The Sensitivity and Detection Distance of a Photosensitive Sensor
The Sensitivity and Detection Distance of a Photosensitive Sensor
Photoelectric sensors utilize the primary properties of light to detect the presence or absence of an object. They include a light source, an emitter and a receiver that combine to detect changes in light intensity.
Sensors operate based on several primary properties of light: intensity, propagation direction, frequency and polarization. They may also incorporate additional concepts to solve specific applications.
The sensitivity of a photosensitive sensor is the amount of information it can detect when it is exposed to a specific light wavelength. It is an important factor in determining the accuracy of a test and its usefulness for detecting a disease or condition.
Sensitivity is often used in clinical practice as a way to judge the confidence in the results of a diagnostic test. It is also used by researchers to evaluate the effectiveness of new medical procedures and techniques.
A sensitivity value is calculated by subtracting the number of false negatives from the number of true positives, and then dividing this by the number of tests performed. The more accurate the test is, the higher the sensitivity value.
This sensitivity value is a valuable way of comparing the reliability of different types of tests, and can be especially useful for screening tests, which only provide a general indication of whether or not a particular condition is present. In addition, it can be useful in evaluating the effectiveness of treatments for conditions that are difficult to diagnose or have serious side effects.
Depending on the type of condition, a test’s sensitivity may be lower or higher. It can also be influenced by the level of sampling, the type of material examined, the timing of sampling in the disease process and other factors.
Some tests, like those for coronavirus, have a low initial sensitivity because it takes a long time for antibodies to develop. However, once the body’s immune system has developed a response to the molecule being tested for, the test’s sensitivity increases.
Another example is a blood test. The sensitivity of a blood test is often measured by how many people who have the disease test positive for the test. It is also influenced by the level of testing and the time of year when a person is tested.
Those who are sensitive to their surroundings are more prone to physical and emotional distress, but they tend to be particularly receptive to positive experiences. This phenomenon is referred to as environmental sensitivity and is photosensitive sensor largely due to genetic differences, although it can also be shaped by the environment and life experiences of a person.
Detection distance is the distance between the light source of the photoelectric sensor and the object being detected. The distance can vary with different types of sensors, but typically ranges between one and several meters.
There are three primary modes of detection for photoelectric sensors: direct reflection, diffused mode and opposed or through-beam (thru-beam). Direct reflection consists of a transmitter and receiver that housed in a single unit. When an object breaks the beam of light between the two, it interrupts the output of the receiver, triggering the sensor.
Diffused mode consists of two separate housings, one for the transmitter and one for the receiver. The light from the transmitter is reflected off the target and returned to the receiver. When an object is within the range of the diffused beam, it is picked up by the receiver and the signal is sent to the PC via a signal processor.
The sensitivity of a diffused mode sensor is dependent on the amount of light reflected from the target and how much of it gets absorbed by the dark surface of the object. The amount of light reflected from the target determines the threshold required to trigger the sensor, and this can greatly vary with different types of objects and their reflectivity.
Some sensors also use a time of flight measurement of the light as it bounces back from the target and is received by the sensor. This allows the sensor to triangulate the distance between the light and the target without relying on a single, fixed sensing window like the convergent beam mode method.
A specialized type of diffuse reflective sensor, Perfect Prox, combines extremely high sensing power with a sharp optical cutoff to reliably detect targets. This means it can detect targets regardless of color, reflectance, contrast or surface shape, while ignoring background objects that are just slightly beyond the target’s range.
In addition to diffused mode, there are a number of other modes of operation for photoelectric sensors. Some of them are application specific, meaning they solve unique problems in automation.
Mechanical Background Suppression
Detection distance with diffuse sensors can be affected by the color, shape or surface finish of a background object. These can cause the sensor to report false detection results, resulting in an error in the output signal.
This can be mitigated with a photoelectric sensor that features background suppression. A background suppression sensor consists of an emitter and receiver that is embedded into one housing. The receiver then measures the angle of reflected light to determine how much it is impacted by the background.
In most cases, background suppression is accomplished mechanically. This is typically achieved by aligning the sensor receiver with an adjusting screw to ensure that only reflected light from the desired working range strikes the sensor receiver.
The sensitivity of the sensor can also be adjusted to “tune out” the background. This can be done by reducing the sensitivity to a very low value.
These types of sensors are used in a wide variety of applications. These include packaging machinery, automatic door systems, factory automation, and car washes.
A photoelectric sensor with mechanical background suppression uses a ‘triangulation principle’ to detect the position of a specified target by measuring the angle of reflected light from the target. This ‘triangulation’ is different than a diffuse sensor which only measures the amount of reflected light and thereby ignores the background.
This type of sensor can be manufactured in both an electromechanical and an electronic variant. The electronics allow the sensor to be reprogrammed via software.
The mechanical variant is generally less expensive than the electronic version, but requires a larger unit to achieve a higher sensitivity setting. This is a tradeoff that may be worth making, depending on the application.
In general, this type of sensor is used when the target and background are close together and are similar in reflectivity. This is particularly common in industrial applications that require tamper-proof detection.
Pepperl+Fuchs offers a number of models of photoelectric sensors with background suppression. These include the O200, which is a miniature sensor that can be installed in tight spaces and features increased functional reserves.
Electronic Background Suppression
Unlike fixed-focus and sharp-cutoff sensors that achieve background suppression through the inability photosensitive sensor to see the background, true background-suppression photoelectrics actively sense both target and background. Using light triangulation, these photoelectrics create a distinct focal plane that effectively excludes targets outside of this region.
The sensor emits light at a specific intensity and frequency, which is then reflected back to the receiver by a reflector installed on the opposite side of the sensor housing. The reflected light is then electronically evaluated at a particular point, which determines the presence of the target object.
Most photoelectric sensors use pulse modulated or non-modulated light that continuously emits and reflects at set intervals to detect objects located some distance away from the sensor. The ability to accurately control the sensing range of these devices is critical for many applications.
One disadvantage of the standard diffused mode is that it has a relatively high black-white differential. This is because the more reflective side of the target is able to reflect more light back to the receiver than the less reflective side.
In order to overcome this problem, some companies have introduced a new type of diffused mode that is much less sensitive to the color of a target: it uses background suppression to ignore the most highly reflective surfaces almost directly behind the object being detected. This type of sensing mode is very useful in many applications and can help reduce the number of reflective surfaces that the sensor needs to detect.
Another benefit of this type of sensor is that it offers an adjustable sensing range. Some models allow the user to adjust the detection range using a mechanical or electronically calibrated knob.
The most important aspect of background suppression is that it is a far better option than the standard diffused sensor when it comes to detecting small objects in difficult situations. It also provides a very bright and clearly defined light spot to aid in alignment, particularly if the background suppression sensor is powered by a visible red light source.
A few of the most common issues that can affect the operation of a background suppression sensor are the cross-eyed effect, blinding effects and the Pepita effect (see Figure 8). The cross-eyed effect occurs when the LED light spot of the sensor is smaller than the surface the target is trying to cover. This causes the target to be unable to block the light beam and thus confuses the sensor!