Infrared thermal imager is suitable for non-contact temperature measurement projects of all enterprises in the world. Point thermometer is another widely used non-contact temperature measurement tool in industrial applications. Its working principle is the same as that of thermal imager: it detects infrared radiation, and then converts it into temperature reading. However, compared with the point thermometer, the infrared thermal imager has the following advantages

The pyrometer only displays numbers, and the infrared thermal imager can generate images.

The temperature meter can only read the temperature of a single point, and the infrared thermal imager can display the temperature readings of all pixels in the thermal image.

Due to the advanced optical lens, the infrared thermal imager can detect the temperature at a longer distance, which helps to inspect large areas.

Several advantages and performance analysis of infrared thermal imager

The point temperature instrument is also known as the point temperature cabin or infrared thermometer. Because its working principle is the same as that of infrared thermometer, it can be considered as an infrared thermal imager with only one pixel. This tool can complete many tasks, but because it can only measure the temperature of a single point, the operator will miss a lot of key information and can not pay attention to some high temperature critical components which are about to fail and need to be repaired.

Using thousands of pyrometers at the same time

Similar to the point thermometer, the infrared thermal imager can also provide non-contact temperature reading. The difference is that the thermal imager can display thousands of temperature readings at the same time, and each pixel corresponds to a temperature reading.

One infrared thermal imager is equivalent to thousands of pyrometers.

FLIR e40sc infrared thermal imager has a resolution of 160 x 120 pixels and can read 19200 temperature readings at a time. FLIR t1050sc, as a high-end thermal imager for industrial R & D / scientific application, has a resolution of 1024 x 768 and can get 786432 temperature readings at a time.

It can save time and detect heat

Thermal imager can not only measure the temperature of thousands of points, but also convert the temperature reading into thermal image. The generated thermal image can fully reflect the overall condition of the equipment to be inspected, and the operator can immediately find the subtle hot spots that are not easy to be found by the pyrometer.

In addition, the thermal imager can save a lot of time. After all, it is time-consuming and laborious to measure a large area with a large number of components, because each component needs to be scanned separately.

Thermal imager can be used to check the heat dissipation problem of printed circuit board, complete quality inspection or check the thermal effect of automobile industry, or carry out error analysis in the laboratory.

In order to accurately measure the temperature of an object with a pyrometer, the target object needs to be completely covered with light spots. This limits the distance of accurate temperature measurement.

Compared with the point thermometer, another advantage of infrared thermal imager is that it can accurately measure the temperature of the object at a longer distance. The distance that can measure a given size target is called “distance coefficient ratio” (D: s) or “spot ratio” (SSR). But where does this ratio come from and what does it mean?

The spot size of the pyrometer is the smallest area that the equipment can measure accurately. This means that the object to be measured (also known as the “target”) needs to cover the entire light spot. The infrared radiation emitted by the target is projected onto the detector through the optical lens of the pyrometer. If the target is smaller than the spot, the detector may detect the radiation around the target. At this time, the thermometer reads not only the temperature of the target, but also the comprehensive temperature of the target and its surrounding environment.

According to the properties of the optical lens, the farther away the pyrometer is from the measurement target, the larger the spot will be. Similarly, the smaller the target is, the closer the thermometer should be to the target in order to measure its temperature accurately. Therefore, it is very important to pay attention to the size of the facula to ensure that the measuring point is close enough to the target to cover the whole facula. If it can be a little closer to form a certain safety boundary, the effect will be better.

For example, if the SSR of the thermometer is 1:30, it means that the temperature of the spot with a diameter of 1cm can be accurately measured at a distance of 30cm. The temperature of the spot with a diameter of 4cm can be accurately measured at 120cm (1.2m). The SSR of most pyrometers is between 1:5 and 1:50, in other words, most pyrometers can measure the temperature of 1cm target at 5-50cm.

The infrared thermal imager is similar to the point thermometer. Its infrared radiation is projected onto the detector matrix, and each pixel in the image corresponds to a temperature value. When thermal imager manufacturers describe the spatial resolution of their products, they usually do not explicitly indicate the SSR value, but use the spatial resolution (IFOV). IFOV is the field angle of view of a single pixel in the detector array of a thermal imager.

In theory, IFOV directly determines the spot ratio of the thermal imager. When the infrared radiation emitted by the target passes through the optical lens and then is projected to the detector, the projected infrared radiation should completely cover at least one pixel of the detector, which corresponds to one pixel of the thermal image. Therefore, in theory, one pixel covering the thermal image should be enough to ensure the correct temperature measurement value.

IFOV is usually expressed in milliradians (one thousandth of a radian). Radian is the ratio of arc length to radius. 1 radian mathematically represents the angle formed when the length of an arc is equal to the radius of a circle. Because the circumference of the circle C = 2 π R (R is the radius), 1 radian is equal to 1 / (2) of the circumference π), 296 °, That is 1 milliradian 0.057 °。

When using thermal imager to measure the temperature of a target, we assume that the distance between the target and the target is equal to the radius of the circle. At the same time, we assume that the target is quite flat. Because the viewing angle of a single detector pixel is small, we can assume that the tangent value of the angle is approximately equal to its radian value.

Ideally, the projected target should cover at least one pixel. In order to ensure accurate reading and explain the light dispersion during projection, it is recommended to cover a slightly larger area.

In this formula, the unit of spot size and target size is cm, IFOV is mrad. When the distance is 100 cm and IFOV is 1 mrad, the spot size is 0.1 cm. If the spot size of 0.1 cm can be measured at 100 cm, then the spot size of 1 cm can be measured at 1000 cm, indicating that the distance coefficient ratio is 1:1000.

