solving the problem of displaying high dynamic images is the use of high dynamic light monitors, called HDR monitors, which allow a much wider number of grey levels (<NUM>-<NUM>-bit) than traditional monitors to be displayed.

It is important to underline that maximizing information within the scene is critical especially for image processing techniques which have the primary objective of extracting information from the scene and not displaying the images themselves. For these techniques, the informative content of the image is much more important than the image quality itself.

In this regard, neuromorphic vision sensors are known in the state of the art which boast of operating like the human eye because they do not detect the image itself but the events occurring in the scene. These sensors are based on high dynamic image acquisition with logarithmic technique and have the advantage of producing a much smaller volume of data when dealing with complete and detailed images, but have the inherent disadvantage of containing less information, demonstrating less versatility with respect to different application fields.

An object of the present invention is to provide an electro-optical device which can be integrated into a support element, or substrate, suitable to realize small electronic circuits, for example of silicon, and which is suitable for providing good quality images at a high repetition frequency both in the case of low brightness and in the presence of a wide range of brightnesses present in the observed scene.

A specific object is to realize an electro-optical device which is capable of operating both in controlled and non-controlled light conditions, allowing to adapt the response dynamics of the sensor both to lighting conditions and to the needs of the user and/or the application.

A further object of the present invention is to realize an electro-optical sensor which comprises a plurality of photosensitive elements arranged according to a matrix, or in another desired arrangement, and which allows to independently manage the reset state even of a single photosensitive element, or of a desired subset, variable in size and shape, going to control the discharge of the photosensitive elements individually or in subgroups.

In order to overcome the drawbacks of the known art and to achieve these and further objects and advantages, the invention outlined below has been studied, tested and implemented.

The present invention is defined by claim <NUM>. The dependent claims show other features of the present invention or variants of the main solution idea.

In accordance with the aforementioned objects, an electro-optical image acquisition device comprises: a plurality of light-sensing sub-blocks (<NUM>), each comprising at least one photosensitive element (<NUM>) which comprises a photodetector element for the conversion of light information into an electrical signal and electronic media controllable by reset signals and capable of resetting the relevant photodetector; a detected signal reading block for reading the output signal from the photosensitive elements; row and column selection elements, which enable access to different photosensitive elements of the matrix and their reading by means of the aforementioned detected signal reading block; and pixel discharge control circuitry.

According to a characteristic aspect of the present invention, the pixel discharge control circuits are configured so as to combine a plurality of input voltage signals, to generate one or more reset curves which are transported to the photosensitive elements by means of reset curve distribution lines. Said pixel discharge control circuits are configured to generate a plurality of reset curves and to dynamically select, for each photosensitive element, an appropriate reset curve among those available transported by said reset curve distribution lines while storing such a selection until the next selection.

The system outlined above, rather than using reset pulses as occurs in the closest known technique, previously mentioned, is based on the creation of real reset curves. From a structural perspective, the technique is implemented by means of a multiplexer which allows to pass on the same line from one voltage to another.

Advantageously, the pixel discharge control circuits comprise reset curve generation elements configured to allow the selective propagation in succession of the various line-by-line reset curves over time, so as to ensure a high dynamic response even in the event of operation with uncontrolled light.

When the pixel discharge control circuits are configured to generate a plurality of reset curves, the device outlined above allows to obtain zones of the matrix of photosensitive elements with different light dynamics within the scene itself and to obtain so-called "multidynamic" images. This allows to both optimize the quality of the image by enabling an overall dynamic response which can even exceed the dynamics of the individual photosensitive elements and to enable schemes which allow to maximize the information content if the image is subsequently processed by a computer.

In fact, the proposed technique allows to obtain images with a higher number of bits than the native number of the electro-optical sensor by virtue of the application of one or more flags which identify the associated dynamics within the information extracted from each photosensitive element. Such information is used during the reconstruction of the image to increase the contrast of the image as a whole. The image thus constructed can then be displayed on HDR monitors or exploited with image processing techniques to optimize the information processing and extraction processes. By way of example, for a better understanding of the concept, we can make the following example: if two subsections of the image have a depth of <NUM> bits but in two different light ranges or with one having reduced dynamics compared to the other, they can be recombined into a single image with a depth of <NUM>-<NUM> bits, exploiting the information linked to the reset curve, encoded with additional bits, which has been associated with the different photosensitive elements.

Furthermore, the disclosed circuit has the following advantages with respect to the prior art:.

These and other advantages and features associated with the device of the present invention will moreover be easier to understand by means of the illustration of non-limiting embodiments, as described below with the aid of the attached drawings, in which:.

