What Are Optical Sensors? Optical Sensor Fundamentals and Characteristics of Each Distance Measurement Method

What are optical sensors and what are they used for?

Optical sensors are devices that detect light intensity and convert it into electrical signals. They operate not only with visible light but also with invisible light, such as near-infrared rays, which cannot be perceived by the human eye. Optical sensors are utilized in various detection tasks, including factory automation, security systems, automatic doors, and screen brightness adjustment in smartphones.

There are other types of sensors such as pressure sensors, angle sensors, and acceleration sensors for various target objects. However, optical sensors are especially well-suited for detecting objects.

Optical sensors primarily consist of light emitters and light receivers. Light from the light emitter hits an object, and the light receiver detects the intensity of the reflected or transmitted light to detect the object’s presence. Sensors use this principle to confirm whether a product is on a factory production line, measure the number of motor rotations, measure dimensions, and more. They perform high-precision detection in place of human eyes to contribute to quality confirmation and industrial machinery automation.

Five major characteristics of optical sensors

Optical sensors have the following five characteristics.

1. Non-contact measurement
   Optical sensors detect objects without physical contact. This enables measurement without damaging an object, making them especially well-suited for use with delicate materials and surfaces as well as in environments where hygiene management is important. They are also effective in detecting moving objects and in challenging environments such as high temperatures or pressure. Compared to contact sensors, they undergo less friction and deterioration and have the advantage of longer service lives. They are widely used in fields that require clean environments such as the food industry, pharmaceuticals, and semiconductor manufacturing.
   
2. Fast response
   Since they operate at the speed of light, optical sensors can respond at extremely fast speeds, on the level of microseconds. This makes them well-suited for use on high-speed production lines and quality control systems. For example, they can detect small flying particles or  accurately position fast-moving products. They are also suitable for real-time feedback control and high-speed counting. This high speed is also leveraged to control industrial robots and detect paper jams in high-speed printers.
   
3. Wide detection range
   Optical sensors make use of the rectilinear propagation of light to detect objects  across a wide range, from short to long distances. The use of lenses and mirrors makes it possible to widen detection ranges even further or focus them on smaller areas. This characteristic allows optical sensors to perform a variety of tasks, from detecting small parts to measuring the position of large objects. And because a single sensor can cover a wide area, the number of sensors can be reduced to contribute to cost savings. They are used in various fields such as entry detection systems for buildings and object detection in large warehouses.
   
4. Various detectable properties
   Optical sensors can detect not only the presence of an object but various properties such as its color, transparency, reflectivity, and shape. This versatility makes them suitable for complex tasks such as product quality control, categorization, and identification. For example, the food industry uses them to detect foreign objects and the automotive industry uses them to check quality of paint jobs, etc. They are also used to optically read information such as barcodes and QR codes. Moreover, by selecting light wavelengths, optical sensors can utilize not only visible light but also infrared and ultraviolet light for detection.
   
5. Sensitive to environmental factors
   Optical sensors are easily affected by environmental factors such as surrounding light conditions, dust, moisture, and vibration. Appropriate positioning and protection are necessary. Especially in places where lighting conditions can change such as outdoors or in factories, measures to prevent detection errors are important. Filters and careful sensor positioning can reduce the effects of these factors. Measures such as improving the sensor’s dust and water resistance and adding temperature compensation can also help. In addition, regular maintenance and calibration are important for maintaining stable performance.

Different distance measurement methods for optical sensors

Next, we will outline the primary distance measurement methods for optical sensors and their characteristics. These methods are broadly divided into two types: “through-beam” and “reflective.” Reflective sensors are further categorized into four distinct methods. In the following sections, we will explore each method in detail.

-Through-beam type

With through-beam optical sensors, the light emitter and light receiver are placed in opposing positions. The object is detected as it passes between them. This method enables high-precision detection and is less affected by background influences. It can detect transparent objects and is suitable for detection at a wide range of distances from dozens of centimeters to dozens of meters. For these reasons and its stable operation, this method is used in a variety of applications. It is necessary to precisely align the emitter and receiver, and it should be noted that the detection area is limited to a straight line.

Primary uses: detecting the presence of products on manufacturing lines, elevator door safety devices, and human detection for automatic doors, etc.

-Reflective type

With reflective optical sensors, the emitter and receiver are on the same side and objects are detected by using light reflected from the target objects. There are four methods used by reflective sensors.

a. Diffuse-reflective type

Diffuse-reflective sensors use light reflected by the target object itself for detection. Installation is easy because they only require a single unit, and they can detect objects of various shapes and materials. Detection distance is short compared to through-beam sensors, making them suitable for close-range detection. There are cases in which detection performance varies depending on the target’s color or surface conditions.

Uses: detecting the presence of parts on production lines, detecting products on packaging lines, and positioning of robotic arms, etc.

b. Distance-setting type

Distance-setting optical sensors accurately measure the distance to the target object and determine whether targets exist within a set range. These sensors measure distance using triangulation. The sensor emits light at a specific angle, and the light that comes back after reflecting off the test object enters the receiver. It detects the position where the reflected light entered the receiver and calculates the distance to the test object using the position information, entry angle, and trigonometric functions. This enables highly accurate distance measurement that is not significantly affected by the color or contours of the test object.

Uses: positioning of parts on assembly lines, inventory management in warehouses, and parking assistance systems for automobiles, etc.

c. Retro-reflective type

Retro-reflective sensors detect objects when the light reflected from a dedicated reflector is obstructed by a target object. They can detect objects at long ranges, offer stable detection performance, and are suited to the detection of small objects.

Uses: position detection of large-scale machinery in factories, cargo passage detection in distribution centers, and railroad crossing control systems

d. Limited-reflective type

Limited-reflective sensors detect only target objects at specified distances. The emitters and receivers are  separate and are positioned so the optical axes intersect at an angle. This allows them to detect reflected light only within a limited detection area where the emitter’s axis crosses the receiver’s axis. In the diagram above, the sensor can detect object (a) but not object (b). This enables stable detection even in complex environments, as the sensor can ignore backgrounds and objects outside the detection area.

Uses: detecting objects on conveyor lines, etc.

Selecting the optimal optical sensor

Each of the distance measurement methods above has their own characteristics, advantages, and disadvantages to consider when selecting the optimal sensor for a use case. For example, diffuse-reflective sensors are well-suited for general object detection on production lines, but retro-reflective or distance-setting sensors are often selected when long-range detection or high-precision positioning is required.

In addition, recent technological advances have led to the development of hybrid sensors that combine these basic methods as well as high-precision distance measurement sensors that use lasers. These sensors can meet the needs of increasingly complex environments and use cases. When selecting optical sensors, it is necessary to comprehensively consider the characteristics of the target objects (such as size, shape, material, and color), detection distance, the surrounding environment (such as light, temperature, and humidity), the required precision and response speed, and more. Durability and reliability are also important factors to consider for industrial uses.

Selecting the optimal optical sensors and installing and adjusting them appropriately can significantly contribute to improved productivity, enhanced quality control, improved safety, and more. Optical sensor performance and functionality continue to steadily evolve  with technological advances, and expectations are high for even broader applications in the future.