Patent Description:
The present disclosure relates to the field of infrared cameras and in particular to a device and method for thermography.

In the field of thermography, infrared (IR) cameras, such as microbolometers or cooled IR imaging devices, are used to capture thermal images of an image scene. Such IR cameras generally comprise an arrangement of IR-sensitive detectors forming a pixel array.

Each pixel of the pixel array converts a measured temperature at the pixel into a corresponding voltage signal, which is converted by an ADC (analog to digital converter) into a digital output signal.

The temperature present at each pixel is a function of the scene temperature, but also of various other thermal components, such as the temperature of the substrate of the pixel array, and also parasitic heat received from the other heat sources. The substrate temperature is usually relatively uniform across the pixel array, and thus it can generally be estimated relatively precisely using one or more temperature sensors in the substrate. However, the parasitic heat received by each pixel from other sources is far more challenging to estimate, and can lead to relatively high imprecision in the temperature readings measured by each pixel. Indeed, while a temperature probe could be added to the housing, estimations of the parasitic heat affecting each pixel of the pixel array based on the reading from such a probe is far from accurate. Thus the use of such a probe does not permit the production of thermal images of high precision, for example accurate to within a few degrees Celsius. Furthermore, such a temperature probe is a relatively high cost component.

There is thus a need in the art for a low cost solution for accurately estimating and compensating for the parasitic heat received by pixels in an infrared camera.

<CIT> relates to an infrared imaging device comprising a pixel array having a plurality of parasitic heat sensing pixels orientated in different directions to detect noise from different portions of an interior surface a housing of the infrared imaging device. <CIT> further discloses a signal correction method using readings from the parasitic heat sensing pixels.

<CIT> relates to an infrared imaging sensor and method of producing the same.

<CIT> relates to an infrared ray imaging element and infrared ray imaging device.

<CIT> relates to an image sensor and manufacturing method thereof.

<CIT> relates to noise correction in an infrared imaging system.

It is an aim of embodiments of the present description to at least partially address one or more problems in the prior art.

The foregoing and other features and advantages will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings, in which:.

While embodiments are described in the following description in relation with a pixel array of the microbolometer type, it will be apparent to those skilled in the art that the methods described herein could be equally applied to other types of IR cameras, including cooled devices.

Throughout the present disclosure, the term "substantially" is used to designate a tolerance of plus or minus <NUM>% of the value in question. Furthermore, the following terms are considered to have the following definitions in the present disclosure:.

<FIG> illustrates an IR imaging device <NUM> comprising a pixel array <NUM> sensitive to IR light. For example, in some embodiments, the pixel array is sensitive to long-wave IR light, such as light with a wavelength in the range <NUM> to <NUM> or higher.

The pixel array <NUM> is indicated by a dashed rectangle in <FIG>, and comprises an image sensor <NUM> formed of image pixels <NUM>, and one or more additional pixels <NUM> for detecting parasitic heat.

In the example of <FIG>, the image sensor <NUM> comprises <NUM> image pixels <NUM> arranged in <NUM> rows and <NUM> columns. In alternative embodiments, the image sensor <NUM> could comprise any number of rows and columns of pixels. Typically, the image sensor for example comprises <NUM> by <NUM>, or <NUM> by <NUM> image pixels.

In the example of <FIG>, there are four parasitic heat sensing pixels <NUM> positioned along one edge of the image sensor <NUM>. However, in alternative embodiments, there could be any number of parasitic heat sensing pixels <NUM> positioned anywhere in or around the image sensor <NUM>. The parasitic heat sensing pixels are for example formed in the same image plane as the image pixels <NUM> of the image sensor.

In the example of <FIG>, each column of pixels of the array <NUM> is associated with a corresponding reference structure <NUM>. Though not functionally a picture element, this structure will be referred to herein as a "reference pixel" by structural analogy with the imaging (or active) pixels <NUM>. Furthermore, an output block (OUTPUT) <NUM> is coupled to each column of the pixel array <NUM> and to each of the reference pixels <NUM>, and provides a raw image IB comprising the signals or readings captured by the image sensor <NUM> and also readings PR from the parasitic heat sensing pixels <NUM>. Indeed, a same output block <NUM> is for example used to read out pixel values from all of the pixels <NUM>, <NUM> of the pixel array <NUM>.

