Through-beam, retro-reflective or diffuse - which sensor?
Photoelectric sensors detect objects without contact using light - but through-beam, retro-reflective and diffuse types work in fundamentally different ways. This guide explains the three sensing principles, their ranges and when each design is the right choice for reliable object detection.
View photoelectric sensorsHow do the three sensing principles differ?
Photoelectric sensors emit light and evaluate whether and how much of it returns. The key difference is where the emitter and receiver sit and what the light reflects off. This gives three designs: the through-beam sensor, the retro-reflective sensor and the diffuse sensor (diffuse-reflective).
In a through-beam sensor the emitter and receiver are two separate housings facing each other; the object breaks the beam. A retro-reflective sensor combines emitter and receiver in one housing and uses a reflector on the opposite side. The diffuse sensor needs no reflector and evaluates the light scattered straight back from the object itself.
- Through-beam: separate emitter and receiver, object breaks the beam.
- Retro-reflective: sensor plus reflector (corner-cube film), object breaks the reflected beam.
- Diffuse: emitter and receiver in one housing, the object itself scatters the light back.
What range and detection does each design deliver?
Range depends directly on the principle. The through-beam sensor reaches the longest distances and highest operating reliability because the full beam hits the receiver. The diffuse sensor has the shortest range because only the small scattered fraction from the object returns.
The through-beam sensor detects objects reliably regardless of colour, gloss or surface - ideal for dust, contamination and long distances. The retro-reflective sensor needs a polarising filter version for shiny or mirrored objects so the part is not mistaken for the reflector. The diffuse sensor is sensitive to light and dark surfaces; an adjustable switching distance helps suppress the background.
When should I choose which design?
Selection depends on distance, object properties, mounting effort and environment. If you can mount on both sides and need long range or maximum reliability, choose the through-beam sensor. If only one side is accessible, pick a retro-reflective or diffuse sensor.
- Long distance, dust or dark objects: through-beam sensor.
- One mounting side, medium range, defined objects: retro-reflective sensor with reflector.
- Short distance, no room for a reflector, varying objects: diffuse sensor.
- Shiny or transparent parts: polarising filter or dedicated clear-object sensor.
- Harsh environment: check for IP67 rating and red light or laser for small targets.
Inductive or capacitive instead of optical - when the switch pays off.
Read the guideFrequently asked questions
What is the difference between a retro-reflective and a diffuse sensor?
A retro-reflective sensor uses a reflector on the opposite side and triggers when the object breaks the reflected beam. A diffuse sensor has no reflector and evaluates light scattered straight back from the object - giving shorter range but simpler mounting.
Which design has the longest range?
The through-beam sensor, because the full emitted beam hits the receiver directly opposite. Standard devices reach up to 60 m, and special versions go further.
Why does my diffuse sensor detect dark objects poorly?
Dark, matte surfaces reflect less light, so less signal returns. A sensor with background suppression helps, or switch to a through-beam sensor, which works independently of colour.
How do I reliably detect transparent objects?
Use special clear-object retro-reflective sensors. They evaluate the smallest attenuation of the light returning from the reflector and detect thin, transparent parts such as films or bottles.
Looking for the right photoelectric sensor?
We supply through-beam, retro-reflective and diffuse sensors for every range and object type - including reflectors and accessories.
Right range
Designs from a few centimetres to 60 m distance.
Rugged in the field
IP67 rating and ambient-light immune evaluation.
Reliable detection
Even with dust, gloss or transparent parts.
Expert advice
Specialists help with design and alignment.