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.
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.
