Infrared Thermal Imaging Knowledge Collection

Infrared principle

1.   Infrared Definition

In nature, any object with a temperature above absolute zero (-273℃) can emit electromagnetic waves. Infrared is the most widespread form of electromagnetic waves in nature, it is a kind of energy, and this energy is invisible to our naked eyes. Any object in the conventional environment will produce its own molecules and atoms irregular movement, and constantly radiate thermal infrared energy.

2.  Infrared band range

The light waves emitted by the sun are also called electromagnetic waves. Visible light is an electromagnetic wave that can be felt by the human eye. After being refracted by a prism, seven colors of red, orange, yellow, green, cyan, blue, and purple can be seen.

Infrared light is part of these electromagnetic waves, which together with visible light, ultraviolet light, X -rays, gamma rays and radio waves make up a complete continuum of the electromagnetic spectrum.

As shown above, electromagnetic radiation with wavelengths ranging from 0.76μm to 1000μm is called infrared radiation.  

3.  Infrared "atmospheric window"

Infrared radiation electromagnetic waves propagate in the air and are absorbed by the atmosphere, so that the energy of the radiation is attenuated. If the absorbed energy is too much,  it cannot be observed by thermal imager.  

The absorption of infrared ray by atmosphere and smoke cloud is also related to the wavelength of infrared radiation, The infrared ray is transparent for 3~5 microns and 8~14 microns. Therefore, these two wavelengths are called the "atmospheric window" of the infrared. Using these two windows, the infrared thermal imager can observe in the normal environment without changing the situation of infrared radiation attenuation.

As shown in figure:

The car can't be seen clearly in the smoke, but it can be clearly seen by an INFRARED thermal imager. 

The principle of infrared thermal imaging

1. Principle of thermal imaging

In popular terms, infrared thermal imaging is the transformation of invisible infrared radiation into visible thermal images.   

Different objects or even different parts of the same object have different radiation capabilities and their reflection strengths of infrared rays. By using the radiation difference between the object and the background environment as well as the radiation difference of each part of the scene, the thermal image can show the radiation fluctuation of each part of the scene, so as to show the characteristics of the scene.   

The thermal image is actually an image of the temperature distribution on the target surface.

Figure: Thermal images can distinguish the difference in thermal radiation on the surface of the object.

2.  Infrared thermal imaging system

A thermal imaging system is a whole system that receives infrared thermal radiation through a series of optical components and photoelectric processing technology,  and then converts the thermal image that can be seen by human eyes to be displayed on the screen. 

3.  Composition of infrared thermal imager

The basic working principle of an infrared thermal imager is: infrared rays pass through a special optical lens and are absorbed by an infrared detector. The thermal image observed by the eye is displayed on the screen. The block diagram is as follows:       

Glossary

Infrared thermal imager is divided into refrigeration and non-refrigeration according to the working temperature.

Refrigerated thermal imager:

The detector is integrated with a cryogenic cooler, which can  lower the temperature of the detector so that the thermal noise signal is lower than the imaging signal and the imaging quality is better.  

Uncooled thermal imagers:

The detectors do not need cryogenic refrigeration, and the detectors used are usually based on microbolometers, mainly polysilicon and vanadium oxide detectors.

Infrared thermal imagers are divided into temperature measurement type and non-temperature measurement type according to their functions.

Temperature measuring infrared thermal imager:

The temperature measurement infrared thermal imager can directly read the temperature value of any point on the surface of the object from the thermal image. This system can be used as a non-destructive testing instrument, but the effective distance is relatively short.

Non-temperature measuring infrared thermal imager can only observe the difference of thermal radiation on the surface of the object. This kind of system can be used as an observation tool, and the effective distance is relatively long.

Infrared Detectors:

The infrared detector is a device that converts invisible infrared radiation into measurable signals, and it is the core and key component of infrared whole system.

Detector size:

The size of detector refers to the size of a single detection element on the detector, and the general specifications are 25 μm, 35 μm, etc. The smaller the detection element is, the better the imaging quality is.

