Patent Description:
Thermographic cameras are known. Most thermographic cameras use a lens to focus IR radiation onto a chip with multiple sensors. Such sensors are expensive and bulky. There is a need for a simple and low-cost thermographic camera.

<CIT> describes an imaging device which has a thermal sensor to remotely measure respective temperatures of regions within an imaging field and to generate temperature information signals. A motion tracking system tracks motion of the thermal sensor and generates position information signals representing positions of the thermal sensor during the temperature measurements. An image construction processor uses the position and temperature information signals to generate a two-dimensional image representative of the imaging field including respective temperature indications at different locations within the two-dimensional image, and stores the two-dimensional image within a memory. The two-dimensional image may be used as an output image for display to a user.

<CIT> describes a thermal image sensor including several infrared detector elements that detect infrared light in a detection area, and rotors that scan the detection area in a scanning direction to detect, with the infrared detector elements, infrared light in an area to be captured as a single thermal image. The infrared detector elements include infrared detector elements arranged in mutually different positions in a rotational direction corresponding to the scanning direction of the plurality of infrared detector elements.

<CIT> describes a device for generating thermal images. The device includes a low resolution infrared (IR) sensor supported within a housing and having a field of view. The IR sensor is configured to generate thermal images of objects within the field of view having a first resolution. A spatial information sensor supported within the housing is configured to determine a position for each of the thermal images generated by the IR sensor. A processing unit supported within the housing is configured to receive the thermal images and to combine the thermal images based on the determined positions of the thermal images to produce a combined thermal image having a second resolution that is greater than the first resolution.

In a first aspect of the invention, there is provided a method for providing thermal image data according to claim <NUM>. Favourable modifications are defined in the dependent claims.

In particular, the temperature for a pixel in the thermal image data is determined by using the temperature values thus determined.

The design with one sensor provides a cost-efficient way of providing thermal imaging in a low-cost device with a single IR sensor, for example a mobile phone.

In one embodiment it is assumed that the entire temperature difference between two partially overlapping fields is caused by one or both of the non-common fields. In one embodiment it is assumed that the entire temperature difference between two partially overlapping fields is caused by one of the non-common fields.

The field of view may be swept in at least two different sweep directions, resulting in a first temperature value for a non-common field from a first sweep direction and a second temperature value for a non-common field from a second sweep direction and the first and the second temperature values are used to determine the temperature for an area in the image or pixels that represent the area that is shared by both the two non-common fields. Using a plurality of sweep directions increases the resolution of the image. Adding even more sweep directions increases resolution.

The data from the various sweep directions can be used in any suitable manner to determine the temperature of the shared region. In one embodiment the temperature for a area or pixel in the image is determined by averaging the temperatures of the shared region.

The IR sensor may be provided in a handheld device, for example a mobile device such as a mobile phone. The user may be provided with instructions to sweep the device in at least two different directions that are not the direction of detection of the field of view of the IR sensor. The instructions to the user may comprise instruction to move the handheld device in a pre-defined direction, and where the certain direction is updated depending on data from the position and orientation determining means. When the device has a display, the display may be used to display indications that prompts the user to sweep, and also used to display a thermal image created with the use of the thermal image data. The display may be used to display the image as it is formed in real time. This has the advantage that the user is provided with feedback for how to sweep to improve the image in real time.

In a second aspect of the invention there is provided a system comprising an IR sensor with the features of claim <NUM>.

In particular, the temperature for a point in the image is determined by using the temperature values thus determined for non-common fields to which the point belongs.

In a third aspect of the invention there is provided software carrying out a method according to claim <NUM> when executed on a system according to claim <NUM>.

In particular, temperature for a pixel in the thermal image data is determined by using the temperature values thus determined.

With reference to <FIG>, system <NUM> comprises an IR sensor <NUM> which is arranged to receive IR radiation in a field of view <NUM>. The field of view <NUM> has a direction of detection <NUM>. The IR sensor <NUM> may be any useful type of IR sensor for example a thermopile or a bolometer, where a thermopile is preferred because of the short response time. It is preferred that the IR sensor is of a type that provides absolute temperature values, such as a thermopile. The angle α of the field of view <NUM> may be from <NUM>° to <NUM>°, where <NUM>° to <NUM>° is more preferred and <NUM>° to <NUM>° is even more preferred. The field of view <NUM> may have any suitable shape but is preferably circular as shown in <FIG>, <FIG>, <FIG>.

