TECHNICAL NOTE

Using one thermal imaging camera TEMPERATURE GUNS VERSUS THERMAL therefore corresponds to thousands of ­IMAGING TECHNOLOGY spot pyrometer measurements. The FLIR E40sc thermal imaging camera has an image resolution of 160 x 120 pixels, resulting in 19,200 temperature readings at a glance. The FLIR T1030sc, one of the top models for industrial R&D/Science applications, has an image resolution of 1024 x 768 pixels, giving you 786,432 temperature readings at once.

SAVE TIME AND ‘SEE’ THE HEAT A thermal imaging camera not only The FLIR E40sc gives you thousands of temperature thermal imaging A temperature readings, it also translates these camera measures gun measures the the temperature of readings into a thermal image. This temperature of one 19,200 spots. conversion into an image results in a spot. complete overview over the inspected equipment and allows the operator to Thermal imaging cameras are used for non-contact temperature measurements immediately see small hot spots that in companies all over the world. Another common tool for non-contact would be easily missed with a spot temperature measurements widely used in industrial applications is the pyrometer. Using a thermal imaging spot pyrometer. Both spot pyrometers and thermal cameras work according camera also saves time. Scanning to the same principle; they detect infrared radiation and translate it into a large areas with many components temperature reading. Thermal cameras however, have several advantages using a spot pyrometer is a very time compared to spot pyrometers. consuming task, because you have • A spot pyrometer just gives you a number - thermal imaging cameras generate to scan every component separately. an image. A thermal imaging camera can be • A spot pyrometer reads the temperature of one single spot - a thermal used to check heat dissipation on imaging camera gives you temperature readings for each pixel of the entire printed circuit boards, to do quality thermal image. checks or inspect thermal impact in • Because of advanced optics, thermal imaging cameras can also resolve temperatures the automotive sector, or to do failure from a longer distance. This allows you to quickly inspect large areas. analysis in the lab.

The spot pyrometer is also known as temperature gun or infrared thermometer. Because it works according to the same physical principle as a thermal camera, a spot pyrometer can be seen as a thermal camera with only one pixel. Such a tool can be very useful for many tasks, but because it only measures the temperature of one single spot the operator can easily miss crucial information. The high temperature of certain critical components that are near failure and need repair What a spot pyrometer ‘sees’. What a thermal imaging camera ‘sees’. might go unnoticed.

USE THOUSANDS OF SPOT PYROMETERS AT THE SAME TIME A thermal imaging camera also provides non contact temperature readings, just like a spot pyrometer does. Unlike a spot pyrometer thermal imaging cameras produce not one, but thousands of temperature readings at the same time, one for each pixel in the thermal image. What a spot pyrometer ‘sees’. What a thermal imaging camera ‘sees’.

FLIR.COM TECHNICAL NOTE

ACCEPTABLE INCORRECT Theoretically the IFOV directly determines BEST (ONLY JUST) the spot size ratio of a thermal imaging camera. As the infrared radiation that is emitted by the target passes through the optics and is projected on the detector, the projected infrared radiation should completely cover at least one detector element, which corresponds to one pixel in the thermal image. So in theory covering one pixel in the thermal image In order to accurately measure an object's temperature with a spot pyrometer the target object needs to entirely cover the measurement spot. This limits the distance from which should be sufficient to ensure correct temperatures can be measured accurately. temperature measurements.

