Everyone · Technology Frontier
Li Bo
Professor at the College of Physics and Electronic Science, East China Normal University, PhD Supervisor
When we go shopping, many stores have automatic doors that open to welcome customers; faucets in restrooms automatically dispense water when needed, creating a more hygienic environment; smoke detectors tirelessly monitor for potential dangers all year round, ensuring safety; when we choose our favorite products, scanning a code makes checkout more convenient and quick… all of this relies on the help of sensors. In fact, sensors have long been deeply integrated into our lives, with extremely important applications in military, agriculture, science, and more. If you look carefully, you can almost always find the shadow of sensors wherever you look.
Limitations of Human Senses
Sensors, as the name suggests, are devices that sense and transmit information. To better understand the world, we have evolved innate perceptual abilities, allowing us to see the sea, hear the wind, and smell fragrances… We acquire information through our five senses: sight, hearing, smell, taste, and touch. Without these senses, we would lose the bridge connecting us to the outside world, our brains would receive no information, and we would fall into endless darkness, unaware of our “existence”.
With advances in cognition, people have gradually realized that the five senses are not perfect. We cannot sense magnetic fields like migratory birds; we cannot hear ultrasonic waves beyond the limits of human hearing like bats… Let’s consider three questions regarding vision.
First question: Light, which we see, is a form of electromagnetic wave. Can we see all electromagnetic waves? Clearly, the answer is no, and many counterexamples can be given. We cannot see the electromagnetic waves of mobile signals that fill the surrounding space; we cannot see the microwaves emitted when the microwave oven heats food; we cannot see ultraviolet light, which can tan skin and kill bacteria; we cannot see infrared light used for temperature measurement, therapy, remote control, and night vision…
In fact, the range of electromagnetic waves visible to the human eye is very limited. We refer to the electromagnetic waves with wavelengths between 400 nm and 760 nm as visible light, which occupies a very narrow window in the entire electromagnetic spectrum. We can use the analogy of a person wearing colored glasses. If someone has worn green-tinted glasses since birth, they will only see wavelengths between 492 nm and 577 nm, perceiving everything as shades of green, from light green to dark green. If one day they remove the glasses, they will be astonished by the new, colorful world before them. So, is there a way to remove the “visible light” glasses?
Second question: Can we see all visible light? You might find it strange; isn’t visible light just light that can be seen? Consider whether you can distinguish the details of flower petals under the moonlight at night? Can you look directly at the sun to observe any sunspots? The results tell us that if the light source is too dim or too bright, we cannot see its true shape. Our eyes also have a certain range of response to light intensity.
Third question: If it is visible light with moderate intensity, can we see the true appearance of things? We cannot count how many feathers a hummingbird has while it is flying. This is because the light signals our eyes respond to take time to transmit to the brain, which is the time resolution ability of the human eye. We cannot see things that happen too quickly.
At this point, you may feel a bit disappointed, realizing that we can only see a small part of the real world. However, we need not be disheartened; this is the result of following the principles of energy optimization and maximum efficiency during human evolution. We naturally chose the most “rich” wavelengths in sunlight, which correspond to the colors of the rainbow. This allows us to obtain as much information as possible at the least cost under limited conditions. Therefore, this limited range of perceived wavelengths is actually the most advantageous choice for us.
Electromagnetic Wave Spectrum
One of the innate human traits is curiosity, driving us to understand and explore the unknown world. As times change, the information our five senses can perceive has been unable to keep pace with human exploration of the world. The development of technology has given us the possibility to enhance and expand our senses. Devices that sense different information have emerged one after another. We have not only enhanced our sensory abilities vertically, “seeing” more colors, “hearing” more sounds, “feeling” weaker information, and “detecting” finer differences… but we have also horizontally increased our senses to six, seven, or more, such as the ability to sense magnetic fields and the ability to perceive others’ thoughts…
These tools that help us see, hear, and feel the unknown world are called “sensors”, devices that can perceive and transmit information. Sensors are the keys to opening new doors, allowing us to perceive worlds that were previously unreachable.
Sensors Realizing Information Perception
The mission of sensors is to perceive and transmit information. When we want to obtain specific information, we need to find the corresponding sensor. For example, when we want to record the beautiful scenery in front of us, we need a photoelectric sensor that senses light signals; when we conduct health checks, we rely on temperature sensors and pressure sensors to measure temperature and weight.
