Optical sensors are electronic components designed to detect and convert incident light rays into electrical signals. These components are useful for measuring the intensity of incident light and converting it into a form readable by an integrated measuring device, depending on the sensor type.
This article outlines the principles of operation of optical sensors, optical sensor types, considerations for optical sensor selection, and key applications.
Example of an optical sensor. Image courtesy of ROHM
Applications of Optical Sensors
Optical sensors are ubiquitous components in electronic devices and equipment utilized in the industrial, consumer, healthcare, and automotive fields.
Medical and Healthcare
With the unprecedented need for contactless sensing due to the global pandemic, optical sensors have been utilized in sanitizer dispensers at long-term healthcare facilities to ensure health and safety compliance.
Other medical applications include biomedical devices for breath analysis and heart rate monitoring. Breath analysis can be achieved using a tunable diode laser, while the reflection of light back to the sensor through the skin can accurately monitor the human heart rate in a process known as photoplethysmography. Portable wearable sensors utilize optical sensors for both automated and manual tracking of users’ health status and vital signs.
In industrial and commercial applications, optical sensors are also being used for distance and temperature sensing and automation in Industry 4.0 applications. For example, optical sensors can detect liquid levels in process engineering facilities, such as petroleum levels in tank farms and hydrocarbon refineries, by integrating an infrared LED, light transistor, and a transparent prism tip.
Optical sensors also enable automated control by detecting the presence of components on factory floors.
Optical sensors are also being used for ambient light sensing in consumer electronics such as smartphones, with advantages such as extending battery life and optimizing screen brightness to match the amount of lighting in the environment.
The schematic below (Figure 1) integrates a microcontroller and an auto-luminous control-equipped LED driver IC to achieve an output current proportional to the amount of ambient light and mimics the spectral sensitivity of the human eye.
Figure 1. Ambient light sensor IC block Diagram. Image courtesy of ROHM
Photo-interrupters and reflective-type photosensors are used for optical sensing in printers and 3D scanners for industrial and retail applications. Optical sensors are also used in surveillance equipment in commercial and residential buildings to detect intruders.
Types of Optical Sensor
The most common types of optical sensors include:
- Transmission-type photo-interrupters detect the presence of objects by intercepting light and are widely utilized in applications such as position sensing and speed of rotation measurements
- Reflective photosensors detect the motion of objects by measuring the reflection of light across them
- Photoconductive devices become electrically conductive by absorbing incident light rays
- Photodiodes convert incident light into electric current
- Phototransistors achieve similar results as photodiodes when the base-collector junction is exposed to light
Operation of Optical Sensors
Optical sensing technologies require monochromatic, compact, and reliable light sources to function effectively. Common light sources suitable for optical sensor lighting include LEDs and lasers.
Light-emitting diodes (LEDs) produce light when electrons combine with holes at a junction of n- and p-doped semiconductors to aid the release of photons. On the other hand, a laser is produced by the electrical excitation of electrons in the atoms of certain materials, such as glass or crystals.
Figure 2. Optical proximity sensor block diagram. Image courtesy of ROHM
Varying types of optical sensors, however, operate slightly differently.
The maximum current an output stage-based phototransistor can drive in photo-interrupters depends on the amount of light it receives. When light shines on the phototransistor (i.e., no object in the gap), photo-interrupters exhibit LOW output.
Figure 3. Photointerrupter construction. Image courtesy of ROHM
Conversely, photo-interrupters exhibit HIGH output with the presence of an object. Engineers can harness the capabilities of photo interrupters by connecting the output to a microcontroller or logic device for optical control.
Response times, cost, size, and sensitivity are essential considerations for engineers looking to integrate optical sensors into their designs.
Response time refers to the time it takes an optical sensor to respond to incident light and is critical in several applications. Faster response times typically result in higher optical sensing efficiencies. Many optical sensors (Figure 4) incorporate response time measurement circuits into their designs to account for their delay, rise, and fall time capabilities.
Figure 4. Product image of the RPI-246 (left) and the RPI-44C1E (right). Image courtesy of ROHM
Similarly, the cost is an essential requirement for designing optical sensors. Many factors affect the overall optical sensor design cost, including hardware/software purchasing, testing, and research and development.
Sensors also come in various sizes, depending on their types and specific applications. For example, typical photo-interrupter package sizes range from 3.6 x 3.3 mm to 8 x 4.2 mm. Due to rapid miniaturization, designers will often opt for smaller optical sensors with a balance of high performance and lower costs.
Moreover, designers favor sensors sensitive to a wider spectrum of light, including visible and infrared. Higher sensitivities of up to ±40 can achieve up to four times faster proximity and ambient light sensing measurements.
Benefits of Optical Sensors
Optical sensors offer several benefits in various applications, including:
In addition, they are well suited for monitoring multiple chemical and physical phenomena and are chemically inert, which is critical in hazardous and combustible environments.
Moreover, in light of the pandemic, the need for non-contact sensing is at an all-time high. Optical sensors can be used to design innovative solutions in industrial and commercial environments to facilitate safety and health compliance.
ROHM Solutions for Optical Sensing in Electronic Applications
ROHM is a provider of high-performance optical sensor solutions. ROHM sensors offer a high degree of sensitivity, which is critical in a wide range of applications, e.g., automation, motion sensing, measurement, security, surveillance, and many more.
The optical sensing solutions include proximity and ambient light sensors, photo-interrupters, infrared LEDs, photosensors, photodiodes, phototransistors, ambient light sensor ICs, and 4-direction detectors. ROHM’s optical sensors offer a wide operating temperature range (-25 to +85°C) and come in small-footprint packages for optimum space savings.
For more information about ROHM’s optical sensing solutions, please visit the website.
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