Retroreflective Mode Sensing

How it Works:
Unlike an opposed-mode sensor, a retroreflective sensor contains both the emitter and receiver elements. The effective beam is established between the emitter, the reflector, and the receiver.  As with an opposed-mode sensor, an object is sensed when it interrupts or "breaks" the effective beam. Most reflectors are made up of many small corner-cube prisms. A light beam enters a corner cube prism through its hypotenuse face and is reflected from the three surfaces. In this way, the reflector returns the light beam to its source.  Most corner-cube reflectors resemble bicycle reflectors, and are molded using clear acrylic plastic, manufactured in various sizes, shapes, and colours. If an opposed-mode sensor is not an option, then a retroreflective-mode sensor may be a good second choice. Retroreflective mode sensors offer relatively long ranges.

Reliable sensing
Retroreflective sensing is a beam-break mode. So, it is generally not dependant upon the reflectivity of the object to be detected. For this reason, the retroreflective mode is a relatively reliable sensing mode.

Reflector Prisms

Pros: Convenience
A retroreflective-mode sensor offers a convenient alternative to opposed mode when sensing is possible only from one side, or if electrical connections are only possible on one side.

Convenient Connections
-At one end only

Minimising "Proxing"

Cons: Less Excess Gain
Retroreflective-mode sensors lose excess gain twice as fast as opposed-mode sensors, due to dirt build-up on both the reflector and the sensor lenses. This is because the light travels through four lenses, once from the emitter to the reflector and back from the reflector to the receiver.  There is also much less available excess gain in a retroreflective mode sensing beam, due to the inefficiencies of the reflector and because the light must travel twice as far to reach the receiver, as compared to the opposed mode.

Cons: Effective Beam
It's difficult to create a small effective beam with a retroreflective mode sensor, so avoid using this mode for detecting small objects or for precise positioning control. We can offer some retroreflective sensors that have an effective beam of less than 25mm.

Effective Beam

Cons: Clear Materials
In the retroreflective mode, an object must interrupt the beam to be detected. As with opposed mode, it is not recommended that you use a retroreflective mode sensor to detect transparent or translucent objects.
However there are several sensors designed specifically as clear object detectors in low excess gain environments.

Clear Objects

Cons: Shiny Materials
The optics of a good quality retroreflective sensor are designed and assembled with great care to minimize "proxing" (undesirable reflection of the sensing beam directly back from an object that is supposed to break the beam).  However, an object with a shiny surface that presents itself perfectly parallel to a retroreflective sensor may return enough light to cause that object to pass by the sensor, undetected. This problem can be compensated for by angling the sensor relative to the object to avoid direct reflection or by using an anti-glare or polarizing filter.

Cons: Reflector Size and Type
Except at close range, the size of the reflector becomes important. The width of the beam pattern for each retroreflective sensor serves as an estimate of how much reflector area should be used to return the maximum amount of light. Reflector size does affect the range. The smaller the target size, the smaller the effective beam, and the shorter the range.  
Also the efficiency of different reflector material types varies

Reflector Size

Cons: Blind Spot at Close Range
Most retroreflective sensors are designed for long-range sensing, and suffer a "blind spot" at close range.  A "Blind Spot" is an area close to a sensor lens, where light energy is returned to the emitter rather than the receiver, rendering the sensor effectively blind. This effect is most pronounced with some retroreflective sensors. Check the excess gain curve of your retroreflective mode sensor to see where the "blind spot" occurs.

Blind Spot

Application - Cardboard Box Detection
 

Objective:
To reliably count boxes moving on a high-speed conveyor line.

The retroreflective optics of a MINI-BEAM sensor are excellent for reliably sensing boxes on a conveyor belt. This sensor generates one solid count from each box moving through the beam.

Review

A retroreflective sensor contains both the emitter and receiver element.  The effective beam is established when the emitter sends a light beam which is bounced off a retroreflector, back to the emitter. An object is detected when it breaks this effective beam. Retroreflective-mode sensors offer reliability, and are convenient in applications where sensors can be mounted only on one side of a process.  However, retroreflective sensors can lose gain twice as fast as opposed mode sensors, and they aren't always the best choice for sensing shiny, clear, or very small objects.

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