GB2478708A

GB2478708A - Measuring the temperature of an object with an image sensor

Info

Publication number: GB2478708A
Application number: GB1004219A
Authority: GB
Inventors: Stuart Clarke; Laurie Linnett; Tommy Marshall; Steven Morrison; Wayne Rudd
Original assignee: BIOS TECHNOLOGIES LLP
Current assignee: BIOS TECHNOLOGIES LLP
Priority date: 2010-03-15
Filing date: 2010-03-15
Publication date: 2011-09-21
Also published as: GB201004219D0

Abstract

A temperature measurement apparatus 2, for measuring a temperature of an object 12, comprises an arrangement of cameras 4, 6, 8, 10 where each camera comprises an image sensor (22, Fig.2) having a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object 12. Such a temperature measurement apparatus may comprise a charged coupled device (CCD) image sensor (22, Fig 2) or a complementary metal oxide semiconductor (CMOS) image sensor (22, Fig 2). The apparatus is particularly suitable for the measurement of the temperature of an object such as a steel pipe 12 during welding to a further steel pipe (400, Fig 7). A method of measuring a temperature of an object comprises measuring a first and second electrical signal generated by the image sensor as a function of temperature of a first and second object respectively and determining the temperature of the second object from the first and second electrical signals.

Description

TEMPERATURE MEASUREMENT APPARATUS AND METHOD

FIELD OF THE INVENTION

The present invention relates to an apparatus and method for remotely measuring a temperature profile of an object and, particularly though not exclusively, for remotely measuring temperature around an object such as a pipe during a welding process.

BACKGROUND OF THE INVENTION

Image sensors are well known for imaging applications in the visible and infrared spectral ranges.

Visible image sensors such as charge coupled device (COD) or complementary metal oxide semiconductor (CMOS) image sensors are well known 1 5 for use in consumer electronic devices such as digital cameras including digital video cameras.

Infrared (IR) image sensors are known for imaging applications in the infrared, typically from one of the near infrared (NIR), short wave infrared (SWIR), medium wave infrared (MWIR) or long wave infrared (LWIR) bands.

IR image sensors have a wide range of applications including security applications where it is important to detect subjects in dark or low visibility conditions, for the detection of localised over-heating in power lines indicative of faulty joints, for the assessment of insulation in building construction applications, for monitoring gas leaks, for medical imaging applications, for remote temperature measurements in the steel industry and the like.

In general, visible image sensors use photon detection while IR image sensors use either energy detection or photon detection.

The terms "photon detector" and "photodetector" as used herein refer to a detector that generates free electrical carriers as a result of absorption of radiation.

Relative to thermal detectors, photodetectors have more limited spectral responses, higher peak sensitivities, and faster response times. Photodetectors include photodiodes such as p-n junction or p-i-n photodiodes, photovoltaic intrinsic detectors, photoconductive intrinsic detectors, extrinsic detectors, photo-emissive detectors, quantum well photodetectors, phototransistors and the like.

The terms "energy detector" and "thermal detector" as used herein refer to a detector that senses temperature changes induced internally in the detector by the 1 0 absorption of thermal energy. Such detectors typically have broad and generally uniform spectral responses with relatively low sensitivities and relatively slow response times. There a number of known energy detectors, including thermocouples/thermopiles, pyroelectric detectors, ferroelectric detectors, thermistors/bolometers/microbolometers and microcantilevers and the like.

1 5 IR photodetectors are typically based on semiconductor materials fabricated with Group Ill-V elements such as indium, gallium, arsenic, antimony; or with Group Il-VI elements such as mercury, cadmium and tellurium; or with Group IV-Vl elements such as lead, sulphur and selenium. There are a number of permutations, including the binary compounds GaAs, InSb, PbS, PbSe or others including InGaAs and HgCdTe.

State of the art lR photodetector image sensor may use semiconductor materials such as HgCdTe or InSb that require cooling in order to stabilize their IR sensitivity while increasing the contrast of the acquired images. For example, one IR device utilizes an LWIR image sensor and a display screen to detect and display thermal energy. However, the LWIR image sensor requires cryogenic cooling to maintain image sensor efficiency and stability. The cooling adds substantial cost and bulk to the LWIR sensor, thus limiting the applications where cryogenically equipped LWIR sensors may be used. Thus, in general, IR image sensors that generate signals based on photon absorption are relatively complex, costly, and are not highly

portable.

A bolometer is a well-known type of energy detector that may be designed to operate across a relatively wide spectral range. A bolometer comprises an absorber for absorbing radiation incident thereon and a temperature sensor for monitoring the temperature of the absorber. The temperature of the absorber varies according to the incident radiation and is monitored by the temperature sensor. The absorber is connected to a heat sink through a thermally insulating link such that the absorber temperature is only sensitive to energy inside the absorber. Bolometers are typically 1 0 slow to respond and are slow to reset i.e. slow to return to thermal equilibrium with their surrounding environment.

Cryogenically cooled silicon bolometer image sensors are used for high-sensitivity imaging applications. Such sensors are, however, particularly expensive.

State of the art thermal imaging applications typically use uncooled 1 5 microbolometers comprising two-dimensional arrays of thermal IR detector elements.

Each thermal IR detector element comprises an lR absorber element and a temperature measurement element in thermal contact with the IR absorber element.

The temperature measurement element measures the temperature rise due to absorption of lR radiation in the lR absorber element. Examples of an lR absorber element include an amorphous silicon absorber element, a Si3N4 absorber element or a thin metal film absorber element. The temperature measurement element senses any temperature rise induced by any IR radiation absorbed in the IR absorber element and converts it into an electric signal. Each thermal IR detector element is designed to be thermally isolated from an underlying read-out integrated circuit. The most common temperature measurement mechanism is the thermoresistive effect, but various other mechanisms can be used, such as the pyroelectric effect, thermoelectric junction, p-n junction conductivity or thermal stress induced mechanical deflection. Examples of materials that are typically used as the temperature measurement element in a thermoresistive microbolometer are materials such as amorphous silicon, vanadium oxide (VOx) or titanium oxide.

Although uncooled IR microbolometer sensors may be less expensive than cooled IR photodetectors or cooled bolometers, uncooled microbolometer sensors are still relatively expensive and may be limited in terms of sensitivity, requiring relatively long exposure times and typically restricting their optical design to relatively low f-numbers in the range fi.0 -1.5.

IR image sensors are also known that incorporate visible COD or CMOS image sensors but which rely on the indirect detection of IR radiation. For example, RedShift Systems Inc. have developed IR image sensors comprising a COD or CMOS image sensor, an array of thermo-optic pixels and a light source that generates a probe beam at 850nm. The IR thermal radiation in the wavelength range 8 -15im that is to be detected is directed onto the thermo-optic pixel array and the 850nm probe beam is directed through the thermo-optic pixel array onto the 1 5 COD or CMOS image sensor. The thermo-optic pixel array comprises an amorphous silicon layer sandwiched between two silicon nitride/amorphous silicon mirror layers.

Any thermal radiation incident on a pixel is absorbed in the pixel resulting in heating of the pixel and variation in the optical transmission of the pixel to the 850nm probe beam. An image of the 850nm probe beam is thereby created on the COD or CMOS sensor, the image of the 850nm probe beam being representative of the thermal IR radiation incident on the thermo-optic pixel array.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided a temperature measurement apparatus for measuring a temperature of an object comprising an image sensor having a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object.

The apparatus may, for example, comprise a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor.

The apparatus may, for example, comprise a monochromatic or a colour image sensor.

The apparatus may, for example, be arranged to measure a temperature profile of an object.

The apparatus may, for example, be arranged to measure a temperature profile of a non-planar surface of an object.

The apparatus may, for example, be arranged to measure a temperature profile of a curved surface of an object.

The apparatus may, for example, be arranged to measure a temperature 1 5 profile of a cylindrical object such as a pipe.

The apparatus may, for example, be configured to measure a temperature profile of a metal object such as a steel object.

The apparatus may be configured to measure a temperature profile of an object before, after or during welding of the object to a further object.

The apparatus may measure a temperature profile of a pipe before, after or during welding of the pipe to a further pipe.

