Photoelectric Sensor Types: Through-beam, Retro-reflective, Diffuse, BGS
A photoelectric sensor detects an object by what it does to a beam of light: it either blocks the beam or reflects some of it back. That sounds simple, but the way the emitter, receiver and target are arranged decides the sensing range, how the sensor copes with shiny, dark or transparent objects and how much installation work is involved. This guide covers the five sensing modes you will meet on Indian shop floors and how to pick between them.
1. Through-beam
The emitter and receiver are separate units mounted facing each other. The output switches when an object interrupts the beam. Because the receiver sees the full light from the emitter, through-beam gives the longest range and the highest operating margin, so it keeps working through dust, mist and dirty lenses better than any other mode.
- Range: standard-size sensors such as the Optex Z3 Series reach 25 m in through-beam, and the Optex V2 Series reaches 70 m for crane bays, gates and long conveyors.
- Strengths: reliable on opaque objects of any colour or finish, tolerant of contamination, precise switching point.
- Trade-offs: two units to mount, align and wire, which adds cost and cabling, especially across wide machines.
2. Retro-reflective
Emitter and receiver share one housing, and the beam is returned by a reflector on the far side. You wire only one side of the conveyor, and the reflector needs no power. Range is shorter than through-beam because the light travels twice the distance and loses energy at the reflector.
The weak point is shiny objects: a glossy carton, foil or polished metal can reflect enough light to look like the reflector. Polarised retro-reflective sensors filter out this direct reflection. Coaxial designs such as the Optex ZR-X Series put the emitting and receiving axes on the same line, so detection does not depend on mounting direction and there is no dead zone close to the reflector.
3. Diffuse (proximity)
The sensor detects light scattered back from the object itself. There is no reflector and only one unit to install, which makes diffuse sensors the quickest to fit. The catch is that the amount of light returned depends on the target: a white carton returns far more than a black rubber part, so the effective range changes with colour and surface. A bright background behind a dark object can also cause false triggering.
Use diffuse sensors for short-range presence checks where the objects are consistent in colour and there is nothing reflective behind them.
4. Background suppression (BGS)
A background suppression sensor is a diffuse sensor that measures distance by triangulation instead of light quantity. It switches only for objects closer than the set distance and ignores everything behind it, so black and white objects switch at nearly the same point and a shiny conveyor frame behind the part does not matter.
The Optex BGS-Z detects objects regardless of colour or gloss with adjustable sensing up to about 300 mm, with IP69K stainless variants for washdown areas. For precise height-difference checks on dark or shiny parts, laser BGS models are the better fit. BGS is usually the right default when you would otherwise choose a diffuse sensor.
5. Fibre optic
A fibre optic sensor separates the optics from the electronics. Small fibre heads, available in through-beam and diffuse versions, mount in tight spaces, high temperatures or moving fixtures, while the amplifier sits on a DIN rail in the panel. Digital amplifiers such as the Optex D3RF offer fast response and crosstalk prevention when several amplifiers sit side by side, which suits small-part detection on electronics, pharma and press lines.
Trade-offs: shorter range than standard sensors, and fibres need protection from sharp bends and abrasion.
Quick comparison
| Mode | Range | Installation | Watch out for |
|---|---|---|---|
| Through-beam | Longest | Two units, alignment | Transparent objects that do not block enough light |
| Retro-reflective | Medium | One unit plus reflector | Shiny objects, reflector damage or dirt |
| Diffuse | Short, varies with colour | One unit | Dark targets, bright backgrounds |
| Background suppression | Short, set by distance | One unit | Very small or very uneven targets |
| Fibre optic | Short | Fibre head plus amplifier | Tight fibre bends, mechanical damage |
Difficult objects
Transparent objects
PET bottles, glass vials and clear film let most light through, so a standard through-beam sensor may not see them. Dedicated transparent-object sensors use a retro-reflective coaxial design and detect the small loss of light as the beam passes through the object twice. The Optex Z3R-Q/ZR-QX Series is built for this, with detection up to 2 m regardless of conveyor travel direction.
Shiny and metallic objects
Avoid plain retro-reflective sensors. Choose background suppression, a polarised or coaxial retro-reflective sensor, or through-beam.
Dark and matt objects
Black rubber, dark plastics and matt-finished parts return little light. Use through-beam or background suppression rather than plain diffuse.
Harsh environments
Steel plants, foundries and cement plants combine heat, dust, vibration and electrical noise. Remote sensing heads keep the electronics away from the hazard. Telco remote sensors work with Telco amplifiers in the control panel, and the Telco SM 8000 covers through-beam, diffuse, retro-reflective and fibre modes with 10 to 30 V DC or 20 to 250 V AC supply and an optional ATEX approval for hazardous areas.
How to choose
- Can you mount units on both sides? If yes and range or reliability matters most, use through-beam.
- Only one side available but a reflector can go opposite? Retro-reflective, polarised or coaxial if the objects are shiny.
- Only one side and no reflector? Background suppression for mixed colours, diffuse for simple, consistent targets.
- Clear bottles, film or glass? A dedicated transparent-object sensor.
- Very small parts or no space for a sensor body? Fibre optic.
- Then check supply voltage, output (NPN, PNP, relay), response time, IP rating and connection type against your machine.
Photoelectric sensors from Rajdeep
Rajdeep Automation supplies Optex FA and Telco photoelectric sensors across India, from general-purpose through-beam and BGS sensors to fibre amplifiers and remote sensing systems for heavy industry. Browse the full range under photoelectric sensors.
Share the object, its colour and finish, the distance to the sensor and the mounting space through the enquiry form, and an application engineer will recommend a sensing mode and model with the datasheet and price.