The Role of Reflectors in Sensor Technology
Reflectors are used in sensor technology for safe and reliable operation of retro-reflex sensors. These are usually retro-reflectors, which are built up from a large number of triple mirrors as a reflection surface. The reflection surface is usually built into a housing and is protected from environmental influences by a pane. A special feature of the so-called retro-reflectors is that the three-dimensional triple mirror structures reflect light almost completely in the direction of the light source. In addition to the retro-reflectors, there is also the reflector foil, which is a reflector in foil form and is therefore flexible and frequently self-adhesive. The reflective surface is arranged under a transparent top layer for protection against environmental influences.
The Following Types of Reflection Exist
We differentiate between three different types in the reflection of light. These are dependent on the surface finish the light beam hits.
Diffuse Reflection
Diffuse reflection, also known as scatter, occurs on rough, uneven surfaces. The light beams are remitted irregularly in all directions. Only a small portion of the light is reflected back towards the source.
Mirror Reflection
Regular reflection occurs on mirror-smooth and shiny surfaces. The incident light beam is reflected according to the reflection law (angle of incidence corresponds to the angle of reflection).
Retro-Reflection
With retro-reflection , the light beam is reflected back in the direction it came from. This type of reflection is typically used for retro-reflex sensors and in combination with time-of-flight sensors.
How a Retro-Reflector Works
A retro-reflector is a reflector that reflects the incident light in the direction of incidence, i.e. back to the light source, regardless of the angle of incidence. The illuminated surface (reflector structure) has a particularly fine angular structure consisting of many small triple mirrors.
The Principle of the Triple Reflector
With a triple reflector, three mirror surfaces are perpendicular to each other and form the inner corner of a cube, also known as a “corner cube”. The light beam hits the first planar mirror surface and is reflected onto an adjacent reflection surface of the triple structure according to the reflection law (mirror reflection). The light beam is then reflected onto two further adjacent reflection surfaces according to the same principle. With a slight beam offset (relative to the incident beam), the light beam is thus deflected back parallel to the light source. This type of reflection is called retro-reflection.
The Operating Principle of a Retro-Reflex Sensor with Reflector or Reflector Foil
The functional principle of the retro-reflex sensor with retro-reflector is based on polarization and polarizer.
What is polarization?
A light beam can be regarded as an electrometric wave. The light waves oscillate in different vertical and horizontal directions. The polarization of light describes the direction of oscillation. If the light has no preferred direction, it is known as non-polarized light.
A light beam can be regarded as an electrometric wave. The light waves oscillate in different vertical and horizontal directions. The polarization of light describes the direction of oscillation. If the light has no preferred direction, it is known as non-polarized light.
What is a linear polarizer?
A linear polarizer is a filter that transmits the light in a specified direction of oscillation (vertically polarized, for example), while light is not transmitted orthogonally in the direction of oscillation. The transmitted light is thus linearly polarized according to the orientation of the polarizer.
A linear polarizer is a filter that transmits the light in a specified direction of oscillation (vertically polarized, for example), while light is not transmitted orthogonally in the direction of oscillation. The transmitted light is thus linearly polarized according to the orientation of the polarizer.
How a Retro-Reflex Sensor with Reflector Works
The functional principle of a retro-reflex sensor uses the properties of the polarizer in combination with the optical properties of the retro-reflector.
- A sensor with a built-in polarizer emits light in the vertical direction of oscillation.
- The light hits a retro-reflector with triple-mirror structure. The light beam is reflected on the three sides of the mirror, whereby the polarization of the light is rotated to a certain extent from the vertical to the horizontal direction.
- A horizontal polarization filter is installed on the receiver side in the sensor. The polarization filter transmits light with horizontal polarization (coming from the reflector) to the receiving element in the sensor.
- If an object is now inserted into the beam path between the sensor and reflector, the signal on the receiver side is attenuated and the object is thus detected. By using the polarization of the light, a retro-reflex sensor also works with glossy objects. In contrast to the retro-reflector, these do not rotate the direction of polarization.
This Must be Taken into Account when Selecting the Reflector
There are several factors to consider when selecting the reflector. It is an interplay between the reflector structure, the light source as well as the range and the sensor optics (single-lens optics and two-lens optics). The reflector size should also be taken into account when making the decision. The ambient conditions, to which a reflector is exposed, also play a major role in the choice of suitable reflector and reflector foil.
Where reflectors and reflector foils are concerned, structure describes the shape of the elements (triple, cube corners) on the reflective surface. These can have different triple structure sizes. From very small continuous structures, triple structures in the micrometer range (microstructure), to macroscopic triple (macrostructure) or honeycomb structures.
For many applications, it is important that the light from the emitter hits as many triple structures of the reflector as possible so that as much light as possible can be retro-reflected and the received signal is stable. If a light beam with a small beam diameter (e.g. laser light) encounters a small number of triples, for example, the received signal may not be stable if vibrations occur in the application. This, in turn, can lead to malfunctions.
For many applications, it is important that the light from the emitter hits as many triple structures of the reflector as possible so that as much light as possible can be retro-reflected and the received signal is stable. If a light beam with a small beam diameter (e.g. laser light) encounters a small number of triples, for example, the received signal may not be stable if vibrations occur in the application. This, in turn, can lead to malfunctions.