The apparatus may comprise an imaging system. The imaging system may comprise a lens or a mirror or the like for imaging radiation emitted from a surface area of an object onto a corresponding photodetector element.

Radiation emitted from a surface area of an object may be incident on a photodetector element so as to generate an electrical signal such as an electrical current or a voltage. Each photodetector element may, for example, generate an electrical signal that is a function of temperature of a corresponding surface area of an object.

Each photodetector element may generate an electrical signal that is a known function of temperature of a corresponding surface area of an object. Such an electrical signal as a known function of temperature may, therefore, be used in combination with a measurement of the electrical signal generated by the photodetector element to determine a temperature of the corresponding surface area of the object.

The electrical signal generated by a photodetector element as a known 1 0 function of temperature may, for example, be determined as part of a calibration procedure in which the electrical signal generated by a photodetector element is measured together with the temperature of a corresponding surface area of an object over a temperature range of interest. During such a calibration procedure, the temperature of the corresponding surface area of the object may be measured using 1 5 a temperature sensor such as a thermocouple, thermistor, resistance temperature detector (RTD) or the like.

The apparatus may comprise a memory. The memory may be configured to store a photodetector element electrical signal value as a known function of temperature as part of a calibration procedure. For example, the electrical signal generated by a photodetector element as a known function of temperature may be stored as data in the memory as a look-up table or as a functional form in combination with a series of associated coefficients or the like.

The apparatus may comprise an aperture. The aperture may be arranged between an object and the image sensor so as to spatially filter radiation emitted from the object. Using an aperture may be advantageous because the amount of radiation incident on the image sensor is thereby reduced.

Where the radiation emitted by an object exceeds a saturation level so as to saturate a photodetector element, inserting an aperture in front of the image sensor may reduce the radiation incident on the photodetector element to a level below the saturation level. In addition, an aperture may be used to increase a depth of field associated with the image sensor. This may be particularly advantageous when measuring the temperature profile across a non-planar surface of an object such as a curved surface, for example, a surface of a cylindrical object such as a pipe.

The apparatus may, for example, comprise an adjustable aperture. The adjustable aperture may be adjustable to vary the attenuation of radiation incident on the image sensor and/or a depth of field of the image sensor. For example, the aperture may be adjustable so as to provide an associated range of f-numbers when combined with an imaging system. The range of f-numbers may, for example, extend from fi.4 to fi 6.

The image sensor may be sensitive over an associated image sensor spectral range. For example, at least one of the photodetector elements may be sensitive over an associated spectral range. The image sensor spectral range may be defined 1 5 in terms of a cut-on wavelength and a cut-off wavelength wherein the sensitivity of the image sensor reduces for wavelengths shorter than the cut-on wavelength and the sensitivity of the image sensor reduces for wavelengths longer than the cut-off wavelength.

The apparatus may comprise a spectral filter.

The spectral filter may be transmissive over a spectral filter range.

The spectral filter may be arranged between an object and the image sensor so as to spectrally filter radiation emitted from the object.

The apparatus may have an associated spectral range defined by the combination of the spectral filter range and the image sensor spectral range.

The spectral filter range may be chosen to ensure that a spectral bandwidth of the apparatus is smaller than a spectral bandwidth of the image sensor.

Using a spectral filter may be advantageous because the amount of radiation incident on the image sensor is thereby reduced.

Where the radiation emitted by an object exceeds a saturation level so as to saturate a photodetector element, inserting a spectral filter in front of the image sensor may reduce the radiation incident on the photodetector element to a level below the saturation level.

The use of a spectral filter to ensure that the apparatus is only sensitive to radiation over a reduced spectral band may also be advantageous for the calibration of the apparatus because emissivity is, in general, a function of wavelength and different objects typically have different emissivity spectra. Thus, where a first emissivity of a first object is known over a reduced spectral band and a second 1 0 emissivity of a second object is known over the same reduced spectral band, then measurement of a first electrical signal generated by a photodetector element corresponding to a surface area of the first object at a known temperature may be used to predict a second electrical signal generated by the same photodetector element corresponding to a corresponding surface area of the second object at the same temperature. This may be advantageous during a calibration procedure in which the electrical signal generated by a photodetector element is measured together with the temperature of a corresponding surface area of the first object over a temperature range of interest. The electrical signal generated by the same photodetector element for the corresponding surface area of the second object may then be predicted over the temperature range of interest from the ratio of the known second emissivity to the known first emissivity without having to repeat the calibration procedure for the second object.

The apparatus may comprise a long-pass filter, a short-pass filter or a band-pass filter.

The apparatus may comprise a long-pass filter having a cut-on wavelength that falls within the image sensor spectral range. Such an apparatus may have a band-pass spectral response having an associated bandwidth that falls within the spectral range of the image sensor.

The apparatus may comprise a short-pass filter having a cut-off wavelength that falls within the image sensor spectral range. Such an apparatus may have a band-pass spectral response having an associated bandwidth that falls within the spectral range of the image sensor.

The apparatus may comprise a band-pass filter having an associated filter spectral transmission range that falls within the spectral range of the image sensor.

The apparatus may comprise an infrared (IR) pass filter. The IR pass filter may be arranged between an object and the image sensor so as to spectrally filter radiation emitted from the object.

1 0 The apparatus may, for example, comprise an IR pass filter having a cut-on wavelength that falls within the spectral range of the image sensor. For example, the apparatus may comprise an IR pass filter having a cut-on wavelength between the cut-on and cut-off wavelengths of the image sensor or substantially equal to the cut-on or cut-off wavelength of the image sensor.

1 5 The apparatus may comprise an IR pass filter having a cut-on wavelength substantially equal to a cut-off wavelength of the image sensor.

The cut-off wavelength of the image sensor and the cut-on wavelength of the IR pass filter may be in the IR. This may ensure that the apparatus is essentially only sensitive to radiation over a reduced spectral band within the IR. Thus, using an IR pass filter in this way may allow the detection of IR radiation emitted by an object using an image sensor having silicon photodetector elements primarily designed for detection of visible light. Thus, a low-cost nominally-visible image sensor such as a COD or CMOS image sensor may be used for the detection of IR radiation. Using an IR pass filter in this way when measuring objects at temperatures up to l2OOO also means that the power radiated by the object over the apparatus spectral range is greater than the power radiated over a shorter wavelength spectral range such as a visible spectral range having the same bandwidth as the apparatus spectral range.

This may be advantageous because an electrical signal of a given magnitude may be generated by a photodetector element for a smaller aperture or a higher f-number for an IR apparatus spectral range compared to a shorter wavelength apparatus spectral range resulting in an improved depth of field. This may be particularly important when measuring the temperature of a non-planar surface such as a curved surface or a surface of a cylindrical object.

The apparatus may comprise a visible band-pass filter. The visible band-pass filter may be arranged between an object and the image sensor so as to spectrally filter radiation emitted from the object.

The apparatus may, for example, comprise a visible band-pass filter having a 1 0 transmission band that falls within the spectral range of the image sensor.

This may ensure that the apparatus is essentially only sensitive to visible radiation over a reduced spectral band. The image sensor may have at least one photodetector element that is sensitive over a first associated spectral range and at least one photodetector element that is sensitive over a second associated spectral 1 5 range wherein the first and second spectral ranges are different.

For example, the image sensor may comprise a colour filter array having a colour filter element corresponding to each photodetector element of the image sensor. Different colour filter elements may have different transmission bands. This may be advantageous for enhancing the dynamic range of the temperature measurement apparatus. For example, a colour filter element having a red transmission band may be used to detect longer wavelengths at lower object temperatures and a colour filter element having a green transmission band may be used to detect shorter wavelengths at higher object temperatures. Furthermore, a colour filter element having a blue transmission band may be used to detect even shorter wavelengths at even higher object temperatures.

The apparatus may comprise a neutral density filter. The neutral density filter may be arranged between an object and the image sensor so as to attenuate radiation emitted from the object.

Using a neutral density filter may be advantageous because the amount of radiation incident on the image sensor is thereby reduced.