The IR sensor <NUM> is preferably of a type that senses one value, and one value only, at one given moment. The thermal image data is produced by scanning the field of view <NUM> of the IR sensor <NUM> across an area of interest <NUM> and repeatedly gather IR information during scanning, and then compiling and processing the thus gathered information using the position and/or orientation of the IR sensor <NUM> to produce the image data.

The system <NUM> also comprises a memory <NUM>, a processor <NUM>, and a communication bus <NUM>. In a preferred embodiment the system <NUM> comprises a digital display <NUM> for showing digital images. The digital display <NUM> may be for example an LCD display. System <NUM> may comprise any suitable combination of software and hardware. System <NUM> may for example comprise an operating system and a device for making input such as a touch display.

The memory <NUM> is used for storing data, such as for example software, for example software carrying out the method and also necessary data such as IR sensor values, positions, orientation, time points, time stamps, overlapping fields and their positions, and image data.

The system <NUM> also comprises positioning and/or orientation determining means <NUM> referred to as "position determining means <NUM>" herein, able to determine a position and/or the orientation of the IR sensor <NUM>. Orientation of the IR sensor <NUM> means the orientation of the direction of detection <NUM>. Preferably, both position and orientation are determined by position determining means <NUM>, but in certain embodiments only position or orientation is determined. Position and orientation are determined at fine resolution, preferably such that a movement of a few millimetres (for translation) or degrees (for orientation) can be detected.

Position and orientation are determined with methods known in the art. Hence, position determining means <NUM> may comprise any suitable technology including tracking cameras, marker-based or markerless tracking, inertial tracking, digital model building, gyroscopes and accelerometers, which technologies may be combined in any suitable manner. Present models of mobile phones such as iPhone X and Android phones are able to determine position and orientation with a high resolution. Apple iOS provides an API called Core Motion for providing position and orientation data, and Android provides a similar API.

Preferably the IR sensor <NUM> and positioning determining means <NUM> are comprised in a device <NUM>, which preferably is a device that can be handheld. The device <NUM> may be a mobile phone, such as an iPhone or an Android phone. Nevertheless, parts of system <NUM> may located outside device <NUM>, such as on a server. In particular, memory <NUM> and processor <NUM> or parts of or memory <NUM> and processor <NUM> may be located on a server which is in digital communication, for example wireless communication, with device <NUM>.

The direction <NUM> of detection of the field of view <NUM> is preferably fixed in relation to device <NUM> The position determining means <NUM> is able to determine the direction <NUM> of detection of the field of view <NUM> and the position of the IR sensor <NUM>. The position and the direction <NUM> are preferably determined in relation to object in the surroundings such as objects in the area of interest <NUM>. The position and direction may preferably be determined in relation to a stationary object. The direction of detection <NUM> may be the same as a line of observation perpendicular and directed towards surface of a display <NUM> on device <NUM> (see <FIG>). A user that holds device <NUM> with display <NUM> in the hand will then observe the display <NUM> in the same direction as the direction of detection <NUM> of IR sensor <NUM>, which is an advantage during real time display of thermal images (see below).

System <NUM> may also comprise suitable components for processing the signal from the IR sensor <NUM> known in the art, such as for example, amplifiers, filters, A/D converters, etc. System <NUM> is powered by a suitable power source such as for example a battery. The IR signal is provided from the IR sensor <NUM> to rest of system <NUM> which may be able to process the signal, store data representing the signal and carry out computations using this data using any suitable combination of software and hardware.

The system <NUM> is used for production of thermal image data in the form a dot matrix <NUM> representing a thermal image, an example of which is shown in <FIG>. The dot matrix <NUM> comprises pixels <NUM> which makes up the image. The dot matrix image may be a bitmap image. The image data is suitable for rendering a thermal image <NUM> on a display, for example display <NUM> of system <NUM>. The image data may be arranged as dot matrix <NUM> where a temperature value is stored for each pixel <NUM>. The number of pixels in thermal image <NUM> may be from, for example, <NUM><NUM> to several millions.

Each pixel <NUM> in the dot matrix <NUM> may assume at least two different colours (for example black and white) but it is preferred that more colours can be shown. Each colour represents a temperature value or a range of temperature values. The colour scheme is selected depending on the application. Typically, a heat image uses the colour scale: black-blue-green-yellow-red, where black indicates the lowest temperature and red indicates the highest temperature. Each colour indicates a certain temperature range. The colours are pseudo-colors and any useful colour scheme may be used. The temperature range for each colour is selected based on the intended use. The display <NUM> of device <NUM> may be used for rendering the thermal image based on the dot matrix <NUM>. Hence a pixel <NUM> may display a colour representing a temperature value.