Another advantage of thermal imaging The IFOV is usually expressed in cameras compared to temperature guns milliradians (one thousandth of a radian). is that they can accurately measure The term radian describes the ratio temperatures from larger distances. between the length of an arc and its The distance at which a certain spot radius. One radian is mathematically pyrometer is able to measure a target defined as the angle formed when the of a given size is often described with length of a circular arc equals the radius the “Distance to spot size ratio” (D:S) of the circle. Since the circumference or ‘Spot Size Ratio’ (SSR). But where equals 2 π times the radius, one does that value come from and what Thermal imaging cameras allow you to 'see' the heat. radian equals 1/(2 π) of the circle, or does it stand for? approximately 57.296 angular degrees and one mrad 0.057 angular degrees. The ‘spot size’ of a spot pyrometer is the distance to the target. If the SSR of a smallest area that still can be measured spot pyrometer is 1:30, for instance, In the situation where a thermal accurately with the device. That means this means that the temperature of a imaging camera is used to measure that the object of which you want to spot with a size of 1 cm in diameter the temperature of a certain target measure the temperature, also referred can be accurately measured at a we assume that the distance to the to as the target, needs to cover the distance of 30 cm. The temperature of target equals the radius of the circle entire spot size. The infrared radiation a spot having a size of 4 cm can be and we also consider the target to be that is emitted by the target passes measured from a distance of 120 cm rather flat. Since the viewing angle of a through the spot pyrometer’s optics (1.2 meters). Most spot pyrometers single detector element is small, we can and is projected onto the detector. If have an SSR between 1:5 and 1:50. assume that the tangent of this angle is the object is smaller than the spot size, This means that most spot pyrometers approximately equal to its value in radian. the detector will also be hit by parts of can measure the temperature of a the radiation coming from the object’s target of 1 centimeter in diameter from Therefore, the spot size calculates as surroundings. Hence, the device will a distance of 5 - 50 cm. IFOV (in mrad) divided by 1000 and not read the object’s temperature only multiplied by the distance to the target: but a mixture of the temperatures of Thermal imaging cameras are very the object and its surroundings. similar to spot pyrometers in that infrared radiation is projected onto a The farther away you hold the spot detector matrix, with each single pixel pyrometer from the object that you in the image corresponding want to measure, the larger the spot to a temperature measure­ size will become, due to the nature of ment. Thermal imaging optics. Consequently, the smaller the camera producers usually target, the closer you need to hold the do not specify SSR values spot pyrometer in order to accurately to describe the spatial measure its temperature. It is therefore resolution of their products; very important to keep an eye on the but use instead the spot size and make sure that you stand Instantaneous Field of View close enough to cover the entire spot (IFOV). The IFOV is defined Instantaneous size with the target, preferably even a as the field of view of a field of view (IFOV) bit closer to create a safety margin. single detector element of the camera’s detector array. Field of view (FOV) The Spot Size Ratio defines a spot pyrometer’s spot size for any given Perfect optics Realistic optics

detector detector

Projected target Projected target

Focal Plane Array (FPA) IFOV, the field of view of a single pixel. Detector element A commonly used guideline for microbolometer cameras is that the Temperature measurement target needs to cover an area at least 3 times the IFOV to take into account optical aberrations. This means that in the thermal image the target should not only cover one pixel, which in 3x IFOV IFOV an ideal situation would have been sufficient for the measurement, but also the pixels around it.

In the ideal situation the projected target should cover at least one When this guideline is observed the pixel. To ensure accurate readings it formula to determine the spot size ratio is advisable to cover a wider area to can be adapted to take into account account for the optical dispersion of the projection. the factor of real optics. Instead of using 1xIFOV we can use the 3xIFOV guideline, which leads to the following, Where the spot size and distance to infrared radiation can also ‘spill over’ more realistic, formula: target are expressed in cm and IFOV from neighbor detector elements. In in mrad. other words: the temperature of the surfaces surrounding the target might For a distance of 100 cm and an IFOV of influence the temperature reading. 1 mrad the spot size would be 0.1 cm. If Where the IFOV is expressed in mrad. a spot size of 0.1 cm can be measured at Just like with a spot pyrometer, where a distance of 100 cm, then the distance the target should not only cover the Based on this formula a camera with at which a spot size of 1 cm can be spot size entirely but should also cover an IFOV of 1.4 mrad will have an SSR measured is 1000 cm. This means that a safety margin around the spot size, it of 1:238, which means that you should the spot size ratio is 1 : 1000. is advisable to employ a safety margin be able to measure an object of 1 cm in when using a microbolometer thermal diameter at a distance of just under 2.4 If we put the above calculations into a imaging camera for temperature meters. This theoretical value is likely formula to express the SSR as 1 : x, with measurements. This safety margin is on the conservative side, because of 1 standing for the spot size and x for the captured in the term Measurement the safety margin observed. The real distance, then the formula for x would Field of View (MFOV). The MFOV life SSR might therefore be higher, be as follows: describes the real measurement spot but using these conservative SSR size of a thermal camera, in other values the accuracy of the temperature words: the smallest measureable area readings is safeguarded. for correct temperature readings. It is usually expressed as a multitude of the Where the IFOV is expressed in mrad.

