Nowadays most photoelectric sensors use LEDs as light sources. An LED is a semiconductor, electrically similar to a diode, but with the characteristic that it emits light when a current flows through it directly.

LEDs can be built to emit green, blue, yellow, red, infrared, etc. The most commonly used colors in detection applications are red and infrared, but in applications where contrast needs to be detected, the choice of emission color is essential, with green being the most used color.

Phototransistors are the most widely used components as light receivers, because they offer the best relationship between light sensitivity and response speed, compared to photoresistive components, and they also respond well to visible and infrared light.

Photocells are used when great sensitivity is not necessary, and a visible light source is used. On the other hand, photodiodes where extreme response speed is required.

Common light sources.

With the exception of infrared, LEDs produce less light than the incandescent and fluorescent sources that commonly illuminate the environment. Modulation of the light source provides the sensing power necessary to reliably detect at these low light levels. Many photoelectric sensors use modulated light emitting diodes and phototransistor receivers.

The LEDs can be “on” and “off” (or modulated) with a frequency that is normally around one kiloHertz).

The operation of sensors that do not have modulated light is limited to areas where the receiver does not receive ambient light and only receives light from the emitter. A modulated receiver ignores the presence of ambient light and responds only to the modulated light source.

Infrared LEDs are the most effective and are also the ones with the spectrum that work best with phototransistors; It is for this reason that they are used in many applications. However, photoelectric sensors are also used to detect contrast (mark detection) or color, and for this the light is required to be visible.

The excess gain curve is specified in each type of photoelectric sensor, and it is a function of the sensing distance.

This curve is used when selecting the sensor, to predict the reliability of detection in a known environment.

light barrier

The emitter-receiver type barriers are composed of two parts, a component that emits the light beam, and another component that receives it. A detection area is established where the object to be detected is recognized when it interrupts the light beam. Because the mode of operation of this class of sensors is based on interruption of the light beam, the detection is not affected by the color, texture or brightness of the object to be detected. These sensors operate precisely when the emitter and receiver are aligned.

The light only has to pass through the workspace once, so long operating distances, up to 60 meters, are favored. They are appropriate for unfavorable environmental conditions, such as dirt, humidity, or outdoor use, as well as, regardless of the color of the object, they perform precise detection of the object.

Installation is made difficult by having to place two separate devices with the optical axes aligned precisely and delicately, since the detector emits in infrared. In addition to the impossibility of the objects to be detected being transparent.

When mounting, the reflective surfaces close to the devices must be taken into account, causing the photocell to malfunction. It is also necessary to take into account possible mutual interference due to the proximity of several of these devices, in addition to controlling dirty environments, since dirt negatively affects the emitting lens.

Reflection on mirror

They have the emitting component and the receiving component in a single body, the light beam is established by using a catadioptric reflector. The object is detected when the beam formed between the emitting component, the reflector and the receiving component is interrupted. Because of this, detection is not affected by its color. The advantage of reflex barriers is that the wiring is on one side only, unlike emitter-receiver barriers which are on both sides.[1]​.

In these photocells the light beam travels twice the detection distance, which means that the working distances achieved are average (about 15 meters). The mirror is easy to install, and no wiring is required to it, so you only have to wire one detector. In addition to being valid for detection of opaque objects, they also efficiently cover applications with detection of objects with a certain degree of transparency.

The most striking problem is that the object to be detected has to be larger than the mirror and, if possible, non-reflective, in addition to the alignment having to be precise.

An object with a reflective surface can cause detection errors. This can be avoided by ensuring that the reflection of the object to be detected does not have the same inclination as the detector beam.

Reflection on object

Infrared light travels in a straight line. When an object is in the way, the light beam bounces against it and changes direction, allowing the light to be sent to the receiver and the element to be sensed. A black object is not detected since this color absorbs the light and the sensor does not experience changes. There are two types of object reflection photocells, those with diffuse reflection and those with defined reflection.

In diffuse reflection photocells on the object, the emitter launches a beam of light; The rays of the beam are lost in space if there is no object, but when an object is present, its surface produces a diffuse reflection of light, part of which falls on the receiver and thus changes the output signal of the photocell.

The reflection on the surface of the object to be detected by the defined reflection photocells is normally diffuse in nature, as in diffuse reflection sensors, that is, the reflected light comes out without a determined path.

This is very important, so as not to fall into the false idea that the difference with diffuse reflection sensors is in the type of reflection; It is in the type of optics used.

In defined reflection photocells the light source is at a distance greater than the focal length, so the beam converges to a point on the optical axis.

Object reflection photocells consist only of an emitter and a receiver mounted under the same housing, so assembly is simple and quick. In these photocells the light beam travels twice the detection distance and the object may also have low reflectivity, so only small detection distances are achieved (generally less than one meter.