For generating the image data, the field of view <NUM> is swept over the area of interest <NUM>, i.e. the area of which a thermal image <NUM> is to be generated. The direction of sweep is indicated with arrow <NUM> in the figures. When the device <NUM> is handheld, sweeping is done manually, by moving the hand. However, powered and/or automatically controlled sweeping may be used in some embodiments, whereby for example a motor controlled by a computer moves the field of view <NUM>. Hence, the system <NUM> and method described herein may be used as an alternative to multipixel thermal imaging chip sets.

The IR sensor <NUM> determines the IR value in the field of view <NUM> with at least a predetermined frequency. For a handheld device, a suitable frequency may be from <NUM> to <NUM>, where <NUM> to <NUM> is more preferred.

The position determining means <NUM> repeatedly determines the position and/or the orientation of the IR sensor <NUM> with a suitable time interval, which may be simultaneous or with the same frequency as capturing IR data. However, different sampling frequencies may be used. The sampling frequency for position and/or orientation may be at least the frequency for sampling IR data. Time-pairing algorithms may be used to pair the correct IR data with the correct data for position and/or orientation. For example, time stamps may be used. Hence, the position determining means <NUM> detects the respective current relative position and/or orientation of the IR sensor <NUM> for each IR value during the course of the field of view <NUM> is being swept.

Sweeping the direction of field of view <NUM> may be done by translating the field of view <NUM>, for example by translating the device <NUM> so that the field of view <NUM> moves across the area of interest <NUM> (shown in <FIG> where direction of translation is indicated with arrow T), or by changing the direction of detection <NUM>, for example by rotating the device <NUM> (i.e. turning the device <NUM> around an axis that is not corresponding to the direction of detection <NUM>, direction of turning is indicated with arrow R) as shown in <FIG>, or combinations of these. In a preferred embodiment, both translation and change of direction are used.

Hence, each IR value detected during scanning becomes associated with data representing position and/ or orientation. The IR image data may be compiled by using temperature values and positioning them in a dot matrix <NUM> with the use of the position and/or orientation data. The position and/or orientation data obtained by position determining means <NUM> may be used to position the various captured fields of view in three-dimensional space. As shown in <FIG>, the field of view <NUM> (or parts of field of view, see below) may assign temperature values to a plurality of pixels <NUM>. Thus, a pixel <NUM> in the image <NUM> typically correspond to a smaller area than the field of view <NUM>. More than one pixel <NUM> may be used to represent the area for which a temperature value has been determined and one pixel may be used to represent more than one area for which the temperature has been determined.

Sampling may be used to map the various areas for which temperature has been determined to pixels <NUM> of image <NUM>. For example, one or more pixels <NUM> in image <NUM> may be used to depict a shared region <NUM> (see below). Any suitable bitmap digital image file format may be used, including but not limited to, JPEG, TIFF, or GIF. Vector graphics can also be used, although it seems more straightforward to use bitmap/dot matrix file formats.

An edembodiment is shown in <FIG>. According to the invention, the sampling speed and the sweep speed is such that there are partial overlaps between the fields of view 4a 4b. The two fields of view 4a, 4b are associated with two fields <NUM> and <NUM> not common for the two overlapping fields of view 4a, 4b.

In <FIG>, the direction of sweep is again indicated with arrow <NUM>. "Old" non-common field <NUM> is a part of field of view 4a captured before field of view 4b. "New" non-common field <NUM> is a part of field of view 4b which is captured after field of view 4a. In various embodiments different assumptions are made regarding what affects any detected change in temperature between area 4a and area 4b. The difference in temperature between the old field of view 4a and the new field of view 4b may be attributed to the old non common area <NUM> or the new non-common field <NUM>, or both. A temperature value for at least one non-common field <NUM><NUM> is determined by using the temperature difference between the two partially overlapping fields of view 4a 4b and the proportion of the area of the non-common field <NUM><NUM> in relation to the area of the field of view 4a 4b. The temperature for a pixel <NUM> in the image <NUM> is determined by using the temperature values thus determined for at least one non-common field <NUM><NUM> to which the pixel <NUM> belongs.

