Far infrared rays in the infrared rays
Invisible light with the power to warm things was named “infrared rays”, because they exist outside of red in visible light. Infrared rays are electromagnetic waves, as well as X-rays, UV, visible light, microwaves and radio waves. Electromagnetic waves have characteristics depending on their wavelength. Infrared rays are divided into two (2) wavelength regions, “near infrared rays” and ”far infrared rays”.
Many materials in our personal belongings except metal (plastics, paints, textiles, wood, rubber, food etc.) absorb the electromagnetic wave from 2.5μm to 30μm in wavelength (mainly in the far infrared region) well. The far infrared rays with above wavelength are mainly generated from heated ceramics. That is the reason why far infrared ceramic heater is widely used as a heat source of heating and drying in industrial and commercial fields. The oxygen and nitrogen in air do not absorb far infrared rays, however, carbon dioxide (CO2) and water vapor (H2O) absorb far infrared rays well.
What are far infrared radiations to?
All materials more than absolute zero (-273ºC) radiate energy of the far infrared region, the higher the temperature is the higher the radiation dose (energy) is. It is known that the amount of radiation in a certain temperature is affected by the material and by the surface condition. Ceramics are the kind of materials that radiate a lot of far infrared rays. Radiation of far infrared rays is low in metal, however metal reflects it well. Therefore metals with high reflectance are usually used as reflectors.
How do far infrared rays warm materials?
Far infrared rays emitted from ceramic heaters go straight on in space and shoots the material surface with the speed (about 300,000 km/second = the speed to make 7.5 laps around the earth in 1 second) same as the speed of light. Frequencies (velocity of light ÷ wavelength) of far infrared rays match the vibrations of molecules forming water, woods, plastics, fibers, paints, foods and animals including human being. These materials absorb irradiated far infrared rays and a temperature rise occurs because of molecule vibration. This effect becomes remarkable when natural frequencies of material and frequencies of irradiated rays are harmonized. It is the reason that far infrared rays are widely used in the field of heating and drying. As the natural frequencies of material do not match the irradiated electromagnetic waves, the effect to warm is less.
Will FIR infiltrate the body of person deeply? Will FIR penetrate glass?
The energy of far infrared rays is almost absorbed in about 200μm depth by the skin surface and changes to heat. The heat transfers by blood efficiently inside the body and warms the body. The near infrared rays seep from the skin surface to depth of several millimeters. Using this characteristic, methods to certify a person by examining the vein pattern of a finger or a palm using near infrared rays, are recently introduced in several banks. Water and alcohol absorb far infrared rays well.
Far infrared rays are almost absorbed if the water layer thickness is more than 1mm. Then the rays do not penetrate the water layer. Metal, which has a glittering surface (not oxidized) reflects far infrared rays well. The reason why a metal plate is installed back of the heater in the far infrared heating equipment is that it centralizes far infrared rays to frontside as much as possible.
Figure: Infiltration characteristics of human skin in infrared region. (N.Terada et al,”Spectral radiative proper of a living human body”, International Journal of Thermophys., vol.7, pp.1101-1113, 1986)
Figure: Infrared absorption characteristics of water (渡辺敦夫,清水賢,”食品工業における電磁波の知用(1),” 化学技術誌MOL, pp.120-128, 昭和63年2月. (Japanese)
What is emissivity?
It is important to understand the physical meaning of ”emissivity” in order to understand far infrared rays. Emissivity is defined as the ratio of energy radiated from the material surface in certain temperature and a black body (the virtual object which completely absorbs radiated energy) in the same temperature. Emissivity depends on the material and its surface conditions (for example roughness). In addition it depends also of the wavelength. The ceramics including metal oxide are widely used as a radiation material because they can effectively transfer the given energy radiation because they have high emissivity (about 0.7-0.9) in infrared region. Emissivity of the metal surface that is not oxidized is generally very low (about 0.05 for abrasion aluminum).
Figure: Spectral radiation characteristics of quartz.
Figure: Spectral emissivity of the radiator material (2 examples)
Three (3) methods of Heat Transfer
Heat transfers from high temperature to low. This is a basic principle. There are three (3) methods (conduction, convection and radiation) how the heat can transfer. Heat transfer is usually performed by combination of these three methods.
Heat transfer by conduction : Heat gradually comes when one end of the iron bar is heated and becomes hot to the end. It is called conduction heat transfer that heat is transmitted through the material in this way. Thermal conductivities are different by a material. Metal is a good conductor of the heat. Gas is generally a low heat conduction body. Therefore, heat conduction becomes lower in the material with many apertures.
