Patent Description:
The present teachings are generally related to thermal imaging systems, and more particularly, to a readout integrated circuit included in a thermal imaging system.

Bolometer pixels are used in a wide variety of infrared or thermal imaging applications. When they are exposed to heat sources, bolometer pixels change their resistance to provide an output signal. A readout integrated circuit (ROIC) is typically utilized to detect and measure the output signal from one or more of the bolometer pixels. However, bolometer pixels are vulnerable to excessive heat sources which can fundamentally change the mechanical properties and response of the pixels and degrade their resulting imaging capability. In addition, extreme heat sources can completely destroy the pixels.

For further background, <CIT> describes a device for detecting infrared radiation comprising a resistive imaging bolometer intended to be electrically connected to a circuit for measuring a resistance of the bolometer. It comprises means of controlling the resistance of the imaging bolometer by injecting current into the bolometer.

<CIT> describes techniques for binning (e.g., clustering or grouping) two or more infrared sensors of a focal plane array (FPA) to permit configuration of the FPA to various dimensions and/or pixel sizes. For example, switchable interconnects may be implemented within the FPA, wherein the switchable interconnects comprise a plurality of switches adapted to selectively connect or disconnect infrared sensors of the FPA to/from column lines, row lines, and between each other. The switchable interconnects may also comprise another set of switches adapted to selectively connect adjacent column lines together. By selectively opening and closing appropriate switches of the switchable interconnects, two or more neighboring infrared sensors may be binned together to form a binned detector.

Non-limiting embodiments of the present invention are directed to a thermal imaging system comprising a pixel array including a plurality of pixel groups, each pixel group comprising: a plurality of pixel rows containing a plurality of bolometer pixels. The pixel array further comprises a trigger sense circuit including a pixel group input line in signal communication with the plurality of bolometer pixels; and a selector switch that selectively establishes an electrical connection between the pixel group and the trigger sense circuit. An integration unit is configured to generate an image based on a resistance of the plurality of bolometer pixels, wherein the selector switch operates in a first state to disconnect the pixel group from the pixel group input line while connecting the pixel group to the integration unit such that the integration unit generates the image, and wherein the selector switch operates in a second state to disconnect the pixel group from the integration unit while connecting the pixel group to the pixel group input line such that the trigger sense circuit monitors the pixel group for a high temperature bolometer.

Non-limiting embodiments of the present invention are directed to a trigger sense circuit comprising a current source configured to generate a trim current; and a pixel group input line in signal communication with a pixel group included in a pixel array. The pixel group includes a plurality of pixel rows, with each pixel row containing a plurality of bolometer pixels. The trigger sense circuit further comprises an overheating condition detector circuit in signal communication with the pixel group input line; and a selector switch. The selector switch operates in a first state to connect the pixel group to the pixel group input line to establish a signal path between the pixel group and the overheating condition detector circuit, and a second state to disconnect the pixel group from the pixel group input line to open the signal path between the pixel group and the overheating condition detector circuit.

Non-limiting embodiments of the invention are directed to a method of detecting thermal energy delivered to a bolometer pixel, the method comprising arranging a plurality of pixel groups in a pixel array, each pixel group comprising a plurality of pixel rows, each pixel row containing a plurality of bolometer pixels; connecting each pixel group to a pixel group input line, and connecting the pixel group input line to a trigger sense circuit. The method further comprises operating a group selector switch in a first state that disconnects a selected pixel group among the plurality of pixel groups from the pixel group input line and connects the selected pixel group to an integration unit while remaining groups are connected to the pixel group input line, and a second state that disconnects the pixel group from the integration unit and connects the selected pixel group to the pixel group input line such that the selected pixel group is in signal communication with the trigger sense circuit. The method further comprises generating an image based on a resistance of the plurality of bolometers connected to the integration unit; and monitoring a temperature of the remaining pixel groups via the trigger sense circuit.

