Infrared radiation consists of electromagnetic waves in the wavelength region from 0.75 µm to 1000 µm, lying between visible light and microwave light.
The wavelength region of 0.75 µm to 3 µm is often called the near-infrared, the wavelength region of 3 µm to 6 µm the middle infrared, and the wavelength region of 6 µm to 15 µm the far infrared. Also, even longer wavelength regions are sometimes referred to as ultra-far infrared, but this is not a universally accepted term.
Infrared radiation has the following characteristics:
- Invisible to human eyes – This is useful for security applications but sometimes makes measurements and optical system design difficult.
- Small energy – Infrared radiation equals molecules’ vibrational or rotational energy. This phenomenon makes it possible to identify molecules.
- Long wavelength – This means infrared radiation is less scattered and offers better transmission through various mediums.
- Emitted from all kinds of objects
Infrared radiation is emitted by all objects with an absolute temperature greater than 0 K. Infrared radiant energy depends on an object’s temperature and surface quality. Assume there is a substance that completely absorbs all radiant energy and has a wavelength appearance of total blackness. This thing is referred to as a “blackbody.” Numerous uses exist for infrared radiation, and more are continuously being created.
1. Optical power meters
Lasers and optical fiber communications are just two examples of the many uses for optical power meters, which measure the intensity of the light. Systems for short-, middle-, and long-distance communications over optical fibers are distinguished. Infrared beams with wavelengths between 1.3 and 1.5 m that have lower optical fiber transmission loss are used in long-distance optical communications systems. For optical power meters to measure optical fiber transmission loss, relay quality, laser power, etc., InGaAs PIN photodiodes are employed. Linearity and uniformity are the qualities that the optical power meter specifically needs. It is occasionally necessary to use a cooled-type detector to lower the noise level to measure low-power light with minimal noise.
2. LD monitors
A laser diode’s (LD) output power and emission wavelength are temperature-dependent. An automatic power control (APC) is carried out to stabilize the LD. The APC has two methods: one tracks the total amount of light pulses coming from the LD, and the other tracks their peak value. High linearity has become crucial for monitoring light pulses with the development of high-power LD. Additionally, a quicker reaction is needed to track the peak value of light pulses. InGaAs PIN photodiodes are mounted using an optical system either in the same package as the LD or outside the package. Also, longer wavelength lasers use both InAs and InSb detectors.
3. Radiation thermometers
Depending on their temperatures above absolute zero, objects emit infrared radiation. Temperature is not the only factor that affects how many infrared rays an object emits. It is necessary to account for the emissivity e. e = 1 in the case of a black body. Care should be taken when measuring an object’s absolute temperature because the emissivity e is dependent on the object’s temperature and wavelength.
4. Flame monitors (flame detection)
The flame monitor is used to track the burning of the flames and detect light coming from them. Flames emit a wide spectrum of light, ranging from ultraviolet to infrared. A PbS photoconductive detector is used to detect infrared light, a two-color detector (K1713-01) is used to detect a wide spectrum from the UV to the infrared, and PbSe and pyroelectric detectors are used to detect 4.3 m wavelengths.
5. Moisture analyzers
The wavelengths (1.1 m, 1.4 m, 1.9 m, and 2.7 m) that are absorbed by moisture in the infrared region are used by the water content or moisture analyzer. Light beams at these wavelengths and a reference wavelength are directed at an object like a vegetable or a piece of coal. To measure the moisture, the moisture analyzer calculates the ratio to the reference wavelength light using the light reflected or transmitted from the object. PbS photoconductive detectors and InGaAs PIN photodiodes are suitable for this application’s sensors.
6. Gas analyzers
Gas analyzers use the infrared absorption of gases to calculate the density of the gas. The dispersive method and the non-dispersive method are two different approaches. To measure the components and quantity of a sample, the dispersive method divides the infrared light emitted from a light source into spectra using spectroscopy. Although it is more frequently used than the dispersive method, the non-dispersive method only measures absorption characteristics. Non-dispersive infrared gas analyzers, for instance, are used to measure fuel leaks (CH4, C3H2), CO, SO, and NO2 emissions controls, intake gas (CO2), and vehicle exhaust gases (CO, HC, and CO2). Carbonated beverages’ CO2 (4.3 m) and saccharine (3.9 m) content are measured using ingredient analyzers (soft drinks, beer, etc.).
7. Infrared imaging devices
Infrared imaging device development is divided into first, second, and third generations as it proceeds. Since there is only one detector element in the first generation, an image must be produced by rotating the optical system around the X and Z axes. In the second generation, a linear array (1D array) is utilized, necessitating only a Z-axis rotation for the optical system. The third generation eliminates the need to scan an infrared image using the optical system using an area array (2D array). The optical system of the infrared imaging device can be made simpler by using an element array, which will result in a smaller, lighter device. The fact that each element has a unique characteristic might be an issue. The infrared imaging tool is employed in several scientific, medical, and industrial applications.
8. Remote sensing
Light is emitted or reflected by objects, and depending on the wavelength, that light contains various information. Learning specific details about the objects is possible by measuring each wavelength. We can gather more precise data using infrared remote sensing, such as the surface temperatures of solids and liquids, the kinds and temperatures of gases, etc. We can now obtain macroscopic information such as the temperatures of land and seawater, the gas concentration in the atmosphere, etc., thanks in part to the growing use of remote sensing by space satellites and aircraft in recent years. Such data has been used for resource discovery, environmental pollution monitoring, and meteorological observation.
9. Sorting devices
It is possible to separate crops (like potatoes, tomatoes, onions, and garlic) from clumps and stones using the organic matter’s natural absorption wavelength. For this sorting, PbS photoconductive detectors and InGaAs PIN photodiodes are employed. Additionally, those detectors are employed to sort goods on a factory belt conveyor by spotting variations in temperature, emissivity, and transmittance.
10. Human body detection
This device emits a warning signal when it detects the temperature of a moving human body. The detector uses a mirror or a Fresnel lens to collect the intruder’s infrared radiation. A Fresnel lens array or multiple-surface mirror covers a wide range. If the movement’s speed is converted to frequency, tiptoeing has a frequency of about 0.3 Hz, and a dash has a frequency of about 10 Hz.
The temperature emitted from the human body can vary from 20 to 30 °C depending on clothing and the season. It is equivalent to blackbody radiation with a 9.5 m peak. To take advantage of a window in the atmosphere, choose the wavelength region appropriate for the human body temperature, and prevent effects from external disturbance light like sunlight, optical filters that transmit wavelengths from 7 to 15 m are used as the window material.