Infrared sensors are a technology that goes beyond simple lighting to provide safety and convenience. How are these sensors, which utilize superconducting materials, transforming our lives? In this article, we’ll explore the principles behind infrared sensors and their various applications.
After finishing my school classes as usual, I had dinner and returned to my empty apartment. Though the room was dark and cramped, a good friend was waiting there, always shining a bright smile to help me avoid stumbling in the darkness. This friend is known as an infrared sensor light—a product of materials engineering that helps illuminate our lives.
These infrared sensors, which we commonly see in bathrooms, hallways, and home entryways, operate by detecting the infrared radiation emitted by our bodies. Infrared light has a longer wavelength than visible light, and our bodies emit infrared radiation with a wavelength of approximately 9.4 micrometers. However, unlike visible light, it is invisible to the naked eye, so we are not usually aware of it. Nevertheless, this sensor detects the infrared radiation from an object when it enters the detection range at a speed between 30 cm/s and 2 m/s, provided the object’s temperature differs by at least 3 degrees from the ambient temperature. If nothing is detected after a certain period, the sensor stops operating, thereby preventing power waste. For this reason, during the summer, when there is little difference between our body temperature and the ambient temperature, the sensor may not detect movement effectively.
The advent of infrared sensors has brought about significant progress not only in terms of convenience but also in safety and efficiency. For example, automatic lighting systems in home entryways or bathrooms reduce unnecessary power waste and contribute to energy savings by providing light only when needed. Furthermore, this technology plays a crucial role in security systems. When an intruder is detected, warning lights automatically turn on or an alarm sounds, significantly enhancing the safety of residential spaces. In this way, infrared sensors are improving our lives in various ways in our daily routines.
So, what materials are these sensors made of that allow them to detect the infrared radiation emitted by our bodies? It is thanks to “pyroelectric materials”—a product of materials engineering mentioned earlier—that such sensors have become possible. The various molecules that make up a material possess diverse properties depending on their molecular structure or the atoms they contain. Among these, certain crystalline molecules exhibit changes on their surface when the crystal is subjected to temperature changes, due to alterations in the atomic motion caused by heat or shape changes resulting from thermal expansion. This results in a change in electric potential. Simply put, this refers to the property that allows electricity to be generated due to temperature changes; this is called piezoelectricity, and materials possessing this property are called “piezoelectric materials.”
To function effectively as a piezoelectric material, the molecules themselves must possess a high degree of polarization (the presence of polarity within the molecule) in their natural state. Such materials are called ferroelectrics and are widely used as materials for piezoelectric sensors in our daily lives. A representative example of a ferroelectric is ceramics. Most of these ceramic piezoelectric materials are based on PZT (a ceramic molecule with high piezoelectricity), and by adding various components, it is possible to improve durability or create materials with specific properties.
Piezoelectric sensors utilize these materials. A piezoelectric sensor measures the intensity of incident light by utilizing the piezoelectric effect, in which the magnitude of electrical polarity (polarization) changes when light strikes a material and the object’s temperature rises due to the light’s energy. Based on this principle, piezoelectric sensors are used as temperature sensors and infrared sensors. Temperature sensors can detect temperature either through direct contact or by measuring the infrared radiation emitted by an object. Infrared sensors, on the other hand, measure the light emitted by the object itself. This offers the advantage of enabling observation even at night. Furthermore, since infrared light can penetrate most types of smoke or haze, it is possible to capture an image even when the object is obscured. All of these applications were made possible by pyroelectric materials.
Thanks to these principles, sensors can now greet us at our front doors. Beyond this, they can be used in automatic door systems, motion sensors, intrusion alarms, fire detectors, and infrared photography. For example, in the case of gas detectors, a pyroelectric sensor that detects wavelengths around 4.3 μm—emitted by CO₂—is used to determine the concentration of CO₂ in the air. Another example is the vidicon. A vidicon is what we commonly refer to as an infrared camera; it uses pyroelectric materials to capture an image of an object and then acquire a thermal image. It then focuses the light emitted by the object into an image, much like a conventional camera, allowing us to see it with our eyes.
Furthermore, pyroelectric materials and sensor technology are also being utilized in the medical field. For example, non-contact thermometers use pyroelectric sensors to accurately measure body temperature from a distance, and these have served as crucial tools for contactless temperature screening, particularly during the pandemic. Additionally, infrared sensors are used in smart mattresses that reduce discomfort caused by temperature fluctuations at night, thereby contributing to improved sleep quality.
Thanks to the invention of these pyroelectric materials, scientists have been able to develop pyroelectric sensors, which have provided us with countless benefits in various aspects of our lives, often in ways we don’t even realize. As I finish writing this today and step into my empty home, the pyroelectric sensor—which has now become somewhat familiar to me—will greet me and brightly illuminate the floor beneath my feet.