A control circuit (CTRL) <NUM> for example provides control signals to the pixel array <NUM>, to the reference pixels <NUM>, and to the output block <NUM>.

The raw image IB and the readings PR from the parasitic heat sensing pixels <NUM> are for example provided to an image processing circuit (IMAGE PROCESSING) <NUM>, which for example applies 2D signal correction to the pixels of the image to produce a corrected image IC. In particular, the image processing circuit <NUM> for example applies correction of parasitic heat in the captured image based on the readings PR from the parasitic heat sensing pixels <NUM> and based on a conversion matrix MCpix stored in a non-volatile memory (NVM) <NUM>, which for example permits a conversion of the readings PR into a correction value for each pixel of the captured image.

Indeed, a voltage reading VOUT from each image pixel <NUM> of the image sensor <NUM> can be modelled by the following equation: <MAT> where Tpix is the temperature of the pixel, P<NUM> is a vector representing the parameters of the pixel array effecting the temperature to voltage conversion, such as the conversion gain, losses in the readout path, etc., and <IMG> is the function linking the output voltage VOUT to the parameters P<NUM> and the temperature Tpix.

The temperature Tpix of each pixel will be influenced by the various thermal components, and can for example be modelled by the following equation: <MAT> where φscene is the luminous flux arriving at the pixel from the image scene via the optical elements of the IR camera, φparasitic is the luminous flux arriving at the pixel from sources other than the image scene, such as from the interior surfaces of the housing of the IR camera, TCMOS is the temperature of the focal plane, in other words the temperature of the substrate on which the image sensor is formed, P2 is a vector representing the parameters of the image pixels effecting the conversion of the received luminous flux to the temperature Tpix of the pixel, and g is the function linking the temperature Tpix, to the parameters P<NUM> and variables φscene, φparasitic and TCMOS.

By estimating the parameters P<NUM> and P<NUM> and the variables φparasitic and TCMOS, and by approximating the functions <IMG> and g, it is possible to isolate the component φscene and thereby generate a thermographic image of the scene. Among these parameters, variables and functions, it is the component φparasitic that is the most challenging to estimate accurately. Indeed, this component can vary for each image pixel based on the temperature of several different interior surfaces in the IR camera, and the effect on each pixel will depend on the distance and sensitivity of the pixel with respect to the relevant surfaces.

The present inventors have found that, by using readings from parasitic heat sensing pixels positioned in the image plane, it becomes possible to generate a relatively precise estimation of the luminous flux φparasitic received by each image pixel, without the use of a temperature probe, as will be described in more detail below.

<FIG> is a flow diagram illustrating operations in a method of correcting images captured by an image sensor of a pixel array. For example, the method is implemented by the image processing circuit <NUM> of <FIG>. For example, the image processing circuit <NUM> is a hardware circuit, such as an ASIC (application specific integrated circuit), and thus implements the method entirely in hardware. Alternatively, at least part of the method could be implemented in software. For example, the image processing circuit <NUM> comprises one or more processors under the control of instructions stored in an instruction memory (not illustrated), the execution of these instructions causing at least part of the method of <FIG> to be executed.

In an operation <NUM>, the readings PR captured by the parasitic heat sensing pixels <NUM> are received by the circuit <NUM>.

In an operation <NUM>, signal correction values are generated based on the readings PR. For example, the conversion matrix MCpix, and optionally one or more further matrices stored by the non-volatile memory <NUM>, are used to convert the readings PR into a signal correction value for each pixel of the image IB, as will now be explained in more detail.

In some embodiments, the readings PR are first processed in order to extract an estimate of the temperature of a plurality q of zones of a model of the interior surface of the IR camera housing, wherein each zone of the model is for example considered to have a uniform temperature. These estimates form a luminance vector Vlum of the form [φ<NUM>. φq], each of the values φ<NUM>. φq representing a luminous flux from the q zones of the model. For example, the readings PR form an output vector Vout of the form [Out<NUM>. Outn], which can for example be characterized as follows: <MAT> where MClum defines the relationship between the luminance values φ<NUM>. φq and the n readings PR of the output vector Vout, and is for example of the form: <MAT> wherein the parameters <MAT> to <MAT> represent the relation between the readings Out<NUM> to Outn and the luminance φi of each zone i.

