What a photoelectric sensor is
A photoelectric sensor detects an object by putting light across the space where the object will be and watching what comes back or what fails to arrive. On a production line that makes it the device for part-present checking, counting, edge detection and guard positions, and it is usually the least expensive way to answer a yes-or-no question about position.
It differs from an inductive or capacitive sensor in a way that decides both its strength and its weakness. There is no field to be damped, so almost any material can be detected, at distances an inductive sensor cannot reach, and a transparent object or a printed mark can be distinguished as well as a solid one. The same optical path that makes this possible is also what dust, mist, washdown and a blocked sight line interrupt.
Selection turns on four questions: where the sensor can be mounted relative to the object, what the object is and how much light it returns, what the background behind it looks like, and what the controller input expects. The second and third of those are what separate the sensing modes rather than the sensing distance alone.
How a photoelectric sensor measures
The sensing modes differ in where the light goes, and each answers a different problem. A plain diffuse sensor sends light out and switches when enough of it returns, which needs nothing on the far side of the object but makes the switch point depend on how reflective the object is. A photoelectric sensor with background suppression works out where the light came from rather than how much arrived, using the different angles at which the emitter and the receiver see the object, so it switches on distance and can be set to ignore what lies behind the target.
A retro-reflective sensor sends a beam to a reflector and detects the object when the beam is broken, which needs only one cable run and one mounting position but a reflector opposite. A through-beam pair puts the emitter and the receiver at opposite ends and detects when the beam between them is interrupted, which gives the longest range and the most reliable detection at the cost of wiring and aligning both ends. The optical path in the last two is fixed, so the switching point does not depend on the object at all.
What limits all four in practice is the contrast between the object and its surroundings. A reflective object can return more light from an angle than a diffuse one returns head-on, and a dark object returns very little, so a switching point set on one part may not hold on another. A machine frame, a conveyor belt or a shiny guard behind the object can contribute more reflected light than the object itself, which is the problem background suppression exists to solve and the reason a plain diffuse sensor sometimes cannot be made to work at all on a given installation.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Sensing mode | Diffuse, diffuse with background suppression, retro-reflective, through-beam | What has to be mounted on the far side of the object, and whether the switching point depends on the object or on a fixed optical path. |
| Sensing distance | From a few millimetres to several metres, depending on the mode | Where the sensor can be mounted. The published figure is measured against a reference target, so the working distance is shorter on a dark or rough object. |
| Light source and spot size | Visible red or infrared, and the diameter of the spot at the working distance | Whether the beam can be aimed, and how small an object can be detected reliably. Spot sizes quoted at different distances are not directly comparable. |
| Output type | PNP, NPN, or an analogue output on some models | How the sensor connects to the controller input, and whether the value is a limit or a measurement. |
| Output action | Normally open, normally closed, or switchable | What the controller sees when no object is present, which is a wiring decision rather than a preference. |
| Sensitivity adjustment | Single-turn potentiometer, push-button teach, or a fixed setting | How the switching point is set on site, and whether it can be reset quickly during a changeover. |
| Housing and sealing | Rectangular or cylindrical, with an ingress protection rating | Whether the sensor survives where it is mounted. A rating suitable for splash is not a rating for high-pressure washdown. |
| Connection | Integral cable, or a connector | How quickly a damaged sensor can be replaced, which matters on a machine that carries many of them. |
How to choose a photoelectric sensor
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| What the object is made of and how it reflects | The sensing distance is measured against a white matte target, and a dark, glossy or tilted object behaves differently | Describe the object, its colour and its surface, and whether the same sensor has to handle more than one type. |
| Background behind the object | A reflective background can contribute more light than the object, which is what background suppression is for | State what is behind the object and how far away it is, because that decides whether a plain diffuse sensor can work. |
| Mounting positions available | Some modes need one mounting point and some need two, and the reach differs by mode | Give the distance between where the sensor can sit and where the object passes, and the space on the far side. |
| Object size and speed | A small or fast object needs a spot that fits inside it and a response time that resolves the pass | Give the smallest dimension of the object across the beam and the line speed. |
| Ambient light and atmosphere | Strong ambient light interferes, and dust, mist and washdown interrupt the optical path | Describe the cell the sensor sits in, including whether sunlight reaches it and whether it is hosed. |
| Controller input | The input type sets the output form, and the logic decides the output action | Confirm the input type and what the program expects when no object is present. |
| What happens on a missed detection | The consequence decides how much margin and how much alignment stability are needed | Say whether a missed part causes a rework or a stoppage, because it changes how the mode is chosen. |
A photoelectric sensor detects the interruption or the return of light, not the object itself. Anything that enters the beam or blocks the sight line can trigger it, which is why a guard or an access panel in the optical path is part of the mounting consideration rather than an afterthought.
The nearest alternatives
These are the types an enquiry for a photoelectric sensor usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a photoelectric sensor |
| Proximity sensors | Inductive and capacitive types ignore the optical path entirely, so dust, mist and a blocked sight line do not affect them, and they can be embedded flush in a metal bracket. They respond only to metal, however, and over a much shorter distance, so a plastic part or a target several hundred millimetres away calls for a photoelectric sensor. |
| Position sensors | A position sensor reports where something is, usually as a continuous value over a travel. A photoelectric sensor reports that something has arrived at one point. Where the controller needs the actual position rather than a confirmation, a photoelectric sensor supplies only one part of the answer. |
| Displacement sensors | These measure a small movement as a continuous distance, which an optical sensor can also do in some forms but not in the switching versions described here. Where the amount of movement matters rather than the fact of it, a displacement sensor is the instrument, and a photoelectric switch cannot be substituted. |
| Level sensors | Level detection is a special case of presence detection and is sometimes done with optical parts, but a level sensor is specified against the material, the vessel and the conditions in it. Where the requirement is a fill limit in a tank, the level sensor family is the one to look at rather than a general-purpose optical switch. |
| Encoder and counting systems | Counting parts on a line can be done by counting pulses from the machine drive rather than by detecting each part. Where the parts are evenly spaced and the drive is reliable, that removes the sensor altogether; where they are not, a photoelectric sensor detecting each part is the more direct measurement. |
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- What the object is, its colour and its surface finish
- The smallest object dimension across the beam, and the line speed
- What sits behind the object and how far away it is
- The distance between the mounting position and the object, and the space available on the far side
- Whether the same sensor has to handle more than one type of object
- The controller input type, and what the program expects when no object is present
- The ambient conditions: sunlight, dust, mist, washdown
- How quickly a sensor has to be replaceable in service
- Whether the consequence of a missed detection is a rework or a stoppage