Nowadays most photoelectric sensors use LEDs as light sources. An LED is a semiconductor, electrically similar to a diode, but with the characteristic that it emits light when a current flows through it directly.

LEDs can be built to emit green, blue, yellow, red, infrared, etc. The most commonly used colors in detection applications are red and infrared, but in applications where contrast needs to be detected, the choice of emission color is essential, with green being the most used color.

Phototransistors are the most widely used components as light receivers, because they offer the best relationship between light sensitivity and response speed, compared to photoresistive components, and they also respond well to visible and infrared light.

Photocells are used when great sensitivity is not necessary, and a visible light source is used. On the other hand, photodiodes where extreme response speed is required.

Common light sources.

With the exception of infrared, LEDs produce less light than the incandescent and fluorescent sources that commonly illuminate the environment. Modulation of the light source provides the sensing power necessary to reliably detect at these low light levels. Many photoelectric sensors use modulated light emitting diodes and phototransistor receivers.

The LEDs can be “on” and “off” (or modulated) with a frequency that is normally around one kiloHertz).

The operation of sensors that do not have modulated light is limited to areas where the receiver does not receive ambient light and only receives light from the emitter. A modulated receiver ignores the presence of ambient light and responds only to the modulated light source.

Infrared LEDs are the most effective and are also the ones with the spectrum that work best with phototransistors; It is for this reason that they are used in many applications. However, photoelectric sensors are also used to detect contrast (mark detection) or color, and for this the light is required to be visible.

The excess gain curve is specified in each type of photoelectric sensor, and it is a function of the sensing distance.

This curve is used when selecting the sensor, to predict the reliability of detection in a known environment.

light barrier

The emitter-receiver type barriers are composed of two parts, a component that emits the light beam, and another component that receives it. A detection area is established where the object to be detected is recognized when it interrupts the light beam. Because the mode of operation of this class of sensors is based on interruption of the light beam, the detection is not affected by the color, texture or brightness of the object to be detected. These sensors operate precisely when the emitter and receiver are aligned.

The light only has to pass through the workspace once, so long operating distances, up to 60 meters, are favored. They are appropriate for unfavorable environmental conditions, such as dirt, humidity, or outdoor use, as well as, regardless of the color of the object, they perform precise detection of the object.

Installation is made difficult by having to place two separate devices with the optical axes aligned precisely and delicately, since the detector emits in infrared. In addition to the impossibility of the objects to be detected being transparent.

When mounting, the reflective surfaces close to the devices must be taken into account, causing the photocell to malfunction. It is also necessary to take into account possible mutual interference due to the proximity of several of these devices, in addition to controlling dirty environments, since dirt negatively affects the emitting lens.

Reflection on mirror

They have the emitting component and the receiving component in a single body, the light beam is established by using a catadioptric reflector. The object is detected when the beam formed between the emitting component, the reflector and the receiving component is interrupted. Because of this, detection is not affected by its color. The advantage of reflex barriers is that the wiring is on one side only, unlike emitter-receiver barriers which are on both sides.[1]​.

In these photocells the light beam travels twice the detection distance, which means that the working distances achieved are average (about 15 meters). The mirror is easy to install, and no wiring is required to it, so you only have to wire one detector. In addition to being valid for detection of opaque objects, they also efficiently cover applications with detection of objects with a certain degree of transparency.

The most striking problem is that the object to be detected has to be larger than the mirror and, if possible, non-reflective, in addition to the alignment having to be precise.

An object with a reflective surface can cause detection errors. This can be avoided by ensuring that the reflection of the object to be detected does not have the same inclination as the detector beam.

Reflection on object

Infrared light travels in a straight line. When an object is in the way, the light beam bounces against it and changes direction, allowing the light to be sent to the receiver and the element to be sensed. A black object is not detected since this color absorbs the light and the sensor does not experience changes. There are two types of object reflection photocells, those with diffuse reflection and those with defined reflection.

In diffuse reflection photocells on the object, the emitter launches a beam of light; The rays of the beam are lost in space if there is no object, but when an object is present, its surface produces a diffuse reflection of light, part of which falls on the receiver and thus changes the output signal of the photocell.

The reflection on the surface of the object to be detected by the defined reflection photocells is normally diffuse in nature, as in diffuse reflection sensors, that is, the reflected light comes out without a determined path.

This is very important, so as not to fall into the false idea that the difference with diffuse reflection sensors is in the type of reflection; It is in the type of optics used.

In defined reflection photocells the light source is at a distance greater than the focal length, so the beam converges to a point on the optical axis.

Object reflection photocells consist only of an emitter and a receiver mounted under the same housing, so assembly is simple and quick. In these photocells the light beam travels twice the detection distance and the object may also have low reflectivity, so only small detection distances are achieved (generally less than one meter.