Thus the luminance vector Vlum can for example be generated from the readings of the output vector Vout based on the following multiplication: <MAT> where M-<NUM>Clum is the inverse of the matrix MClum.

The parasitic luminance present at each of the p pixels of the image sensor will be represented herein by a vector Vparasitic of the form [φparasitic_1. φparasitic_p]. The conversion matrix MCpix is for example adapted to convert the luminance vector Vlum into an estimation of the parasitic luminance present at each pixel in accordance with the following equation: <MAT>.

The conversion matrix MCpix is for example of dimensions p by q, where p is the number of pixels in the image sensor and q is the number of zones of the model of the interior surface of the housing.

In an operation <NUM>, the signal correction values are applied to the pixels of the captured image. For example, this correction may be performed directly to the signals forming the raw image IB, or after other forms of offset and/or gain correction have been applied to the raw image IB.

In one embodiment, the signal correction is applied by subtracting, from each of the p pixels of the captured image IB, the corresponding correction value from the vector Vparasitic. In alternative embodiments, the signal correction is based on an estimation of the inverse of the function g described above in order to determine the scene component φscene.

<FIG> is a plan view of the pixel array <NUM> according to an alternative embodiment to that of <FIG>. In the example of <FIG>, there are <NUM> parasitic heat sensing pixels <NUM>, two being positioned along each edge of the image sensor <NUM>, and one at each corner of the image sensor <NUM>.

<FIG> is a cross-section view, without showing the optics, of an IR camera <NUM> comprising the pixel array <NUM> of <FIG>. The cross-section in <FIG> is taken along a dashed line A-A in <FIG> passing through two of the parasitic heat sensing pixels <NUM> on opposite sides of the image sensor <NUM>.

The pixel array <NUM> is mounted on a substrate <NUM>. A housing <NUM> of the IR camera is also mounted on the substrate <NUM>, and houses the pixel array <NUM>. For example, the housing <NUM> is formed of moulded plastic, or of metal. In the example of <FIG>, the housing <NUM> has a substantially cylindrical portion <NUM> extending from the substrate <NUM>, an annular portion <NUM> extending from a top edge of the cylindrical portion inwards and substantially parallel to the surface of the substrate <NUM>, a portion <NUM> corresponding to a section of a cone extending upwards and inwards from an inner edge of the annular portion <NUM>, an annular portion <NUM> extending from a top edge of the portion <NUM> inwards and substantially parallel to the surface of the substrate <NUM>, a substantially cylindrical portion <NUM> extending from an inner edge of the annular portion <NUM> away from the substrate <NUM>, and an annular portion <NUM> extending from a top edge of the cylindrical portion <NUM> inwards and substantially parallel to the surface of the substrate <NUM>. An inner edge of the annular portion <NUM> delimits an aperture <NUM> of the housing <NUM>, centred with respect to the image sensor <NUM>, and via which light from the image scene enters the IR camera. The cylindrical portion <NUM> for example forms a lens barrel in which one or more lenses are positioned (not illustrated in the figures).

It should be noted that the particular form of the housing <NUM> of <FIG> is merely one example, and many different shapes would be possible, including non-cylindrical shapes.

An arc <NUM> in <FIG> extending between dashed lines <NUM> represents an example of the field of view of the image pixels of the image sensor <NUM>, which is for example relatively large, for example of substantially <NUM>° or more. The fields of view of the parasitic heat sensing pixels <NUM> are for example limited with respect to that of the image pixels, such that they receive an increased portion of parasitic heat from the interior of the housing <NUM>. For example, the pixel <NUM> illustrated in <FIG> have fields of view respectively represented by arcs <NUM> extending between dashed-dotted lines <NUM> and <NUM>, each parasitic heat sensing pixels <NUM> having a field of view limited in at least one plane to less than <NUM>°. However, more generally, each parasitic heat sensing pixel has its field of view modified such that it receives a higher proportion of parasitic heat from the housing <NUM> than each image pixel. For example, each parasitic heat sensing pixel receives a luminous flux φpix of which at least <NUM> percent, and in some cases at least <NUM> percent, of the energy originates from the housing <NUM>. In some embodiments, each parasitic heat sensing pixel is designed such that a majority, for example at least <NUM>%, of their received flux originates from a certain zone of the housing, and the sensitivity of the pixel to flux originating from outside this zone decreases rapidly.