Various non-limiting embodiments of the invention are described herein with reference to the related drawings. Alternative embodiments of the invention can be devised without departing from the scope of this invention. Various connections and positional relationships (e.g., over, below, adjacent, etc.) are set forth between elements in the following description and in the drawings. These connections or positional relationships, unless otherwise specified, can be direct or indirect, and the present invention is not intended to be limited in this respect. Moreover, the various tasks and process operations described herein can be incorporated into a more comprehensive procedure or process having additional steps or functionality not described in detail herein, or one or more tasks or operations may be optional without departing from the scope of the invention.

As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having," "contains," or "containing," or another variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus.

Additionally, the terms "at least one" and "one or more" may be understood to include a number greater than or equal to one (e.g., one, two, three, four, etc.). The term "a plurality" may be understood to include a number greater than or equal to two (e.g., two, three, four, five, etc.). The terms "about," "substantially," or "approximately," or variations thereof, are intended to include a degree of error associated with measurement of the particular quantity based upon the equipment available.

For the sake of brevity, conventional techniques related to making and using aspects of the invention may or may not be described in detail herein. In particular, various aspects of computing systems to implement the various technical features described herein may be well known. Accordingly, in the interest of brevity, some conventional implementation details are only mentioned briefly herein or are omitted entirely without providing the well-known system or process details.

Turning now to an overview of technologies that are more specifically relevant to aspects of the invention, readout integrated circuits (ROICs) are typically implemented in thermal imaging system to measure the resistivity of one or more bolometer pixels. Conventional bolometer ROICs typically perform a measurement on a "biased" group of bolometer pixels for a small portion of a frame period while disregarding the "unbiased" bolometer pixels for the majority of a frame period. As a result, the ROIC may not identify an overheated bolometer pixel until the pixel group containing the over-heated bolometer pixel is biased and read out, which may take a considerable fraction of a frame period, ultimately resulting in irreversible damage to the over-heated bolometer pixel.

One or more non-limiting embodiments of the invention address the above-described shortcomings of the prior art by providing a trigger sense circuit to the ROIC that performs a pulsed-bias measurement of a bolometer pixel array. The bolometer pixel array contains a multitude of bolometer pixels arranged in selectable rows. The bolometer pixels are thermally sensitive to a change in temperature. As the temperature of a given bolometer pixel increases, its resistance decreases (see <FIG>). A subset of the rows included in the pixel array are biased at any given time, while the a majority of remaining unbiased pixel rows remain exposed to thermal energy detection but are electrically floating to reduce power consumption and prevent bolometer temperature increases due to Joule heating.

Unlike conventional bolometer ROICs, the ROIC described herein utilizes the formerly unbiased or unread detectors as resistance reference sources, which in turn can be used as high-energy thermal detectors. Accordingly, an instantaneous value of a parallel combination of a group of resistors (e.g., <NUM> rows) can be provided. When this value crosses a threshold, a comparator in the trigger sense circuit is tripped and outputs a trigger signal to the controller which determines that one or more bolometer pixels are overheated.

Turning now to <FIG>, a bolometer pixel <NUM> is illustrated according to a non-limiting embodiment. The bolometer pixel <NUM> includes a substrate <NUM> that supports a bolometer <NUM> via a pair of opposing support beams 104a and 104b. The support beams 104a, 104b each include an electrode 105a and 105b. The bolometer <NUM> includes a photosensitive region <NUM> (sometimes referred to as a mesa), which is interposed between the electrodes 105a, 105b, and thermally isolated from the substrate <NUM> by the support beams 104a, 104b.

The photosensitive resistive region <NUM> is formed from a thermoelectric conversion material (e.g. amorphous silicon) having a thermal resistance coefficient value. In some embodiments, an absorber layer <NUM> is formed on an upper surface of the photosensitive resistive region <NUM> and is configured to selectively pass wavelengths of light (e.g., infrared light). The thermal resistance coefficient value provides a bolometer thermal resistance <NUM>, which can be utilized to sense thermal energy delivered to the bolometer pixel <NUM>. For example, when thermal energy (e.g., infrared light) is delivered to the photosensitive resistive region <NUM>, the resistance of the thermoelectric conversion material decreases. When the ROIC <NUM> applies a current, the change in bolometer electrical resistance is converted into a voltage, which can be read out via a ROIC <NUM> such that the bolometer <NUM> can be utilized as a thermal sensor.

