Opposed Mode Sensing

How it Works:
In opposed-mode sensing, the sensor's emitter and receiver are housed in two separate units. The emitter is placed opposite the receiver, so that the light beam goes directly from the emitter to the receiver. An object is detected when it breaks the working part of the light beam, known as the effective beam.

Pros: Reliability
The opposed mode should be used whenever possible, because it is the most reliable sensing mode. This is because light passes directly from the emitter to the receiver.  Opposed-mode sensing offers the highest level of sensing energy to overcome both contamination in the environment and possible sensor misalignment from shock and vibration

 Pros: High Excess Gain
Because the light beam goes directly from the emitter to the receiver, this means that opposed-mode sensors have high excess gain. Opposed-mode sensing offers much higher excess gain (the measurement of the amount of light falling on the receiver of a sensing system over and above the minimum amount of light required to just operate the sensor’s amplifier) than any other mode of sensing, making them ideal in areas of high optical attenuation (Lessening or loss of signal intensity during transmission).

High excess gain is great for applications that need:

• Long range sensing

• Sensing through heavy dirt, dust, mist, condensation, oil, film, etc.

• Precise position sensing

• Small part detection using lens apertures (the aperture refers to the size of a lens opening which can sometimes be reduced by using a mechanical part attached to a lens used to restrict the size of the opening)

Pros: Impervious to Surface Reflectivity*
It doesn't matter how shiny or dark your object is, or even what color. The object must physically pass between the emitter and the receiver in order to be detected. Therefore, variables such as surface reflectivity, color, and finish don't affect opposed-mode sensing.  The object either "breaks" or interrupts the effective beam, or it doesn't.

*Surface Reflectivity:  A measure of the efficiency of any material surface as a reflector of light, as compared to a Kodak white test card, which is arbitrarily rated at 90% reflectivity. 

Pros: Parts counting
If you need a sensor to accurately count parts, then an opposed-mode sensor is perfect. Due to the well-defined effective beam*, opposed-mode sensors are most reliable for accurate parts counting, as long as the effective beam is no larger than the part.

*Glossary Term
Effective Beam: The "working" part of a photoelectric beam. Not to be confused with the actual radiation pattern of the emitter, or with the field of view of the receiver

Aperturing for Small Parts
If your object doesn’t completely block the effective beam, it runs the risk of not being detected.
You can compensate for this by using apertures, lenses, or fiber optics to shape the effective beam to match your part’s profile.

Cons: Clear Materials
In order to be detected, an object must be able to block the effective beam. Therefore, opposed-mode sensing should be avoided for the detection of translucent or transparent objects.

There are two exceptions:
MINI-BEAM clear plastic detection sensors can differentiate clear plastics from other materials.
The thick bottom section of glass containers may be used to block an opposed beam that has been shaped using rectangular apertures.

Cons: High Excess Gain at Short Range
Some opposed-mode sensors can "burn through" materials like paper, cloth, or plastics. It can be difficult to set a sensitivity control operating point on opposed-mode sensors for the same reason that opposed mode offers the best sensing contrast: too much excess gain.

You can lessen the signal strength by adding apertures, or by intentionally misaligning the emitter and the receiver.

Apertures

Objective: To separate air mail letters (by height) from other parcels in an automated airfreight operation.

Three vertical opposed beams are spaced across the width of the roller conveyor, so that one or more of the beams is interrupted by any passing parcel. The fourth opposed beam is positioned horizontally across the width of the conveyor, to inspect each parcel for height. If a parcel slips under the height inspection beam, the diverter mechanism is energized

Review

In opposed-mode sensor will have a separate emitter and receiver pair. The emitter sends the light beam to the receiver, which is positioned opposite the emitter.
For an object to be detected, it must pass between the two, and interrupt or "break" the beam of light.
Opposed-mode sensing is highly reliable, provides a large amount emitted energy, is impervious to surface reflectivity, and is excellent for parts counting.
However, opposed-mode sensing may not be the best solution for sensing clear materials.

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