In some embodiments, the field of view of one or more of the parasitic heat sensing pixels <NUM> is restricted such that it does not encompass the aperture <NUM>, and thus these pixels are not directly lit by the image scene. It should be noted that even if a parasitic heat sensing pixel targets a zone of the housing close to the aperture <NUM>, such as the zone <NUM> in <FIG>, the position of the parasitic heat sensing pixel in the pixel array and the lens design (light ray curvature) mean that only a relatively small quantity of light from the image scene risks being absorbed by the pixel.

The parasitic heat sensing pixels <NUM> are for example each oriented, in at least the plane of the pixel array, in a different manner from each other in order to detect parasitic heat from different areas of an interior surface of the housing <NUM> of the image sensor. For example, one of the parasitic heat sensing pixels <NUM> is configured to directly receive infrared light only from a first area of the interior surface of the housing, and another of the parasitic heat sensing pixels <NUM> is configured to directly receive infrared light only from a second area of the interior surface of the housing, the first and second areas being non-overlapping.

Examples of the structure of the parasitic heat sensing pixels <NUM> will now be described with reference to <FIG>, <FIG>, <FIG>.

<FIG> is a plan view of a portion of the pixel array <NUM> of <FIG>, and illustrates two image pixels <NUM> and one parasitic heat sensing pixel <NUM> according to an example embodiment.

In the example of <FIG>, the pixels are implemented by microbolometers. Each image pixel <NUM> for example comprises a membrane <NUM> suspended by arms <NUM> between support pillars <NUM>. The parasitic heat sensing pixel <NUM> for example comprises a similar structure, but is partially shielded by a light shield <NUM>, which restricts its field of view.

<FIG> is a cross-section view of the structure of <FIG> taken along a dashed line B-B in <FIG> passing through the two image pixels <NUM> and through the parasitic heat sensing pixel <NUM>. As illustrated, each of the image pixels <NUM> and the parasitic heat sensing pixel <NUM> for example comprises a portion <NUM> of a reflective layer between the corresponding pillars <NUM> and over which the membrane <NUM> is suspended at a distance d. Furthermore, the parasitic heat sensing pixel <NUM> for example comprises a further portion <NUM> of the reflective layer on a side of the pixel adjacent to one of its pillars <NUM>.

The partial light shield <NUM> for example comprises a support layer <NUM>, for example formed of Si, SiN, SiON, or another material, covered by a reflective layer <NUM>. The support layer <NUM> is for example suspended over the pixel <NUM> by a support wall <NUM>, which also for example blocks light from entering from one side of the pixel. The opposite side of the pixel is open, such that light at a certain angle can enter the space between the shield <NUM> and the reflective layer <NUM>, and be absorbed by the membrane <NUM>. This is aided by the portion <NUM> of the reflective layer, which for example directs light at a certain angle onto the underside of the partial light shield <NUM>, from which it reflects onto the membrane <NUM> of the bolometer.

<FIG> is a plan view of the pixel array <NUM> according to a further example embodiment in which parasitic heat sensing pixels <NUM> are formed in a sub-array <NUM> adjacent to the image sensor <NUM>, and the field of view of these pixels is partially restricted by a light shield in the form of a mask <NUM>. In the example of <FIG>, the sub-array comprises four parasitic heat sensing pixels <NUM> arranged two-by-two, and the mask <NUM> comprises an opening <NUM> over each pixel to give each pixel a restricted field of view.

<FIG> is a cross-section view of the structure of <FIG> taken along a dashed line C-C in <FIG> passing through two parasitic heat sensing pixels <NUM> of the sub-array and through one image pixel <NUM> of the image sensor <NUM>. The bolometer of each of the pixels <NUM>, <NUM> for example has a structure similar to that of the pixels of <FIG>, and like features have been labelled with like reference numerals and will not be described again in detail.