Turning now to <FIG>, a trigger sense circuit <NUM> located on a ROIC is illustrated being in signal communication with a pixel group <NUM> included in a larger pixel array (not shown). The pixel array therefore includes a plurality of pixel groups, where each pixel group is connected to a respective trigger sense circuit <NUM>. Accordingly, the trigger sense circuit <NUM> of a given pixel group <NUM> can detect one or more overheated bolometer pixels in a given pixel group <NUM>.

Each pixel group <NUM> includes a plurality of pixel cells <NUM> arranged in selectable rows. A first row, for example, includes a first pixel cell including bolometer pixel 100a and second pixel cell including bolometer pixel 100b, while a second row includes a third pixel cell including bolometer pixel 100c and a fourth pixel cell including bolometer pixel 100d. Each pixel cell <NUM> further includes a bolometer selector switch 301a, 301b, 301c, 301d, respectively.

Each trigger sense circuit <NUM> is connected to a pixel group input line <NUM>. The pixel group input line <NUM> is connected to one or more pixel group selector switches 303a, 303b. Although two group selector switches 303a, 303b are shown, the embodiments described herein are not limited thereto. In this example, a first group selector switch 303a is interposed between bolometer pixel 100a included in the first row and bolometer pixel 100c included in the second row. Similarly, a second group selector switch 303b is interposed between bolometer pixel 100b included in the first row and bolometer pixel 100d included in the second row. The pixel group selector switches 303a, 303b are configured to operate in a first state and a second state to selectively establish electrical connection between the pixel group <NUM> and either the pixel group input line <NUM> or an integration signal line <NUM>.

When operating in a first state (see <FIG>), the pixel group selector switches 303a, 303b disconnect the pixel group <NUM> from the pixel group input line <NUM> (and thus the trigger sense circuit <NUM>), while establishing an electrical connection with the integration signal line <NUM>, which is connected to an integration unit <NUM>. When operating in a second state (see <FIG> and <FIG>), the pixel group selector switches 303a, 303b disconnect the pixel group <NUM> from the integration signal line <NUM>, while establishing electrical connection to the pixel group input line <NUM> and thus the trigger sense circuit <NUM>.

A thermal imaging system reads an image generated by a pixel array by integrating the measurements of each row in the larger pixel array. For example, to read an image generated by pixel bolometers 100c and 100d included in the second row of the pixel group <NUM>, the thermal imaging system invokes the first state of the pixel group selector switches 303a, 303b so that bolometer pixel 100a, 100b, 100c, 100d are connected to their respective integration signal lines <NUM>. The thermal imaging system then activates bolometer selector switches 301c and 301d to connect pixel bolometers 100c and 100d to a ground reference. Accordingly, the integration unit <NUM> determines the resistance of pixel bolometers 100c and 100d to read their respective images.

Although not illustrated, the thermal imaging system performs a similar operation to read the image generated by pixel bolometers 100a and 100b included in the first row of the pixel group <NUM>. For example, while the pixel group selector switches 303a, 303b are in the first state, the thermal imaging system deactivates bolometer selector switches 301c and 301d to disconnect pixel bolometers 100c and 100d from the ground reference, and activates bolometer selector switches 301a and 301b to connect pixel bolometers 100a and 100b to a ground reference. Accordingly, the integration unit <NUM> integrates the current from bolometers 100a and 100b to generate an integrated voltage inversely proportional to the resistance of pixel bolometers 100a and 100b, which can be referred to as read their respective images. After reading the image of each pixel bolometer 100a, 100b, 100c, 100d in the pixel group <NUM>, the thermal imaging system invokes the second state of the pixel group <NUM> such that the pixel group <NUM> is disconnected from the integration unit <NUM> and connected to the trigger sense circuit <NUM> (see <FIG> and <FIG>). Accordingly, the trigger sense circuit <NUM> can determine whether one or more of the pixel bolometers 100a, 100b, 100c, 100d are overheated as described in greater detail below. The thermal imaging system continuously repeats the operations described above for each pixel group included in the pixel array. That is, the thermal imaging system sequentially connects and disconnects each pixel group to and from the integration unit over an image integration time period in order to integrate the images generated by each bolometer pixel included in the pixel array. However, the majority of each frame period the pixels are connected to the trigger sense circuit so any overheating can be detected.

