An optical sensor is a device that uses a beam of light to detect the presence, position, or status of an object without physical contact. Thanks to their fast response time, high durability, and ability to detect a wide range of materials, optical sensors are used in almost every type of automated production line, from packaging conveyors to automatic doors in buildings. This article provides a detailed explanation of optical sensor construction, operating principles, types, and practical applications in industrial systems.

WHAT IS AN OPTICAL SENSOR?

An optical sensor, also known as a photoelectric sensor, emits light and monitors changes in that light when it encounters an object. When an object passes through or enters the detection area, the amount of light reaching the receiver changes. The sensor detects this change and converts it into an electrical signal, which is then sent to a PLC, relay or counting device.

The light source may be visible red light, infrared light or laser light, depending on the product series. Since optical sensors do not require physical contact with the target, they do not cause scratches, experience no mechanical wear and can operate continuously at high switching frequencies.

Compared with inductive proximity sensors, which only detect metal objects over very short distances, optical sensors can detect almost any type of material, including plastic, paper, glass, wood and metal, and generally offer much longer detection ranges.

OPTICAL SENSOR CONSTRUCTION

A typical optical sensor consists of the following components:

• Light emitter: Usually a red LED, infrared LED, or laser diode. The emitter generates a light beam modulated into rapid pulses, allowing the sensor to distinguish its own light from ambient light.
• Light receiver: Usually a photodiode or phototransistor, which detects incoming light and converts it into an electrical signal.
• Optical lens: Focuses the emitted light beam and collects incoming light, helping increase detection distance and accuracy.
• Signal processing circuit: Amplifies the signal from the receiver, compares it with the preset threshold and determines when the output state should change.
• Output circuit: Provides an external signal through an NPN transistor output, PNP transistor output or relay contact.
• Sensor housing and adjustment components: Includes a protective plastic or metal housing, status indicators and sensitivity adjustment knobs or buttons on models that support these functions.

In through beam sensors, the emitter and receiver are housed in two separate units positioned opposite each other. In other types, the emitter and receiver are integrated into a single housing.

See also: What is a background suppression photoelectric sensor?

HOW DOES AN OPTICAL SENSOR WORK?

The operating principle of an optical sensor is based on monitoring the amount of light reaching the receiver:

Step 1: The emitter generates a continuous light beam in the form of short pulses.

Step 2: The light reaches the receiver directly or after being reflected by an object or reflector.

Step 3: The receiver converts the detected light into an electrical signal.

Step 4: The processing circuit compares the signal with the preset threshold. When the amount of light exceeds or falls below this threshold, the output changes state.

Step 5: The output signal is sent to a control device to trigger the next operation, such as stopping a conveyor, counting products, or rejecting defective items.

Depending on the wiring and configuration, the output can operate in two modes:

• Light ON mode: The output turns ON when the receiver detects light.
• Dark ON mode: The output turns ON when an object blocks the light beam.

These modes are typically selected using configuration wires, switches or predefined settings depending on the product model. Pulsed light modulation helps reduce interference from fluorescent lighting, sunlight and factory lighting, allowing the sensor to operate reliably in environments with high ambient light levels.

COMMON TYPES OF OPTICAL SENSORS

Optical sensors are primarily classified according to the arrangement of their emitters and receivers. Each type offers specific advantages.

Through-beam photoelectric sensors

The emitter and receiver are positioned opposite each other. When an object passes between them, it blocks the light beam and changes the output state. This type offers the longest detection range and highest stability, with minimal influence from the object's color or surface characteristics. However, installation and wiring are required on both sides, and completely transparent objects can be difficult to detect.

Through beam photoelectric sensors are suitable for counting products on conveyors, detecting people or vehicles passing through gates, and monitoring object positions over long distances.

Retroreflective photoelectric sensors

The emitter and receiver are integrated into a single housing. Light is directed toward a dedicated reflector positioned opposite the sensor and then returned to the receiver. When an object passes between the sensor and reflector, it interrupts the light beam and changes the output state. Since wiring is only required on one side, installation is more compact than with through-beam sensors.

For objects with shiny surfaces, such as polished metal or glossy plastic, reflected light may return directly to the sensor. In this case, a retroreflective sensor with a polarizing filter is recommended to ensure that the sensor detects light returned from the reflector. These sensors are commonly used on conveyor systems, packaging lines and automatic doors.