The mask <NUM> for example comprises a support layer <NUM> covered by a reflective layer <NUM> and through which the openings <NUM> over each pixel <NUM> are formed. The support layer <NUM> and reflective layer <NUM> are for example suspended over the pixels <NUM> of the sub-array by lateral walls <NUM>.

The openings <NUM> over each pixel <NUM> are for example misaligned with respect to the membrane <NUM> of each bolometer such that only light at certain angles falls on the membrane <NUM> of each bolometer. Each pixel <NUM> is for example configured to receive light from a different portion of the interior of the housing.

<FIG> is a plan view of the sub-array <NUM> of parasitic heat sensing pixels of the image sensor of <FIG> according to a variant to that represented by <FIG>. In the example of <FIG>, the sub-array <NUM> comprises nine parasitic heat sensing pixels <NUM> arranged <NUM>-by-<NUM>, although a larger or smaller array could alternatively by provided. The pixels <NUM> are spaced apart from each other.

The cover or mask <NUM> is represented by dashed lines line <FIG>, and is for example at least partially opaque to infrared light, but comprises openings <NUM>, which are circular in the example of <FIG>. Each opening <NUM> has a width dimension (diameter in the case of a circular opening) that is for example between <NUM>% and <NUM>% of the width of the membrane <NUM> of each pixel <NUM>. The openings <NUM> are positioned according to a pattern such that each pixel has a different angular view of the interior of the housing, and the field of view of each pixel can thus be de-convolved in a relatively simple manner. This has the advantage of leading to a good signal to noise ratio and a large coverage area of the interior of the housing.

In the example of <FIG>, the sub-array <NUM> is arranged in a <NUM>-by-<NUM> grid in which the nine locations in the <NUM>nd, <NUM>th and <NUM>th columns and rows contain the pixels <NUM>. An opening <NUM> associated with each pixel <NUM> is for example positioned entirely or at least partially in the area of the <NUM>-by-<NUM> grid in which each pixel is formed, these <NUM>-by-<NUM> grids being delimited by thicker lines in <FIG>.

In the example of <FIG>, a single opening <NUM> is associated with each pixel <NUM>. However, in alternative embodiments, more than one opening <NUM> could be associated with some or all of the pixels <NUM>, and/or some or all of the pixels <NUM> could receive light from more than one of the openings <NUM>.

<FIG> is a plan view of a portion the pixel array <NUM> of <FIG>, and illustrates two image pixels <NUM> and one parasitic heat sensing pixel <NUM> according to a further example embodiment. The pixels of <FIG> are for example implemented by bolometers having a structure similar to that of the image pixels <NUM> of <FIG>, and like features have been labelled with like reference numerals and will not be described again in detail.

In the embodiment of <FIG>, a wall <NUM> is for example positioned adjacent to the parasitic heat sensing pixel <NUM> for restricting its field of view, as will now be described with reference to <FIG>.

<FIG> is a cross-section view of the structure of <FIG> taken along a dashed line D-D in <FIG> passing through the two image pixels <NUM> and through the parasitic heat sensing pixel <NUM>.

In the example of <FIG>, the parasitic heat sensing pixel <NUM> has a distance d' separating the membrane <NUM> of its bolometer from its reflective layer <NUM>, the distance d' being greater than the distance d in the bolometers of the image pixels <NUM>. For example, the distance d' is equal to substantially twice the distance d. This increased distance results in a modification of the cavity Fabry-Perot of the bolometer, increasing the angular absorption. Furthermore, the wall <NUM> for example permits the azimuthal angle of the pixel to be restricted.

As described above, the signal correction applied to images captured by the image sensor <NUM> based on readings from the parasitic heat sensing pixels <NUM> is for example based on an approximation of the interior surface of the camera housing. For example, the conversion matrices M-<NUM>Clum and MCpix described above are based on a model representing the interior surface of the IR camera housing. Examples of models for approximating the interior surface the housing <NUM> of <FIG> will now be described with reference to <FIG>.