Where the radiation emitted by an object exceeds a saturation level so as to saturate a silicon photodetector element, inserting a neutral density filter in front of the image sensor may reduce the radiation incident on the photodetector element to a level below the saturation level.

The apparatus may comprise a processor. The processor may be arranged to convert an electrical signal generated by a photodetector element to a digital value. The processor may, for example, be arranged to convert the electrical signal 1 0 generated by a silicon photodiode element to a two-byte number comprising 16 bits.

Using a processor in this way may be advantageous because a grey-level may be constructed from 10 of the 16 bits of the two-byte number thus increasing the dynamic range of the apparatus compared to a standard 8-bit grey level that is traditionally employed in digital imaging devices.

The apparatus may comprise a shutter. The shutter may be implemented mechanically and/or electronically. The shutter may be arranged between an object and the image sensor. The shutter may be controllable so as to vary an exposure time of a photodetector element of the image sensor to radiation. The exposure time may be continuously varied or varied between discrete values. For example, the apparatus may comprise a processor wherein the processor controls the shutter so as to vary the exposure time.

The apparatus may comprise a shutter having an exposure time that may be varied in response to an electrical signal generated by a photodetector element. For example, the exposure time may be reduced in response to the electrical signal generated by the photodetector element reaching a predetermined upper threshold level. Additionally or alternatively, the exposure time may be increased in response to the electrical signal generated by the photodetector element reaching a predetermined lower threshold level. The control of the exposure time in this way may be used to maintain the longest possible exposure time without allowing the electrical signal generated by a photodetector element to reach a saturation level, thereby effectively increasing a dynamic range of the temperature measurement apparatus.

When the exposure time is variable between a set of discrete values, a calibration procedure may be performed for each discrete exposure time value. In such a calibration procedure, an electrical signal generated by a photodetector element is measured for each exposure time value together with the temperature of a corresponding surface area of an object across a temperature range. During a 1 0 subsequent temperature measurement procedure, the appropriate exposure time value may be selected in response to an electrical signal generated by the photodetector element. For example, a lower exposure time value may be selected in response to the electrical signal generated by the photodetector element reaching a predetermined upper threshold level. Additionally or alternatively, a higher 1 5 exposure time value may be selected in response to the electrical signal generated by the photodetector element reaching a predetermined lower threshold level. Once the appropriate exposure time value is selected, the appropriate electrical signal generated by the photodetector element as a known function of temperature as measured during the calibration procedure for the same selected exposure time value may be used to determine the temperature of the object.

An electrical signal value generated by a photodetector element may be adjusted to compensate for variations in the emissivity of an object with emission angle. This may, for example, be necessary if the emissivity of the object with emission angle is relatively large and/or an angular field of view associated with the image sensor is relatively large.

The apparatus may, for example, comprise a processor which is arranged to adjust the electrical signal value generated by a photodetector element of the image sensor to account for variations in the emissivity of an object with emission angle.

The apparatus may comprise a memory which is programmable with information relating to at least one of the object from which radiation is being emitted, the relative spatial arrangement of the radiating surface and the image sensor, information relating to any imaging system and any aperture or optical filter inserted between the object and the image sensor. The information relating to the object may, for example, relate to at least one of the material, size, shape, surface finish, colour, and emissivity as a function of angle or wavelength for the object. The information relating to any imaging system may comprise characteristics of any lenses or mirrors constituting the imaging system.

The apparatus may comprise a memory arranged to store image data captured by the image sensor.

The apparatus may comprise a processor arranged to process image data captured by the image sensor.

The image sensor may have a frame rate of at least 10 frames per second.

1 5 Such a frame rate may allow the capture of at least 5 frames over a 500ms time period which is a typical time period required for forge welding objects such as steel pipes together. Thus a temperature profile of an object may be monitored during forge welding and temperature trends may be evaluated for a surface area of the object during forge welding.

The image sensor may have a frame rate of 30 or 60 frames per second.

The apparatus may comprise a processor that may be programmable so as to transform image data captured from a non-planar radiating surface of an object so as to provide transformed image data comprising a plurality of pixel values in which each pixel value represents a quantity of radiation emitted from an equal area of the object. Such a spatial transformation may, for example, be used to "unwrap" data captured from a curved surface of an object or a cylindrical object such as a pipe.

Similarly, the apparatus may comprise a processor that may be programmable so as to transform image data captured from a radiating surface of an object when an imaging system is inserted between the object and the image sensor, for example, to correct for any distortion associated with a lens.

A method of measuring a temperature of an object may comprise positioning the object in a field of view associated with the image sensor of the apparatus and measuring an electrical signal generated by the image sensor.

The apparatus may comprise one or more further image sensors wherein each image sensor is arranged for the photodetection of radiation emitted by the object.

The apparatus may, for example, comprise one or more further image sensors, each further image sensor having a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object.

The apparatus may comprise one or more further image sensors, each further image sensor comprising a COD or a CMOS image sensor.

1 5 Each image sensor may, for example, be arranged to detect radiation emitted from a different portion of an object. A plurality of image sensors may, for example, be arranged to detect radiation emitted from a plurality of different portions of an object wherein the plurality of different portions of the object collectively extend continuously around the object.

Each image sensor may have an associated field of view that includes a different angular portion of the object. For example, the apparatus may comprise four image sensors arranged generally in a plane and each image sensor may have an associated field of view that includes an angular portion of the object that subtends a 9O angle about an axis perpendicular to the plane.

The plurality of image sensors may be synchronised so that each image sensor captures a respective image of a corresponding portion of an object at substantially the same instant. For example, each image sensor may be synchronised to capture a respective image by a trigger signal common to the plurality of image sensors.

A method of measuring a temperature of an object may comprise positioning the object in a field of view associated with each of the image sensors and measuring an electrical signal generated by each of the image sensors.

According to a second aspect of the present invention there is provided a temperature measurement apparatus for measuring a temperature of an object comprising an image sensor for the photodetection of radiation emitted by the object and a spectral filter.

1 0 The image sensor may be sensitive over an associated spectral range. For example, at least one of the photodetector elements may be sensitive over an associated spectral range. The image sensor spectral range may be defined in terms of a cut-on wavelength and a cut-off wavelength wherein the sensitivity of the image sensor reduces for wavelengths shorter than the cut-on wavelength and the 1 5 sensitivity of the image sensor reduces for wavelengths longer than the cut-off wavelength.

The spectral filter may be transmissive over a spectral filter range.

The apparatus may have an associated spectral range defined by the combination of the spectral filter range and the spectral range of the image sensor.

The apparatus may, for example, comprise an IR pass filter having a cut-on wavelength that falls within the spectral range of the image sensor. For example, the apparatus may comprise an IR pass filter having a cut-on wavelength between the cut-on and cut-off wavelengths of the image sensor or substantially equal to the cut-on or cut-off wavelength of the image sensor.

The apparatus may comprise an IR pass filter having a cut-on wavelength substantially equal to a cut-off wavelength of the image sensor.

The cut-off wavelength of the image sensor and the cut-on wavelength of the IR pass filter may be in the IR.

The apparatus may comprise at least one of an aperture and a neutral density filter.

1 0 The image sensor may comprise a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object.

The image sensor may comprise a COD or a CMOS image sensor.

According to a third aspect of the present invention there is provided a a 1 5 camera arrangement configured to measure a temperature of an object wherein the camera arrangement comprises a camera arranged for the photodetection of radiation emitted by the object.

The camera arrangement may comprise a camera. For example, the camera arrangement may comprise a COD or a CMOS camera.

The camera may be sensitive over an associated spectral range. For example, the camera may comprise a plurality of silicon photodetector elements wherein at least one of the photodetector elements may be sensitive over an associated spectral range. The camera spectral range may be defined in terms of a cut-on wavelength and a cut-off wavelength wherein the sensitivity of the camera reduces for wavelengths shorter than the cut-on wavelength and the sensitivity of the camera reduces for wavelengths longer than the cut-off wavelength.

The camera arrangement may comprise a spectral filter.

The spectral filter may be transmissive over a spectral filter range.

The spectral filter may be arranged between an object and the camera so as to spectrally filter radiation emitted from the object.

