Photo Sensors
Photo sensors are devices that emit visible light beams or infrared rays using a light-emitting element. Typically, a reflective photo sensor is employed to detect the light beam reflected from an object, while a Thrubeam sensor is utilized to measure changes in light intensity that occur when an object passes through its optical axis. The emitted light beam is transmitted from the light-emitting element and captured by a light-detecting component. Photo sensors can be categorized into three main models:
Reflective Model
This model includes both light-emitting and light-receiving elements housed in one compartment, where the sensor captures the light reflected from the target.
Thrubeam Model
This model consists of a separate transmitter and receiver. When an object is positioned between the transmitter and receiver, the light beam is interrupted.
Retroreflective Model
This model integrates both light-emitting and light-receiving components within a single housing. The light emitted from the light source travels to a reflector, where it reflects back to the receiving element. If an object is present, the light beam is interrupted.
Thus, photo sensors are classified as photovoltaic devices that convert light energy, in the form of photons—whether visible or infrared—into electrical signals represented by electrons. These devices are categorized into various types, including:
- Photovoltaic Cells: These are optical devices that release free electrons from a light-sensitive material when struck by a sufficiently energetic photon. The energy of the photon is determined by the light’s frequency; higher frequencies correspond to higher photon energy, which facilitates the conversion of light energy into electrical energy.
- Photoresistor Cells: These operate by altering their resistance when exposed to light. Conductivity changes occur when light interacts with a semiconductor material, which regulates current flow. Consequently, increased light levels lead to higher current production at a certain voltage. Cadmium sulfide is one of the most commonly used conductive materials in photoresistor cells (LDR).
- Photovoltaic Cells: These cells generate power proportional to the energy of the incoming light. When light energy strikes two semiconductor materials, it results in a potential difference of 0.5 volts. Selenium is frequently used in solar cells.
- Photoelectric Devices: These semiconductor devices utilize light to control electron flow through their junction. An example of this type is the photodiode.
The photovoltaic sensor acts as a type of switch, operating based on the presence or absence of light, and is characterized by its ability to detect the presence of an object without any direct contact. Additionally, these sensors have no mechanical components that wear out and can be installed at significant distances. It is worth noting that these sensors find extensive applications in the industrial sector.
Fiber Optic Sensors
The various types of photo sensors include point sensors, distributed sensors, external sensors, internal sensors, beam sensors, reflective diffuse sensors, and retroreflective sensors. Their operation relies on two components: the transmission device (light source) and the detection device (light sensor). There are two categories of these devices, classified by the location of light beam changes for detection purposes. An external optical sensor is identified by its ability to intercept a light beam leaving the fiber optic cable, while an internal optical sensor is characterized by changes occurring within the same fiber optic cable.
There are three types of optical sensors based on the sensing method and the arrangement of the transmitter and receiver:
- Unidirectional Optical Sensor: Here, the transmitter and receiver are orientated towards each other, creating a direct path for the light beam. When an object passes through this path, the light intensity changes, indicating the object’s presence.
- Reflective Sensor: In this configuration, the transmitter and receiver are aligned in parallel. As an object passes, it reflects the emitted light, which the receiving device measures.
- Retroreflective Sensor: Both the transmitter and receiver are housed together, allowing the transmitter to send a light beam that reflects off a reflector and is received by the detector. If an object enters the path of the light beam, it results in a diffraction, enabling detection based on variations in light intensity and other factors.
Key applications of photo sensors include:
- Satellite technology
- Imaging systems
- Medical devices
- Quality control and process monitoring applications
Ultraviolet Sensors
These sensors measure the intensity or strength of ultraviolet radiation. This type has a shorter wavelength than visible light but is longer than X-rays. They capture only a single type of energy signal, generating electric signals that are directly forwarded to an electric meter. These signals can be converted into an analog-to-digital converter and subsequently to a computer for data collection and report generation, finding utility in various fields including pharmaceuticals, automotive industries, robotics, and chemical manufacturing.
Notable types of ultraviolet sensors include:
- Ultraviolet Tubes: Used for monitoring air treatment with UV rays, water treatment using the same radiation, and solar radiation.
- Light Sensors: Employed to measure the intensity of incident light.
- UV Spectrum Sensors: Devices used for scientific imaging and measuring the spectrum of radiation harmful to human skin.
Infrared Reflective Sensors
This electronic device measures and detects infrared radiation. There are two main types: active and passive. Active sensors emit rays from active sensing devices. They consist of two components: a light-emitting diode and a receiver. When an object approaches the sensor, it reflects infrared light from the LED, which is then detected by the receiving device. These sensors are widely utilized in obstacle detection systems, such as in robotics. In contrast, passive sensors detect infrared radiation without emitting light from the diode.
Key applications include motion detection systems, particularly in home security setups. When a moving object enters the sensor’s detection range, it generates infrared rays; the device measures the differences in infrared levels between the two thermal elements, triggering the sensor to send an electronic signal to a computer, which may issue a warning alarm.
Laser Sensors
Laser sensors are distinguished by their high-intensity light, allowing for precise measurements or positioning. They are recognized for their fast measurement capabilities. Types of laser sensors include distance sensors, laser projection devices, laser light curtains, and photovoltaic laser sensors.