<FIG> is a cross-section view of the housing <NUM> illustrating an example of a model that is close to the actual form of the housing <NUM>. For example, the model corresponds a surface represented by a dashed line <NUM> in <FIG>, which generally follows the interior surface of the housing <NUM>, but rather than incorporating the lens barrel <NUM>, it has a planar portion <NUM> at the level of the annular portion <NUM> of the housing.

<FIG> is a cross-section view of the housing <NUM> illustrating an example of a model represented by a dotted curve <NUM> that is semi-spherical in shape, in other words in the form of a dome. The radius R of the dome <NUM> is for example chosen to correspond to the average distance of the interior surface of the housing <NUM> from image sensor <NUM>. While in the example of <FIG> the dome <NUM> extends from the image plane IP of the image sensor <NUM>, in the case that the field of view of the image sensor <NUM> is less than <NUM>°, the model could extend from a level of the housing higher that the image plane IP.

According to some embodiments, the model of the interior of the housing is divided into q discrete zones, each zone being considered to have a uniform temperature, as will now be described with reference to <FIG>.

<FIG> represents a 3D model of the parasitic heat surfaces of the interior of the housing of an IR camera according to an example embodiment in which the model corresponds to the dome <NUM> of <FIG>.

The surface of the model is divided into q discrete zones <NUM>, two of which are shown shaded in the example of <FIG>. The discrete zones <NUM> are for example chosen such that they have substantially the same area as each other. In the example of <FIG>, the dome is divided horizontally into slices, and each slice is subdivided into a number of segments of equal width. The height of each slice, and the width of the segments in the slice, for example varies from the bottom to the top of the dome in order to achieve zones of substantially equal area. Of course, <FIG> represents only one example of the division of a model into zones, there being many possible ways in which this could be achieved.

The number q of zones is for example equal to at least two, and in some embodiments to at least eight. It will be apparent to those skilled in the art that the greater the number of zones, the better the precision, but the more complex the image processing for correcting the signals of the images based on the luminance vector Vlum.

According to embodiments of the present disclosure, the readings from the parasitic heat sensing pixels are used to estimate an average heat of each zone <NUM> of the model, as will now be described in more detail with reference to <FIG> and <FIG>.

<FIG> represent angular sensitivity of a parasitic heat sensing pixel in terms of elevation θ and azimuth ϕ.

As represented by <FIG>, each parasitic heat sensing pixel for example has a field of view extending an angle θ in the vertical plane.

<FIG> represents an example of a radial absorption function of a parasitic heat sensing pixel. In particular, the centre of vision of the pixel is for example targeted at a certain angle in the horizontal plane, which is <NUM>° in the example of <FIG>, and the sensitivity of the pixel decreases for flux received at angles moving away from this point in the horizontal plane. An angle ϕ representing the angular sensitivity of the pixel can for example be defined as the angle over which the sensitivity is above a certain level. For example, in <FIG> the angle ϕ is defined as the angle over which the sensitivity is at <NUM>% or higher.

There are three possible relations between the observation areas of the parasitic heat sensing pixels and the zones of the model.

According to a first relation, there are as many parasitic heat sensing pixels as zones in the model, and each parasitic heat sensing pixel has an angular sensitivity in θ and ϕ adapted to a corresponding one of the zones. Thus the reading from each parasitic heat sensing pixel corresponds directly to a reading for a corresponding zone.

According to a second relation, there is a greater number of parasitic heat sensing pixels than zones of the model, and/or the total areas observed by the parasitic heat sensing pixels is greater than the area of the model. For example, the relation is based on following equation: <MAT> This can be expressed as: <MAT> where the model comprises q discrete zones, there are n parasitic heat sensing pixels w1 to wn, the values φ<NUM> to φq of the vector Vlum correspond to the parasitic luminance from each zone <NUM> to q, which is the vector to be found, the values <MAT> to <MAT> of the matrix MClum represent the contribution of the parasitic heat sensing pixels to each zone <NUM> to q, and the values Out<NUM> to Outn of the vector Vout correspond to the readings from the n parasitic heat sensing pixels. In the simplest case (first relation indicated above), each parasitic heat sensing pixel observes only a corresponding zone, and the matrix MClum is a diagonal matrix. However, in other cases, each zone <NUM> to q is defined by a set of weighted contributions from one or more of the parasitic heat sensing pixels.

