Infrared (IR) radiation is a form of energy that exists just beyond the red end of the visible light spectrum. While invisible to the human eye, this energy is a fundamental tool across science and technology for gathering information about our world. Everything in the universe that has a temperature above absolute zero emits or interacts with this radiation. By detecting and analyzing this emitted or absorbed energy, instruments can measure properties ranging from the temperature of a distant star to the chemical makeup of a pharmaceutical drug. Infrared is primarily used in two distinct ways to measure and quantify data about an object or substance.
The Nature of Infrared Radiation
Infrared radiation is a component of the electromagnetic spectrum, positioned between visible light and microwaves. Its wavelengths are longer than those of red light, generally spanning from about 700 nanometers to 1 millimeter. This range is subdivided into categories based on wavelength, which determines how the radiation interacts with matter and what it can measure.
Near-infrared (NIR) has the shortest wavelengths and is typically used in fiber optics and material analysis. Mid-infrared (MIR) is strongly absorbed by molecular bonds, making it the primary range for chemical identification. Far-infrared (FIR) has the longest wavelengths and is closely associated with heat, making it extensively used in thermal imaging.
Measuring Temperature via Infrared Emission
The most common application of infrared technology is the non-contact measurement of temperature, which relies on the principle of thermal emission. Any object with a temperature above \(-273.15\) degrees Celsius (absolute zero) radiates energy. The intensity of this emitted energy is directly related to the object’s temperature, a relationship described by physical laws like Planck’s Law.
Instruments like thermal cameras and non-contact thermometers measure the intensity of the infrared radiation emitted from a surface. Hotter objects emit more intense radiation, and their peak emission shifts to shorter wavelengths. The sensor converts the detected radiant power into an electrical signal, which is then processed using physics equations to calculate the object’s surface temperature.
A challenge in accurate temperature measurement is emissivity, which is a measure of a material’s efficiency in emitting thermal energy. A perfect emitter, known as a blackbody, has an emissivity of \(1.0\), but real-world surfaces have values between \(0\) and \(1\). For example, polished metal has much lower emissivity than human skin or black paint. Therefore, infrared thermometers must have their emissivity setting adjusted or use advanced techniques to compensate for this factor to ensure an accurate reading.
Analyzing Materials via Infrared Absorption
Infrared technology is used to determine the chemical identity and structure of a substance through absorption spectroscopy. This method relies on the interaction between mid-infrared light and the molecular bonds within a sample. All molecules are constantly vibrating, exhibiting motions such as stretching and bending.
When a molecule is exposed to infrared light, it selectively absorbs energy at wavelengths that match its natural vibrational frequencies. This absorption causes the molecular bonds to vibrate more intensely. By shining infrared light through a sample and measuring which wavelengths are absorbed, a unique spectral pattern is created.
This pattern of absorption peaks and valleys serves as a distinct “molecular fingerprint.” No two different molecules have the exact same infrared spectrum, allowing for definitive identification of chemical composition and purity. Fourier-Transform Infrared Spectroscopy (FTIR) is the primary instrument for this measurement, used to analyze samples in solid, liquid, or gas phases. The resulting data can identify the presence of specific functional groups, such as the carbonyl bond in a ketone, providing detailed insight into the material’s structure.
Infrared Measurement in Health and Environmental Monitoring
Data collected through IR emission and absorption has broad application in specialized fields, moving beyond simple temperature checks.
Health and Medical Diagnostics
- Advanced IR spectroscopy is used for detailed tissue analysis and the detection of specific molecular signatures in bodily fluids, such as blood serum.
- Thermal imaging captures variations in skin temperature that may indicate underlying circulatory issues or inflammatory responses, aiding in diagnosis.
Environmental and Agricultural Monitoring
- Infrared remote sensing tracks atmospheric gases like carbon dioxide and methane.
- Spectroscopic techniques assess water quality by detecting chemical contaminants, such as nitrates, and biological contaminants, including E. coli bacteria.
- Agricultural and forestry professionals use infrared sensors to measure vegetation health, identifying early signs of water stress or disease in crops and trees.
This ability to gather rapid, non-invasive data makes infrared a powerful tool for ecological management.