Security monitoring infrared camera application principle analysis

Everything in nature, as long as its temperature is higher than the absolute temperature (-273°C), there is an irregular movement of molecules and atoms, and the surface continuously radiates infrared light. Infrared is an electromagnetic wave, its wavelength range is 0.78 ~ 1000um, not seen by the human eye. Infrared imaging devices are devices that detect infrared radiation that is not visible to the human eye. It reflects the infrared radiation field on the surface of the object, ie the temperature field.

Note: Infrared imaging devices can only reflect the temperature field on the surface of an object.

For power equipment, the basic principle of infrared detection and fault diagnosis is to detect the infrared radiation signal of the surface of the device to be diagnosed, so as to obtain the thermal state characteristics of the device. Based on this thermal state and appropriate criteria, whether the device has a fault and Fault diagnosis, location and severity of the diagnosis.

In order to deeply understand the principle of infrared diagnostics for power equipment failures and better detect equipment failures, the following will briefly discuss the relationship and laws between the thermal state of power equipment and the infrared radiation signals it generates, the influencing factors, and the working principle of DL500E.

one. Infrared Radiation Emission and Its Law (I) Blackbody's Infrared Radiation Law

The so-called black body, simply speaking, is an object with an incident radiation absorption rate equal to 1 for all wavelengths in any case, that is to say full absorption. Obviously, because any object actually existing in nature has a certain reflection on the incident radiation with different wavelengths (absorption rate is not equal to 1), the black body is just an idealized object model abstracted by people. However, the basic law of black body heat radiation is the basis of infrared research and application. It reveals the quantitative relationship between infrared heat radiation emitted by blackbody and temperature and wavelength.

Below, I highlight three basic laws.

1. Spectral Distribution of Radiation - Planck's Law of Radiation

A blackbody whose absolute temperature is T(K), the radiant power (abbreviated as spectral radiance) Mλb(T) emitted by the unit surface area to the entire hemispherical space within a unit wavelength interval near the wavelength λ, and the wavelength λ, the temperature T satisfy the following relation:

Mλb(T)=C1λ-5[EXP(C2/λT)-1]-1

Where C1-the first radiation constant, C1=2πhc2=3.7415×108w·m-2·um4

C2-second radiation constant, C2=hc/k=1.43879×104 um·k

Planck's radiation law is the basis for all quantitative calculations of infrared radiation. The introduction is rather abstract and will not be elaborated here.

2. Radiative Power Variation with Temperature - Stephen Boltzmann's Law

The Stephen-Boltzmann law describes the variation of the total radiant power Mb(T) (abbreviated as total irradiance) of all wavelengths emitted by a blackbody unit surface area to the entire hemispherical space with its temperature. Therefore, the law is obtained by integrating the Planck radiation law with the wavelength:

Mb(T)=∫0∞Mλb(T)dλ=σT4

In the formula, σ=π4C1/(15C24)=5.6697×10-8w/(m2·k4), which is called Stefan-Boltzmann constant.

Stefan-Boltzmann's Law states that any object that has a temperature above zero degrees Celsius will spontaneously emit infrared heat radiation. Moreover, the total radiated power emitted by a blackbody per surface area is proportional to the fourth power of the Kelvin temperature. Moreover, as long as there is a small change in temperature, it will cause a large change in the radiated power emitted by the object.

Then, can we imagine that if we can detect the total radiant power emitted by the blackbody surface area per unit, can we not determine the temperature of the black body? Therefore, Stephen Boltzmann's law is the basis of all infrared thermometry.

3. The law of spatial division of radiation - Lambert's cosine law
The so-called Lambertian cosine law is that the radiation intensity of a blackbody in any direction is proportional to the cosine of the observation direction with respect to the normal angle of the radiation surface. As shown in the figure, Iθ=I0COSθ.

This law shows that the blackbody has the strongest radiation in the normal direction of the radiation surface. Therefore, when actually doing infrared detection. Should be selected as far as possible in the normal direction of the surface to be measured, if detected in the direction of the angle of θ with the normal, the received infrared radiation signal will be reduced to the COSθ times the maximum value of the normal direction.

(b) Infrared radiation law of actual objects Kirchhoff's law
The ratio of the radiant exit M(T) of the object to the absorptivity α of the object is independent of the properties of the object and is equal to the radiant emittance M0(T) of the black body at the same temperature. It shows that the ability to absorb large objects has a large emission power. If the object cannot emit radiant energy of a certain wavelength, it must not absorb radiant energy at this wavelength.