According to a third relation, there are less parasitic heat sensing pixels than discrete zones in the model. In this case, the above matrix MClum is under-defined, as will now be described with reference to an example of <FIG>.

<FIG> is a Lambert azimuthal projection of the observation area <NUM> of each parasitic heat sensing pixel according to an example embodiment. In particular, the intersection between the dome and a solid angle cone of each pixel creates a measured observation area of each pixel. The luminous flux received from each zone can then be estimated based on local measurements and on a hypothesis relating to the thermal diffusion in each zone. For example, each reading Mk from a parasitic heat sensing pixel k can be evaluated as follows: <MAT> wherein Ω is a 2D surface representing the model divided into discrete zones i,j, Si,j is the intersection surface between each zone and the observation area of the pixel k, and φi,j is surface luminance flux of zones i,j.

In the case that the entire surface Ω is not fully observed by the collection of parasitic heat sensing pixels as shown in <FIG>, a simple hypothesis can be assumed, which is that the variation of the luminance across the surface of each zone is minimal, which can be expressed as: <MAT> where Δ represents the Laplacian of the luminance. The non-uniformity repartition of the luminance is then for example solved for each zone φi,j based on the above hypothesis, and an a priori hypothesis for the thermal diffusion in any white zones, i.e. zones that are not intersected by any observation area <NUM>.

A method of calibrating an IR camera comprising parasitic heat sensing pixels in order to construct the conversion matrices M-<NUM>Clum and MCpix will now be described with reference to <FIG>.

<FIG> schematically illustrates a computing device <NUM> configured to perform the calibration of an IR camera according to an example embodiment of the present disclosure. The device <NUM> for example comprises a processing device (P) <NUM> comprising one or more processors or CPU cores under control of computing instructions of a computer program stored for example in a memory (RAM) <NUM> coupled to the processing device <NUM> by a bus (BUS) <NUM>. The computing device <NUM> for example further comprises an IR camera interface (IR CAMERA INTERFACE) <NUM> permitting reception, from the IR camera under calibration, of a captured image from the image sensor and the readings from the parasitic heat sensing pixels. The computing device <NUM> for example further comprises a display (DISPLAY) <NUM>, and input devices (INPUT DEVICES) <NUM> such as a keyboard and mouse.

<FIG> is a flow diagram representing operations in a method of generating at least one conversion matrix, such as the matrices M-<NUM>Clum and MCpix, for converting readings from parasitic heat sensing pixels into signal correction values according to an example embodiment of the present disclosure. This method is for example implemented by the processing device <NUM> of the computing device <NUM> of <FIG>.

The generation of the at least one conversion matrix involves determining the correlation between the outputs of the parasitic heat sensing pixels and the parasitic luminous flux received by each image pixel. In other words, a relative map of the response by each parasitic heat sensing pixel and each image pixel to an exact same luminance variation should be estimated. This can be represented by the following equation: <MAT> where ΔVout(x,y) is the variation of the output voltage of each pixel at position (x,y), ∂φi is the variation in the luminance φi at each zone i of the model of the interior surface of the housing, Ti(x,y) is the etendue of each pixel with respect to each zone i, and Resp(x,y) is the responsivity of each pixel.

When calibrating a standard infrared image pixel array, a gain map is generally used in a process known as a <NUM>-point non-uniformity-correction. In the case of the pixel array of the present disclosure, in practice, exposing the parasitic heat sensing pixels and image pixels to a same luminance variation would be difficult, and the calibration process would be long. Instead, the present inventors propose to perform the calibration using two main operations (<NUM> and <NUM>), as will now be described in more detail.

In an operation <NUM>, relative transfer functions are determined between the surface contribution of the interior surface of the camera housing and the luminous flux received by the parasitic heat sensing pixels and by the image pixels. This corresponds to the etendue between each pixel and the various zones i of the model. In this operation, it is assumed that all of the pixels have the same response in terms of their voltage generated for a given received luminous flux of a given power (watts, W) and for a given solid angle (steradian, sr). Based on the geometry of the camera housing and of the pixels of the pixel array, the etendue Ti(x,y) of each parasitic heat sensing pixel and of each image pixel at position (x,y) with respect to each zone i can for example be estimated, as will now be described.