IDEAL AND REAL OPTICS Using the formula you can calculate Infrared energy (A) coming from an object is focused by the optics (B) onto an infrared detector (C). The detector sends the that a camera with an IFOV of 1.4 information to sensor electronics (D) for image processing. The mrad will have a theoretical SSR of electronics translate the data coming from the detector into an image (E) that can be viewed in the viewfinder or on a standard 1:714, so in theory you should video monitor or LCD screen. be able to measure an object of 1 cm in diameter at a distance of more than 7 meters. However, as was stated previously this theoretical value does not correspond to real life situation, because it does not take into account the fact E that real life optics are never completely perfect. The lens that projects the infrared radiation onto the detector can cause dispersion and other forms of optical aberration. You can never be sure A B that your target is exactly projected onto E one single detector element. Projected C D with a15micron lens. image was lens, taken thebottomimage witha4xclose-up extremely difficulttodowithaspotpyrometer. The top and allow you tomeasure smallspots. This would be andmicroscopicClose-up lensesprovide great imagedetail Most thermal imaging 1 cm target. away than10 to50cm,assuminga pyrometers cannotbeheldany farther ishuge.Mostspot measuring distance cameras andspotpyrometers in the difference betweenthermalimaging realistic opticsistakenintoaccount Even whenthefactor ofidealversus DISTANCE A FROM DETECT TEMPERATURES the optics. is compensatedfor theimperfection of theoretical value ortotheonewhich know iftheSSRnumberrefers tothe pyrometer specificationsyou must comes tooptics. When comparingspot as thermalimagingcameraswhenit pyrometers alsohave thesameproblem even 1:50.Note,however thatspot reaching SSRvalues upto1:40or and therefore moreexpensive, models 1:5 to1:10, whilethemoreadvanced, of theaffordable modelshave anSSRof usually falls between 1:5and1:50.Most Spot pyrometers have anSSRthat TECHNICAL NOTE the temperature ofaspotthissize with anIFOV of2.72mrad,canmeasure the FLIRE40thermalimagingcamera, (1 cm) fromseveral meters away. Even reading iscorrect. image, ensuringthatthe temperature coverstarget morepixels inthethermal can move closertomake sure that the hot spotisdetectedandtheoperator reading mightnotbeperfect, butthe the thermalimage. The temperature coversthe target onlyonepixel in in thethermalimageeven when reading. The hotspotcanbediscernible always requireanaccuratetemperature life detectingahot spotdoesnot measurement canbemade.Inreal from which an accuratetemperature SSR values onlyrefer tothedistance comes totheSSRvalues, butthe outperform spot pyrometers when it Thermal imagingcamerasclearly IN CLOSER MOVE NEEDTO WHETHER YOUSEE 17 meters. ofalmost size fromadistance target measure thetemperature ofthesame thermal imaging cameracanaccurately milliradians. thislens, thesame With T1030sc thermalimagingcamerais0.20 12° telephotolenstheIFOV ofaFLIR size ratioissignificantlylarger. the With thespot for observations, longdistance this lens,which isdeveloped especially lens, butalsoa12° telephotolens. With FLIR not onlyoffers 28° the standard thermal imagingcamera,for instance, the spotsize ratio.For theFLIR T1030sc changes theIFOV, which inturn affects When a different lens is used this cameras feature interchangeable lenses. of themoreadvanced thermalimaging lensisused.Manythat thestandard These values arecalculatedassuming 28°lens. meters withastandard size ofmorethanseven atadistance measure temperature ofthis ofatarget models for industrialinspections, can camera, oneofFLIR’s mostadvanced away. The FLIR T1030sc thermalimaging (1 cm) from more than 120 centimeters temperature ofthissize ofatarget cameras canaccuratelymeasurethe

faster andeasier. Systems finding problems and start a thermalimagingcamerafromFLIR accurately. upgradeto Stopguessing, enables you tofindfaults quickly and accurate temperature measurements combination ofvisualinformation and image of heat distribution. instant This temperature measurements,butalsoan to ‘see’ the heat, giving you not only A thermalimagingcameraallows you inaccurate, which leaves you guessing. a number. That numbermightbe A spotpyrometer canonlygive you STOP GUESSING, START SEEING smallscale. a very technicians tomake measurementson spot size. This allows engineersand less than5µm(micrometers)perpixel with close-upopticscanfocus down to too large. However, thermal cameras temperature, butitsspotsize issimply gun caneffectively detectandmeasure spots isarealproblem. A temperature to dissipate the heat and identify hot fit intosmallerpackages, findingways processing speed,yet arerequiredto As devices continuetogetfaster in for electronicsinspection.important objects. This isincreasingly capability with measuringtemperature onsmall Spot pyrometers also have challenges ­illustrative purposesonly. the actualresolution of thecamerashown.Imagesfor The imagesdisplayed maynotberepresentativeof www.flir.com/research cameras oraboutthisapplication,please visit: For moreinformationaboutthermalimaging

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