The proportion of the area of the non-common parts <NUM><NUM> in relation to the field of view 4a4b depend on the sampling frequency and the speed with which the sweep is carried out. The sweep speed may be varying, in particular when a handheld device <NUM> is used. The proportion of area of non-common field <NUM><NUM> in relation to the area of the field of view <NUM> may be determined by a area proportion determining means. The area proportion determining means may use the current sweep speed as determined by position determination means <NUM>. Area proportion determining means is a part of system <NUM> and is preferably implemented as software that uses information from position determining means <NUM> and the sampling time points. Hence, area proportion determining means uses information from position determination means <NUM> to determine the speed with which the field of view <NUM> moves over the area of interest <NUM> and can use that information to determine the proportion between the area of a non-common field <NUM><NUM> in relation to a field of view 4a 4b. High scanning speed results in large non-common fields <NUM><NUM> and slow sweep speed results in small non-common fields. Low sweep speed results in improved resolution of the thermal information. A suitable proportion of the non-common field in relation to the total field of view <NUM> may be from <NUM> to <NUM> %.

In one embodiment sampling frequency of IR sensor and position determining means <NUM> automatically increases when the sweep speed is high.

The change in temperature between field of view 4a and 4b is divided with the area proportion and the resulting ratio is used to calculate a temperature for the old and/or new non-common field <NUM><NUM>. In one embodiment it is assumed that the entire temperature difference between overlapping fields 4a, 4b is due to new non-common field <NUM>. In one embodiment it is assumed that the entire temperature difference is due to old non-common field <NUM>. In further embodiments it is assumed that the temperature difference is due to both new non-common field <NUM> and old non-common field <NUM>, for example in fixed proportions, for example <NUM>% due to new non-common field <NUM> and <NUM>% due to old non-common field <NUM>.

In the following is an example where it is assumed that the entire change in temperature from field of view 4a to field of view 4b is due to new non-common field <NUM>. Say for example that the temperature of field of view 4a is <NUM> and that the temperature of area 4b is <NUM>. The difference in temperature (-<NUM>) will be attributed to non-common fields <NUM><NUM>. If we assume that the non-common field <NUM> has a size of <NUM>% of field of view 4b, there will be a change that is -<NUM>/<NUM>= -<NUM>, i.e. the temperature value for the new non-common field <NUM> will be <NUM>-<NUM>= <NUM>. Using data from more directions of sweep as described below increases the accuracy.

A method is shown in <FIG>. In step <NUM>, IR sensor data from two overlapping fields of view 4a 4b from the same sweep direction is collected. In step <NUM> the proportion of the area of the non-common field <NUM> and <NUM> to the field of view 4a 4b is determined by area proportion determining means. It is noted that the proportion between non common area <NUM> and field of view 4a will be the same as the proportion of noncommon area <NUM> in relation to field of view 4b. Steps <NUM> and <NUM> can be carried out in any order. In step <NUM> the temperature for non-common field <NUM> or <NUM>, or both, is determined.

The method is preferably applied for a series of overlapping field of views 4a, 4b, 4c, 4d. as shown in <FIG> such that the overlap between 4a and 4b is used, the overlap between 4b and 4c is used, the overlap between 4c and 4d is used, and so on. In this way substantial parts of area of interest <NUM> may be covered by non-common fields. A starting point <NUM> may be used for relating the positions and orientations of the different fields of view 4a, b and their non-common fields <NUM>, <NUM> to each other. The starting point <NUM> is preferably assigned to the first sampled field of view 4a. The position determining means <NUM> is be able to build a model of how the field of view <NUM> moves, and map the IR sensor values in relation to the starting point <NUM> to create a thermal image <NUM>. The starting point <NUM> is preferably a point in space defined in relation to the surrounding or the area of interest <NUM>. The starting point <NUM> is preferably associated with an initial direction of observation <NUM>.

A larger area of interest <NUM> may be covered by sweeping across the area of interest <NUM> multiple times, each time with a suitable offset (<FIG>). The offset should preferably at most be the diameter of the field of view <NUM>.

With reference to <FIG> it is preferred that the field of view <NUM> is swept across area of interest <NUM> in at least two different directions 15a, b. For example, the point <NUM> is covered by new non-common field 103a from the first direction 15a and a new non-common field 103b from the second direction 15b. The temperature for point <NUM> or area <NUM> is determined by using the temperature values for the first and new second non-common fields 103a,b as above and then use the temperature values for the point <NUM> or area <NUM> from the at least two sweep directions 15a,15b to determine a temperature value for an area in the thermographic image <NUM>, such as a pixel <NUM> or a number of pixels in the thermographic image <NUM>.

