How does photoelectricity detect transparent objects ?

How does photoelectricity detect transparent objects

A photoelectric sensor is a small electronic device and a key component in various photoelectric detection systems. It mainly detects the presence or absence of objects and changes in surface conditions by utilizing the various properties of light. A photoelectric sensor has characteristics such as non-contact, fast response, and reliable performance, and is therefore widely used in industrial automation equipment and robots.

Classification of Photoelectric Sensors

According to the detection method, photoelectric sensors can be classified into slot-type photoelectric sensors, diffuse reflective photoelectric sensors, reflective photoelectric sensors, and through-beam photoelectric sensors.

1. Slot-type Photoelectric Switch

It usually employs a standard U-shaped structure, with its emitter and receiver located on the opposite sides of the U-shaped slot to form an optical axis. When the detected object passes through the U-shaped slot and interrupts the light path, the photoelectric switch generates a switching signal.

2. Through-Beam Opto Sensor

It comprises an emitter and a receiver that are structurally separated and placed with their optical axes in opposite directions. The light emitted by the emitter directly enters the receiver. When the detected object passes between the emitter and the receiver and interrupts the light path, the photoelectric switch generates a switching signal.

3. Reflective-type Photoelectric Switch

It is also a sensor that integrates an emitter and a receiver. The light emitted by the opto switch emitter is reflected back to the receiver by a reflector. When the detected object passes through and completely blocks the light path, the photoelectric switch generates a detection signal.

4. Diffuse Reflective Photoelectric Switch

It is a sensor that integrates an emitter and a receiver. When the measured object passes by, it reflects a sufficient amount of light emitted by the opto switch back to the receiver, and the photoelectric switch generates a switching signal accordingly. When the surface of the detected object is bright or has a high reflectivity, the diffuse reflective opto switch is the preferred detection mode.

The principle of photoelectric sensor detecting transparent objects is as follows

After understanding the basic classification of photoelectric switches, is it possible for them to detect transparent objects? And how to choose the best photoelectric sensor for detecting transparent objects.

1. Retroreflective photoelectric switch

Detection principle

The working principle of the opposed type photoelectric sensor is based on the emitter emitting a beam of light while the receiver detects changes in the light signal, thus achieving the detection and identification of objects. Transparent objects cause changes in the refracted and transmitted light beams, affecting the reception of the light beam by the receiver. When a transparent object approaches the photoelectric switch, the light beam may be refracted or transmitted by the transparent object, thus not directly hitting the receiver. This makes it difficult for the opposed type photoelectric switch to accurately detect transparent objects. Therefore, additional measures need to be taken to enhance the detection capability of opposed type photoelectric switches for transparent objects, such as adding reflectors, illuminators or diffusers on the light beam path to change the light beam path or enhance the beam intensity, thereby increasing the influence of transparent objects on the light, and improving the accuracy of detection.


Features

Transparency detection requires additional measures, and has higher requirements for environmental and auxiliary equipment installation. The device has high consumption and both units require cable laying. It is more suitable for identifying opaque reflective objects, is less susceptible to interference, and can be used in outdoor or dusty environments.

2. Reflective-type photoelectric switch

Detection principle

The reflective-type photoelectric switch consists of a transmitter, a receiver, and a reflective plate. The transmitter emits visible light (usually an LED light source) that is projected onto the reflective plate. The reflective plate redirects the light beam back to the receiver. An object passing between the sensor and the reflective plate will block the detection beam and be detected.

The principle behind detecting transparent objects with a reflective-type photoelectric switch is based on the transmission characteristics. When light passes from one medium (such as air) to another medium (such as a transparent object), it undergoes refraction and reflection. If the refractive index of the transparent object is different from that of the surrounding medium, the light will be refracted, thereby changing the direction of propagation. In the application of detecting transparent objects using an LED light source, the light is emitted from the LED light source and received by the photoelectric switch. If a transparent object is present, the light will be refracted, and a portion of the light will be received by the receiver, generating a signal. If a transparent object is not present, the light will not be refracted, and the receiver will not receive any light, thus no signal will be generated. By measuring the intensity and time of the received signal, the presence or absence of a transparent object can be determined. This method can be used to detect the presence of transparent objects such as transparent films, glass, and so on.