Still referring to <FIG>, the trigger sense circuit <NUM> includes a resistance transimpedance amplifier circuit (RTIA) <NUM>, and an electronic comparator <NUM>. The combination of the pixel group selector switches 303a, 303b, pixel group input line <NUM>, RTIA <NUM>, and electronic comparator <NUM> provides one example of a trigger sense circuit <NUM> capable of detecting whether one or more of the bolometer pixels 100a, 100b, 100c, 100d have become overheated due to exposure to an excessive amount of thermal energy (e.g., solar radiation, laser energy, etc.). In <FIG>, bolometer pixels 100a, 100b, 100c, 100d are operating at an expected normal temperature, e.g., at about <NUM> (i.e., not overheated by thermal energy), and their resistance is referenced as "RH".

The RTIA <NUM> includes an operational amplifier (OpAmp) <NUM>, a feedback resistor (RTIA) <NUM>, and a current source <NUM>. A first input terminal <NUM> (e.g., positive terminal) of the OpAmp <NUM> is connected to a bias voltage (Vbias) (e.g., <NUM> V), while a second input terminal <NUM> (e.g., negative terminal) is connected to the pixel group input line <NUM>. A first end of the feedback resistor <NUM> is connected to the second input terminal <NUM> (e.g., negative terminal) of the OpAmp <NUM>, while the opposing second end is connected to the output <NUM> of the OpAmp <NUM>. The input of the current source <NUM> is connected in common with the second end of the feedback resistor <NUM> and the output <NUM> of the OpAmp <NUM>, while the output of the current source <NUM> is connected in common with the first end of the feedback resistor <NUM> and the second input terminal <NUM> of the OpAmp <NUM>.

Turning to <FIG>, the trigger sense circuit <NUM> is shown operating in an armed state. The current source <NUM> generates a direct current (DC)-sourced trim current (Itrim) to source the "untriggered" bias current to the bolometers. Because none of the bolometer pixels 100a, 100b, 100c, 100d are overheated, they are all operating in an "untriggered" state such that approximately an equal amount of current flows through each pixels 100a, 100b, 100c, 100d. The amount of current (ILRH) flowing through a given untriggered pixel 100a, 100b, 100c, 100d can be calculated as: ILRH = Vbias/RH.

The difference in total current through the bolometers in a trigger group and the trim current (Itrim) induces an initial voltage (e.g., 0V) across RTIA <NUM>, which is applied to the second input terminal <NUM> of the OpAmp <NUM>. Accordingly, the OpAmp output <NUM> generates a first RTIA output voltage (e.g., 1V). In this manner, the RTIA <NUM> serves as a current monitor by converting the sum of the currents minus the trim current level (Itrim) into an output voltage signal.

The comparator <NUM> includes a first input terminal <NUM> (e.g., a reference voltage terminal <NUM>) and a second input terminal <NUM> (e.g., a RTIA voltage terminal <NUM>). The reference voltage terminal <NUM> is connected to a voltage source to receive a reference voltage (e.g., 2V). The RTIA voltage terminal <NUM> is connected to the OpAmp output <NUM> to receive the RTIA output voltage. When the trigger sense circuit <NUM> is operating in the normal armed state as shown in <FIG>, the RTIA output voltage (e.g., <NUM> V) applied to the RTIA voltage terminal <NUM> is less than the reference voltage (e.g., <NUM> V) applied to the reference voltage terminal <NUM>. Accordingly, the comparator <NUM> outputs a trigger signal <NUM> having a first signal state (e.g., binary "<NUM>" bit value).