Heat transfer by convection: When liquid and gas, such as water and air, are heated from the bottom, the warmed part rises because its density lightens by expanding. On the other hand, the cold upper part drops. These actions are performed repeatedly, and total temperature rises. This kind of heat transfering method by moving liquid and gas is called convection.
Heat transfer by radiation: Heat transfer methods that do not need a medium, are called radiation heat transfer, as solar heat directly arrives at the earth and warms the ground. The heat is directly absorbed to a material in the form of electromagnetic waves and the temperature of the material raises (radiation activates the vibration of material atoms). Heat transfer by far infrared rays is based on the radiation heat transfering itself.
If the medium is gas, in case of nitrogen (N2) and oxygen (O2), the far infrared rays are not absorbed. However, radiation is absorbed in case of gas with the polarity such as carbon dioxide (CO2) and water vapor (H2O).
Three (3) physical laws related to the radiation
Planck’s law: The material emits the energy according to the temperature in the form of the electromagnetic waves. The relationship between the radiated power and the wavelength depends on the temperature of the material. These relationships are called Planck’s law. The emissivity depends on the material and its surface condition. The emissivity of a material is less than 1. The emissivity curve of a material is shifted from one of a black body. A material with the same temperature as a black body has a spectrum curve of radiated energy located below the curve of a black body.
Figure: Radiant energy of the blackbody in each temperature (Planck’s law)
Stefan Boltzmann law: As the temperature of the material becomes higher, the amount of radiated energy from a material becomes larger. The amount of energy (E) emitted by a black body of kelvin (K) is in proportion to the biquadrates of the absolute temperature (T). It is shown as follows：Ｅ＝5.6697×10－８・Ｔ４ [W/m2]. This is known as Stefan Boltzmann law.
Wien’s displacement law: As the absolute temperature of the radiator rises, the peak wavelength (most high energy point) shifts to a short wavelength. The peak wavelength (λ) of a black body in absolute temperature T (K) is given as follows: λ＝2897／Ｔ[μｍ]. This is called Wien’s displacement law. For example, the peak wavelength (λ) of a person with the temperature of 36 degrees Celsius (absolute temperature T = 36 + 273 = 309K) becomes 2,897/309=9.4μm. The person emits the far infrared rays with about 9.4μm as a peak. About peak wavelength shown in Wien’s displacement law, what you should pay attention to, is as follows: An integrated area of the short wavelength side holds 25% of all energy, the long wavelength side holds 75%. The long wavelength side (the far infrared side) emits energy of 3 times of the short wavelength side. Then, the wavelength (λ) to divide the radiated energy of the black body of absolute temperature T (K) into two is given as follows: λ=4,108/ T [μm]. For example, at wavelength 3μm of the border of near infrared and the far infrared regions, the black body temperature T divided into by 50% of radiated energy becomes =1,096 degrees Celsius T =4,108/3=1,369 (K) (=1,369-273). It was shown that a lot of far infrared radiated energy is considerably included in the object of the high temperature. The peak wavelength is of course in near infrared region at 2,898/1,369=2.1μm.
Features of the far infrared heating (Radiation heat transfer)
In far infrared heating, heat flows from a heater to receiving material in proportion to the difference of the biquadrates value of each temperature (absolute temperature (K)). As the temperature of the heater is able to maintain considerably higher than the receiving material, the heat flux is maintained during a heating period without largely changing. As heat flows into the receiving material steadily, efficient heating is possible.
On the other hand, in case of hot wind heating, heat flux flows in proportion to a difference of hot wind temperature and the surface temperature of the material to be heated. In this case, the surface temperature of receiving material nears hot wind temperature immediately. The difference in temperatures disappears and the heat flux falls.
Figure: Differences of heat flux in far infrared heating and hot wind heating
Features of the far infrared processed fiber
Fiber is material that the absorption and the characteristics of the re-radiation of the far infrared rays are high in personal materials. ”The far infrared processed fiber” is the fiber which improved heat retention by raising absorption and re-radiation characteristics in wider far infrared regions than original through kneading the materials such as the ceramics into the chemical fiber, or through coating them on the outside of the natural fiber. In comparison of the far infrared processed fiber and the non-processed fiber with same quality, absorption and the re-radiation characteristics of the far infrared processed fiber improves obviously, therefore, the heat retention of the far infrared processed fiber becomes higher than that of the non-processed fiber.
Figure: Difference of skin temperatures between non-infrared processing