With reference to <FIG>, an electro-optical device for the acquisition of digital images <NUM> according to the present invention comprises: a plurality of light-sensing sub-blocks, <NUM>, arranged in rows and columns to form a matrix structure, which could have any geometry, even linear; elements for generating a reset curve, <NUM>, capable of appropriately combining input voltage signals, <NUM>, available and transported along the entire matrix, so as to generate appropriate reset curves transported by means of reset curve distribution lines <NUM> to the light-sensing sub-blocks <NUM>. By way of example, such input voltage signals <NUM> can be constants, such as mass, power or other intermediate values generated inside or outside the electro-optical device, or be programmable voltages, such as those generated by digital-analog converters, called DACs. The various input voltage signals <NUM> are brought to the various reset curve generation elements <NUM> by means of matrix-level voltage signal transport lines <NUM>, and row-level voltage signal transport lines <NUM>. In particular, in the exemplary embodiments depicted and described, the reset curve generation elements <NUM> act as selectors in that they select, from time to time, one of the input voltage signals <NUM>; however, they can be suitable to perform a combination or alteration of said input voltage signals <NUM> to generate an appropriate reset curve.

The device <NUM> further comprises a control block <NUM> adapted to generate an appropriate sequence of signals useful to the reset curve generation elements <NUM> and the light-sensing sub-blocks <NUM> for generating and propagating the reset voltage within the device in an appropriate manner. The aforementioned signals are transmitted from the control block <NUM> to the reset curve generation elements <NUM>, and to the light-sensing sub-blocks <NUM>, by means of lines of the reset curve generation control signal <NUM> (hereinafter also referred to simply as control signal lines), and lines of the reset curve selection signal at the pixel level <NUM>, respectively. In the example shown, the distribution of the reset curves occurs, by means of the reset curve distribution lines <NUM>, per row, but this is only one implementation example. In fact, it can occur by column or other configuration without thereby departing from the objects of the present invention. For the sake of simplicity, row and column selection elements are not depicted in the figure because they are known to the state of the art, but are present in that they are necessary for the operation of the device; they enable access to the different photosensitive elements of the matrix and the reading thereof by means of a detected signal reading block <NUM>, whose implementation mode is also known in the state of the art. The various light-sensing sub-blocks <NUM> are connected to the detected signal reading block <NUM> by means of detected signal reading lines <NUM>.

The circuits shown in <FIG> depict a light-sensing sub-block <NUM> (indicated with the same reference numeral because the relevant elements for the purposes of the present disclosure are common to both embodiments). Each sub-block <NUM> includes a reset curve selection circuit <NUM>, and a photosensitive element <NUM>, or pixel. The photosensitive element <NUM> has at least one reset terminal <NUM>, one selection terminal <NUM> and one reading terminal <NUM>. The reset terminal <NUM> is connected to the reset curve selection circuit <NUM>.

With reference to <FIG>, a light-sensing sub-block <NUM>', includes a reset curve selection circuit <NUM>, to which a plurality of photosensitive elements <NUM> are connected. Within the sub-block <NUM>', the individual photosensitive elements <NUM> can then be directed independently or in groups for reading the relevant output signal <NUM>.

With reference to <FIG>, a reset curve generation element <NUM> in the device <NUM> according to the invention comprises:.

The circuits adapted to realize the AND <NUM>, and NOT <NUM> logic ports are of purely digital type, as well as the switches <NUM> and can advantageously be implemented in CMOS technology as known in the state of the art.

With this structure it is possible to create, by means of an appropriate combination of the input voltage signals <NUM>, several reset curves which are distributed from the relative distribution line of the reset curves <NUM>, which develop over time and allow the discharge of the photosensitive elements <NUM> to be appropriately controlled. The advantage of transporting the different voltage signals <NUM> along the entire electro-optical device <NUM> and not generating them locally per row or subset of photosensitive elements lies in the fact that the voltage used will be exactly the same for all rows while, for example, a digital/analog conversion at the row/sub-block level would lead to image artefacts due to the voltage generation tolerance with respect to that actually desired, as is known in the state of the art. This advantage is also maintained in the variant where a single reset curve is generated and transported along the entire matrix.

The reset curve generation element <NUM> shown in <FIG> is an example which allows the management of four input voltage signals <NUM>, indicated with V1, V2, V3 and V4, and directed by the control signals, RowRES1 and RowRES2, corresponding to two lines of the control signal <NUM>, but can be easily extended to a variant of <NUM>n input voltage signals <NUM>, and n lines of the control signal <NUM>. In such a case, <NUM>n AND ports <NUM>, n NOT ports <NUM>, and <NUM>n switches <NUM> will be required. The structure presented in <FIG> allows to bring the reset curve to all the photosensitive elements of the matrix at the same time and therefore continues to ensure appropriate operation with controlled light.