Turning now to <FIG>, the trigger sense circuit <NUM> is shown operating in a triggered state according to a non-limiting embodiment. Bolometer pixel 100b, for example, receives an excessive amount of thermal energy <NUM> (e.g., incident infrared light, solar energy, etc.) such that its resistance (RL) changes. In this example, the resistance (RL) of bolometer pixel 100b decreases as its temperature increases. Accordingly, current flow through bolometer pixel 100b increases, thereby invoking a "tripped" state. In <FIG>, an overheated tripped bolometer pixel (e.g., bolometer pixel 100b) is referenced as "RL", indicating that its resistance reduced allowing an increased level of current to flow therethrough.

In response to tripping bolometer pixel 100b, the difference between the sum of the currents and the fixed trim current (Itrim) <NUM> increases due to the reduced resistance of the bolometer pixel 100b, thereby invoking a "tripped" state of the trigger sense circuit <NUM>. The amount of current (ILRL) flowing through an overheated bolometer pixel can be calculated as: ILRL = Vbias/RL. The increased current also changes the voltage (e.g., 2V) across RTIA <NUM>, which is applied to the second input terminal <NUM> of the OpAmp <NUM>. Accordingly, the OpAmp output <NUM> generates a second RTIA output voltage (e.g., 3V). In this manner, the RTIA <NUM> indicates a change in the current level based on the changed output voltage signal (e.g., 3V).

In the triggered state, the second RTIA output voltage (e.g., 3V) applied to the RTIA voltage terminal <NUM> of the comparator <NUM> is now greater than the reference voltage (e.g., 2V) applied to the reference voltage terminal <NUM>. Accordingly, the comparator <NUM> outputs a trigger signal <NUM> having a second signal state (e.g., a binary "<NUM>" bit value). A controller <NUM> can receive the trigger signal <NUM> in either case (armed or triggered), and in response to detecting the second signal state (e.g. a binary "<NUM>" bit value) can output a protection control signal that invokes a protection operation to protect the overheated bolometer pixel 100b. The protection operation can include, but is not limited to, initiating a fast-acting mechanical shutter or a variable transmission window mounted in front of the bolometer array to block the excessive energy delivered to the over-heated pixel(s).

Turning now to <FIG>, and referencing <FIG>, <FIG> and <FIG>, a method of detecting thermal energy delivered to a bolometer pixel included in a thermal imaging system is illustrated according to a non-limiting embodiment. The method begins at operation <NUM>, and at operation <NUM> a pixel array is armed by delivering a DC trim current (Itrim) to a plurality of pixel groups <NUM>, each pixel group <NUM> including a plurality of bolometer pixels 100a, 100b, 100c, 100d operating at an expected or nominal temperature. At operation <NUM>, a pixel group <NUM> included in the pixel array is selected. The selected pixel group <NUM> is connected to an integration unit <NUM>, while the remaining pixel groups in the pixel array are connected to respective trigger sense circuit <NUM>. At operation <NUM>, the integration unit <NUM> reads an image generated by the bolometer pixels 100a, 100b, 100c, 100d included in the selected pixel group <NUM>. At operation <NUM>, the next pixel group in the pixel array is sequentially selected for image integration, and the method returns to operation <NUM> to connect the next selected pixel group to the integration unit <NUM>.

While the integration unit <NUM> reads the image generated by the bolometer pixels 100a, 100b, 100c, 100d included in the selected pixel group <NUM> at operation <NUM>, the temperature of the bolometer pixels 100a, 100b, 100c, 100d included in the remaining pixel groups is monitored by a respective trigger sense circuit <NUM> at operation <NUM>. The temperature is monitored by comparing a voltage that represents a thermal resistance change of one or more of the bolometer pixels 100a, 100b, 100c, 100d with a reference voltage. At operation <NUM>, a determination is made as to whether any of the bolometer pixels included in the remaining pixel groups are overheated. When one or more overheated bolometer pixels are detected, an output trigger signal <NUM> is generated by the trigger sense circuit <NUM> for that pixel group at operation <NUM> indicating that corresponding pixel group contains an overheated bolometer pixel.