Features

Reflective-type photoelectric sensors are a common choice for transparent object detection applications. Each sensor consists of a transmitter and a receiver. The transmitter guides a light beam onto a reflective plate that redirects the beam back to the receiver. The object passing between the sensor and the reflective plate will block or attenuate a percentage of the detectable emitted light. These sensors are cost-effective, easy to install and power, and offer fast response and high accuracy.

However, the high light sensitivity required for reliably detecting transparent objects makes these sensors susceptible to false detections caused by reflected light. The reflective plate is typically a specialized mirror with polarization filtering capabilities.

3. Diffuse reflective photoelectric switch

Detection principle

This type of switch utilizes the reflected light from the target object itself. The presence of the object causes a change in the received light intensity, allowing the switch to detect the presence of the object. The switch's emitter emits a beam of light, which reflects from the surface of the transparent object and is received by the switch's receiver. The switch can employ phase-locked loop narrowband filtering technology to filter out interference from other light sources, enabling it to resist strong light interference such as sunlight. Additionally, this technology enhances the sensitivity to the returning light, so even if the returning light is weak, it can still be detected to a great extent. The presence of the object causes a change in the received light intensity, enabling the switch to detect the presence of the object.


Features

Sensitivity to scattered light: The diffuse reflective photoelectric switch exhibits a high sensitivity to scattered light, enabling it to achieve exceptional performance in detecting transparent objects. It can identify subtle changes in light intensity, allowing for precise recognition and differentiation.

Wide detection angle: This sensor provides a wide detection angle, allowing for comprehensive coverage and efficient detection of transparent objects within its field of view. This extensive viewing angle ensures enhanced reliability and reduces blind spots.

Adjustable sensing range: The sensing range of the diffuse reflective photoelectric switch is adjustable, making it flexible to adapt to transparent objects of different sizes and distances. This adaptability enhances its versatility and applicability in diverse scenarios.

4. Slot-type photoelectric switch

Detection principle

It typically adopts a standard U-shaped structure, with the emitter and receiver positioned on opposite sides of the U-shaped slot, forming an optical axis. When an object passes through the U-shaped slot and interrupts the beam of light, the photoelectric switch generates a switch signal. In the case of transparent objects, as they pass through the optical axis, the refraction and transmission of light cause changes in the amount of light received by the receiver, resulting in variations in the received signal quantity, enabling the detection of transparent objects.


Features

Fast response time, compact, portable, less constrained by the object being detected, and not affected by external light interference.

5.Conclusion

The detection of transparent objects presents unique challenges due to the minimal absorption or scattering of transmitted light. There are different types of photoelectric sensors available in the market, including retro-reflective, through-beam, diffuse reflective sensors, and slot-type photoelectric sensors. Among these options, retro-reflective photoelectric sensors are preferred for transparent object detection applications due to their excellent detection accuracy, versatility for various transparent objects, and simplified installation process. They are considered the optimal choice for transparent object detection.

5.1 Detection Accuracy

Retro-reflective photoelectric sensors exhibit superior accuracy in detecting transparent objects. Their close proximity to the object ensures precise and reliable detection, reducing false alarms and missed detections. In comparison, through-beam sensors may encounter alignment issues and signal loss, while diffuse reflective sensors may struggle to accurately differentiate between transparent and opaque objects.

5.2 Versatility

Retro-reflective photoelectric sensors offer greater versatility in detecting various types of transparent objects. Their adjustable detection range and sensitivity settings allow for easy adaptation to different sizes, shapes, and distances. Through-beam sensors have limitations in detecting non-transparent materials, while diffuse reflective sensors may exhibit reduced performance when faced with highly transparent surfaces.

5.3 Installation Convenience

The reflective photoelectric sensor boasts a simplified installation process, requiring only the installation of a single sensor unit. In contrast to the through-beam sensor, which necessitates separate installation of transmitter and receiver units, or the diffuse reflection sensor, which requires meticulous positioning and alignment, this installation convenience saves both time and effort. Moreover, when compared to slot-type photoelectric sensors, it offers a broader detection range.