This Light Source is Well Suited for Microstructures
A laser beam typically has a very small beam divergence and a small beam diameter (up to less than one millimeter). For this reason, small triple structures with a micro or continuous structure are generally the recommended reflector variant here. If, on the other hand, the laser light meets a small number of triples, the received signal may be unstable and interference may occur under the influence of vibrations in the application.
A reflector with microstructure is also advisable for retro-reflex sensors for transparent materials, as slight changes in light intensity must be detected.
A reflector with microstructure is also advisable for retro-reflex sensors for transparent materials, as slight changes in light intensity must be detected.
This Light Source is Well-Suited for Macrostructures
For many applications, it is important that the light from the emitter hits as many triple structures of the reflector as possible so that as much light as possible can be retro-reflected and the received signal is stable. A retro-reflex sensor with red light typically has a larger beam diameter (several cm) and can therefore be combined with a reflector with larger triple structures, such as macrostructure or honeycomb structure. The advantage of a large triple structure is the degree of reflection, because the larger the triple, the more incident light is reflected from the surface and the greater the achievable range.
The choice of reflector is largely influenced by the shape of the light beam. The beam path and the distance between the sensor and reflector are important influencing factors. The reflector is selected to match the beam diameter. For long distances, laser sensors and reflectors are combined with macrostructures, provided that the reflector is placed far behind the focal point (and the beam diameter is correspondingly larger).
Divergent Light Beam
The beam diameter of a divergent light beam becomes larger in the direction of propagation (for example, emitted light from an LED)
An important characteristic of a retro-reflector is that as much light as possible is reflected back towards the light source – regardless of the triple size. There are effects that should be taken into account during positioning, depending on the sensor type.
How Does Single-Lens Optics Work?
Sensors with single-lens optics have only one lens which is used by the emitter as well as the receiver. They are characterized by a very small minimum distance between sensor and reflector. This is possible because the light is reflected directly back to the light source where the receiver lens picks up the reflected light.
How Does Two-Lens Optic Work?
With two-lens optics, the emitter and receiver lenses are housed separately in a housing. To avoid reflected light hitting the emitter and not the receiver, the sensor and reflector must be positioned at a sufficient distance from each other. This applies in particular to applications in close range. With triple structures with an aperture angle of ≠ 90°, light beams are reflected back into a larger spatial angle so that light hits the receiver. It should be noted that the specifications in the data sheet must be observed when installing a reflector, as the range of the sensor (reference reflector) can be smaller or larger with other reflectors. The minimum distance of the reflector (lower limit of the range) should not be confused with a so-called blind spot, which refers to the object to be detected.
Reflector Position for Two-Lens Optics
The correct distance between the sensor and reflector determines how much light is reflected back to the light source. If the reflector is too close to the sensor (yellow area), the reflected light does not hit the receiver and no signal is generated. If the reflector is too far away from the sensor (red area), the reflected light is too weak to trigger a signal. If the reflector is placed at a sufficient distance from the sensor (green area), an object located in the gray area can be detected, because the signal on the receiver side is weakened.
The size of the reflector should be adapted to the light spot of the incident light beam.
• The larger the reflector or reflector foil, the easier it is to align the sensor at large distances.
• Small reflectors can be used in close range and where space is limited.
• The larger the reflector or reflector foil, the easier it is to align the sensor at large distances.
• Small reflectors can be used in close range and where space is limited.
If the light beam is exactly the same size as the reflector and there is a large distance between the sensor and the reflector, even small vibrations can move part of the beam away from the reflector. This causes a change in signal strength, which also occurs in the event of temperature fluctuations (e.g. in the area where the sensor or reflector is mounted). It is therefore advisable to choose a slightly larger reflector compared to the beam diameter.
Mounting Options for Reflectors and Reflector Foils
Different attachment options can be selected depending on the application and intended use of the reflectors.
Mounting Holes
Slotted or round holes are built into the reflector housing diagonally, opposite or next to each other.
Screw
Round reflectors can be screwed directly into drill holes with an injected metric screw in the housing.
Fixing Plugs
Round reflectors with clip attachment using an integral plug enable easy installation in holes.
Self-Adhesive
Reflectors and, above all, reflector foils have a self-adhesive backing, which can be stuck onto walls, sheet metal sides or other surfaces.
Influential Factors on Reflectors and Reflector Foils
Rough high-pressure cleaning, high temperatures, intensive cleaning processes and external mechanical shocks and vibrations – in industrial environments, sensors and reflectors are often subject to unusual requirements. wenglor offers detergent-resistant and sturdy reflectors for these cases.
- Detergent-resistant reflectors are suitable for use in washdown areas and can be used in a temperature range of up to 150 °C.
- ECOLAB-certified reflectors are also suitable for washdown applications. The reflectors are distinguished by their blue color, which makes it easier to see housing chips in the event of damage.
- An anti-fog coating prevents the reflectors from misting up.
- Reflectors with a robust housing design are made of 316L stainless steel and have a glass cover over the reflection surface.