The camera arrangement may have an associated spectral range defined by the combination of the spectral filter range and the spectral range of the camera.

The spectral filter range may be chosen to ensure that a spectral bandwidth of the camera arrangement is smaller than a spectral bandwidth of the camera.

The camera arrangement may comprise an infrared pass filter.

The camera arrangement may comprise at least one of an aperture or a neutral density filter.

The camera arrangement may comprise a shutter. The shutter may be implemented mechanically and/or electronically. The shutter may be controllable so as to vary an exposure time of a photodetector element to radiation.

The camera arrangement may comprise an imaging element for imaging radiation emitted by an object onto the plurality of silicon photodetector elements.

1 5 The camera arrangement may have an associated frame rate of at least 10 frames per second. Such a frame rate may allow the capture of at least 5 frames over a 500ms time period which is a typical time period required for forge welding objects such as steel pipes together. Thus a temperature profile of an object may be monitored during forge welding and temperature trends may be evaluated for a surface area of the object during forge welding.

The camera arrangement may have an associated frame rate of 30 or 60 frames per second.

According to a fourth aspect of the present invention there is provided a temperature measurement apparatus comprising a plurality of image sensors wherein each image sensor comprises an array of photodetector elements and each image sensor is arranged for the photodetection of radiation emitted from a different portion of an object.

At least one of the image sensors may comprise an array of silicon photodetector elements.

At least one of the image sensors may comprise a COD or a CMOS image sensor.

The plurality of image sensors may, for example, be arranged to detect radiation emitted from a plurality of different portions of an object wherein the plurality of different portions of the object collectively extend continuously around the object.

Each image sensor may have an associated field of view that includes a different angular portion of the object. For example, the apparatus may comprise 1 0 four image sensors arranged generally in a plane and each image sensor may have an associated field of view that includes an angular portion of the object that subtends a 9O angle about an axis perpendicular to the plane.

The apparatus may comprise a plurality of spectral filters.

Each spectral filter may be transmissive over a corresponding spectral filter range.

Each spectral filter may be arranged between an object and a corresponding image sensor so as to spectrally filter radiation emitted from the object.

The spectral filter range of each spectral filter may be the same or different.

Each image sensor and corresponding spectral filter combination may have a spectral range defined by the combination of the spectral filter range and the image sensor spectral range.

Each spectral filter range may be chosen to ensure that a spectral bandwidth of the corresponding image sensor and spectral filter combination is smaller than a spectral bandwidth of the corresponding image sensor.

According to a fifth aspect of the present invention there is provided a temperature measurement apparatus comprising a plurality of camera arrangements wherein each camera arrangement is arranged for the photodetection of radiation 1 0 emitted from a different portion of an object.

Each camera arrangement may comprise a camera having a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by an object.

Each camera arrangement may, for example, comprise a COD or a CMOS camera.

Each camera may have an associated spectral range. For example, at least one of the photodetector elements of each camera may be sensitive over an associated spectral range.

Each camera arrangement may comprise a spectral filter.

Each spectral filter may be arranged between an object and the corresponding camera so as to spectrally filter radiation emitted from the object.

Each camera arrangement may have an associated spectral range defined by the combination of a corresponding spectral filter range and the spectral range of a corresponding camera.

Each spectral filter range may be chosen to ensure that a spectral bandwidth of the corresponding camera arrangement is smaller than a spectral bandwidth of the corresponding camera.

The camera arrangement may comprise an infrared pass filter.

The camera arrangements may have substantially the same spectral range.

Alternatively, the camera arrangements may have substantially different spectral ranges.

According to a sixth aspect of the present invention there is provided a method of measuring a temperature of an object, the method comprising: providing a temperature measurement apparatus comprising an image sensor; positioning a first object in a field of view associated with the image sensor; measuring a first electrical signal generated by the image sensor as a function of temperature of the first object; 1 5 positioning a second object in the field of view associated with the image sensor; measuring a second electrical signal generated by the image sensor; and determining the temperature of the second object from the measured value of the second electrical signal and the first electrical signal as a function of temperature.

The image sensor may comprise a plurality of photodetector elements. The method of measuring a temperature of an object may comprise generating the first and second electrical signals from the same photodetector element.

The method of measuring a temperature of an object may comprise measuring the temperature of the first object using a temperature sensor such as a thermocouple, thermistor, resistance temperature detector (RTD) or the like.

The method of measuring a temperature of an object may comprise measuring the temperature of the first object across a temperature range such that the temperature of the second object to be measured lies within the temperature range.

The method of measuring a temperature of an object may further comprise storing the temperature of the first object measured across a temperature range.

The image sensor may have an adjustable exposure time. The exposure time may be continuously adjustable or adjustable in discrete steps.

For example, the exposure time may be selected from a set of discrete exposure times.

The method of measuring a temperature of an object may comprise 1 0 measuring the first electrical signal for each of the discrete exposure times.

The method of measuring a temperature of an object may comprise storing a measured value of the first electrical signal as a function of temperature of the first object for each of the discrete exposure times.

The method of measuring a temperature of an object may comprise selecting 1 5 an appropriate exposure time from the set of discrete exposure times in response to a value of the second electrical signal generated by the image sensor.

For example, the method of measuring a temperature of an object may comprise selecting a lower exposure time in response to the second electrical signal reaching a predetermined upper threshold level.

The method of measuring a temperature of an object may comprise selecting a higher exposure time in response to the second electrical signal reaching a predetermined lower threshold level.

The method of measuring a temperature of an object may comprise using the first electrical signal measured using a first exposure time equal to a second exposure time selected for the measurement of the second electrical signal to determine the temperature of the second object from a measured value of the second electrical signal.

Alternatively, the first and second exposure times may have different known values and the method of measuring a temperature of an object may comprise compensating a measured value of the second electrical signal so as to account for the different known values of the first and second exposure times.

The temperature measurement apparatus may comprise an aperture inserted between the object and the image sensor. The aperture may be adjustable, for example, the aperture may be continuously adjustable or adjustable in discrete steps.

The method of measuring a temperature of an object may comprise using a first aperture or aperture setting for measuring the first electrical signal and using a second aperture or aperture setting for measuring the second electrical signal wherein the first and second apertures or aperture settings are equal.

Alternatively, the first and second apertures or aperture settings may have different known values and the method of measuring a temperature of an object may comprise compensating a measured value of the second electrical signal so as to 1 5 account for the different known values of the first and second apertures or aperture settings.

The first object may have a first emissivity spectrum and the second object may have a second emissivity spectrum wherein the first and second emissivity spectra are substantially identical.

The first object may have a first emissivity spectrum and the second object may have a second emissivity spectrum wherein the first and second emissivity spectra may have different known values and the method of measuring a temperature of an object may comprise compensating a measured value of the second electrical signal so as to account for the different known values of the first and second emissivity spectra.

The image sensor may be sensitive over an associated image sensor spectral range. For example, at least one of the photodetector elements may be sensitive over an associated spectral range. The image sensor spectral range may be defined in terms of a cut-on wavelength and a cut-off wavelength wherein the sensitivity of the image sensor reduces for wavelengths shorter than the cut-on wavelength and the sensitivity of the image sensor reduces for wavelengths longer than the cut-off wavelength.

The temperature measurement apparatus may comprise a spectral filter.

The spectral filter may be transmissive over a spectral filter range.

The spectral filter may be inserted between the object and the image sensor.

Thus, the method of measuring a temperature of an object may comprise spectrally filtering radiation emitted by the first and second objects.

1 0 The apparatus may have an associated apparatus spectral range defined by the combination of the spectral filter range and the image sensor spectral range.

Thus, the method of measuring a temperature of an object may comprise 1 5 spectrally filtering radiation emitted by the first and second objects so as to reduce the spectral bandwidth of the radiation.

The apparatus may, for example, comprise an IR pass filter having a cut-on wavelength less than or equal to a cut-off wavelength of the image sensor.

Thus, the method of measuring a temperature of an object may comprise spectrally filtering radiation emitted by the first and second objects so as to limit the spectral range of the radiation to a reduced spectral range within the lR.

