Reflector-based photoelectric sensors: selection, wiring and practical use

A reflector-based photoelectric sensor has its emitter and receiver in one housing and detects objects by monitoring light returned from a separate reflector mounted opposite it. Within photoelectric sensing, this arrangement is known as retro-reflective sensing. When a target enters the path between the sensor and reflector, it sufficiently reduces or interrupts the return. The output then changes according to the selected operating mode.

In industrial automation, applications include box passage detection, package counting and product presence checks. In robotic applications, a suitable installation can detect a part at a required location. The target does not have to reflect light back. The reflector provides the return. When selecting from the wider photoelectric sensor family, first identify which component returns the light.

How does reflector-based sensing work?

The light emitter, often using an LED, directs light towards the reflector. Returned light is converted into electrical information by the receiver arrangement, which may contain a photodiode. The electronics compare that information with a switching threshold. There must be sufficient return light without a target and sufficient light loss when the target is present.

Receiver construction varies by model; not every photoelectric sensor simply consists of a phototransistor. Similarly, red LEDs, infrared LEDs and laser light sources describe a separate property from the sensing method. A laser-based sensor can operate in retro-reflective, through-beam or diffuse mode.

How does it differ from diffuse photoelectric sensing?

The clearest distinction is the surface that returns the light. Retro-reflective sensing uses a separate reflector. With diffuse photoelectric sensors, light returns from the target's surface. In through-beam sensing, light travels directly to a receiver in a separate housing.

Retro-reflective sensing can be useful when target colours vary and the target sufficiently blocks the beam. Consider diffuse sensing when nothing can be mounted opposite the sensor. For long distances and a high optical margin, investigate suitable through-beam sensor models. No single method is the best choice for every target. The comparison of detection sensors helps relate these differences to target material and mounting conditions.

Does “reflective proximity” describe a separate sensor type?

Proximity sensor alone does not specify an optical sensing method. Depending on the datasheet's terminology, reflective proximity may describe diffuse sensing. It does not establish a general category that combines separate “diffuse” and “reflector” signals. Look for retro-reflective, diffuse-reflective or through-beam in the sensing-mode specification and check whether a separate reflector is required.

Some diffuse models offer background suppression, BGS. This is designed to reduce the influence of surfaces beyond a defined distance. Target size, surface characteristics and separation from the background still need to be checked.

Choosing the reflector, working distance and polarization

Reflector type and size must match the selected sensor. A prismatic reflector directs light back towards its source; an ordinary mirror does not provide the same result in every installation. Check which reflector was used for the maximum-range specification and whether a minimum sensor-to-reflector distance applies. Distance limits depend on the model's optical arrangement.

Shiny metal or glossy packaging may return light in place of the reflector and cause a target to be missed. Polarized retro-reflective sensors help distinguish the compatible reflector's return from these misleading reflections. Test actual target angles and surfaces. For small targets, the effective beam area and the proportion blocked by the target matter; a larger reflector does not always improve small-object detection.

A conventional retro-reflective sensor usually provides presence/absence information. If the application requires a product's distance in millimetres, assess suitable distance-measuring photoelectric sensors separately. Sensing range and distance measurement are different specifications.

Is a standard model sufficient for clear bottles or film?

Transparent targets may not completely block the beam. A clear-object detection model designed to respond to small light losses is therefore required. Polarization alone does not guarantee suitability for transparent targets. Test empty and filled bottles, labels, curved surfaces and film thickness under actual operating conditions.

Where provided, teach-in adjustment must follow the manufacturer's procedure. Check the setting again if the sensor or reflector is moved or replaced. Particularly with sensitive models, stable mounting, a clean reflector and a fixed light path matter. Contamination compensation offered by some models does not eliminate the need for regular cleaning.

PNP/NPN, NO/NC and PLC wiring

A PNP or NPN output specifies the electrical output arrangement. NO/NC describes switching behaviour relative to a stated reference condition. In light-on mode, the output becomes active when sufficient return light is received. In dark-on mode, it becomes active when the return falls below the threshold. Define the required output state when a target arrives.

The output can connect to a PLC or a suitably specified smart relay input. Supply voltage, output type and the signal levels accepted by the input module must be compatible. Wiring requirements for two-wire sensors should not be assumed to match three- or four-wire models; always read the product's pin assignment.

Even in a panel using a 24 V DC power supply, check the sensor's permitted supply range. The choice of cable or connector affects maintenance and environmental suitability. The same connector shape does not guarantee the same pin functions.

Conveyor counting: is sensor speed enough?

The pulse signal produced by each passing product must be generated by the sensor and captured by the counter input. At a conveyor speed of 1.5 m/s and a centre-to-centre product spacing of 100 mm, or 0.1 m, the required counting frequency is f = v / p = 1.5 / 0.1 = 15 Hz. This spacing is not simply the gap between products.

If the effective beam-blocking length in the direction of travel is 30 mm, the interruption lasts approximately t = 0.03 / 1.5 = 0.02 s, or 20 ms. This initial estimate assumes a narrow beam and neglects sensor delays. Check the actual pulse width against sensor response time, input filtering and the counter's minimum high/low durations.

Specify whether the rising or falling edge is counted. Counting both edges of one product pulse can double the reported quantity unless the appropriate conversion is applied. Use a suitable PLC high-speed counter input when required. If conveyor travel distance or speed must also be tracked, information from an encoder can be evaluated alongside the photoelectric signal.

Practical mounting and troubleshooting checks

Start with the optical path. With no target present, the sensor must receive sufficient light from the reflector; the target must then produce the expected change. Reflector contamination, lost alignment or vibration can mimic product presence. Excess gain is the received-light margin relative to the switching threshold. Maximum-range performance in clean conditions does not guarantee equal stability in a dirty installation.

If the sensor detects the target but the PLC does not, investigate output mode, the common connection, PNP/NPN compatibility and input filtering. Electromagnetic interference near power cables and drives can affect the electrical signal chain. Optical interference from another nearby sensor is a separate issue; follow the manufacturer's spacing and alignment guidance.

Consider the IP protection rating, operating temperature, cable movement and cleaning chemicals together. Verify operation by passing a known quantity of real products at the lowest and highest operating speeds. Include shiny and transparent products, different passing angles and the smallest product gap.

When should another sensor be considered?

In confined spaces, a suitable fibre-optic sensor arrangement can use optical fibres to carry light to compact sensing tips. An inductive sensor may suit metal detection at short range. Depending on material and environmental conditions, a capacitive sensor may suit certain proximity or level applications.

Sound-based ultrasonic sensors can be alternatives in some applications where optical appearance makes photoelectric detection difficult. Check target angle, acoustic reflection and working range. A standard product-detection photoelectric sensor must not replace a safety-rated device for personnel protection. A machine function intended to protect people requires suitable protective equipment and safety-system design.