2. Emissivity

Experiments have shown that the radioactivity of an actual object depends not only on the temperature and the wavelength, but also on the nature of the material and surface conditions that make up the object. Here, we introduce an emissivity coefficient that varies with the nature of the material and the surface state, so that the basic laws of blackbody can be applied to real objects. This emissivity, which is often referred to as the emissivity, or as the emissivity, is defined as the ratio of the actual object to the radiation performance of the black body at the same temperature.

Here, we do not consider the influence of wavelength, we only study the full emissivity of an object at a certain temperature:

ε(T) = M(T)/M0(T)

Then Stephen Boltzmann's law applied to real objects can be expressed as:

M(T) = ε(T).σT4

(c) Emissivity and its effect on the monitoring of equipment status information

The object must have absorption, reflection and transmission for a given incident radiation, and the absorption rate α, the sum of the reflectivity ρ and the transmittance τ must be equal to 1:

α+ρ+τ=1

Moreover, its reflection and transmission parts do not change. Therefore, under the condition of thermal equilibrium, the radiation energy absorbed by the object must be converted into the radiation energy emitted by the object. It can be concluded that under the condition of thermal equilibrium, the absorptivity of the object must be equal to the emissivity of the object at the same temperature:

α(T)=ε(T)

In fact, by Kirchhoff's law, we can also infer the above formula:

M(T)/α(T)=M0(T)

ε(T) = α(T)

ε(T) = M(T)/M0(T)

Then for an opaque object ε(T)=1-ρ(T)

Effect of radiation transfer between objects Based on the above formula, we can understand qualitatively the following factors affecting the size of emissivity:

1. Influence of different material properties

Different materials have different radiation absorption or reflection properties, so their emission performance should also be different. Generally, when the temperature is lower than 300K, the emissivity of the metal oxide is generally greater than 0.8.

2. Surface conditions affect

any actual surface of the object is not absolutely smooth, always showed different surface roughness. Therefore, this different surface morphology will affect the reflectivity and thus affect the emissivity value. The size of this effect depends on the type of material.

For example, for non-metal dielectric materials, the emissivity is less or less dependent on the surface roughness. However, for metallic materials, the surface roughness will have a large effect on the emissivity. Such as wrought iron, when the surface condition is a rough surface, the temperature is 300K, the emissivity is 0.94; when the surface condition is polished, the temperature is 310K, the emissivity is only 0.28.

In addition, it should be emphasized that in addition to the surface roughness, some human factors, such as the application of lubricating oil and other deposits (such as paint, etc.), will significantly affect the emissivity of the object.

Therefore, when testing, we should first determine the emissivity of the measured object. Under normal circumstances, we do not understand the emissivity, then only use the phase comparison method to determine the failure. For electrical equipment, the emissivity is generally between 0.85-0.95.

3. Temperature effect

The influence of temperature on objects of different nature is different, it is difficult to make quantitative analysis.

Only pay attention during the test.

(d) Effects of radiation transfer between objects

Above, we have discussed that an object must have absorption and reflection for a given incident radiation, and when the heat balance is reached, the radiation energy absorbed by the object must be converted into emitted radiation energy. Therefore, when we detect an arbitrary target in a substation, the detected temperature must have the influence of other nearby objects.

Therefore, we should pay attention to the direction and time of detection when testing, so as to minimize the impact of other objects.

(5) Influence of atmospheric attenuation

The atmosphere has radiation, scattering, refraction, and other physical processes on the object's radiation, which will attenuate the radiation intensity of the object. We call this extinction.

The extinction effect of the atmosphere is related to the wavelength and there is obvious selectivity. Infrared in the atmosphere there are three bands can be basically completely through, we call the atmospheric window, divided into near infrared (0.76 ~ 1.1um), mid-infrared (3 ~ 5um), far infrared (8 ~ 14).

For power equipment, most of its temperature is low, concentrated in the 300K ~ 600K (27 °C ~ 327 °C) or so, in this temperature range, according to the basic law of infrared can be derived, the device emits infrared radiation signals, in the far Infrared 8~14um is the largest percentage of the interval, and the radiation contrast is also the largest. Therefore, most of the power system's infrared detection equipment works within the wavelength of 8 ~ 14um.

However, please note that even if you work in the atmosphere window, the atmosphere still extinguishes infrared radiation. In particular, water vapor has the greatest impact on infrared radiation. Therefore, when testing, the humidity is preferably less than 85%, and the closer the distance, the better.