The method of measuring a temperature of an object may comprise compensating a measured value of the second electrical signal according to a ratio of a second emissivity value for the second object to a first emissivity value for the first object wherein the first and second emissivity values correspond to the reduced spectral range of the temperature measurement apparatus.

The temperature measurement apparatus may comprise a neutral density filter. The neutral density filter may be inserted between the object and the image sensor. The neutral density filter may be adjustable, for example, the neutral density filter may be continuously adjustable or adjustable in discrete steps.

The method of measuring a temperature of an object may comprise using a first neutral density filter or neutral density filter setting for measuring the first electrical signal and using a second neutral density filter or neutral density filter setting for measuring the second electrical signal wherein the first and second neutral density filters or neutral density filter settings are equal.

1 0 Alternatively, the first and second neutral density filters or neutral density filter settings may have different known values and the method of measuring a temperature of an object may comprise compensating a measured value of the second electrical signal so as to account for the different known values of the first and second neutral density filters or neutral density filter settings.

1 5 The method of measuring a temperature of an object may comprise converting the first electrical signal to a first two-byte number and the second electrical signal to a second two-byte number which may be the same as or different from the first two-byte number.

The emissivity of the first and/or second objects may vary with emission angle. Thus, the method of measuring a temperature of an object may comprise using a first object having an emissivity as a first function of emission angle and using a second object having an emissivity as a second function of emission angle wherein the first and second emissivity functions are equal.

Alternatively, the method of measuring a temperature of an object may comprise using a first object having an emissivity as a first known function of emission angle, using a second object having an emissivity as a second known function of emission angle wherein the first and second emissivity functions are different, and compensating a value of the second electrical signal to account for the known differences between the first and second emissivity functions.

The method of measuring a temperature of an object may comprise using a first imaging system for measuring the first electrical signal and using a second imaging system for measuring the second electrical signal wherein the first and second imaging systems are substantially identical.

Alternatively, the method of measuring a temperature of an object may comprise using a first known imaging system for measuring the first electrical signal, using a second known imaging system for measuring the second electrical signal 1 0 wherein the first and second imaging system are different, and compensating a value of the second electrical signal to account for the known differences between the first and second imaging systems.

The method of measuring a temperature of an object may comprise using a first relative spatial arrangement between the first object and the image sensor and 1 5 using a second relative spatial arrangement between the second object and the image sensor wherein the first and second relative spatial arrangements are substantially identical.

Alternatively, the method of measuring a temperature of an object may comprise using a first known relative spatial arrangement between the first object and the image sensor, using a second known relative spatial arrangement between the second object and the image sensor wherein the first and second relative spatial arrangements are different, and compensating a value of the second electrical signal to account for the known differences between the first and second relative spatial arrangements.

The method of measuring a temperature of an object may comprise capturing an image of the object on the image sensor. For example, the method of measuring a temperature of an object may comprise capturing an image of the object on the image sensor at a rate of at least 10 frames per second, for example, 30 or 60 frames per second.

The method of measuring a temperature of an object may comprise capturing a first image on the image sensor and measuring pixel values of the first image.

The method of measuring a temperature of an object may comprise capturing a second image on the image sensor and measuring pixel values of the second image.

The method of measuring a temperature of an object may comprise transforming image data captured from a non-planar radiating surface of the object 1 0 so as to provide transformed image data comprising a plurality of pixel values in which each pixel value represents a quantity of radiation emitted from an equal area of the object.

The method of measuring a temperature of an object may comprise transforming image data captured from the object when an imaging system is inserted between the object and the image sensor, for example, to correct for any distortion associated with a lens or other component of the imaging system.

The method of measuring a temperature of an object may comprise providing a temperature measurement apparatus comprising one or more further image sensors.

The method of measuring a temperature of an object may comprise positioning a first object in a field of view associated with the one or more further image sensors.

The method of measuring a temperature of an object may comprise measuring a first electrical signal generated by each of the one or more further image sensors as a function of temperature of the first object.

The method of measuring a temperature of an object may comprise positioning a second object in a field of view associated with the one or more further image sensors.

The method of measuring a temperature of an object may comprise measuring a second electrical signal generated by each of the one or more further image sensors as a function of temperature of the second object.

The method of measuring a temperature of an object may comprise using the first electrical signal as a function of temperature for each of the one or more further image sensors to determine one or more temperatures of the second object from the measured value of the second electrical signal generated by each of the one or more further image sensors.

The method of measuring a temperature of an object may comprise capturing an image of the object on each of the one or more further image sensors. For example, the method of measuring a temperature of an object may comprise capturing an image of the object on each of the one or more further image sensors at a rate of at least 10 frames per second, for example, 30 or 60 frames per second.

The method of measuring a temperature of an object may comprise capturing 1 5 a first image on each of the one or more further image sensors and measuring pixel values of each of the one or more corresponding first images.

The method of measuring a temperature of an object may comprise capturing a second image on each of the one or more further image sensors and measuring pixel values of each of the one or more corresponding second images.

The method of measuring a temperature of an object may comprise transforming image data captured from a non-planar radiating surface of the object on each of the one or more further image sensors so as to provide transformed image data comprising a plurality of pixel values in which each pixel value represents a quantity of radiation emitted from an equal area of the object.

The method of measuring a temperature of an object may comprise transforming image data captured on each of the one or more further image sensors when one or more further imaging systems are inserted between the object and the one or more further image sensors, for example, to correct for any distortion associated with a lens or other component of each of the one or more imaging systems.

According to a seventh aspect of the present invention there is provided a welding apparatus for welding an object to a further object comprising an image sensor having a plurality of photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object and/or the further object.

For example, the photodetector elements may comprise silicon photodetector elements.

1 0 The image sensor may comprise a COD or a CMOS image sensor.

The image sensor may be sensitive over an associated image sensor spectral range. For example, at least one of the photodetector elements may be sensitive over an associated spectral range. The image sensor spectral range may be defined in terms of a cut-on wavelength and a cut-off wavelength wherein the sensitivity of 1 5 the image sensor reduces for wavelengths shorter than the cut-on wavelength and the sensitivity of the image sensor reduces for wavelengths longer than the cut-off wavelength.

The welding apparatus may comprise a spectral filter.

The spectral filter may be transmissive over a spectral filter range.

The spectral filter may be inserted between the object and the image sensor.

The spectral filter may be inserted between the further object and the image sensor.

The apparatus may have an associated apparatus spectral range defined by the combination of the spectral filter range and the image sensor spectral range.

A method of welding may comprise positioning an object in a field of view associated with the image sensor and measuring an electrical signal generated by the image sensor while the object is being welded to a further object.

The method of welding may, for example, comprise capturing a first image on the image sensor and measuring pixel values of the first image. The method of welding may comprise capturing a second image on the image sensor and measuring pixel values of the second image.

The method of welding may comprise capturing an image at a rate of at least frames per second, for example, 30 or 60 frames per second. Such a frame rate may allow the capture of at least 5 frames over a 500ms time period which is a typical time period required for forge welding objects such as steel pipes together.

Thus a temperature profile of an object may be monitored during forge welding and temperature trends may be evaluated for a surface area of the object during forge welding.

1 5 The welding apparatus may comprise one or more further image sensors.

The welding apparatus may, for example, comprise one or more further image sensors, each further image sensor having a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object.

The welding apparatus may comprise one or more further image sensors, each further image sensor comprising a COD or a CMOS image sensor.

The welding apparatus may comprise one or more further spectral filters.

Each of the one or more further spectral filters may be transmissive over one or more further spectral filter ranges.

The one or more further spectral filter ranges may be the same or different.

Each of the one or more further spectral filters may be inserted between the object and the image sensor.

Each of the one or more further spectral filters may be inserted between the further object and the image sensor.

The apparatus may have an associated apparatus spectral range defined by the combination of each of the one or more further spectral filter ranges and a corresponding image sensor spectral range.

Each of the one or more spectral filter ranges may be chosen to ensure that a spectral bandwidth of the apparatus is smaller than a spectral bandwidth of the image sensor.

