Mlx90640 infrared thermal imager temperature sensor module development notes (VIII)

MLX90640 Infrared thermal imager temperature sensor module development notes （ 8、 ... and ） Expanding knowledge - Radiation rate 、 sensitivity 、 precision 、 Detection distance

 

    The final result is a miniature USB Interface infrared imaging module （ Miniature infrared imager 30*30mm）, Can be connected to Android Mobile phone or computer USB Interface , Real time display of thermal video , It's similar to a mobile camera , It's just thermal infrared imaging , So called “ Red eye camera ”. The picture of the module is as follows ：

    Red Eye Camera（ hereinafter referred to as “IDF-x” or “ equipment ” ） It is a non-contact thermal imaging instrument based on infrared array high-precision temperature sensor and advanced software algorithm , Infrared imaging can be performed on any object within the field of view , Imaging resolution 512*384 Pixels , Temperature sensitivity 0.1℃, Absolute accuracy ±1.5℃, The refresh rate is the highest 64Hz. With storage and real-time clock , With real-time data output display 、 Photo storage function , Digital interfaces include UART and USB, It can be directly connected to the computer and Android mobile phone , Cooperate with upper computer software or mobile phone APP Program , It's very convenient to use .

Radiation rate

It is a physical quantity that describes the characteristics of surface radiation sources . It means that the heat radiated per unit area of an object is at the same temperature as the blackbody 、 The ratio of radiant heat under the same conditions . namely ： Generally speaking, emissivity is the ability of an object to convert its temperature into radiation and spread out , 1 It means that its temperature can be converted into 100% Radiation of , 0.9 Indicates that it can reduce its temperature 90% Convert to thermal radiation and spread out . In fact, the emissivity is 1 The material （ In black ） It doesn't exist , So the emissivity of any material is 0~1 Value between .

Any object above absolute zero （-273.15℃） When , The surface of the object will have infrared energy, that is, infrared radiation , Higher temperature , The stronger the infrared energy emitted . Infrared thermometer and infrared thermal imager measure the temperature of the object surface according to this feature , Because infrared thermometers and infrared thermal imagers measure the temperature of the surface of objects , Therefore, when measuring, it will be affected by the smoothness of the object surface . Experimental proof ： The closer the surface of the object is to the mirror （ The stronger the reflection ）, The more the infrared energy emitted by its surface decays , It is necessary to compensate the attenuation of infrared energy on the surface of different objects , That is, set a compensation coefficient , This compensation coefficient is the emissivity .

The emissivity of different materials is still very different , For example, the human body is 0.95~0.98, And smooth stainless steel is 0.16, Rusty iron is 0.65 about , When you want to get a more accurate temperature value in the detection process , It is necessary to set the radiation coefficient of the detected object .（ If it's just for imaging, it doesn't need to be so strict ）

 

Sensitivity and temperature detection accuracy

Noise equivalent temperature difference （NETD） It refers to the minimum temperature difference that can be detected by the infrared detector , namely ： When the temperature of the measured object changes, the infrared detector can detect . One of the main indicators to measure the performance of infrared detectors .

The noise equivalent temperature difference of the heat detector is 100mK about （0.1℃）. The second generation of photodetectors is 20mK about （0.02℃）. The third generation detector aims at 1mK（0.001℃）.

The absolute temperature value calculated by infrared detection is directly related to the emissivity parameter of the measured object , The emissivity values of different materials are different , What's more serious is that even the same material , Surface finish 、 Moisture content 、 The influence of temperature and other factors will also directly change the emissivity , This leads to the absolute temperature of infrared detection cannot be absolutely accurate , The problem lies not in the accuracy of the infrared detector's perception of the amount of radiation, but in the fact that the emissivity of the material changes in a small range at any time . therefore , In general, absolute temperature cannot be used to measure the performance of infrared detectors , Instead, use temperature sensitivity , namely ： Noise equivalent temperature difference （NETD）. This can also draw a conclusion , The main function of infrared thermal imager is to distinguish the temperature differences in different areas as far as possible , Show it in numbers , Then it is displayed as a colored image , Only after the temperature is separated , The image will be detailed and clear .

Infrared detection range

Infrared thermal imager uses optical lens to collect the thermal radiation energy of the measured object , Therefore, the detection distance will be affected by the lens field angle and the resolution of thermal imaging pixels . If the imaging resolution of an imager is 32*32 Pixels , The angle of view is 75 degree , It can be understood as emitting from the lens 32*32=1024 Laser detection 1024 The temperature of a point （32 That's ok *32 Column ）, Each row 32 A little bit , Each column 32 A little bit . The included angle of each adjacent two laser lines is 75/31=2.4193° Spread out . As the distance grows , The distance between the two laser lines will become larger , When the measured object is small enough , It may be between two laser lines and not detected , This is the problem of detection distance .

namely ： When the number of pixels and field angle of view of the imager are fixed , Its effective detection distance is related to the size of the measured object . When the size of the measured object is known , The theoretical maximum distance to detect it is ：

Calculation formula .