When we go shopping, many store doors automatically open to welcome customers; faucets in restrooms automatically dispense water when needed, providing a more hygienic environment; smoke detectors tirelessly check for hazards all year round to ensure safety; and when we choose our favorite products, scanning to pay makes the checkout process more convenient and quick… All of these rely on the help of sensors. In fact, sensors have long been deeply integrated into our lives, playing extremely important roles in military, agriculture, science, and many other fields. If you look carefully, you can almost find the shadow of sensors everywhere you look.

The Limitations of Human Senses

A sensor, as the name suggests, is a device that senses and transmits information. To better understand the world, we have evolved innate perceptual abilities, allowing us to see the sea, hear the wind, and taste aromas… We acquire information through the five senses: sight, hearing, smell, taste, and touch. Without these senses, we would lose our bridge to the outside world, our brains would receive no information, and we would fall into endless darkness, unaware of our own existence.

As our understanding has progressed, people have gradually discovered that the five senses are not perfect. We cannot sense magnetic fields like migratory birds; we cannot hear ultrasonic waves that are beyond the limits of human hearing like bats… Take vision as an example; here we raise three questions.

The first question: The light we see is a form of electromagnetic wave. Can we see all electromagnetic waves? Obviously not, as there are numerous counterexamples. We cannot see the electromagnetic waves of mobile signals that fill the surrounding space; we cannot see the microwaves emitted by microwaves when heating food; we cannot see ultraviolet light that can tan skin and kill bacteria; and we cannot see infrared light used for temperature measurement, therapy, remote control, and night vision…

In fact, the range of electromagnetic waves that the human eye can see 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 spectrum of electromagnetic waves. We can make a comparison to 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 green, from light green to dark green. If one day they take off their glasses, they will surely be amazed by the new, colorful world before them. Is there a way for us to take off the ‘visible light’ glasses?

The second question: Can we see all visible light? You might find this strange; isn’t visible light just light that can be seen? Consider whether you can distinguish the details of petals under the moonlight at night? Can you look directly at the sun to observe possible 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 response range to light intensity.

The third question: If it is visible light and the intensity is moderate, does that mean we can definitely see the true appearance of things? We cannot count how many feathers are on a hummingbird’s wings while it is flying. This is because the light signals that our eyes respond to take some time to transmit to the brain, which is the time resolution capability of the human eye. If the scene is too fast, we cannot see it clearly.

Having read this, you might feel a bit disappointed; it turns out we can only see a small part of the real world. In fact, we need not be discouraged; this is the result of human evolution following the principles of energy optimization and efficiency. We naturally chose the richest band of wavelengths in sunlight, which corresponds to the colors of the rainbow. This way, we can obtain as much information as possible at the least cost under limited conditions. Therefore, this limited perception of wavelength is actually the most advantageous choice for us.

Sensors in Daily Life: Understanding Their Importance

Electromagnetic Spectrum

One of humanity’s innate curiosities drives us to understand and explore the unknown world. With the development of the times, the information that our five senses can perceive has been unable to keep pace with our continuous exploration of the world. Technological advancements have given us the possibility to enhance and expand our five senses. Devices that sense different information have emerged one after another, enhancing our sensory capabilities vertically; we have “seen” more and more colors, “heard” more sounds, “felt” weaker signals, and “noticed” finer differences… At the same time, we have also horizontally expanded our senses to six, seven, or more, such as our ability to sense magnetic fields or discern others’ thoughts…

These tools that help us see, hear, and feel the unknown world are called sensors, devices that can sense and transmit information. Sensors are the key that opens new doors, allowing us to perceive worlds that were once unreachable.

Sensors Achieve Information Perception

The mission of sensors is to perceive and transmit information. When we want to obtain specific information, we need to find the matching sensor. For example, when we want to record the beautiful scenery before us, we need to use a photoelectric sensor that senses light signals; when we conduct health checks, we will use temperature sensors and pressure sensors to measure temperature and weight.

So how do sensors achieve information perception?

Energy can convert among forms such as light, sound, heat, and electromagnetic waves. A little observation will reveal many such instances around us.

When we turn on the light switch, the bulb lights up, which is the conversion of electrical energy into light energy; sunlight shining on solar cells causes current to flow in the circuit, corresponding to the conversion of light energy into electrical energy; sunlight dispels cold because light energy is converted into heat energy, bringing warmth; when we turn on the speakers to listen to beautiful music, electrical energy is converted into the mechanical energy of the speaker diaphragm, producing sound; when we pick up the microphone to sing loudly, the energy carried by sound is converted into electrical energy; electric kettles boiling water and rice cookers cooking rice convert electrical energy into heat energy, while thermal power plants generate electrical energy through heat energy.

The conversion of energy has certain physical mechanisms corresponding to it. This ensures that regardless of what information we wish to observe, we can almost always find a mechanism to achieve the direct or indirect conversion of the signal to be observed into an electrical signal, thus realizing the sensing function of the sensor.

According to incomplete statistics, there are approximately 30,000 different types of sensors with various functions and purposes.

We can classify them according to the functions they achieve: there are visible light sensors, infrared light sensors, and ultraviolet light sensors for “seeing”; dynamic microphones, condenser microphones, ultrasonic sensors, and infrasound 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-level molecular sensors to various 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 Daily Life: Understanding Their Importance

Sensors in Mobile Phones

The diverse and numerous sensors have long entered our daily lives. Taking the indispensable mobile phone as an example, let’s 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 photo 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 photo, can’t a similarly sized photo be used to unlock the phone instead? And how does facial recognition work in the dark or when taking a photo is not possible?

