1. Infrared radiation
Infrared radiation, also known as infrared thermal radiation, has a strong thermal effect. Substances above absolute zero (0 K, that is, -273.15℃) can all generate infrared rays, whose frequency is lower than that of visible light and is invisible to the naked eye.
The Infrared light spectrum is located outside the visible spectrum on the spectral graph, with wavelengths ranging from 0.8μm to 50μm, which is longer than the visible spectrum (0.4μm to 0.8μm).

So what is the relationship between wavelength and frequency?

λ=c/f, where c is the speed of light 3.0×108 m/s and λ is the wavelength.
For example: The frequency of visible light is approximately between 4x1014Hz ~ 8x1014Hz.


2. Infrared radiation-Atmospheric window
The various gases that make up the Earth's atmosphere absorb most of the infrared radiation, leaving only some detectable infrared radiation.
Among them, the part with a higher transmittance is called the "infrared radiation atmospheric window".
In the short-wave, medium-wave and long-wave spectral bands, the main atmospheric Windows are 0.7~2.5μm, 3~5μm and 8~14μm respectively;
The detection band of an uncooled long-wave infrared detector is 8~14μm.



3. Infrared thermal imager composition:
①. Infrared lens: is mainly used to receive and focus the infrared light emitted by the object under test.
②. Infrared detector assembly: is primarily used to convert infrared radiation signals received by infrared lenses into electrical signals.
③. Electronic components: are primarily used for processing electrical signals.
④. Display component: is primarily used to display electrical signals as visible light images.
⑤. Software: is primarily used to process collected data to form temperature readings in images.

4. Near Infrared (NIR)
The near-infrared (NIR) band (0.8μm~1μm) is next to the visible light band, just beyond the recognizable range of the human eye, and NIR imaging exhibits additional image detail information than visible light imaging; 
Like visible light, NIR light is also reflected, so the images we see from NIR sensors are mostly reflected sunlight;
Applications: 
Nowadays, CMOS sensors can mostly cover the near-infrared band, with infrared fill light, laser fill light, you can realize the night vision function, mostly used in security cameras and night vision devices; 
Near-infrared is also appearing on the current smart phones, to enhance the facial recognition ability of the cell phone camera;


4. Short Wave Infrared (SWIR) 
Short Wave Infrared (SWIR) is similar to visible light and can be reflected and absorbed by objects to form images with shadows and contrasts between light and dark; 
Water vapor, fog, and certain materials such as silicone are good mediums for SWIR images; 
SWIR also has the ability to penetrate glass and plastics; 
Hot spots can be detected, with typical temperatures ranging from 500 to 3000 degrees Celsius. Typical temperature is between 500~3000℃; 
Applications: 
Can be used in surveillance camera through smoke, haze and fog; 
Can be used in the field of machine vision to provide inspection, classification and quality control; 
Silicon semiconductor imaging detection and failure analysis; 
Can be used in the military field.


5. Mid Wave Infrared (MWIR) 
Advantages: 
High sensitivity and resolution: Cooled detector with low noise, thermal sensitivity (NETD) <20mK, and excellent detail resolution; 
Strong atmospheric penetration: MWIR has high transmittance in specific atmospheric windows (e.g., 3-5 μm), and is suitable for long-distance observation because it is less affected by interference from fog, smoke and soot.
Anti-stray light interference: Compared with LWIR, mid-wave is less affected by the reflection of sunlight, which makes the image more stable during the daytime and reduces the problem of “sun glare and burns”.
Wide dynamic range: suitable for capturing both high and low temperature targets.
Fast response time: Cooled detector with short response time, can do 100hz high frame rate.

Disadvantages: 
High sensitivity and resolution: Cooled detectors need to be paired with a Stirling cooler, complex structure, high maintenance costs, price is usually 5-10 times that of an uncooled thermal imaging camera.
Large size and power consumption: The refrigeration system results in bulky equipment, poor portability, and requires pre-cooling time for start-up (usually a few minutes), making it unsuitable for rapid deployment.
Environmental limitations: Mechanical components of the cooler are prone to failure in extreme temperature environments and may be less reliable than uncooled cameras.
Complicated maintenance: chillers have a lifetime (e.g., about 10,000 hours for a Stirling chiller) and require regular maintenance or replacement, increasing the cost of ownership.

Applications: 
For machine vision, gas detection, environmental and air quality monitoring; 
Missile guides, airborne infrared search and tracking (IRST).


6. Long Wave Infrared (LWIR) 
Advantages: 
No need for refrigeration, low cost: eliminates the need for refrigeration device, simple equipment structure, small size, light weight, affordable price.
Extremely adaptable to the environment: Wide range of operating temperature (-40°C~+85°C), no pre-cooling time, ready to use right out of the box, vibration-resistant, suitable for field or harsh environments.
Low power consumption and long life: power consumption can be as low as 1W or less (e.g. cell phone integrated thermal imaging camera), detector life up to 100,000 hours, very low maintenance costs.
All-weather capability: not affected by day and night lighting, strong ability to penetrate smoke and dust (but weaker than MWIR), suitable for night monitoring or search and rescue.

Disadvantages: 
Low sensitivity: usually 30~50mK, lower than cooling type (<20mK), weak detail resolution, easy to overexpose high-temperature targets (need dynamic range adjustment).
Slow response speed: frame rate is usually ≤60Hz, not suitable for ultra-high-speed dynamic scenes (such as ballistic tracking).
Obvious interference by the environment: susceptible to strong sunlight reflection interference (such as water, glass reflection), rain or high humidity environment performance degradation.
Restricted performance at long distances: atmospheric absorption (water vapor, CO? absorption bands) leads to greater attenuation of long-wave transmission at long distances (>1km), and the observation effect is weaker than that of medium-wave cooling.

Applications: 
Civilian: building thermal inspection, electrical equipment inspection, smartphone thermal imaging plug-in, medical temperature measurement, surveillance cameras, drone night vision, firefighting search and rescue.
Military: man-portable night vision device, low-cost reconnaissance equipment.