If we substitute the above calculation into the formula, and express the SSR as 1: X, use 1 to represent the spot size, and X to represent the distance, then the formula about X is as follows

Where IFOV is expressed in milliradians (mrad).

Ideal and practical optical lens

Using the above formula, the thermal imager with IFOV of 1.4 mrad can be calculated, and the theoretical SSR is 1:714. Therefore, the object with diameter of 1 cm can be measured at a distance of 7 m in theory. However, as mentioned above, the theoretical value does not represent the real situation, and it has not considered that the optical lens used in reality is not perfect. If infrared radiation is projected onto the detector lens, it will cause dispersion and other optical anomalies, which can not ensure that the target can be accurately projected onto a single detector pixel.

The projected infrared radiation may also come from adjacent detector pixels. In other words: the surface temperature around the target may affect the temperature reading.

Like the pyrometer, the target should not only cover the light spot completely, but also cover the safety boundary near the light spot. When using the infrared detector thermal imager to measure the temperature, it is recommended to use the safety boundary. The safety boundary is obtained by measuring the angle of view (MFOV). MFOV describes the actual measurement spot size of the thermal imager, in other words, the minimum measurement area to obtain the correct reading.

MFOV is usually represented by many ifovs (the field angle of view of a single pixel). The common practice of infrared detector thermal imager is: considering the optical anomaly, the target should cover at least 3 times of IFOV. This means: in a thermal image, the target should cover not only one pixel, but also the surrounding pixels. Under ideal conditions, the pixels should be enough to meet the measurement requirements.

When using this Convention, the formula for determining the spot ratio can take into account the coefficients of the real optical lens. In order to get closer to the real value, three times IFOV can be used instead of one time IFOV. The formula is as follows:

Where IFOV is expressed in milliradians (mrad).

Based on this formula, the SSR of a thermal imager with IFOV of 1.4 mrad is 1:238, which means that an object with a diameter of 1 cm can be measured at 2.4 M. Due to the existence of safety boundary, the theoretical value may tend to be conservative. The true SSR may be higher, but using these conservative SSR values can ensure the accuracy of temperature readings.

The infrared energy (a) from the object is focused by the optical lens (b) and projected onto the infrared detector (c). The detector sends information to the sensor electronics (d) for image processing. The electronic component converts the data from the detector into an image (E) that can be read on a viewfinder, standard video display or LCD display.

The SSR value of the thermometer is usually between 1:5 and 1:50. The SSR value of most affordable models ranges from 1:5 to 1:10. The more advanced the function is, the higher the price is. The highest SSR value can be 1:40 or even 1:50. Note: when referring to the optical lens, the same problem exists between the pyrometer and the infrared thermal imager. When comparing the technical specifications of the pyrometer, it must be clear whether the SSR value refers to the theoretical value or the compensation value of the lens.

Temperature detection at a distance

Even considering the coefficient of ideal and actual optical lens, there is a great difference between thermal imager and point thermometer in measuring distance. When the measurement target is 1 cm, the distance of most pyrometers is 10-50 cm, which is difficult to be higher than this range.

Close up and micro lens can capture detailed image details, easy to measure tiny hot spots. This is extremely difficult for a pyrometer. The top image is shot with 4x close-up lens, and the bottom image is shot with 15x close-up lens μ M lens.

For the same size target, the thermal imager can accurately measure the target at a distance of several meters. Even if the IFOV is 2.72 mrad, the FLIR E40 infrared thermal imager can still measure the temperature point of 1 cm at a distance of 120 cm. FLIR t1050sc, as a high-end industrial application infrared thermal imager of FLIR, adopts standard 28 ° Lens, which can measure the same size target at a distance of 7m.

These values can be calculated using a standard lens. Many advanced thermal imagers are equipped with replaceable lenses. When using different lenses, IFOV will also change, which in turn will affect the facula ratio. For FLIR t1050sc infrared thermal imager, FLIR not only provides 28 ° Standard lens, also provides 12 ° Telephoto lens. Equipped with a lens specially designed for long-distance observation, the light spot ratio will be larger. If 12 ° The IFOV of FLIR t1050sc infrared thermal imager is 0.20 milliradian. With this lens, the same thermal imager can accurately measure targets of the same size at a distance of 17m.

Judge if you need to get closer to the target

In terms of SSR value, the performance of the infrared thermal imager is significantly higher than that of the point thermometer, but the SSR value only refers to the distance that can accurately measure the temperature. In the actual detection, the hot spot does not need accurate temperature reading. In the thermal image, even if the target only covers one pixel, the hot spot is still clear. The temperature reading may not be perfect, but it can be used to detect hot spots. The operator can get closer to the target to ensure that the target can cover more pixels in the thermal image and the temperature reading is accurate.

When measuring small targets, the pyrometer is also facing great challenges. This function is becoming more and more important in the detection of electronic components. As the processing speed of the equipment continues to accelerate, and it needs to be installed in a smaller space, it is a very practical problem to find the method of heat dissipation and hot spot identification. The pyrometer can effectively detect and measure temperature, but its spot size is too large. However, the focal length of a thermal imager with a close-up lens can be adjusted down to 5 per pixel spot size μ m. It is convenient for engineers and technicians to measure the subtle targets.

Eliminate guesswork, seeing is believing

The thermometer can only display one reading, and the reading may not be accurate, which is easy to make people guess. Infrared thermal imager can accurately display the heat, not only can realize the temperature measurement, but also can display the transient image of temperature distribution. The perfect combination of visible light information and accurate temperature measurement helps to find the fault point quickly and accurately. Immediately upgrade to FLIR systems’ infrared thermal imager, find problems in a faster and more convenient way, and eliminate all kinds of guesswork caused by uncertainty.

Editor in charge: GT

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