Whether or not overheated bolometer pixels are detected at operation <NUM>, the next pixel group is selected at operation <NUM>, and the method proceeds to operation <NUM> to connect the next selected pixel group to the integration unit to provide essentially continuous pixel monitoring.

Turning now to <FIG>, a method illustrates that a trigger detection signal output by the trigger sense circuit <NUM> and operation <NUM> can be utilized to perform a protection operation to protect overheated bolometer pixels in a thermal imaging system. The method begins at operation <NUM>, and at operation <NUM> a determination is made as to whether a trigger sense circuit <NUM> outputs a trigger signal <NUM> indicating that a one or more bolometer pixels 100a, 100b, 100c, 100d is overheated. When the trigger signal <NUM> is not output from the trigger sense circuit <NUM>, the method returns to operation <NUM> and continues monitoring for the trigger signal <NUM>.

When, however, the trigger signal <NUM> is detected, a separate controller <NUM> outputs a protection control signal at operation <NUM>, and a protection operation is initiated at operation <NUM> to protect the bolometer pixels 100a, 100b, 100c, 100d in the thermal imaging system. The protection operation can include, but is not limited to, initiating a mechanical shutter or a variable transmission window in front of the bolometer array to block the excessive energy delivered to the over-heated bolometer pixel(s). At operation <NUM>, a protection control signal can be output (e.g., via an imaging system controller), and the direction(s) and/or location(s) of the thermal energy source(s) can be determined (via the imaging system controller) at operation <NUM> before the method ends at operation <NUM>.

In one or more embodiments, a ROIC is provided that performs a pulsed-bias measurement of a bolometer pixel array. The bolometer pixel array contains a multitude of bolometer pixels arranged in selectable rows. The ROIC utilizes unbiased detectors as resistance reference source, which in turn can be used as high-energy thermal detectors. The ROIC includes a trigger sense circuit capable of providing an instantaneous value of a parallel combination of a group of resistors. When this value crosses a threshold, a comparator in the trigger sense circuit is tripped and outputs a trigger signal to the controller which determines that one or more bolometer pixels are overheated. In response to detecting one or more overheated pixels, the controller can output a protection control signal that invokes a thermal protection operation and/or initiates a thermal mitigation device such as, for example, a shutter or voltage-controlled window, that blocks the thermal energy source from delivered damaging radiation to the affected pixels.

Claim 1:
A thermal imaging system comprising:
a pixel array including a plurality of pixel groups (<NUM>), each pixel group comprising:
a plurality of pixel rows, each pixel row containing a plurality of bolometer pixels (<NUM>);
a trigger sense circuit (<NUM>) including a pixel group input line (<NUM>) in signal communication with the plurality of bolometer pixels (<NUM>), the trigger sense circuit configured to detect one or more overheated bolometer pixels included in the pixel group; and
a group selector switch (303a, 303b) that selectively establishes an electrical connection between the pixel group (<NUM>) and the trigger sense circuit (<NUM>); and
an integration unit (<NUM>) configured to generate an image based on a resistance of the plurality of bolometer pixels (<NUM>),
wherein a group selector switch (303a, 303b) of a selected pixel group operates in a first state to disconnect the selected pixel group (<NUM>) from the pixel group input line (<NUM>) and to connect the selected pixel group (<NUM>) to the integration unit (<NUM>) such that the integration unit (<NUM>) generates the image while remaining groups are connected to the pixel group input line, and
wherein the group selector switch (303a, 303b) of the selected pixel group operates in a second state to disconnect the selected pixel group (<NUM>) from the integration unit (<NUM>) and to connect the selected pixel group (<NUM>) to the pixel group input line (<NUM>) such that the trigger sense circuit (<NUM>) monitors the selected pixel group (<NUM>) for a high temperature bolometer.