Optical sensors detect and measure light intensity, converting light rays into electrical signals. These sensors are connected to electrical triggers sensitive to light changes. Optical sensors are used in many devices, including computers and motion detectors. This article will discuss the types of optical sensors that have been developed and their applications.
Types of Optical Sensors
The main principle of optical sensors is the transmission and reception of light. The light reflected or interrupted by the target object is evaluated by various optical devices based on the material type, such as wood, metal, plastic, transparent, or colored products.
Different types of light sources are used in optical sensors, which must be monochromatic, long-lasting, and compact. The two common types of light sources are Light Emitting Diodes (LEDs) and Light Amplification by Stimulated Emission of Radiation (LASER).
Optical sensors can be placed externally or internally in a device. External optical sensors, also known as extrinsic sensors, gather and transmit a required quantity of light. In contrast, internal or intrinsic sensors are commonly used to measure the bend and other changes regarding the direction of light. Intrinsic sensors are typically embedded within an optical fiber or other devices.
Based on the application, different types of optical sensors are used. For instance, in photoconductive devices, optical sensors measure resistance by converting a change in incident light to a change in resistance.
In solar cells, the sensors convert incident light into an output voltage. A photodiode converts incident light to output current. The function of a phototransistor, a type of bipolar transistor, is similar to a photodiode but with internal gain. Some common types of optical sensors are discussed below.
Through-beam Sensors
In through-beam sensors, the two components, namely the transmitter and the receiver, are positioned opposite each other. The interruption of light is interpreted as a switch signal by the receiver.
This sensor covers a considerable operating distance and can detect independent object surfaces, structure, and color.
Retro-reflective Sensors
In retro-reflective sensors, the transmitter and receiver are placed in the same system. Similar to the through-beam sensor, the retro-reflective sensor also initiates via switching operation and covers large operating distances. All objects interrupting the light beam are detected independently and accurately by this sensor.
Diffuse Reflection Sensors
In diffuse reflection sensors, the receiver and transmitter are housed in one system. This sensor detects the light reflected by the objects.
Latest Advancements in Optical Sensor Applications
Optical sensors are applied in various devices, including computers and copy machines. They are also used in light fixtures that automatically turn lights on in the dark. Optical sensors are used in alarm systems and photographic flashes. Some of the latest applications of optical sensors are discussed below:
Biomedical and Healthcare Applications
Photoplethysmography: Heart Rate Monitoring
Optical sensors have been widely used in the biomedical field, including developing an optical heart-rate monitor that uses light. In this device, an LED focuses on the skin and an optical sensor analyzes the reflected light. As blood absorbs more light, fluctuations in light intensity can be translated into the heart rate. This process is known as photoplethysmography.
Contactless Sanitizer Dispensers
The coronavirus disease 2019 (COVID-19) pandemic has created an unprecedented need for contactless sensing. In this regard, a common application of optical sensors includes contactless sanitizer dispensers used in healthcare facilities.
Breath Analyzers
Breath analysis is achieved by using a tunable diode laser.
Detection and Quantification of Biomarkers in Human Serum
Serum is essential for proteomic analysis as it carries key markers of overall health status. However, detecting some diagnostic biomarkers, especially those of the low-molecular-weight (LMW) peptidome and circulatory proteome, is challenging due to their dynamic concentration in serum.
A novel strategy has recently been formulated to improve the detection of LMW amino acids. This method combines mid-IR (MIR) and near-IR (NIR) spectroscopic data to detect LMW compounds in a complex serum matrix.
Agricultural Application
Optical sensors have enabled on-site detection of pesticides in agricultural food products. Recent advancements in optical sensing technology include target-responsive hydrogels, fluorescence sensors, tube enzyme-linked immunosorbent assay, enzymatic fiber-optic biosensors, lateral flow immunoassays, wearable glove-based sensors, double-signal fluorescence strategies, and paper-based sensors. These innovations have facilitated the on-site detection of pesticide residues in food products.
Environmental Application
Food security and human health are significantly affected by heavy metal pollution, such as cadmium, in soil. Although microelectronic sensors based on fluorescence spectrometry (XRF) are frequently used to detect heavy metals in soil, their accuracy and sensitivity are often questioned. A high-detection-efficiency photodiode (HDEPD) has been developed to substantially increase the accuracy of these analyzers.
Industrial and Commercial Applications
Optical sensors are used to detect liquid levels in engineering facilities. For instance, optical sensors detect petroleum levels in hydrocarbon refineries and tanks. An ambient light sensing system is used in smartphone devices to increase battery life. Additionally, optimal screen brightness is achieved in accordance with the amount of light in the environment through the application of optical sensors.
Future of the Optical Industry
By analyzing the increased demand for optical sensors, an overall rise in the compound annual growth rate (CAGR) is estimated to be around 9.5% from 2022 to 2032. With the discovery of automatic lamps, position sensors, smartphones, and photoelectric sensors, optical sensors have garnered significant attention, particularly in the electronics industry.
As various optical sensors for critical biomedical analysis can be developed at a low cost, the future looks promising for this industry. The significant growth of the biometric market, associated with the deployment of fiber optic sensors in aerospace, defense, and the development of smart homes, has triggered increased sales of optical sensors.