To fill these gaps, phones with facial recognition capabilities are also 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 turn on 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 images formed by the infrared sensor, ultimately obtaining three-dimensional facial information. The three-dimensional facial recognition technology greatly enhances security compared to two-dimensional recognition, eliminating the risk of unlocking with a photo. Infrared illumination and photography help us easily unlock the screen even in dark environments.

As we can see, the simple operation of unlocking the screen involvesfingerprint sensors, image sensors (camera), light sensors, proximity sensors, and infrared sensors—five types of sensors. Of course, these sensors are not only for the purpose of unlocking the screen.

For example, the central processor adjusts the screen brightness based on the ambient light intensity measured by the light sensor, allowing the screen brightness to automatically adjust according to different ambient light conditions, making it easier for us to see the screen content clearly in varying lighting conditions. There are also gyroscopes to measure rotation angles, accelerometers and gravity sensors to sense motion posture, pressure sensors to reflect altitude, and sensors to receive satellite signals, all of which give the phone navigation capabilities. There are many other types of sensors in mobile phones; each new sensor added expands or enhances the phone’s functionality, making it smarter and more intelligent! Today’s smartphones are more like intelligent terminals, with making calls no longer their primary function.

The Near-Infrared Camera Sensor on the James Webb Space Telescope (0.6 micrometers to 5 micrometers), with a central part made of cadmium telluride film

In our daily lives, we also encounter sensors that have olfactory functions like a nose. For instance, smoke detectors used to prevent fires, gas sensors specifically designed to detect formaldehyde odors in air purifiers. Additionally, to ensure safe travel, the breathalyzer held by traffic police is a sensor specifically used to detect alcohol; it can “smell” whether there is an alcoholic odor in the breath of the driver.

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. Besides cameras, there are surveillance cameras that can “see” like eyes; these systems rely on CCD and other image sensors. Among the many sensors used to “see” the world, infrared sensors are definitely at the center. From the infrared sensors in automatic doors and electronic thermometers to the infrared focal planes on space telescopes, the core is always the infrared sensor.

The most technologically advanced is the James Webb Space Telescope, launched on December 25, 2021. It has two infrared sensors, one for detecting wavelengths from 0.6 micrometers to 5 micrometers, and the other from 5 micrometers to 28 micrometers. With the support of these two infrared eyes, the James Webb Space Telescope can study the birth and growth of the universe and explore the atmospheric conditions of exoplanets, thus searching for potentially habitable Earth-like planets and seeking information about other life forms in the universe.

Air conditioners, refrigerators, rice cookers, electric kettles, and ovens can all automatically control temperature.They all use temperature sensors that mimic human skin’s ability to perceive heat and cold.

Health is currently the most concerning topic for people, and almost every item in health checks involves sensors. Electronic scales, electrocardiograms, ultrasounds, X-rays, bone density tests, and blood tests all involve corresponding sensors. Now, some sensors have been integrated into wearable devices for real-time monitoring of health. The most representative wearable device today is the smartwatch, which, besides showing the time, also has functions for measuring heart rate, electrocardiograms, blood oxygen levels, and even blood pressure. These functions involve pressure sensors, electrocardiogram (ECG) sensors, and photoelectric sensors. Perhaps in the near future, we could implant some miniature sensor chips in our bodies to monitor our health status around the clock, ensuring the best protection for ourselves.

Mobile signals, wireless networks, Bluetooth transmissions, etc., all belong to electromagnetic signals, so corresponding mobile terminals, wireless network cards, and Bluetooth receivers need to have the ability to sense electromagnetic signals. The information perceived by human senses is transmitted to the brain in the form of electrical signals, and the brain’s processing of this information is accompanied by brainwave activity; therefore, whether in medical auxiliary examinations or in the exploration of the human brain, electromagnetic sensors can be used to capture the brainwaves generated during brain activity.

Compared to conventional sensors, flexible sensors are closer to human tissues and can better adapt to our bodies. They have enormous application potential in medicine, bionics, and other fields. Flexible sensors are an inevitable trend in the future development of wearable devices, sports assistance, health management, and artificial electronic skin. Implanting flexible sensors in the body can better monitor health status. We could even place flexible sensors next to the heart or brain for special care. Flexible sensors are also expected to replace our sensory organs as they age. Artificial electronic skin based on flexible sensors is also key to whether bionic robots can “fool” humans.

Sensors in the Age of Intelligence

Sensors are the cornerstone of the intelligent age, and intelligent systems are the core of this age.Dynamic perception, intelligent recognition, and automatic response are the three key elements for realizing intelligent systems, much like our senses, brain, and limbs.

For example, the emerging field of autonomous driving is a highly focused intelligent system today. The car’s radar dynamically perceives the surrounding situation, and the car’s central processor intelligently recognizes the lanes, pedestrians, vehicles, buildings, and road signs from the dynamically perceived information, as well as their positions and motion trajectories. It then uses this “intelligence” to predict the subsequent positions of these entities. Meanwhile, combining the speed, direction, and other parameters provided by sensors, it automatically adjusts the throttle and steering wheel to stay in the lane, thus not deviating from its path. If a danger is anticipated, 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 perceive the environment accurately.

As we can see, sensors have become increasingly important in the process of human technological development. The intelligent age is approaching us; the future is here, and it has arrived! 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.” In a broader sense, when we equip all objects with corresponding sensors and achieve the Internet of Things, we will usher in the intelligent age, constructing an intelligent ecosystem and creating a wise Earth.

Source: World Science

Editor: Machine No. 7

Reprinted content only represents the author’s views

Not representing the stance of the Institute of Physics, Chinese Academy of Sciences

If reprinting is needed, please contact the original public account

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