Diffuse reflective photoelectric sensors

The sensor emits light directly toward the target, and the light reflected from the object's surface returns to the receiver. This type is the easiest to install because it does not require a separate reflector or receiver. However, its detection range is generally shorter and depends on the object's color and surface reflectivity. Dark objects absorb more light and are therefore more difficult to detect at longer distances than light colored objects.

Diffuse reflective sensors are suitable for confined installation spaces and short range object detection, such as detecting boxes, bottles and components on conveyor belts.

Background suppression photoelectric sensors

This is a variation of the diffuse reflective sensor that can ignore objects located beyond a preset distance. As a result, the sensor only responds to objects within the required detection zone and is less affected by the background, even when it is bright or reflective. This type is useful for detecting objects of different colors on the same production line while maintaining reliable detection.

Fiber optic photoelectric sensors

A fiber optic sensor consists of an amplifier and thin optical fibers that transmit light to the detection point. The fiber optic head is very small and can be installed in confined spaces. Depending on the fiber type, it can also withstand high temperatures, vibration and chemical exposure. This type is suitable for detecting small components in assembly machines, component placement equipment and semiconductor manufacturing systems.

Slot type photoelectric sensors

The emitter and receiver are integrated into a U-shaped housing, with the object passing through the slot to interrupt the light beam. This type provides a fixed and highly accurate detection position. It is commonly used for detecting the position of small components, counting pulses or functioning as a limit switch in compact equipment.

See also: Photoelectric sensor products

PRACTICAL APPLICATIONS OF OPTICAL SENSORS

In industrial applications, optical sensors are widely used across various production stages:

• Counting products, cartons and bottles on conveyor belts.
• Detecting missing caps, missing labels, or incorrectly positioned products on packaging lines.
• Identifying component positions in machining and assembly machines.
• Detecting misplaced or jammed objects on conveyor systems to stop machines promptly.
• Monitoring material levels in hoppers and storage containers at designated detection points.

Beyond manufacturing, these sensors are also commonly used in automatic doors, escalators, parking facilities, ATMs, printers and various consumer devices. Their general function is to detect the presence or movement of people or objects and trigger the corresponding operation.

HOW TO SELECT THE RIGHT OPTICAL SENSOR

To select the right optical sensor, start by considering three questions: What object needs to be detected? How far away is it from the sensor? What are the installation conditions?

• For long detection distances and high reliability, prioritize through-beam photoelectric sensors.
• If installation is only possible on one side but a relatively long detection range is still required, choose retroreflective photoelectric sensors. Add a polarizing filter if the target has a shiny surface.
• For confined spaces and nearby objects, choose diffuse reflective sensors. If the background causes interference, select a background suppression model.
• For very small objects or confined installation spaces, choose fiber optic sensors or slot-type photoelectric sensors.

After selecting the sensor type, check the supply voltage and output type, including NPN, PNP or relay, to ensure compatibility with the controller. Also consider the housing's IP protection rating against dust and water, as well as the wiring or connector type. These specifications are provided in the manufacturer's technical documentation.

FREQUENTLY ASKED QUESTIONS ABOUT OPTICAL SENSORS

What is the difference between an optical sensor and a proximity sensor?

Optical sensors use light and can detect a wide range of materials over longer distances. Inductive proximity sensors use electromagnetic fields, detect only metal objects and generally have shorter detection ranges.

Can optical sensors detect transparent objects?

Conventional through beam sensors can have difficulty reliably detecting transparent objects such as glass bottles or plastic films. Choose a model specifically designed for transparent object detection, typically a polarized retro reflective sensor or a sensor developed for this particular application.

What is the detection range of an optical sensor?

The detection range depends on the sensor type. Slot type photoelectric sensors typically detect objects over distances of only a few millimeters. Diffuse reflective sensors generally have detection ranges from a few centimeters to approximately one meter, while through beam photoelectric sensors can reach several tens of meters.

BUY GENUINE OPTICAL SENSORS FROM SENSORS VIETNAM

Optical sensors are fast, flexible and non-contact detection devices that address a wide range of automation requirements, from product counting to component positioning. Selecting the right sensor based on the target material, detection distance and installation environment helps maintain stable system operation and minimize equipment failures.

Sensors Vietnam is a distributor specializing in photoelectric sensors, proximity sensors, ultrasonic sensors and industrial sensing equipment for factory automation systems. For further information or assistance with selecting the right photoelectric sensor model, please contact Sensors Vietnam team.