In step <NUM>, the field of view is swept over the area of interest <NUM> in a first direction 15a, resulting a in a first temperature value for a first new non-common field 103a. This is done using the method above with reference to <FIG>. In step <NUM> the field of view <NUM> is swept over the area of interest <NUM> in a second direction 15b, resulting in a second temperature for a second new old non-common field 103b, in the same manner. In step <NUM> it is determined that the first and second old non-overlapping fields 103a, b covers the same area to a certain degree, shown as shared region <NUM>. The first and second temperature values are used to determine the temperature for shared region <NUM> and may be used to determine the temperature value for pixels <NUM> in shared region <NUM> of image <NUM>. Shared region <NUM> (dashed) will have the same temperature value.

The shared region <NUM> is determined by using the position information stored for each non-common area 103a 103b. The starting position <NUM> may be used, for example, and all temperature vales may be related in space to starting position <NUM>.

Above it is described how new non-common areas 103a,b are used, but old non-common areas 102a,b may alternatively be used. Both new and old non-common areas may also be used as described above, where, for example any temperature change is divided by old and new areas, for example in a predetermined manner, for example, <NUM>/<NUM>.

In preferred embodiments values from three, four, five, six, seven, eight or more sweep directions <NUM> are used. Hence temperature values for three, four, five, six, seven, eight or more non-overlapping regions <NUM><NUM> may be used. Using temperature data from a plurality (such as two or more) sweeping directions increases the resolution of the thermal image <NUM>. The temperature data from the plurality of sweep directions can be used in any suitable manner. In one embodiment the average of the temperature values from the different directions 15ab are used. In one embodiment, where there are more than two sweep directions, the value that is furthest away from the average is used. In one embodiment the extreme value that is closest to the average is used (if the extreme values are <NUM> and <NUM> and the average is <NUM>, <NUM> is chosen). In one embodiment the median value is used.

Thus, for each pixel <NUM> in the image <NUM>, the temperature value may be the result of using temperature data for a plurality of temperature values, for example the average of the plurality of temperature values.

The directions 15a 15b of sweep of the two datasets is preferably separated with at least <NUM>°. Preferably two directions of sweep are separated by <NUM>°. In a preferred embodiment the field of view <NUM> is swept in at least four mutually different directions <NUM>, resulting in four different sets of determined IR sensor values. The mutually different directions <NUM> are preferably separated by <NUM>°. In an even more preferred embodiment eight different directions <NUM> separated by <NUM>° is used. In one embodiment three different sweep directions <NUM> separated by <NUM>° is used.

The sweeps are preferably carried out in the same plane or planes that are parallel. The planes are preferably perpendicular to the direction of observation <NUM> of the field of view <NUM> at the starting point <NUM>.

In one embodiment, shown in <FIG>, the field of view <NUM> is swept in a first direction 15a several times with perpendicular offsets so that each sweep covers at least partly different areas, and wherein the field of view <NUM> is additionally swept in the second direction 15b several times with different perpendicular offsets so that each sweep <NUM> covers at least a partly different area of the area of interest <NUM>. Each arrow <NUM> indicates the path of the centroid of the projection of the field of view <NUM> as it covers the area of interest <NUM>.

However, random sweeping may be used as shown in <FIG>. For example, a user holding device <NUM> in his or her hand may sweep field of view <NUM> over area of interest <NUM> in a criss-cross manner to gather information from several directions. In one embodiment, the image is shown in real time on display <NUM> providing the user with feedback as the image improves during sweeping. Hence, the user is able to create a thermal image using a "painting"-type motion. This is particularly useful when the image <NUM> is shown on a display <NUM> in real time, see below. Then the user can improve the image <NUM> where needed by sweeping across an area of interest <NUM> that needs improvement and see the result in real time.

A user may be provided with instructions to sweep in different predefined directions as described above, such as for example, two directions separated by <NUM>°. The instructions may be provided in any manner such as text, symbols, or sound such as synthetic speech.

In a preferred embodiment, instructions are provided on display <NUM>. An example is shown in <FIG>. For example, a handheld device <NUM> is used by a user and the user is prompted to move the field of view <NUM>, for example by moving the device <NUM> in different directions 15a 15b. In a preferred embodiment, the device <NUM> has a display <NUM> and the display <NUM> is used to prompt the user. The display <NUM> can be used to indicate to the user the direction <NUM> in which the user should sweep the device <NUM>. The display <NUM> may also provide guidance for orienting the device <NUM> and hence direction of observation <NUM> in the correct direction. The prompting may be provided simultaneous as a real time image is shown as described above.

