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

Why can't infrared thermal imager be used to detect temperature?

Spectral principle

To understand the operation of TIC, what is infrared first? This should also start from the spectral wavelength. Light is an electromagnetic wave, and there are different rays according to the wavelength. The shorter the wavelength, the greater the harm to human body. X-rays (0.00 1 nm-10 nm) and ultraviolet rays (10 nm -380 nm) are commonly heard, which have short wavelengths and will have an impact on human body. Then the visible light range with the wavelength of 380 nm -780 nm; Then there is the infrared block with the wavelength of 780 nm-1mm.

There are many carbon particles in the fire, which ordinary visible light can't penetrate, so smoke will block the internal view. Infrared ray is characterized by long wavelength, which will not be blocked by carbon particles, and can see smoke and airflow, which is the key.

Introduction and application of infrared ray

Infrared rays are divided into near infrared rays (780 nm-1.5 micron), middle infrared rays (1.5 micron -5 micron) and far infrared rays (5 micron-1 mm) according to wavelength. The longer the wavelength, the less affected by visible light.

Infrared has many applications, which can be summarized as follows:

Near-infrared luminous mirror: it can be used to increase sight at night. Its setting value is about 900 nm -2 microns, which belongs to near infrared and close to visible light, so it needs some light sources to image when detecting, and it is almost impossible to image without light sources at all.

Mid-infrared camera: When the setting value is about 2 microns to 7 microns, the infrared signal will be stronger than near infrared, and it will be easily interfered by visible light sources, but it is better than near infrared.

Thermal imager (TIC): NFPA 180 1 is defined as 8 microns-14 microns. The imaging in this range will not be disturbed by visible light, and it can be imaged with infrared energy, which is very suitable for firefighters.

Definition of infrared energy emitter

Briefly introduce the types of transmitters:

Passive emitter: an object that does not actively emit energy, but is easily reflected by heat. Such as doors, windows, floors, etc.

Active emitter: an object that actively emits heat energy, but is easily covered by matter. Such as people and wires, but these energies can be covered by only one piece of clothing.

Direct emitter: flame.

Why talk about emitters? Simply put, it is difficult for a thermal imager to obtain an image without emitting contrast energy. For example, in an empty office, you go in with a thermal camera, because they are all passive emitters, there is no energy difference, and the image is difficult to see clearly; But as long as a flame is lit next to it, the heat source generates energy to heat the passive transmitter, and the image can be clearly seen after comparison.

contrast

Talking about "contrast" is the essence of thermal imager. According to the basic mode of thermal imager, the relative high temperature is white and the relative low temperature is black, but it is only "relative temperature" and does not mean "real high temperature". For example, when a person stands in the freezer, his image is white, but when he stands in the oven in the bathroom, his image is black.

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

Restrictions on the use of thermal imager

There is no way to penetrate the water: so when searching in the water, you can only see the objects on the water; Water mist can easily interfere with the imaging of thermal imager.

Mirror reaction: substances with shiny surfaces will reflect heat source images, such as glass, mirrors, windows, stainless steel surfaces, etc. And it is easy to misjudge the environment, so be careful (you can wave your hand to confirm whether it is a reflection image).

Can't focus in the range of 1 meter: because it is too close to focus, there are operational restrictions in a narrow space.

Generation of condensed water: If the screen is completely white during use, it is likely that the screen, mask or front lens are condensed by water mist. At this time, you can wipe the lens with your fingers.

The taboo of thermal imager-temperature measurement

Recently, it is popular to measure the temperature of 600℃ with a thermal imager to judge whether an explosion or an iron house can enter. I didn't expect this to be a very dangerous practice in America. Bob said that four firefighters died in the disaster relief process in Houston, USA. There is a detailed analysis of various reasons on NIOSH website, but one of them is that "the temperature measurement of TIC is the basis for judging the fire!"

The thermal imager measures the surface temperature, not the "smoke layer temperature". Even if the distance is different, the material medium is different and the measuring angle is different, there will be great errors. For example, in a fire scene, it is very likely that when measuring the temperature, this thermal imager will measure 500 degrees, another will measure 250 degrees and the other will measure 300 degrees. Is it dangerous? Still not dangerous? Will it flash? Really won't collapse? Is there any heat source blocked by the ceiling or decoration that leads to misjudgment? There are too many factors that affect the accuracy of temperature measurement. Remember, the design concept of fire-fighting thermal imager is "contrast imaging", not "temperature measurement".

Judging the fire situation should be based on the existing building structure, ventilation structure, fire load, room space and other fire characteristics, rather than using the temperature of the thermal imager to make a decision. Judging by temperature will kill firefighters.

Bob said that they strongly hope to cancel the function of temperature measurement in the next version of NFPA to avoid danger. He also told us earnestly not to take temperature measurement as the basis for judging fire!