In the device depicted in <FIG>, the number of input voltage signals <NUM>, constant or programmable, is <NUM>n, with the number n defined at the implementation step, and consequently the number of lines of the control signal <NUM>, for each reset curve generation element <NUM>, is n to make all the voltages available at the input of the generation blocks <NUM> accessible. Similarly, the number of reset curves generated and propagated to the various photosensitive elements line by line by means of the reset curve distribution lines <NUM> is <NUM>m, as designed, and consequently the number of lines of the selection signal <NUM> which will go to each light-sensing sub-block <NUM> is m. In particular, when m is <NUM> (zero), it is part of the variant in which the generation and transport of a single reset curve to the entire matrix occurs. The structure composed of <NUM>m reset curve generation elements <NUM> is repeated row by row. It is evident that row propagation is only one variant, while other configurations are possible.

In a device <NUM>' shown in <FIG>, a different group of input voltage signals <NUM> is associated with each reset curve generation element <NUM> of a row. The structure consisting of m reset curve generation elements <NUM> is repeated line by line and the various groups of input voltage signals <NUM> are similarly associated with the m reset curve generation elements <NUM> of each line.

A variant of a reset curve generation element <NUM>' is shown in <FIG>. This variant represents the next stage with respect to the variant of <FIG>. In fact, comparing the two variants, it can be seen that the input voltage signals <NUM> are still present, transported by means of the voltage signal transport lines <NUM> and <NUM>, and the AND ports <NUM>, which allow the combination of the signals <NUM>, as well as the switches <NUM>, which exclusively connect the signal transport lines <NUM> to two terminals, RES1 and RES2, connecting to respective reset curve distribution lines <NUM> exiting from the reset curve generation element <NUM>. Unlike what occurs in the variant of <FIG>, in this case the control signals of the AND ports <NUM> are generated by flip-flops, <NUM>, and transported by the lines of the ports <NUM> and <NUM>. Such flip-flops <NUM> are known in the state of the art and in the proposed implementation are repeated per row/sub-block, allowing, in addition to generating the negated signal, the sliding of a signature along the matrix of light-sensing sub-blocks <NUM> at each clock cycle <NUM>, by means of control signals RowRES1 and RowRES2 transported by the control signal lines <NUM>. Thereby, over time, there will be a delay of a clock cycle <NUM> in the signal configuration for each row/sub-block, thus realizing the sliding of a certain reset curve along the distribution lines <NUM>, and therefore along the matrix of photosensitive elements <NUM>. This enables the possibility of the sensor to work even in uncontrolled light and in the so-called rolling shutter condition.

As in the variant of <FIG>, that of <FIG> also shows an example with four input voltage signals <NUM> (V1, V2, V3 and V4) and two control signal lines <NUM> (RowRES1 and RowRES2 signals) which allow the generation of a reset curve <NUM> for each row/sub-block, where in this case the number of rows/sub-blocks is equal to two. Obviously, the structure shown in <FIG> can also be readily extended and adapted to the embodiments of the device <NUM> and <NUM>' of <FIG> and <FIG>, respectively, in which <NUM>n input voltage signals <NUM>, n control signal lines <NUM> are present, and the entire reset curve generation element <NUM>' will be replicated <NUM>m times so that generally <NUM>m reset curves will be distributed by as many reset curve distribution lines <NUM>, and then selected at the level of each light-sensing sub-block <NUM>, from the related reset curve selection circuit <NUM>, by means of the m control signals transported by the selection signal lines <NUM>.

In the variant where the reset curve generation element <NUM> is configured to generate a single reset curve for the entire matrix, the reset curve selection circuit <NUM> may not be present and the light-sensing sub-blocks <NUM> would then comprise only the photosensitive elements <NUM>.

In the example of <FIG>, the reset curves generated on the reset curve distribution lines <NUM>, starting from the input voltage signals <NUM>, are stepped and decreasing, while those shown in <FIG> are pulsed. Obviously, alternative forms can be applied. <FIG> and <FIG> use four input voltage signals <NUM>, which correspond to fixed or programmable voltages available within the electro-optical device set at different time points, identified with T1, T2, T3 and T4. However, the example can be easily extended to <NUM>n input voltage signals <NUM> enabled at different time points in a number not necessarily equal to the number of signals. In fact, the different voltage signals available may not all be used or be used multiple times.

<FIG> and <FIG> also show the control signals RowRES1 and RowRES2 transmitted by means of the control signal lines <NUM> and used for generating the reset curves shown in the graphs and the related conversion tables.

