What Is a Photosensitive Sensor?

Light sensors are used to detect the presence or absence of objects without direct contact. They can be mounted a long distance away from the object to be detected.

There are many types of photosensors that are used in everyday life. Some generate electricity, such as photovoltaic cells that convert solar radiation into direct current. Others change their electrical properties, such as a photoresistor that varies its resistance depending on the illumination intensity.

Emitter and Receiver

The main components of a photosensitive sensor are the Emitter and the Receiver. Light is emitted from the Emitter and then reflected by the object being detected. The amount of reflected light changes the amount of the signal that is received by the Receiver and then converted into an electrical output. This output is then used to trigger a system or device.

There are many different types of photosensitive sensors. Some generate electricity when illuminated and can be referred to as either photovoltaic (converting solar radiation into direct-current electricity) or photo-emissive (releasing free electrons from light sensitive materials such as cesium which then vary their electrical resistance when exposed to light). Others simply change their electrical properties when illuminated and can be referred to either as photoresistor (whose electrical resistance decreases with increasing incident light intensity) or as photo-conductor (which varies its electrical conductivity with incident light intensity).

These sensors are commonly used in manufacturing, packaging, paper, transportation, and other industrial applications. They are able to detect the presence or absence of objects in a very broad range of distances, making them ideal for applications that require wide area coverage, such as detecting and counting products in assembly line operations or conveyor systems. They are also able to detect more narrow objects than would be possible with magnetic or ultrasonic sensors due to their ability to be more precisely focused in a single point.

Mechanical Background Suppression

Diffuse photoelectric sensors work well for Microwave sensor most applications but there are some that require special features like background suppression. This mode allows the sensor to ignore very reflective backgrounds that are almost directly behind a dark, less-reflective object. Sensors with this feature are more complex and typically cost more than other diffuse models.

Mechanical background suppression uses a pair of receivers built into the sensor to measure the angle of light reflected from a target and a background, a process called optical triangulation. The sensor only activates the output when the reflected beam hits the targeted object. This eliminates the possibility of the sensor detecting the background as an object and not activating its output.

Depending on the model, background suppression photoelectric sensors can be fitted with either visible red or infrared (IR) LEDs as the light source. Visible red allows for easy alignment, while IR reduces the power consumption of the sensor and can operate at longer sensing distances.

GLV18 series photoelectric sensors with background suppression are available in a variety of mounting options, including flush-mount brackets that allow for a clean installation. They also come in an M18 threaded cylindrical housing that is up to 50% shorter than many competitive versions, allowing them to be mounted in tight spaces where other sensors cannot. Moreover, these sensors consume 50% less power than their competitors, reducing total energy costs.

Electronic Background Suppression

Many applications that are difficult to solve using standard diffuse sensors may be addressed with a background suppression sensor. These sensors offer a cost-effective solution to problems such as false triggers caused by ambient light or other reflections from the environment. They work by utilising an internal second receiver to recognise the conditions in which they operate, and can switch between foreground and background modes accordingly.

In addition to reducing the effects of external reflections, a background suppression sensor has the ability to distinguish between a target and similar-textured, coloured backgrounds. This is achieved by a mechanism that measures the distance of a target from a fixed focal point on the sensor, rather than a fixed distance from the lens.

Because of this, these sensors require a separation gap between the target and the background that is equal to the sensor’s sensing range. This separation gap varies depending on the difference in reflectivity between the target and the background, with darker targets requiring a larger gap than lighter ones.

In order to reduce this gap, most of the available models come in either a fixed or adjustable background suppression mode. Fixed-focus sensors have a stationary focal plane that can only be adjusted by changing the angle of the internal mirror, meaning that adjusting the sensor also changes the sensitivity of the sensor. By contrast, adjustable-focus sensors have a focal plane that can be dialed in to match an application, with sensitivity remaining constant.

Two-Part Photodiode

The photodiode is divided into two different parts: the p-type and photosensitive sensor n-type layers. The n-type layer is lightly doped with holes, and the p-type layer is heavily doped with electrons. When reverse bias is applied to the photodiode, electrons from the n-type layer pull towards the positive terminal and holes from the p-type layer move toward the negative terminal. This creates a current that is proportional to the amount of light that reaches the device.

The maximum reverse voltage that a photodiode can tolerate is called the breakdown voltage. It is the largest reverse voltage that can be applied to the device before an exponential increase in leakage current (dark current) occurs. It is important to design a circuit that operates the photodiode well below its breakdown voltage so that it doesn’t suffer damage.

The photodiode’s resistance is determined by the width of its depletion region, which is inversely proportional to the voltage applied to the device. The device’s capacitance is also influenced by the width of this region, and it influences the photocurrent and the response time of the device. The device’s responsivity, which is measured in amps per watt of incident light power, is a good indicator of its performance at high wavelengths and low optical powers. A photodiode with a larger active area will have a higher responsivity and will be more sensitive to light.