As known by those skilled in the art, in the field of optics, the etendue defines the extent to which light is spread out in area and angle.

The etendue Ti(x,y) for each pixel of the pixel array with respect to a zone i of the interior surface of the camera housing, assuming that this surface is in the form of a dome of radius R, can be defined as follows: <MAT> where Spixel is the surface area of the pixel, θ is the elevation angle, ϕ is the azimuth angle, and d is the distance between the pixel and the centre of the dome. Thus, based on the geometry of the pixel array and of the interior of the camera, it is possible to estimate etendue Ti(x,y) of each image pixel and parasitic heat sensing pixel based on the above equation.

The operation <NUM> is for example performed once for a given type of IR camera having a given housing and pixel array, the generated etendues being relevant to any IR camera having the given geometry of the camera housing and of the pixel array.

Optionally, in an operation <NUM>, one or more parameters of the model of the interior of the housing of the IR camera may be determined. For example, in the case that the model is a dome, the radius R of the model of the dome is for example defined based on an estimate of the average level of luminous flux received from the interior of the housing.

In an operation <NUM>, a unitary calibration is for example performed for each IR camera unit in a family of products in order to determine absolute values of the transfer functions between the surface contribution of the model of the interior surface of the camera housing and the pixel readings from the image sensor and from the parasitic heat sensing pixels. In particular, this for example involves determining the relative responsivity Resp(x,y) of each pixel for a same solid angle. For the image pixels of the image sensor, the responsivity Resp(x,y) can for example be determined using known calibration techniques, such as based on <NUM>-point non-uniformity-correction. As regards the characterisation of the parasitic heat sensing pixels, this is for example performed by placing a dome-shaped black-body over the pixel array and obtaining readings from each of the parasitic heat sensing pixels for two different temperatures of the black body.

Once this relative responsivity has been determined for each pixel, the matrices M-<NUM>Clum and MCpix can for example be determined based on the responsivity Resp(x,y) and etendue Ti(x,y) of each pixel.

An advantage of the embodiments described herein is that a parasitic heat component in an image captured by an IR camera can be estimated relatively precisely without the use of a temperature probe. For example, the present inventors have found that a precision as low as +/-<NUM> can be achieved.

Having thus described at least one illustrative embodiment, various alterations, modifications and improvements will readily occur to those skilled in the art. For example, it will be apparent to those skilled in the art that the embodiments of the parasitic heat sensing pixels merely provide one example, and that other pixel structures for limiting the field of view of the pixels would be possible.

Claim 1:
An infrared camera comprising a housing (<NUM>) containing a pixel array (<NUM>), wherein the pixel array comprises:
image pixels (<NUM>) forming an image sensor (<NUM>) arranged to receive infrared light from an image scene;
a plurality of parasitic heat sensing pixels (<NUM>), a first of said parasitic heat sensing pixels being orientated in a different manner from a second of said parasitic heat sensing pixels such that the first and second parasitic heat sensing pixels receive infrared light from different portions of an interior surface of said housing (<NUM>);
a signal correction circuit (<NUM>) configured to receive readings (PR) from the plurality of parasitic heat sensing pixels (<NUM>), and to perform 2D signal correction on signals (IB) captured by said image sensor (<NUM>) based on said readings;
characterised in that the infrared camera further comprises:
a non-volatile memory (<NUM>) storing a conversion matrix (MCpix) for converting said readings (PR) into correction values for performing said 2D signal correction, wherein said signal correction circuit (<NUM>) is configured to convert said readings (PR) into correction values by:
converting, using a first conversion matrix (M-<NUM>Clum), said readings into estimates of the luminous flux received from each of a plurality of zones of a model of the interior surface of said housing, each zone being considered to have a uniform temperature; and
converting, using a second conversion matrix (MCpix), said estimates of the luminous flux received from each of said plurality of zones into the pixel correction values, wherein said model of the interior surface of said housing is a dome.