Infrared light
2023-12-25
Definition:
Invisible light with wavelengths between 750nm and 1mm.
The wavelength of infrared light is greater than 700-800nm, which is the upper limit of the wavelength of visible light. The boundary between the two is not very clear, and the responsivity of the eye decreases very slowly in this area. Although the responsivity is already very low at 700nm, it can still be seen at a wavelength of 750nm when the laser diode light is strong enough. Although these lights are not bright, they are harmful to the eyes. The upper limit of the infrared spectrum is not clearly defined, but it is generally considered to be approximately 1mm.
The different infrared spectral regions are defined as:
Near-infrared region (also called IR-A), with wavelengths from approximately 700 nm to 1400 nm. Lasers operating in this wavelength region are very harmful to the eyes because near-infrared light can penetrate and focus on the retina just like visible light, but it does not cause the protective blink reflection. Eye protection is therefore required.
Mid-infrared light (MWIR) has wavelengths from 3 to 8 microns. The atmosphere has strong absorption of light in this band, with many absorption lines, such as carbon dioxide (CO2) and water vapor (H2O). Many gases have strong mid-infrared absorption lines, so light in this spectral region can be used in gas spectroscopy.
Longwave infrared light (LWIR) has wavelengths from 8 to 15 microns, followed by far infrared (FIR), which ranges in wavelength to 1 mm and is sometimes considered to start at 8 microns. This spectral region can be used for thermal imaging.
It is important to note that the definition of these areas changes somewhat in practice.
Many glasses are transparent to near-infrared light but absorb strongly at longer wavelengths, where photons can be converted directly into phonons. For silica glass, strong absorption occurs at wavelengths around 2 microns.
Infrared light is also called thermal radiation because the thermal radiation of hot bodies is usually in the infrared wavelength range. Even at or below room temperature, the hot body radiates strong mid-infrared and far-infrared light, which can be used for thermal imaging. For example, infrared imaging of a house heated in winter can reveal the presence of heat leaks within the house (e.g., in windows, roofs, etc.), thereby increasing the efficiency of direct detection.
Invisible light with wavelengths between 750nm and 1mm.
The wavelength of infrared light is greater than 700-800nm, which is the upper limit of the wavelength of visible light. The boundary between the two is not very clear, and the responsivity of the eye decreases very slowly in this area. Although the responsivity is already very low at 700nm, it can still be seen at a wavelength of 750nm when the laser diode light is strong enough. Although these lights are not bright, they are harmful to the eyes. The upper limit of the infrared spectrum is not clearly defined, but it is generally considered to be approximately 1mm.
The different infrared spectral regions are defined as:
Near-infrared region (also called IR-A), with wavelengths from approximately 700 nm to 1400 nm. Lasers operating in this wavelength region are very harmful to the eyes because near-infrared light can penetrate and focus on the retina just like visible light, but it does not cause the protective blink reflection. Eye protection is therefore required.
Mid-infrared light (MWIR) has wavelengths from 3 to 8 microns. The atmosphere has strong absorption of light in this band, with many absorption lines, such as carbon dioxide (CO2) and water vapor (H2O). Many gases have strong mid-infrared absorption lines, so light in this spectral region can be used in gas spectroscopy.
Longwave infrared light (LWIR) has wavelengths from 8 to 15 microns, followed by far infrared (FIR), which ranges in wavelength to 1 mm and is sometimes considered to start at 8 microns. This spectral region can be used for thermal imaging.
It is important to note that the definition of these areas changes somewhat in practice.
Many glasses are transparent to near-infrared light but absorb strongly at longer wavelengths, where photons can be converted directly into phonons. For silica glass, strong absorption occurs at wavelengths around 2 microns.
Infrared light is also called thermal radiation because the thermal radiation of hot bodies is usually in the infrared wavelength range. Even at or below room temperature, the hot body radiates strong mid-infrared and far-infrared light, which can be used for thermal imaging. For example, infrared imaging of a house heated in winter can reveal the presence of heat leaks within the house (e.g., in windows, roofs, etc.), thereby increasing the efficiency of direct detection.
