What Is a Photocell? Photoelectric Sensor Selection Guide
A photocell is a sensor that detects interruption or return of light from an object and gives a contactless output; find its types and selection criteria in this guide.
What is a photocell and where is it used
A photocell is a photoelectric sensor that detects the interruption of a light beam by an object, or its return from an object, and produces a contactless output signal. In manufacturer catalogs it appears under the name "photoelectric sensor"; it is an object detection element with no mechanical contact, connected to a PLC input or a control circuit.
It is used for product counting on conveyor lines, pallet or package presence, product position on packaging machines, and checking whether a part is present at a station. Inductive sensors detect metal targets; when the target is a non-metallic material such as plastic or cardboard, a photocell comes into play.
Dusk-to-dawn switches that turn street and garden lighting on and off according to daylight are a different product group; they are listed under Photocell Relays.
Types of photocells
| Sensing Method | Operating Principle | Typical Range and Advantage | Key Limitation |
|---|---|---|---|
| Diffuse-Reflective | Emitter and receiver in one housing; detects light reflected directly from the object | Short–medium range; single-side mounting without reflector | Dark/matte targets reduce effective sensing distance |
| Retro-Reflective | Beam bounces back from a corner-cube reflector; object interrupts the beam | Medium range; single-side wiring, largely independent of target color | Shiny/mirror surfaces require a polarized retro-reflective model |
| Through-Beam | Separate emitter and receiver housings facing each other | Long range; highest excess gain in dusty environments and on any surface | Requires mounting and power wiring on both sides of the conveyor |
Photocells are classified along two axes: sensing method and housing shape.
By sensing method
- Through-beam: the emitter and receiver are in separate housings; when an object interrupts the light between them, the output changes. In the product names on this page it appears as "Through-Beam Photocell".
- Retro-reflective: the emitter and receiver are in the same housing, and a reflector opposite sends the light back. The object is detected when it interrupts the light path between the sensor and the reflector. Reflectors are under Photoelectric Sensor Reflectors.
- Diffuse: no reflector is needed; the sensor detects the light returning from the object itself. The color of the object noticeably affects the sensing distance: a dark object absorbs light and is therefore detected at a shorter distance.
- Background suppression: this is the type of diffuse design that ignores surfaces beyond a set distance. It distinguishes the object by the angle of the light returning to the receiver, that is, by its distance; because it does not look at the amount of light, it is less sensitive to the color of the object. In the category it is found under Background Suppression Rectangular Photoelectric Sensors.
By housing shape
- Cylindrical photoelectric sensors: threaded housings such as M12 and M18; they come in plastic, metal, stainless and 90-degree (angled) options, and there are AC two-wire models.
- Rectangular photoelectric sensors: these are models with a rectangular housing; they have plastic-housing, through-beam, retro-reflective, diffuse and background suppression subcategories.
What to look at when choosing a photocell
- Sensing method: If the object can be reached from both sides, through-beam is chosen; if it can be reached from one side only, retro-reflective or diffuse is chosen. If there are other surfaces behind it, a background suppression model may be suitable.
- Sensing distance: The distance in the product name (for example 400 mm, 4000 mm or 2 m) is the catalog value; the color and surface of the object and the environment can change this distance. Determine the operating margin according to the project and manufacturer data.
- Surface of the object: Shiny objects can act like a reflector on a retro-reflective sensor and give a false result; in this case a model with a polarizing filter and a corner-cube (triple mirror) type reflector is preferred. For transparent objects, look at the manufacturer's transparent object detection data.
- Supply and output: DC three-wire NPN or PNP, or an AC two-wire model; it must be compatible with the input card and the load the output will be connected to.
- Output function: NO, NC or NO+NC. In some product names this information is stated directly.
- Housing, connection and environment: M12 or M18 thread, cable or M12 connector output, plastic or metal housing. For models with connectors, sensor connectors and cables are selected separately.
How to read the codes in the product name
Product names in this category usually give the method, housing and output together. Example: the phrase "Diffuse Photocell, M18, PNP NO, 400 mm, M12 connector" indicates the diffuse method, the M18 housing, the PNP output type, the NO output function, the 400 mm catalog distance and the M12 connector connection. NO means normally open and NC normally closed output. In Lanbao names the sensing distance is written separately, as in "40cm Algılama". The details of the series codes are in the brand catalog.
Difference between a photocell and a light barrier
A photocell detects an object on a single beam line. Light barriers (light curtains), on the other hand, create a field of many parallel beams between the emitter and receiver and detect an object entering this field. Safety use, that is, hazardous area protection, is evaluated separately according to the project and manufacturer data.
Brands in this category
- Lanbao: most of the products in the category are from this brand; through-beam, retro-reflective and diffuse models in cylindrical and rectangular housings.
- Isıso: diffuse and retro-reflective models with M12, M18 and B50 housings; products with NPN and PNP outputs, versions carrying 90-250 V AC information and options with M12 connectors.
Order notes
Photocells are sold per piece. Some products are listed as "made to order"; for these, the lead time is verified separately. You can search by the stock code or series code in the catalog.
Frequently asked questions
Is a photocell the same as a photoelectric sensor?
In industry, both describe the same product: a contactless sensor that detects objects with light. In everyday language, photocell sometimes also refers to the dusk-to-dawn switch that turns street lamps on according to daylight. Those products are listed in a separate category as photocell relays and are not used for object detection.
What is the difference between through-beam, retro-reflective and diffuse photocells?
In a through-beam photocell, the emitter and receiver are in separate housings, and the object interrupts the light between them. In a retro-reflective one, both are in the same housing, and a reflector opposite sends the light back. A diffuse one has no reflector; the sensor detects the light returning from the object itself, so the color of the object affects the sensing distance.
Which photocell is chosen for shiny or transparent objects?
A shiny object can reflect the light back on a retro-reflective sensor instead of interrupting the light path and may not be detected reliably. As a solution, a retro-reflective sensor with a polarizing filter and a corner-cube type reflector are used. For transparent objects, models that can detect a small change in the amount of light are needed; suitability must be verified against manufacturer data.
Should NPN or PNP be chosen for a photocell?
The choice is made not according to the sensor itself but according to the PLC input or control circuit it will be connected to. Whichever output type the input card expects, the sensor's output must be compatible with it. If in doubt, look at the catalog data of the card and the sensor; connection details are applied according to the project and manufacturer data.
How do you choose between a photocell and an inductive sensor?
An inductive sensor detects metal targets. A photocell, because it works with light, can also detect non-metallic opaque objects such as plastic and cardboard. If the object is metal, an inductive sensor is also an option; if it is not metal, an inductive sensor is not suitable. For sensing distance and environmental conditions, refer to the project and manufacturer data.