Each image sensor may, for example, be arranged to detect radiation emitted 1 0 from a different portion of an object. A plurality of image sensors may, for example, be arranged to detect radiation emitted from a plurality of different portions of an object wherein the plurality of different portions of the object collectively extend continuously around the object.

Each image sensor may have an associated field of view that includes a different angular portion of the object. For example, the apparatus may comprise four image sensors arranged generally in a plane and each image sensor may have an associated field of view that includes an angular portion of the object that subtends a 90 angle about an axis perpendicular to the plane.

A method of welding may comprise positioning an object in a field of view associated with each of the image sensors and measuring an electrical signal generated by each image sensor while the object is being welded to a further object.

The method of welding may comprise capturing an image on each image sensor. For example, the method of welding may comprise capturing an image on each image sensor at a rate of at least 10 frames per second, for example, 30 or 60 frames per second.

The method of welding may comprise capturing a first image on each image sensor and measuring pixel values of each of the corresponding first images.

The method of welding may comprise capturing a second image on each image sensor and measuring pixel values of each of the corresponding second images.

The method of welding may comprise transforming image data captured from a non-planar radiating surface of the object on each image sensor so as to provide 1 0 transformed image data comprising a plurality of pixel values in which each pixel value represents a quantity of radiation emitted from an equal area of the object.

The method of welding may comprise transforming image data captured on each image sensor when a respective imaging system is inserted between the object and each image sensor, for example, to correct for any distortion associated with a 1 5 lens or other component of each of the imaging systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further described by way of non-limiting example only with reference to the following figures of which: Figure 1 is a schematic plan view of a temperature measurement apparatus constituting an embodiment of the present invention arranged around a pipe; Figure 2 is a schematic of a camera arrangement of the temperature measurement apparatus of Figure 1; Figure 3 illustrates a method of temperature measurement using the temperature measurement apparatus of Figure 1; Figure 4 shows multiple instances of two calibration functions for an image sensor of the temperature measurement apparatus of Figure 1 wherein each calibration function corresponds to a different exposure time and comprises measured temperature as a function of image intensity measured by the image sensor; Figure 5 schematically illustrates the emission of radiation from different positions on an outer surface of a pipe at different angles and the incidence of such radiation on different photodetector 1 0 elements of an image sensor of the temperature measurement apparatus of Figure 1; Figure 6 illustrates a method of unwrapping an image captured on an image sensor of the temperature measurement apparatus of Figure 1; and 1 5 Figure 7 is a schematic side view of the temperature measurement apparatus of Figure 1 in use measuring pipe temperatures during a welding process.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a temperature measurement apparatus generally designated 2 comprising four COD camera arrangements 4, 6, 8 and 10 arranged generally in a plane around a cylindrical object in the form of a pipe 12. As indicated by the dotted lines in Figure 1, each camera arrangement 4, 6, 8, 10 has a corresponding field of view that at least includes a 9O angular portion of a cylindrical outer surface 14 of the pipe 12 50 that the combined field of view of the camera arrangements 4, 6, 8, 10 is continuous around the outer surface 14 of the pipe 12.

As illustrated by the broken lines in Figure 1, each camera arrangement 4, 6, 8, 10 is arranged for communication with a controller 15 comprising a memory 16 and a processor 18. As illustrated in the case of camera arrangement 4 in Figure 2, each camera arrangement 4, 6, 8, 10 comprises a CCD image sensor device in the form of a board-level Point Grey Firefly MV FFMV-O3MTC COD camera 20. Each COD camera 20 has a COD image sensor 22 comprising an array of silicon photodetector elements wherein each silicon photodetector element is arranged for the photodetection of IR radiation emitted by the pipe 12. Each COD camera 20 comprises a shutter 24. In addition, each camera arrangement 4, 6, 8, 10 comprises 1 0 an aperture 26, and a lens 28. The image sensor 22, the shutter 24, the aperture 26 and the lens 28 are all arranged along a common optical axis 29. The function of the shutter 24 is to vary an exposure time during which the image sensor 22 is exposed to radiation emitted from the pipe 12. The shutter 24 is operable so as to select an exposure time from a set of discrete exposure times. Together the image sensor 22 1 5 and the lens 28 define the field of view of the camera arrangement 4 as is known in the art.

The COD camera 20 additionally comprises communication means (not shown) for communication with the controller 15.

Each camera arrangement 4, 6, 8, 10 also comprises an IR pass filter 30.

The IR pass filter 30 has a nominal 3dB "cut-on" wavelength below which the IR pass filter 30 transmits less radiation. The image sensor 22 has a nominal 3dB "cut-off" wavelength above which the image sensor 22 becomes less sensitive i.e. above which an electrical signal generated by a photodetector element of the image sensor 22 when a given quantity of radiation is incident on the photodetector element reduces. The cut-on wavelength of the IR pass filter 30 and the cut-off wavelength of the image sensor 22 of the temperature measurement apparatus 2 are both substantially equal to 950 nm. Ohoosng the cut-on wavelength of the IR pass filter and the cut-off wavelength of the image sensor 22 so as to be substantially equal results in each camera arrangement 4, 6, 8, 10 having a "band-pass" spectral response and an associated spectral bandwidth. This has the effect of reducing the amount of radiation incident on the image sensor 22. In addition, as described below, this also means that a first object having an emissivity which is a first function of wavelength may be used during a calibration procedure and a temperature of a second object having an emissivity which is a second function of wavelength may be determined from temperature data measured for the first object. From knowledge of an emissivity ratio within the spectral bandwidth for the first and second objects, the temperature of the second object may be derived from the quantity of radiation emitted from the first object measured as a function of temperature during the calibration procedure.

Each camera arrangement 4, 6, 8, 10 also comprises a neutral density filter 32. The use of the neutral density filter 32 and/or attenuation properties of the neutral density filter 32 will, however, depend on properties of the object 12, a temperature to be measured, a size of the aperture 26 and a dynamic range associated with the CCD camera 20. For measuring the temperature of a curved surface such as the cylindrical surface 14 of the pipe 12, for example, it is generally preferable to maximise a depth of field associated with each camera arrangement 4, 6, 8, 10 by selecting a minimum size for the aperture 26. The attenuation properties of any neutral density filter 32 used will then depend on the temperature to be measured and the dynamic range of the COD camera 20. For a given maximum temperature, the attenuation properties of any neutral density filter 32 used are chosen so as to prevent saturation of the COD camera 20 when a minimum exposure time is selected.

Each COD camera arrangement 4, 6, 8 and 10 is operated in raw camera mode to provide 16 bits per pixel. A grey level is constructed using 10 of the 16 bits thus providing an increase in dynamic range over a standard 8-bit grey level.

As illustrated in Figure 3, a method of measuring temperature of a pipe using the temperature measurement apparatus 2 comprises two main steps, namely calibration and measurement. During calibration, a first pipe is placed at an object position in the respective fields of view of camera arrangements 4, 6, 8 and 10 at step 100. For each exposure time selected from the set of discrete exposure times, the first pipe is heated or cooled across a temperature range of interest. As the first pipe is heated or cooled, the temperature 81 of a surface area of the first pipe is measured at step 102 using a thermocouple held in thermal contact with the surface area of the first pipe and a corresponding first electrical signal s generated 1 0 by the image sensor 22 in response to radiation emitted from the surface area of the first pipe is measured at step 104. The values of the exposure time t1, the temperature 81 and the first electrical signal s are all stored in memory 16 at step 106. Steps 102, 104 and 106 are repeated for each temperature across the temperature range of interest to measure and store the temperature 81 as a function 1 5 of the first electrical signal s across the temperature range of interest for each exposure time from the set of discrete exposure times.

