Three sensing modes are available with photoelectric switches - through beam, retro reflection and diffuse reflection. This application note explores the determination of the best sensing mode for a given application.

The best sensing mode is that one which yields the highest sensing contrast i.e. the greatest amount of difference in the intensity of light falling on the receiver between the conditions of target presence and target absence.
For the through beam and retro reflective modes, the intensity of light at the receiver is the highest in the absence of a target. The entry of the target into the light path will either block out or diminish the intensity of the light at receiver. The difference between these two intensities is the contrast.

For the diffuse reflection mode, the intensity of light at the receiver is the lowest in the absence of a target. The presence of the latter diverts some of the transmitted light to the receiver, thus increasing the intensity of light at the receiver. The difference between these two intensities is the contrast. To determine which sensing mode will yield the highest contrast the following application details must be evaluated: target size, shape, opacity and reflectivity.
Each of the three modes has its strengths and weaknesses, which are summarised below.

THROUGH BEAM

Strengths

• Provided the object is opaque (i.e. it completely blocks out the transmitted light from the receiver), this mode is the most reliable one. In any application its use should therefore be always considered first.

• For a given size this mode offers the highest sensing distance. Since the receiver is across from the transmitter and aligned with the latter, the highest amount of transmitted light energy (as compared with the other two modes) is available at the receiver, thus enabling higher distances.

• Since the highest intensity level is available at the receiver, this mode works reliably in highly contaminated environments i.e. those where a lot of dust, dirt, smoke, moisture etc may be present.

• Since the light path of this mode is the best defined of the three modes, it is best suited for parts counting applications. Where the size of the part is smaller than that of the light path, the size of the latter can be reduced by adding apertures to the two sensors (contact factory for details).

• Since this mode is unaffected by the target's colour and reflectivity, it is best suited for applications where targets have varying colours and reflectivities.

• Where transparent or translucent objects need to be detected, this mode should be avoided.

• Small targets (i.e. smaller than the effective beam width) cannot be reliably detected by this mode without the addition of apertures to the two sensors.

• When used at ranges much closer than those rated, this mode may actually sense through thin-walled opaque targets such as paper boxes, cloth, plastic bottles etc. the use of apertures over the two sensors will also help here.

• Where mounting space on both sides of the detection path is unavailable, this mode cannot be used.