Resolution of infrared detectors:

Resolution is an important parameter to measure the quality of thermal imager detector. It shows how many units of detector are in the focal plane of the detector. At present, the mainstream resolutions in the market are 160 × 120, 384 × 288, etc. In addition, there are 320 × 240, 640 × 480 and so on. The higher the resolution, the clearer the imaging effect.      

Infrared optical lens:

The infrared optical lens is usually composed of a group of lenses, which can receive various infrared final focal length to the infrared detector, photoelectric conversion processing.

The germanium crystal with a refractive index of 4 is the most used in infrared optical lenses , which is suitable for the 2-25μm band . Si with refractive index of 3 is commonly used in the band of 1-6μm. The heat-resistant shock of the missile fairing is optimized to use the hot-pressed MgF2 and ZnS.

Field of View ( FOV ):

The field of view Angle is the Angle stretched by the line length of the scene or image plane at the intersection of the main plane of the lens system and the optical axis. Popularly speaking,  the lens has a definite field of view, and the Angle of the lens to the height and width of this field of view is called the field of view Angle.         

Temperature measurement accuracy:

The temperature measurement accuracy refers to the difference between the temperature data read and the actual temperature when the temperature measurement is carried out by the infrared thermal imager.  The smaller the value, the better the performance of the thermal imager.

Temperature measurement range:

The range of temperature measurement refers to the range of the highest and the lowest temperature that can be measured by the infrared thermal imager.

focal length:

The distance from the center of a lens to its focal point, usually denoted by f. The unit of focal length is usually expressed in mm ( millimeters ) . The focal length of a lens is generally marked in front of the lens, such as f=50mm (this is what we usually call " standard lens " ), 28-70mm (our most commonly used lens), 70-210mm (telephoto lens), etc. The larger the focal length, the farther the clear image can be taken.

Spatial Resolution:

Spatial resolution refers to the minimum limit of the spatial geometric length of critical objects identifiable in an image, that is, the resolution of fine structures. The smaller the value, the higher the resolution.     

Minimum Resolvable Temperature Difference ( MRTD ):

In thermal imaging, MRTD is an important parameter for comprehensive evaluation of temperature resolution and spatial resolution. At a certain spatial frequency, the temperature difference between the target and the background is called the minimum distinguishable temperature difference of the spatial frequency when the observer can just distinguish (50% of the time) the four bands.  The smaller the MRTD value is, the better the infrared thermal imager performance is.  

Noise Equivalent Temperature Difference ( NETD ):

The thermal imager observes the measurement pattern. When the RMS ratio of the signal voltage peak output by the reference electronic filter of the system and the noise voltage is 1, the temperature difference between the black-body target and the black-body background is called the noise equivalent temperature difference.  The smaller NETD is, the better the image quality is.

Ghost:

It refers to the bright or dark lines in the infrared image that do not change with the target. It is caused by the uneven response rate of the detection element of the infrared detector to the infrared radiation.

Bad points:

Dead pixels refer to bright and dark spots whose coordinates do not change with the target in the infrared image, which are caused by the high or low response rate of a single detection element of the detector to infrared radiation, also called invalid pixels.

Non-uniformity Correction:

Due to the limitations of the manufacturing process of infrared detectors, the response rate of each detection element of the infrared detector to infrared radiation is different, and the above-mentioned ghost images and dead pixels will appear on the imaging surface, which affects the imaging quality of the thermal imager.

Non-uniformity correction refers to a technical method to effectively reduce the non-uniformity of the responsivity of detector and improve the imaging quality of the thermal imager. After the non-uniformity correction, the imaging image of the thermal imager is uniform, the ghost and dead pixels disappear, and the imaging effect is significantly improved, which can greatly improve the observation ability of the thermal imager.

Compensate:

Compensation is also called correction, in order to obtain the original data required by non-uniformity correction, so as to obtain an ideal infrared image. When the image is unclear, the thermal imager can be compensated. The compensation target can select different objects with uniform temperature according to the scene environment and target characteristics. This object can be a clear cloudless sky, the built-in shutter of a thermal imager, or closed lens cap, etc.