During measurement, a second pipe having the same shape and size as the first pipe is substituted for the first pipe by placing the second pipe at the object position in the respective fields of view of camera arrangements 4, 6, 8 and 10 at step 108. An appropriate exposure time r2 is selected at step 110 by measuring a second electrical signal 52 generated by the image sensor 22 in response to radiation emitted from a surface area of the second pipe corresponding to the surface area of the first pipe for a default exposure time selected from the set of discrete exposure times. If the value of the second electrical signal s2 is greater than 90% of a saturation value associated with the image sensor 22, the next lower exposure time is selected from the set of discrete exposure times and the value of the second electrical signal 2 is measured again. If the value of the second electrical signal 2 is still greater than 90% of the saturation value, the next lower exposure time is selected and the process is repeated until the value of the second electrical signal s2 is less than or equal to 90% of the saturation value. If the value of the second electrical signal 52 is less than 10% of a saturation value associated with the image sensor 22, the next higher exposure time is selected from the set of discrete exposure times and the value of the second electrical signal 2 is measured again. If the value of the second electrical signal s2 is still less than 10% of the saturation value, the next higher exposure time is selected and the process is repeated until the value of the second electrical signal s2 is greater than or equal to 10% of the saturation value.

Once an appropriate exposure time value is selected at step 110, the corresponding measured value of the second electrical signal s2 is determined at step 112. The temperature 82 of the surface area of the second pipe corresponding to the surface area of the first pipe is determined at step 114 from the data stored during calibration for the same exposure time setting 1=2 by identifying the temperature 81 of the surface area of the first pipe corresponding to a first electrical signal value s1 which is equal to the measured value of the second electrical signal value 52 If the measured value of the second electrical signal value 52 faIls between two values of the first electrical signal s1 stored during calibration, the first electrical signal s1 data is interpolated to determine the temperature 82 of the surface area of the second pipe.

By way of illustration, Figure 4 shows the temperature 81 of the surface area of a first pipe measured during calibration as a function of the image intensity i.e. as a function of the first electrical signal s generated by the image sensor for two different consecutive exposure time settings. In Figure 4, each plot of temperature 81 as a function of image intensity comprises multiple traces representing multiple measurements at the same exposure time setting. The noise appearing on the different traces is an artef act of the thermocouple temperature measurement set-up.

Furthermore, although the temperature 9 is the independent variable during calibration, it should be understood that in Figure 4, the image Intensity has been plotted as the independent variable to emphasise that the calibration data of Figure 4 is stored in a look-up table in the memory 16 to allow the temperature 82 of the surface area of the second pipe to be determined from the image intensity value measured for the first pipe i.e. from the measured value of the first electrical signal 5i.

As illustrated in Figure 5, the image sensor 22 comprises a photodetector 1 0 element 202 arranged on the optical axis 29 and further photodetector elements 204 and 206 arranged at extremities of the image sensor array 22 either side of the optical axis 29. Radiation incident on photodetector elements 202, 204 and 206 corresponds to radiation emitted from corresponding cylindrical surface arc 208, 210 and 212 of the pipe surface 14 respectively. It should be understood that in Figure 5 the size of the photodetector elements 202, 204 and 206 and the corresponding cylindrical surface arcs 208, 210 and 212 have been exaggerated in extent for the purposes of illustration only and that, in reality, each photodetector element 202, 204 and 206 would detect radiation emitted from a much smaller arc of the cylindrical surface 14. As illustrated in Figure 5, the size of the cylindrical surface arcs 208, 210 and 212 may be different depending on the position of the corresponding photodetector element 202, 204 and 206 with respect to the optical axis 29. Thus, the size and shape of an outer surface of a first pipe used for calibration are chosen to be substantially the same as the size and shape of an outer surface of a second pipe whose temperature is to be measured. In addition, the relative spatial arrangement of the first pipe and the image sensor 22 used during calibration is substantially the same as the relative spatial arrangement of the second pipe and the image sensor 22 used during measurement.

Where the intensity of radiation varies significantly with emission angle, more radiation may be emitted in a direction perpendicular to the surface 14 of the pipe 12 relative to radiation emitted in other directions. Thus, the electrical signal generated by photodetector element 202 may be greater than the electrical signals generated by photodetector elements 204 or 206 when the surface 14 of the pipe 12 has a uniform temperature. Consequently, the emissivity of the first pipe as a function of emission angle used during calibration is chosen to be substantially the same as the emissivity of the second pipe as a function of emission angle used during measurement. This requires attributes of the first pipe such as surface material, 1 0 colour, finish and the like to be substantially the same as the corresponding attributes of the second pipe.

In order that a circumferential temperature profile may be obtained around the pipe 12, the controller 15 triggers each camera arrangement 4, 6, 8 and 10 to capture an image at substantially the same instant. The captured images are each 1 5 remapped such that each pixel of each captured image represents an equal area of the cylindrical surface 14 of the pipe 12. Figure 6 is a flow diagram illustrating an inverse mapping method used to unwrap each of the captured images. As indicated at step 300, the method of Figure 6 is repeated for each pixel position in the unwrapped image. It will be understood by one skilled in the art that a corresponding pixel position is determined in the true reference frame of the camera arrangement at step 302 from the geometry of the pipe surface 14 and the relative spatial arrangement of the pipe surface 14 and the image sensor 22 of the camera arrangement. A corresponding position in the captured image is determined at step 304 from characteristics of the camera arrangement which may include distortion characteristics of the camera arrangement. As will be appreciated by one skilled in the art, such characteristics of the camera arrangement may be predetermined from image measurements performed using the camera arrangement such as image distortion measurements performed using the camera arrangement in conjunction with a distortion target. One or more of the pixel values in the vicinity of the sub-pixel position determined at step 304 are interpolated at step 306 to determine a pixel value for the unwrapped image position. The pixel value for the unwrapped image position is associated with or assigned to the corresponding unwrapped pixel position at step 308. Subsequently, the unwrapped images corresponding to each camera arrangement 4, 6, 8 and 10 are stitched together to provide a two-dimensional image of the entire surface 14.

Figure 7 illustrates the temperature measurement apparatus 2 in use for monitoring temperature of the pipe 12 and temperature of a further pipe 400 during welding of the pipe 12 to the further pipe 400. As indicated by the dotted lines in Figure 7, the field of view of each camera arrangement 4, 6, 8 and 10 spans a gap 402 that exists prior to commencement of a welding operation between an end 404 of the pipe 12 and an end 406 of the further pipe 400. During the welding operation, the pipes 12, 400 are heated and relative motion between the pipe 12 and the further 1 5 pipe 400 results in reduction of the gap 402 until the pipe ends 404 and 406 come into contact. The temperature measurement apparatus 2 is used to monitor the temperature of the pipes 12, 400 during heating of at least one of the pipes 12, 300 up to a predetermined temperature, for example, up to l2OOC. The temperature measurement apparatus 2 is also used to monitor the temperature of the pipes 12, 400 during a subsequent period when the pipes 12, 400 may be allowed to cool to a further predetermined temperature, for example, GOOC. In particular, the temperature measurement apparatus 2 is used to monitor the temperature of the pipes 12, 400 during the subsequent period when relative motion between the pipes 12, 400 results in the pipe ends 404 and 406 coming into contact. The controller 15 triggers each camera arrangement 4, 6, 8 and 10 to capture an image at substantially the same instant at a frame rate of 30 Hz. The captured image intensity values are subsequently converted into temperature measurements using calibration data as hereinbefore described thus providing a "real-time" temperature profile around a circumference of the pipe 12 and the further pipe 400 in the vicinity of the pipe ends 404, 406.

It should be understood that the embodiments described herein are merely exemplary and that modifications may be made thereto without departing from the scope of the present invention. For example, although embodiments having four camera arrangements have been described which capture a continuous temperature profile around a pipe, the number of camera arrangements may be greater or less than four. The camera arrangement or camera arrangements may not capture a continuous temperature profile around a pipe but rather capture a temperature profile 1 0 across a portion of a surface of an object. For example, the camera arrangement or camera arrangements may capture a temperature profile across a portion of a curved surface of an object such as a cylindrical object.

Although a lens 28 is used for imaging in the embodiments described hereinbefore, an alternative imaging element such as a mirror may be used.

1 5 Additionally or alternatively, one or more further imaging elements may be used, for example, one or more further lenses and/or one or more further mirrors may be used.

It should also be understood that the shutter 24 is shown schematically as a mechanical shutter in Figure 2 for the purposes of illustration only and that control of the exposure time during which the image sensor 22 is exposed to radiation emitted from the object 12 may be may alternatively or additionally be achieved electronically.

