Heat is always infrared

Infrared radiation

We cannot see infrared radiation, but we can feel it as warmth. Their applications range from space heating and thermal imaging cameras to infrared astronomy.

Infrared radiation is electromagnetic radiation that follows the visible radiation in the direction of greater wavelengths and extends to the microwaves. It covers a wavelength range from 780 nanometers to 1 millimeter.

The spectral range of infrared radiation (IR) is not clearly subdivided; the different classifications are motivated by specific applications of the radiation or certain physical phenomena. A very common classification distinguishes, for example, between the “near infrared” (NIR) with wavelengths from 780 nanometers to three micrometers, the “mid infrared” (MIR) from three micrometers to 50 micrometers and the “far infrared” (FIR) from 50 Micrometer to 1 millimeter. The long-wave part of the MIR and the FIR are also known today as the terahertz (THz) range, which extends over the wavelength range from 30 micrometers to 1 millimeter and thus describes electromagnetic radiation with oscillation frequencies of 0.3 THz to 10 THz. Another classification, according to DIN 5030 Part 2, divides the NIR into an IR-A from 780 nanometers to 1.4 micrometers and an IR-B from 1.4 micrometers to 3.0 micrometers. The limit at 1.4 micrometers is due to the fact that above this value the absorption of infrared radiation by water increases significantly. In this classification, the MIR and the FIR are combined under the designation IR-C.

Infrared radiation is not visible to the human eye. However, humans can perceive the warming effect of infrared radiation through their skin.

Discovery and detection methods

Herschel's experiment

Infrared radiation was discovered by the German-English astronomer and musician Friedrich Wilhelm (William) Herschel in 1800. Like Isaac Newton in 1666, he used a glass prism for the spectral decomposition of light. In order to measure the energy distribution of solar radiation, Herschel positioned several blackened mercury thermometers behind the prism in the light broken down into its spectral colors. To his surprise, he found that the greatest increase in temperature was not in the visible spectral range, but beyond the red light. He had discovered the infrared radiation.

The progress of infrared spectroscopy was shaped by the development of increasingly sensitive infrared radiation receivers and new methods for the spectral decomposition of infrared radiation (grating spectrometer, Fourier transform spectrometer). Initially, thermal radiation receivers were used, in which the detection of infrared radiation is based on a warming of the receiver and an associated change in an electrical signal. Thermal receivers, such as thermopiles, bolometers and pyroelectric detectors, are still widely used today to detect infrared radiation.

A more modern group of infrared receivers are the photoelectric detectors, which are detected by the absorption of infrared radiation in a semiconductor material (via the internal photoelectric effect) and the associated change in photoconductivity or photo voltage. In particular, cooled semiconductor receivers achieve a very high sensitivity and accuracy and are used to detect extremely weak infrared radiation, for example in astronomy.

Thermal radiation from a black body

First quantitative measurements of infrared radiation

As early as 1900, the Berlin physicists Heinrich Rubens, Ferdinand Kurlbaum and Friedrich Lummer at the Physikalisch-Technische Reichsanstalt (PTR), the forerunner of today's Physikalisch-Technische Bundesanstalt (PTB), used the residual radiation method developed by Lummer and bolometers as radiation receiver standards to use infrared radiation measured absolute measurements of black bodies up to wavelengths of around 50 micrometers. These precise measurements of the radiation emission from a black body inspired the theoretical physicist Max Planck, who also works in Berlin, to formulate the law named after him for describing the radiation emission from black bodies (Planck's radiation law) and thus ushered in the birth of quantum physics.

Planck's law of radiation describes the emission of thermal radiation emitted by every body at a temperature above absolute zero (0 Kelvin or minus 273.15 degrees Celsius). The thermal radiation emitted by a body is often equated with infrared radiation, but this is not entirely correct, as the spectrum of thermal radiation extends over the entire electromagnetic spectral range. In general, however, infrared radiation forms a very significant proportion of the thermal radiation of a body, for example around 50 percent of the solar radiation hitting the earth's surface. The wavelength at which a black body has the maximum of its thermal radiation emission is described by Wien's law of displacement. At room temperature, this wavelength is around 10 micrometers and therefore in the infrared spectral range.

Diverse applications

Infrared radiation sources for heat generation are widespread and are used, for example, in the private sector for space heating and in the industrial sector in thermal process technology - for example for material drying and paint curing.

The fact that a considerable part of the heat radiation emitted by a body lies in the infrared spectral range is used for non-contact temperature measurement with the aid of infrared radiation thermometers. The rapid fever measurement with infrared ear thermometers is a special application of this method. Infrared radiation thermometers are widely used in the industrial sector for thermal process monitoring and control.

If the infrared radiation emanating from objects is converted into a "thermal image", one speaks of thermography. Thermography devices are increasingly used in diagnosis and maintenance, e.g. in the identification of heat losses in buildings or the detection of a defective component in an electrical circuit.

The Eagle Nebula in the visible light and infrared range

Of great importance for the application of infrared radiation is the fact that infrared radiation can set the molecules of many compounds in oscillation and rotation. As a result, these substances show a characteristic absorption and emission spectrum in this spectral range, which can be used for chemical analysis and concentration determination.

Infrared spectroscopy to identify chemical compounds is also used in astronomy. Earth-based infrared astronomy is limited to individual wavelength ranges because of the absorption of infrared radiation by the earth's atmosphere. Infrared astronomy takes advantage of the fact that infrared radiation is much less attenuated by interstellar dust than visible radiation. It can also be used to observe "cold" objects in space (with temperatures of a few 100 Kelvin) that do not "glow" in the visible spectral range.

In electronics and data transmission, NIR radiation is used for light barriers, optical interfaces and remote controls, among other things. Wavelengths of 1.55 micrometers are used for optical data transmission with glass fibers.