So how do sensors realize information perception?
Energy can convert between forms such as light, sound, heat, and electromagnetic. A little observation reveals many such examples around us.
When we turn on the light switch, the bulb emits light, which is the conversion of electrical energy into light energy; sunlight hitting solar cells causes current to flow in the circuit, corresponding to the conversion of light energy into electrical energy; sunlight can dispel the cold because light energy converts into heat energy, bringing warmth; when we turn on the sound system to listen to wonderful music, electrical energy converts into the mechanical energy of the speaker’s diaphragm, producing sound; when we pick up a microphone and sing loudly, the energy carried by sound converts into electrical energy; electric kettles boiling water and rice cookers cooking rice convert electrical energy into heat energy, while thermal power plants generate electricity through heat energy.
The conversion of energy between different forms has certain physical mechanisms corresponding to them. This ensures that no matter what information we want to observe, we can almost always find a mechanism to directly or indirectly convert the observed signal into an electrical signal, thus realizing the perception function of the sensor.
According to incomplete statistics, there are approximately over 30,000 types of sensors with different functions and uses.
We can classify them according to their functions, such as visible light sensors, infrared sensors, and ultraviolet sensors for “seeing”; dynamic microphones, condenser microphones, ultrasonic sensors, and infrasonic sensors for “hearing”; gas sensors for “smelling”; molecular sensors for “tasting”; and pressure sensors, temperature sensors, and humidity sensors for “touching”.
They can also be classified according to the size of the sensor: from molecular sensors at the molecular level to numerous micro-sensors in mobile phones; from photoelectric sensor arrays in cameras to photoelectric focal planes on space telescopes. There are also the latest wearable flexible sensors, multifunctional composite sensors, and intelligent sensors that can self-diagnose, self-compensate, and self-adapt.
Sensors in Mobile Phones
A wide variety of sensors have long been integrated into our daily lives and work. Let’s take the mobile phone, which we cannot live without, as an example to see what sensors it has. When you pick up your phone to unlock the screen, the fingerprint sensor reads your fingerprint, or the camera takes a picture of your face, then transmits the relevant information to the central processor for comparison to achieve fingerprint or facial recognition unlocking. You might wonder, since facial recognition reads feature information through a captured photo, can’t a photo of similar size also unlock the phone? And how does facial recognition work to unlock in dark environments or when a photo cannot be taken?
To fill these gaps, phones with facial recognition also come equipped with modules such as light sensors, proximity sensors, infrared sensors, infrared illumination, and dot projection. The light sensor senses the intensity of ambient light and informs the central processor. If the processor detects that the light is too dim, it will automatically activate infrared illumination to provide additional light. The dot projector and proximity sensor work together to measure the distance from different positions on the face to the camera, combined with the image formed by the infrared sensor, ultimately obtaining three-dimensional facial information. Compared to two-dimensional recognition, three-dimensional facial recognition greatly enhances security, eliminating the risk of unlocking with a photo. Infrared illumination and photography help us easily unlock the screen in dark environments.
As we can see, just the operation of unlocking the screen involves five types of sensors: fingerprint sensor, image sensor (camera), light sensor, proximity sensor, and infrared sensor. Of course, these sensors do not only serve the function of unlocking the screen.
For example, the central processor adjusts the screen brightness based on the light intensity measured by the light sensor, allowing the screen brightness to automatically adjust according to the ambient light conditions, ensuring we can clearly see the screen display in different lighting environments. There are also sensors in the phone to measure rotation angles, accelerometers to sense motion posture, pressure sensors to reflect altitude, and sensors to receive satellite signals, which together give the phone navigation capabilities. Many other types of sensors exist in mobile phones; each time a new sensor is added, the phone’s functionality expands, making it smarter and more intelligent! Today’s smartphones have become intelligent terminals, and making phone calls is no longer their primary function.
The Near-Infrared Camera Sensor on the James Webb Space Telescope (0.6 microns to 5 microns), with a central part made of mercury cadmium telluride thin film
We also encounter sensors in our daily lives that have olfactory functions like a nose. For example, smoke detectors used to prevent fires, and gas sensors in air purifiers specifically for detecting formaldehyde odors. Furthermore, to ensure safe travel, the breathalyzer used by traffic police is a sensor specifically for detecting alcohol; it can “smell” whether there is an alcohol odor in the driver’s exhaled breath.