In one embodiment, shown in <FIG>, instructions to the user is provided in real time using information from position determining means <NUM>. In particular, the user may be prompted to translate or turn the device <NUM> by translating or turning the field of view <NUM> order to maintain a predefined sweep direction and orientation. In <FIG> the display <NUM> displays an arrow <NUM> and a level tool comprising a fixed cross <NUM> and an orientation dependant cross <NUM> that moves on the display <NUM> with the orientation of the device <NUM>, as determined by orientation determination means <NUM>. The user is prompted to keep orientation dependant cross <NUM> aligned to fixed cross <NUM>. The arrow <NUM> moves to instruct the user to scan in different directions. Arrow <NUM> may hence be controlled by position determining means <NUM> and a predetermined pattern for movement such as for example the pattern of <FIG>. For example, the user may be prompted to first sweep in a first direction 15a with offsets and then sweep in a second direction 15b, where the directions are separated with angles as described above. The position and orientation at the starting point <NUM> may be used to provide guidance to the user, where the user is prompted to hold the device <NUM> in the same orientation or position as at the starting point <NUM>.

The user may also be prompted to sweep in a certain plane, which may be the plane perpendicular to the direction of observation <NUM> at the starting point <NUM> or the plane of the display <NUM> of the device <NUM> at the starting point <NUM>. Command such as "closer" and "further away" may be used on the display <NUM> to keep the device <NUM> in the plane while sweeping.

The user may also be prompted to sweep with a certain speed. As mentioned above, the resolution will be better if the sweep speed is low. Hence the user may be prompted to increase or decrease the sweep speed as the case may be. Feedback may be provided in real time.

In one embodiment, the display <NUM> of the device <NUM> is used to display a thermal image <NUM> using the data for the thermal image processed as above. The method may thus comprise the step of displaying a thermal image <NUM> of the display <NUM> of the device <NUM> that comprises the IR sensor <NUM>. The image <NUM> may be rendered on the display <NUM> in real time as the user sweeps across the area of interest <NUM>. Hence, image data may be provided to display <NUM> for rendering in real time. This provides feedback to the user.

There is also provided imaging software implementing the methods described herein. The imaging software may be provided as an app to be downloaded to a portable device <NUM> with an IR sensor <NUM> and position determining means <NUM>. The imaging software is preferably stored on memory <NUM>. The imaging software may be configured to obtain IR sensor values and position and/or orientation data from position determining means <NUM>. For example, the imaging software may repeatedly query one or more sensors, or switch on one or more sensors, and providing a predetermined configuration to one or more sensor. Such configuration may for example be instructions to the IR sensor <NUM> or position determining means <NUM> regarding sampling frequencies. The imaging software may be configured to pair IR sensor data with data for position and/or orientation. The imaging software includes the proportion determining means described above.

The imaging software is configured to compile data to produce thermal imaging data to produce a thermal image <NUM>. Hence imaging software is configured to determine non-overlapping fields, and positions and orientation for these, and to compile temperature data from different directions as described herein.

Claim 1:
A method for providing thermal image (<NUM>) data using a device (<NUM>) comprising an IR sensor (<NUM>) arranged to receive IR radiation from the surroundings in a field of view (<NUM>), and position determining means (<NUM>) able to determine a position and orientation of the IR sensor (<NUM>), the method involving the steps:
a) the IR sensor (<NUM>) determining, with at least a predetermined frequency, an IR sensor value corresponding to a temperature of the field of view (<NUM>),
b) sweeping the field of view (<NUM>) over the area of interest (<NUM>) for which the thermal image (<NUM>) data is to be produced, where the sweeping is done such that two consecutive IR sensor (<NUM>) value determinations have overlapping corresponding fields of view (<NUM>),
c) the position determining means (<NUM>) determining the respective current relative position and orientation of the device (<NUM>) for each determination in step a), during the field of view (<NUM>) is being swept,
d) using the determined IR sensor (<NUM>) values, together with their respective detected positions and orientations, to determine thermal image (<NUM>) data of the area of interest (<NUM>),
characterised in that
a temperature for an area of the image (<NUM>) is determined by determining temperatures for two partially overlapping fields of view (<NUM>), such that an overlap between two fields of view (<NUM>) are associated with two non-common fields (<NUM>,<NUM>), which are not common for the two overlapping fields of view (<NUM>), and where a temperature value for a non-common field (<NUM>, <NUM>) is determined by using the temperature difference between the two partially overlapping fields of view (<NUM>) and the proportion of the area of the non-common field (<NUM>, <NUM>) in relation to the area of the field of view (<NUM>).