US-12625001-B2 - Thermometric camera comprising a bolometer sensor and method of determining a temperature of an object in motion by a thermometric camera

US12625001B2US 12625001 B2US12625001 B2US 12625001B2US-12625001-B2

Abstract

A thermometric camera for determining a temperature of an object in motion comprises a bolometer sensor and circuitry. The bolometer sensor captures a sequence of image frames of the object while the object is moving. The circuitry executes: an object identifying function configured to identify an area corresponding to the object in each image frame of a series of image frames among the sequence of image frames; a combining function configured to combine the identified areas from each image frame in the series of image frames into a stacked image of the object, wherein pixel values in the stacked image of the object are estimated as a sum of pixel values of the corresponding pixels in the image frames in the series of image frames; and a temperature determining function configured to determine the temperature of the object in motion from pixel values in the stacked image of the object.

Inventors

Thomas WINZELL

Assignees

AXIS AB

Dates

Publication Date: 20260512
Application Date: 20240604
Priority Date: 20230720

Claims (9)

1 . A method of determining a temperature of an object in motion using a stationary thermometric camera, the method comprising: capturing a sequence of image frames of the object while the object is moving using a bolometer sensor of the stationary thermometric camera; identifying an area corresponding to the object in each image frame of a series of image frames among the sequence of image frames; combining the identified area from each image frame of the series of image frames among the sequence of image frames into a stacked image of the object, wherein pixel values in the stacked image of the object are estimated as a sum of pixel values of corresponding pixels image frames of the series of image frames among the sequence of image frames; and determining the temperature of the object in motion from the pixel values in the stacked image of the object.
2 . The method according to claim 1 , further comprising: estimating, from image frames in the sequence of image frames of the object, a pixel velocity of the object in motion; calculating, from the pixel velocity of the object in motion and a frame integration time, a pixel exposure time for the image frames in the sequence of image frames of the object; and determining a number of image frames to be used for the series of image frames among the sequence of image frames based on the calculated pixel exposure time and a thermal time constant of the bolometer sensor of the stationary thermometric camera.
3 . The method according to claim 2 , further comprising: determining the thermal time constant of the bolometer sensor of the stationary thermometric camera by one or more of: obtaining the thermal time constant of the bolometer sensor of the stationary thermometric camera given by the manufacturer of the bolometer sensor of the stationary thermometric camera, and a calibration of the stationary thermometric camera upon installation.
4 . The method according to claim 1 , further comprising: selecting the series of image frames among the sequence of images as image frames depicting the object in a center portion of a field of view of the stationary thermometric camera.
5 . The method according to claim 1 , further comprising: estimating, from image frames in the sequence of image frames of the object, an angular distance moved during a total integration time, α, of the object in motion; estimating blurring of radiance due to motion using a module transfer function, MTF = sin ⁢ π ⁢ α ⁢ f πα ⁢ f , where f is a spatial frequency; and correcting pixels of the series of image frames among the sequence of image frames based on the estimated blurring of radiance due to motion.
6 . A non-transitory computer-readable storage medium having stored thereon computer code instructions being executable by a device having processing capabilities, wherein the computer code instructions when being executed are configured to instruct the device to: receive a sequence of image frames of an object in motion captured using a bolometer sensor of a stationary thermometric camera; identify an area corresponding to the object in each image frame of a series of image frames among the sequence of image frames; combine the identified area from each image frame of the series of image frames among the sequence of image frames into a stacked image of the object, wherein pixel values in the stacked image of the object are estimated as a sum of pixel values of corresponding pixels in image frames of the series of image frames among the sequence of image frames; and determine a temperature of the object in motion from the pixel values in the stacked image of the object.
7 . The non-transitory computer-readable storage medium according to claim 6 , wherein the computer code instructions when being executed are further configured to instruct the device to: estimate, from image frames in the sequence of image frames of the object, a pixel velocity of the object in motion; calculate, from the pixel velocity of the object in motion and a frame integration time, a pixel exposure time for the image frames in the sequence of image frames of the object; and determine a number of image frames to be used for the series of image frames among the sequence of image frames based on the calculated pixel exposure time and a thermal time constant of the bolometer sensor of the stationary thermometric camera.
8 . A thermometric camera for determining a temperature of an object in motion, the thermometric camera comprising: a bolometer sensor configured to capture a sequence of image frames of the object while the object is moving; and circuitry configured to execute: an object identifying function configured to identify an area corresponding to the object in each image frame of a series of image frames among the sequence of image frames; a combining function configured to combine the identified area from each image frame of the series of image frames among the sequence of image frames into a stacked image of the object, wherein pixel values in the stacked image of the object are estimated as a sum of pixel values of corresponding pixels in image frames of the series of image frames among the sequence of image frames; and a temperature determining function configured to determine the temperature of the object in motion from the pixel values in the stacked image of the object.
9 . The thermometric camera according to claim 8 , wherein the combining function is further configured to: based on image data in image frames in the sequence of image frames of the object, estimate a pixel velocity of the object in motion; calculate, from the pixel velocity of the object in motion and a frame integration time, a pixel exposure time for the image frames in the sequence of image frames of the object; and determine, based on the calculated pixel exposure time and a thermal time constant of the bolometer sensor, a number of image frames to be used for the series of image frames among the sequence of image frames.

