The Sensitivity Capabilities of a Photosensitive Sensor
A photosensitive sensor is a device that converts radiant energy from the visible or infra-red parts of the light spectrum into electrical signal output. This makes it useful for a variety of applications.
This simple photosensitive resistance module has an on-board potentiometer for adjusting the sensitivity to detect very faint ambient light. It can be used to trigger relay modules.
Detection
The ability to detect objects or changes in surface conditions is one of the primary capabilities of a photosensitive sensor. The devices are used in a number of different industries to help automate processes and increase company productivity.
A photosensitive sensor is comprised of an Emitter for emitting light and a Receiver to capture that light. When the emitted light is interrupted or reflected by an object, it causes a change in the electrical output from the Receiver. This change is converted into an output signal to indicate the presence of an object.
Unlike Proximity Sensors that are limited to detecting metal objects, these sensors can detect a broad range of materials, including glass and plastic. Several factors can affect the detection capabilities of a Photoelectric Sensor, such as color, texture, angle of incident, and target characteristics.
To expand their sensing capabilities, these devices can be modified with features such as polarization to minimize false reflections, visible or invisible beam, and adjustable response time. They also come in various sizes to accommodate a variety of mounting applications and environmental conditions. Moreover, these sensors can be used with a variety of signal outputs such as pulse, digital, and relay. A basic photosensitive sensor can be built with a simple circuit such as a relay output light activated switch.
Sensitivity
The sensitivity capabilities of a photosensitive sensor determine how much light is required to trigger a signal. This is important as not all light is created equal and different wavelengths are reflected or absorbed differently. For example, long wavelengths like red and infra-red are more absorbed by the body and can be hard to detect.
The more light a photosensitive sensor can detect the higher its sensitivity will be, but this also means it can also produce more false positive test results. For diagnostic and screening tests, a balance is Microwave sensor needed between sensitivity (how many true positives are detected) and specificity (how few false negatives are produced).
This is illustrated in the graph below. The black dotted line marks the point where the test’s sensitivity and specificity are equal at 100%. As you move to the left or right of this line the sensitivity increases but the specificity decreases.
Another light sensor type to mention is the Phototransistor. This is similar to a Photodiode but with the addition of an amplifier. This provides a far greater increase in light sensitivity than Photodiodes alone. The phototransistor consists of a bipolar NPN transistor with photosensitive sensor its large base region electrically unconnected, but when light falls on the base it causes electron/hole pairs to be generated that cause a collector to emitter current to flow.
Electronic Background Suppression
Unlike most photoresistive sensors, which use an LED to transmit light, background suppression sensors use multiple receiver elements to detect and differentiate objects. As shown in Figure 3, the reflected beam from a target is directed toward receiver element E1 while the reflected light from the background is directed to element E2. By observing which element has more of a reflected signal, the sensor determines whether the object is a threat and activates its output.
Because they are far less sensitive to reflected light color, background suppression sensors can achieve much shorter black-white differentials than standard diffuse mode sensors. They are especially useful for transparent and translucent targets placed against a dark or reflective background that might otherwise interfere with detection.
Sensors with background suppression are available in both diffuse and through-beam configurations. They also have either a fixed or adjustable sensing range. The difference is that a fixed-focus background suppression sensor has no sensitivity adjustment, while an adjustable one allows users to change the plane of the set range.
Pepperl+Fuchs GLV18 Series background-suppression photoelectric sensors, for example, have a mechanically adjustable sensing range with a maximum black and white distance of 120 mm. Their small, compact housings meet a variety of mounting and positioning requirements while delivering excellent performance and reliability in harsh environments. They are ECOLAB certified and IP69K sealed.
Convergent Beam Mode
In Beam Break or Beam Make modes, the sensor is capable of sensing an object that breaks or diminishes an existing light beam path between the emitter and receiver lenses. A passing opaque object interrupts or diminishes the intensity of the sensor’s light signal and the output circuit of the photosensitive sensor is activated (see Figure 1-1-1).
In Convergent Beam Mode, a light source, such as an LED, emits unpolarized light. Because light waves oscillate both vertically and horizontally, this emitted light cannot pass through optical filters that constrain oscillations to a particular direction. When this unpolarized light passes through a reflective surface, such as a piece of glass, its plane of polarization is rotated and the nonpolarized return wave is blocked from entering the sensor’s receiver. As a result, the effective beam is narrow and highly focused.
To operate in the convergent beam mode, the sensor is equipped with a grating-based imaging system that generates a convergent beam containing multiple wavelengths. Depending on the desired wavelength(s), the sensor can be designed with different periods of chirped gratings that are mounted in the IMI substrate. The tendencies of these grating periods in the x position for the generation of a convergent beam containing a single wavelength, or involving the red, green and blue light are plotted in Figure 6(a) and (b). When operating in the convergent beam mode, these gratings provide the ability to detect an object that is larger than the effective diameter of the convergent beam and to perform a Beam Make measurement without compromising the sensing range or background suppression capabilities.
