Secret: Why can infrared thermal imager not be used to detect temperature?

The principle of the Thermal Image Camera (TIC ) is to use special lenses to capture the infrared energy released by molecules. As long as any object is above absolute zero ( -273 °C ) it will release infrared energy.

Spectral principle

To understand the operation of TIC , we must first talk about what is infrared? This also starts with the spectral wavelength. Light is an electromagnetic wave. There are different types of light according to the wavelength. The shorter the wavelength, the greater the damage to the human body. The most commonly heard are the X- rays ( 0.001 nm- 10 nm) and the ultraviolet rays ( 10 nm- 380 nm). , very short wavelength, there will be effects on the human body; and a visible light wavelength range of 380 nm -780 nanometers; and is 780 nm to 1 mm wavelength infrared blocks.

There are a lot of carbon particles in the fire field, the general visible light can not penetrate, so smoke will block the internal line of sight. The infrared characteristic is that because of its long wavelength, it will not be blocked by the carbon particles, and smoke flow and air flow will be seen. This is the key.

Infrared Introduction and Application

Infrared wavelength is divided into "near-infrared" ( 780 nm - 1.5 micron), "middle-infrared" ( 1.5 micron - 5 micron) and "far-infrared" ( 5 micron - 1 millimeter ), the longer the wavelength, representing the effect of visible light The less.

There are many applications of infrared, the outline are:

Near-infrared luminescence mirror: can be used to increase the line of sight for the night, its setting value is about 900 nm - 2 microns, is near-infrared, close to visible light, so the detection needs some light to image, if you do not have a light source is almost impossible Imaging.

Mid-infrared camera: The setting value is between 2 μm and 7 μm, and the infrared signal will be stronger than the near infrared light, easily disturbed by the visible light source, but better than the near infrared light.

Thermal Imager ( TIC ): The definition of NFPA1801 is between 8 microns and 14 microns. This range of imaging will not be disturbed by visible light and will allow infrared energy to be completely imaged, making it ideal for firefighters.

Infrared energy emitter definition

Briefly introduce the type of emitter:

Passive emitter (Passive Emitter): not actively emit energy, but easily heat an object and reflected energy. Such as doors, windows, floors, etc.

Active emitter (Active Emitter): active transmitter will be the object of thermal energy, but the material is easily obscured. Such as people, wires, but this energy can be covered by just one piece of clothing.

Flame: body heat (Direct Emitter) direct emission.

Why talk about emitters? In simple terms, there is no environment that emits contrast energy. It is difficult for the thermal imager to see the image. For example, in an empty office, when you carry the thermal imager in, the images are difficult to see because they are both passive emitters and there is no energy difference; but as long as there is a flame next to them, the heat source generates heat for passive emitters. After comparing the energy, we can clearly see the image.

Compared

When it comes to "comparison", it is precisely where the thermal imager comes from. According to the thermal imager's initial mode ( Basic mode ), the relative high temperature is white and the relatively low temperature is black, but it is only the "relative temperature" and does not mean that it is "true high temperature". For example, a person stands In the freezer, he looks white, but when he stands in the bathroom oven, his imaging is black.

In the course of the thermal imager training, it is to teach you how to judge the current situation of the interior through the contrast, difference and shape of the color!

Thermal Imager Use Limits

There is no way to penetrate the water: so when searching in the waters, only objects on the surface of the water can be seen; when the water sprays water, it is easy to interfere with the imaging of the thermal imager.

Mirror reaction: The material on the smooth surface will reflect heat source images such as glass, mirrors, windows, stainless steel, etc. It is easy to misjudge the environment. Care should be taken (you can wave your hand to confirm whether it is a reflection pattern).

Can not focus within 1 meter : because too close to focus, narrow space has operational restrictions.

Condensate production: If the screen is completely white during use, it is likely to be a screen, mask or condensation in front of the lens, then you can wipe the lens with your fingers.

Thermal Imager Uses Large Taboo - Temperature Measurement

Recently, it is popular to say that the temperature measurement with a 600 °C thermal imager is used to determine whether a deflagration or a tin house can enter. I did not expect this to be a very dangerous practice in the United States. Bob said that in the United States, four firefighters in Houston died in the disaster . There are detailed analysis of various reasons in the NIOSH website, but one of the reasons is that “the use of TIC 's temperature measurement as a basis for judging the situation of the fire!”

The thermal imager's temperature measurement is to measure the surface temperature, not to measure the "smoke temperature," and because of different distances, different material media, and different measurement angles, even if the same point is measured, there will be a large error. For example, in a fire scene, it is very likely that when measuring the temperature, the thermal imager measures 500 degrees, the other measures 250 degrees, and the other measures 300 degrees. In the end, it is dangerous. ? Or is it not dangerous? Will flash burn? Really will not collapse? Are there any ceilings or decorations that block the heat source and cause false positives? There are many factors that affect the accuracy of temperature measurement. Remember that the design concept of a fire thermal imager is to "make contrast imaging" rather than "measure temperature."

Judging the situation of the fire field should be judged on the basis of the existing fire field characteristics such as building structure, ventilation structure, fire load, and living space, rather than using the temperature of the thermal imager to make decisions. Using temperature for judgment will kill firefighters.

Bob said that they very strongly hope that the temperature measurement function will be cancelled in the next version of NFPA to avoid danger. He also told us with earnest attention, do not use the measured value of temperature as a basis for the judgment of the fire!

The copyright of this article belongs to "Fire Fighting Frontier" and is compiled and edited by China Rescue Equipment Network.