Description

TECHNICAL FIELD The present invention relates to a thermometric camera for determining a temperature of an object in motion. Additionally, a method of determining a temperature of an object in motion using a thermometric camera is covered by the invention. BACKGROUND In many situations, thermometric cameras are excellent devices for automatic detection and recognition surveillance. Recently, there has been focus on making thermometric cameras for various temperature applications. One application is to monitor vehicles that pass into certain critical road passages such as tunnels, bridges etc., with the purpose to screen for overheated breaks etc., preventing fire incidents. Doing so, temperature thresholds for hazardous vs. non-hazardous events can be monitored. However, such monitoring typically requires that vehicles slow down or even come to a full stop for accurate measurements. This since measurements with a thermometric camera typically will suffer from motion blur in case the object is moving. Furthermore, thermometric cameras have a finite thermal resolution and need to be subjected to thermal radiation for a certain minimum time in order to sense a temperature. SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a thermometric camera and a method for determining a temperature of an object in motion. According to a first aspect, a method of determining a temperature of an object in motion using a thermometric camera is provided. The thermometric camera is preferably stationarily arranged. The method comprises: capturing a sequence of image frames of the object while the object is moving using a bolometer sensor of the thermometric camera; identifying an area corresponding to the object in each image frame of a series of image frames among the sequence of image frames; combining the identified areas from each image frame in the series of image frames into a stacked image of the object, wherein pixel values in the stacked image of the object are estimated as a sum of pixel values of the corresponding pixels in the image frames in the series of image frames; and determining the temperature of the object in motion from pixel values in the stacked image of the object. Such a method of determining a temperature of an object in motion using a thermometric camera facilitates temperature determinations of objects in motion. It is, e.g., safeguarded that enough statistics on thermal radiation for the object is used in order to arrive at a reliable temperature determination for the object. By this method, monitoring for hazardous versus non-hazardous events based on temperature of objects may be provided. As an example, breaks of a train may be monitored during traveling of the train without the need for the train to slow down or much less stop. The method may further comprise: estimating, from image frames in the sequence of image frames of the object, a pixel velocity of the object in motion; calculating, from the pixel velocity of the object and a frame integration time, a pixel exposure time for the image frames of the sequence of image frames; and determining a number of image frames to be used for the series of image frames based on the calculated pixel exposure time and a thermal time constant of the bolometer sensor. The method may further comprise determining the thermal time constant of the bolometer sensor by one or more of: obtaining it as a value given by the manufacturer of the bolometer sensor, and a calibration of the thermometric camera upon installation. The method may further comprise selecting the series of image frames among the sequence of images as image frames depicting the object in a center portion of a field of view of the thermometric camera. The method may further comprise: estimating, from image frames in the sequence of images of the object, an angular distance moved during the total integration time, α, of the object in motion; estimating blurring of radiance due to motion using a module transfer function, M⁢T⁢F=sin⁢ π⁢α⁢fπα⁢f, where f is a spatial frequency; and correcting pixels of the series of image frames based on the estimated blurring. The angular distance moved during the total integration time can be expressed as α=v×t/D, where v is the velocity, t the integration time and D is the distance between the object and the camera. D may also be scaled with the angle β from the optical central axis, i.e. Dcosβ. This angle β is easily estimated as the field of view is known as a function of pixel position. According to a second aspect, a non-transitory computer-readable storage medium is provided. The non-transitory computer-readable storage medium having stored thereon computer code instructions being executable by a device having processing capabilities. The computer code instructions when being executed are configured to instruct the device to: receive a sequence of image frames of an object in motion captured using a bolometer