Although use of an IR-pass filter has been described, an alternative spectral filter may be used provided the spectral filter has the effect of reducing a spectral bandwidth associated with the image sensor 22. For example, a long-pass, short-pass or band-pass spectral filter may be used.

Additionally or alternatively, a thermistor or a resistance temperature detector (RTD) or the like may be used for temperature measurement during calibration.

The electrical signals generated by the image sensor 22 may be averaged, filtered or smoothed. For example, the electrical signals may be averaged, filtered or smoothed as the signal is measured or after the signal is measured.

Rather than storing image intensity and temperature data measured for a first pipe during calibration in a look-up table and subsequently using interpolation to determine a temperature value corresponding to the image intensity value measured for a second pipe during measurement, a functional form may be fitted to the image intensity data measured for the first pipe as a function of temperature during calibration. The functional form and associated fitting constants may be stored in memory 16 and subsequently used to determine a temperature value corresponding to the image intensity value measured for the second pipe during measurement.

When selecting an appropriate exposure time during measurement, rather than selecting the next lower exposure time when an electrical signal generated by the image sensor exceeds a predetermined upper threshold of 90% of an image 1 5 sensor signal saturation value, an alternative predetermined upper threshold value may be selected. Similarly, rather than selecting the next higher exposure time when an electrical signal generated by the image sensor falls below a predetermined lower threshold of 10% of an image sensor signal saturation value, an alternative predetermined lower threshold value may be selected.

Additionally, if a predetermined upper threshold value of x% of the image sensor signal saturation value is selected, a predetermined lower threshold value of (1-x)% may be selected.

Although not explicitly discussed above, it should be understood that the same camera arrangement including the same image sensor, lens, aperture, IR pass filter and neutral density filter components and the same relative spatial arrangement between these components is generally used for calibration and measurement.

However, the camera arrangement may be changed between calibration and measurement provided a relationship between the electrical signals s1 and 2 generated by the image sensor 22 during calibration and measurement respectively is known for each temperature across the temperature range of interest and for each exposure time setting. Such a relationship may subsequently be used to compensate for any changes in the camera arrangement between calibration and measurement.

Furthermore, a first known relative spatial arrangement between the first object and the image sensor may be used for calibration and a second known relative spatial arrangement between the second object and the image sensor may be used for measurement provided a relationship between the electrical signals s and 52 generated by the image sensor 22 during calibration and measurement respectively is known for each temperature across the temperature range of interest and for each exposure time setting. An image intensity value measured during measurement may then compensating to account for the known differences between the first and second relative spatial arrangements.

1 5 In addition, first and second objects having different emissivities as a function of emission angle may be used for calibration and measurement respectively provided a relationship between the electrical signals s and 52 generated by the image sensor 22 during calibration and measurement respectively is known for each temperature across the temperature range of interest and for each exposure time setting. An image intensity value measured during measurement may then be compensated to account for the known differences between the first and second emissivity functions.

First and second objects having different emissivities as a function of wavelength may be used for calibration and measurement respectively provided a relationship between the electrical signals s1 and s2 generated by the image sensor 22 during calibration and measurement respectively is known for each temperature across the temperature range of interest and for each exposure time setting. An image intensity value measured during measurement may then be compensated to account for the known differences between the first and second emissivity functions.

The image data captured from an object may be transformed to compensate for any distortion associated with an imaging system between the object and the image sensor, for example, to correct for any distortion associated with a lens or other imaging element of such an imaging system.

Claims

CLAIMS1. A temperature measurement apparatus for measuring a temperature of an object, the temperature measurement apparatus comprising an image sensor having a plurality of silicon photodetector elements wherein each photodetector element is arranged for the photodetection of radiation emitted by the object.
2. The temperature measurement apparatus as claimed in claim 1 wherein the image sensor is a charged coupled device image sensor or a complementary metal oxide semiconductor image sensor.
3. The temperature measurement apparatus as claimed in claim 1 or 2 comprising a spectral filter that is arranged to spectrally filter radiation emitted by the object before photodetection of the radiation by a photodetector element of the image sensor.
4. The temperature measurement apparatus as claimed in claim 3 wherein the spectral filter has a spectral filter spectral range such that a spectral bandwidth of the apparatus is less than a spectral bandwidth of the image sensor.
5. The temperature measurement apparatus as claimed in claim 4 wherein the spectral filter spectral range is chosen so as to define a spectral range of the apparatus in the infrared.
6. The temperature measurement apparatus as claimed in claim 5 wherein the spectral filter is an infrared pass filter having a cut-on wavelength substantially equal to a cut-off wavelength of the image sensor.
7. The temperature measurement apparatus as claimed in any of the preceding claims wherein the image sensor is arranged for the photodetection of radiation emitted by a portion of the object.
8. The temperature measurement apparatus as claimed in claim 7 comprising one or more further image sensors wherein each further image sensor is arranged for the photodetection of radiation emitted by a corresponding one or more further portions of the object.

1 0
9. The temperature measurement apparatus as claimed in claim 8 wherein the portion of the object and the one or more further portions of the object collectively extend continuously around the object.
10. Use of the temperature measurement apparatus as claimed in any of the 1 5 preceding claims to measure a temperature of a non-planar surface of the object.
11. The use of the temperature measurement apparatus as claimed in claim 10 to measure the temperature of a non-planar object.
12. The use of the temperature measurement apparatus as claimed in claim 11 to measure the temperature of a cylindrical object.
13. The use of the temperature measurement apparatus as claimed in claim 12 to measure the temperature of a pipe.
14. Use of the temperature measurement apparatus as claimed in any of claims 1 to 9 to measure the temperature of an object before, during or after welding of the object to a further object.
15. The use of the temperature measurement apparatus as claimed in claim 14 to measure the temperature of the further object before, during or after welding of the object to the further object.
16. A method of measuring a temperature of an object using the temperature measurement apparatus as claimed in any of claims 1 to 9, the method comprising: positioning the object in a field of view associated with the temperature measurement apparatus; and 1 0 detecting radiation emitted from the object.
17. A method of measuring a temperature of an object, the method comprising: providing a temperature measurement apparatus comprising an image sensor; 1 5 positioning a first object in a field of view associated with the image sensor; measuring a first electrical signal generated by the image sensor as a function of temperature of the first object; positioning a second object in the field of view associated with the image sensor; measuring a second electrical signal generated by the image sensor; and determining the temperature of the second object from the measured value of the second electrical signal and the first electrical signal as a function of temperature.
18. The method of measuring a temperature of an object as claimed in claim 17, comprising: storing the temperature of the first object as a function of the temperature of the first object.
19. The method of measuring a temperature of an object as claimed in claim 18, comprising: measuring the first electrical signal generated by the image sensor as a function of temperature of the first object for each exposure time from a set of discrete exposure times; and storing the first electrical signal as a function of temperature of the first object for each of the discrete exposure times.
20. The method of measuring a temperature of an object as claimed in claim 19, 1 0 comprising: selecting an appropriate exposure time from the set of discrete exposure times for the measurement of the second electrical signal generated by the image sensor.

1 5
21. The method of measuring a temperature of an object as claimed in claim 20, comprising: selecting an initial exposure time for the measurement of the second electrical signal generated by the image sensor from the set of discrete exposure times.
22. The method of measuring a temperature of an object as claimed in claim 21, comprising: selecting an appropriate exposure time from the set of discrete exposure times in response to the second electrical signal reaching a predetermined upper threshold level wherein the appropriate exposure time is lower than the initial exposure time.
23. The method of measuring a temperature of an object as claimed in claim 21, comprising: selecting an appropriate exposure time from the set of discrete exposure times in response to the second electrical signal reaching a predetermined lower threshold level wherein the appropriate exposure time is higher than the initial exposure time.
24. The method of measuring a temperature of an object as claimed in any of claims 20 to 23, comprising: determining the temperature of the second object from the value of the second electrical signal as measured using the appropriate exposure time value and 1 0 the value of the first electrical signal as a function of temperature as measured using the same appropriate exposure time value.