Advantageously, in a light-sensing sub-block <NUM>, a selection circuit <NUM> has a structure similar to that of the reset curve generation element <NUM>' shown in <FIG>, and this structure is used in the sub-block <NUM> to select the appropriate reset curve <NUM> from those transmitted by the reset curve distribution lines <NUM> associated with the sub-block <NUM> itself. Unlike the row-level structure, the sub-block-level structure <NUM> includes a memory element, such as a flip-flop, adapted to maintain the configuration which selects the appropriate reset curve for as long as necessary for the generation and acquisition of an image, i.e., for the integration and reading time, or a multiple thereof. The flip-flop structure facilitates matrix programming prior to the image generation and acquisition step by scrolling a programming signature along the different columns, or rows, or sub-blocks of different shapes. It should be noted that the clock <NUM> of <FIG> will be different if such a configuration is used to select the appropriate reset curve at the sub-block level <NUM>, i.e., in the selection circuit <NUM>, with respect to what occurs in the reset curve generation element <NUM>, <NUM>'.

Furthermore, it is important to note that the structure shown in <FIG> is only one example of an implementation of the structure usable to implement the selection circuit <NUM>. In fact, for example, it can be alternatively considered to combine any memory element in the selection circuit <NUM>, the implementation of which is known in the state of the art in the different forms thereof, with a selection element of the single sub-block <NUM> implemented with a row and column decoder which allows the selection of the single sub-block <NUM> and the consequent programming thereof.

Finally, <FIG> shows an example of signal propagation along the lines, always referring to an example in which there are four input voltage signals <NUM>, two control signal lines, <NUM>, and two rows of sub-blocks for detecting light <NUM>. Thereby, the generated reset curve will slide along the different rows/sub-blocks of the matrix.

As is apparent from the above description, an electro-optical image acquisition device according to the invention has several advantages with respect to the more common devices of the known art. Firstly, it allows to obtain dynamic images of variable light in any lighting condition, generating a response which can be adapted very precisely to lighting conditions and operator needs. Furthermore, different areas of the matrix of photosensitive elements can be programmed with different light dynamics so as to maximize the information contained in the final image, rather than the information available to the operator and/or the computer to extract useful information. It should be noted that the capillarity of the matrix programming can reach, depending on the implementation choices, even the single pixel or be managed by sub-blocks.

The advantages highlighted above remain substantially unchanged also in the presence of further variants. For example, as will be readily understandable, a light-sensing sub-block <NUM>, consisting of a selection circuit, <NUM>, and a plurality of photosensitive elements <NUM> associated therewith could comprise not all of the photosensitive elements <NUM> of a row but any number of pixels <NUM> with any arrangement in the matrix; and the device <NUM> could comprise a plurality of light-sensing sub-blocks <NUM>, not identical to each other, i.e., composed of different numbers of pixels <NUM>.

Claim 1:
Electro-optical image acquisition device (<NUM>) including:
- a plurality of light-sensing sub-blocks (<NUM>), each comprising at least one photosensitive element (<NUM>) which includes a photodetector element for the conversion of light information into an electrical signal and electronic media controllable by reset signals and capable of resetting the relevant photodetector;
- a detected signal reading block (<NUM>) aimed at reading the output signal from said photosensitive elements (<NUM>);
- row and column selection elements, which enable access to different photosensitive elements (<NUM>) of the device and their reading by means of said detected signal reading block (<NUM>); and
- pixel discharge control circuitry;
said pixel discharge control circuits are so configured as to combine a plurality of input voltage signals (<NUM>) to generate at least one reset curve, which is transported to one or more of said light-sensing sub-blocks (<NUM>) by at least one reset curve distribution line (<NUM>),
characterized in that said pixel discharge control circuits comprise:
- reset curve generation elements (<NUM>) designed to combine said input voltage signals (<NUM>) to generate a defined number of reset curves,
- at least one control block (<NUM>) suitable to generate an appropriate sequence of signals useful to the reset curve generation elements (<NUM>) to generate and propagate the reset curves inside the device (<NUM>) in an appropriate way;
each of said reset curve generation elements (<NUM>) comprising:
- AND ports (<NUM>) that pilot switches (<NUM>) present on the voltage signal transport lines (<NUM>) and that exclusively connect said input voltage signals (<NUM>) to a distribution line of the reset curve (<NUM>) exiting from said reset curve generation element (<NUM>);
- NOT ports (<NUM>) for generating negated signal (<NUM>) from the signal of the signal control lines (<NUM>), so that the signal of the signal control lines (<NUM>) and the related negated signal (<NUM>) constitute the input of said AND ports (<NUM>).