Factors Influencing the Detection Accuracy of Photoelectric Switches

1.High sensitivity to subtle changes in light

Transparent materials themselves have very little reflection and absorption of light. Therefore, photoelectric sensors need to be able to detect the subtle disturbances of light caused by transparent objects in order to accurately detect their presence and position. This often requires the use of highly sensitive photoelectric sensors that can detect small changes in light. Additionally, photoelectric sensors need to have high stability to ensure the accuracy and reliability of the detection results. Thus, sensitivity to slight variations in light is necessary.

To overcome the influence of reflections, many transparent objects (such as reflective glass or plastic, faceted containers, reflective films, etc.) possess reflective characteristics. Similarly, equipment and background objects can also cause reflections. The light reflected back from these objects to the receiver of the sensor can trigger erroneous detections. The use of polarizing filters and coaxial optical techniques can help reduce the impact of reflections.

2.Polarizing filter

Many transparent objects, such as reflective glass or plastic, faceted containers, and reflective films, possess reflective characteristics. Similarly, equipment and background objects can also cause reflections. The light reflected from these objects back to the receiver of the sensor can trigger erroneous detections. The use of polarizing filters and coaxial optical techniques can help reduce the impact of reflections. For example, with polarized sunglasses or photoelectric sensors equipped with polarizing filters, only a portion of the light is allowed to enter the receiver. This helps the sensor differentiate between reflective or transparent objects and a reflector plate

3.Coaxial optical technology

Reflector-based sensors that do not employ coaxial optical design have their emitter and receiver adjacent to each other, with the reflector, reflector plate, and receiver forming a triangular path for light propagation. This triangular detection creates a "blind zone" on the surface of the sensor, and the distance between the emitted and reflected light is significant.

In contrast, sensors designed with coaxial optical technology emit and receive detection beams along a narrow axis. Light is emitted and received through a single lens with a small aperture, allowing the sensor to measure the slight angular deviation of light between the emitted and reflected beams. The object being measured can be positioned anywhere between the sensor and the reflector plate, without any blind spots. The distance between the sensor and the reflector plate can be relatively close, making it suitable for installations with limited space. This design greatly limits the opportunity for ambient light to enter the receiver of the sensor. By incorporating polarizing filters, the sensor significantly reduces false detections caused by reflected light.

Coaxial optical design also offers other advantages. The narrow beam, small spot size, and insensitivity to sensor rotation associated with this design make it well-suited for precise edge detection and narrow gap detection between objects commonly found in high-speed counting applications. Additionally, since the detection beam can pass through small apertures unaffected, the sensor is able to be protected by a housing from environmental challenges such as high-pressure washdowns.

Conclusion

The importance of optoelectronic detection for transparent objects lies in its ability to achieve non-contact, high-precision, and efficient detection of such objects.

Reflective optoelectronic detection allows for detection of transparent objects without physical contact, eliminating the issues of damage or disturbance to the objects. Transparent objects may be fragile or require specific environmental conditions, making non-contact detection methods particularly important.

Reflective optoelectronic detection enables high-precision measurement results. By analyzing the reflection and intensity variations of light, information such as the shape, dimensions, and surface characteristics of transparent objects can be obtained. This is crucial for applications such as quality control, dimensional measurement, and shape analysis.

Transparent materials are widely present in various industries and fields, including ceramic manufacturing, optical components, chemical production, medical devices, and more. Through reflective optoelectronic detection, automated detection and processing of transparent materials can be realized, enhancing production efficiency and product quality.

The analysis of transparent materials is crucial in the field of material science and research. Through reflective optoelectronic detection, the optical parameters of refractive index, transparency, absorption characteristics, and more can be studied, allowing for a deeper understanding of the properties and behavior of transparent materials.

In the realm of automation systems and machine technology, detecting transparent obstacles is essential for ensuring safety and the smooth operation of processes. Through beam optoelectronic detection, the position and shape of malfunctioning objects can be identified, preventing collisions and damages.