Taking photos is a basic skill in today’s society. When you press the shutter, whether it’s a landscape or a portrait, it will be “seen” and recorded by the Charge-Coupled Device (CCD) sensor behind the lens. In addition to the cameras used for photography, surveillance cameras also rely on image sensors like CCDs to “see” the world. Among the various sensors used to “see” the world, infrared sensors play a central role. From small infrared sensors in automatic doors and electronic thermometers to large infrared focal planes on space telescopes, they are all centered around infrared sensors.
The most technologically advanced is the James Webb Space Telescope, launched on December 25, 2021. It has two infrared sensors, one for detecting the 0.6 to 5 micron range and the other for 5 to 28 microns. With the support of these two infrared eyes, the Webb Space Telescope can study the birth and growth of the universe and explore the atmospheric conditions of exoplanets, searching for Earth-like planets suitable for human habitation and seeking information about other life in the universe.
Air conditioners, refrigerators, rice cookers, electric kettles, and ovens can automatically control temperature. They all use temperature sensors that mimic human skin to perceive hot and cold.
Health is currently one of the most concerning topics for people. Almost every item in health check-ups involves sensors. Electronic scales, ECGs, ultrasounds, X-rays, bone density tests, and blood tests all involve corresponding sensors. Now, some sensors have been integrated into wearable devices, allowing for real-time monitoring of health conditions. The most representative wearable device is undoubtedly the smart watch, which not only tells time but also has functions for measuring heart rate, ECG, blood oxygen levels, and even blood pressure. These functions involve pressure sensors, ECG sensors, and photoelectric sensors. Perhaps in the near future, we can implant micro sensor chips into our bodies to monitor our health status around the clock, thus providing the best protection for ourselves.
Mobile signal, wireless networks, and Bluetooth transmission all belong to electromagnetic signals, so corresponding mobile terminals, wireless network cards, and Bluetooth receivers need to have the capability to sense electromagnetic signals. The information perceived by the human five senses is transmitted to the brain in the form of electrical signals, and the brain’s processing of information is accompanied by brain electrical activity. Therefore, whether in medical auxiliary examinations or in the exploration of the human brain, electromagnetic sensors can be used to capture the brain waves generated during brain activity.Compared to conventional sensors, flexible sensors are closer to human tissues and can better adapt to our bodies. They hold enormous application potential in fields such as medicine and bionics. Flexible sensors are an inevitable trend in the future development of wearable devices, motion assistance, health management, and artificial electronic skin. Implanting flexible sensors in the body can better detect health conditions. We can even place flexible sensors next to the heart and brain to give them special care. Flexible sensors are also expected to replace our sensory organs that age over time. Artificial electronic skin based on flexible sensors is key to whether bionic robots can be “indistinguishable” from real beings.
Sensors in the Intelligent Era
Sensors are the cornerstone of the intelligent era, and intelligent systems are the core of the intelligent age.Dynamic perception, intelligent recognition, and automatic response are the three key elements to realize intelligent systems, just like our five senses, brain, and limbs.
For example, the emerging autonomous driving technology is a highly regarded intelligent system. The car’s radar dynamically perceives the surrounding conditions, and the car’s central processor intelligently recognizes lanes, pedestrians, vehicles, buildings, and road signs from the dynamically perceived information, as well as their positions and trajectories. Then, using “intelligence” to predict the future positions of these entities. At the same time, combining speed and direction data provided by sensors, it automatically adjusts the throttle and steering to maintain the lane, thus not deviating from the course. If a danger is predicted, measures will be taken immediately to automatically activate the hydraulic control system to brake. The realization of such intelligent systems requires highly sensitive sensors to accurately perceive the environment.
It can be seen that sensors are becoming increasingly important in the course of human technological development. The intelligent era is approaching us; the future has arrived, and it is already here! In a narrow sense, when we concentrate these sensors and processors into independent entities, we can create machines like robots and smart cars that can “think”. Further, in a broad sense, when we equip all objects with corresponding sensors and achieve interconnectivity, we will usher in the intelligent era, constructing an intelligent ecosystem and creating a wise Earth.
-This article is selected from the “World Science” magazine, 2024 Issue 1 “Everyone · Technology Frontier” column; it is based on the author’s report at the “Shanghai Science Popularization Forum” organized by the Shanghai Science and Technology Popularization Volunteer Association-
This article is reprinted from the “World Science” WeChat public account
