Through-Beam vs Retro-Reflective vs Diffuse: Which Photoelectric Sensor Do You Need?

Pick the photoelectric sensor by what sits behind and around the object, not by the range you think you need. A through-beam sensor (separate emitter and receiver) is the most reliable and reaches furthest: Lanbao's PSE-TM10 pair is rated at 10 m. A retro-reflective sensor bounces its beam off a reflector and is rated at 3 m on the PR18S-DM3, with only one side to wire. A diffuse sensor is the cheapest to install because it needs nothing opposite, but it only works at short range (10 cm on the PR18-BC10, 40 cm on the PR18-BC40, 100 cm on the PSE-BC100) and it can be fooled by a shiny or dark object or by a wall close behind the target. If the background is closer than about twice the sensing distance, skip plain diffuse and choose a background suppression model such as the PR18-YC10, which ignores anything beyond its set distance. All four types are in stock at Azar Group from $25.61, with cash on delivery and five day returns.

In this article

• How do the three photoelectric sensor types work?
• Which type is right for your application?
• Why does a diffuse sensor trigger on the wall behind?
• What does the Lanbao part number tell you?
• Lanbao photoelectric sensors compared
• When is a photoelectric sensor the wrong choice?
• FAQ

How do the three photoelectric sensor types work?

Every photoelectric sensor has a light source and a receiver. The three types differ only in where the receiver sits, and that one detail decides the range, the reliability and how many parts you have to buy.

Through-beam (also called opposed mode). Emitter and receiver are two separate housings facing each other, and the object is detected when it breaks the beam. The light crosses the gap once, so this type has the longest range and the most reserve against dust on the lens. The catch is that you install, align and wire two units.

Retro-reflective. Emitter and receiver share one housing, and a prism reflector opposite sends the beam straight back. You wire one side only, but the reflector is a separate purchase and the rated distance is measured against a specific one: Lanbao rate the PR18S-DM3 at 3 m using their TD-09 reflector.

Diffuse (also called proximity mode). One housing, nothing opposite. The sensor detects the small amount of light the object itself scatters back. One part, one cable, no alignment, but the range is short and depends entirely on how well the target reflects. Lanbao rate the PR18-BC10 at 10 cm on a white card with 90% reflectivity.

Which type is right for your application?

Three questions settle it in most installations.

1. Can you mount something on the far side? If yes, use through-beam for anything long, fast or dusty, and retro-reflective when you want the range but only one cable. If no (a wall, a tank, a machine frame), you are choosing between diffuse and background suppression.

2. What is the object like? Matte cardboard, wood, plastic or paper: any type works. Shiny metal, foil or glossy packaging: avoid plain diffuse, because a mirror-like surface can bounce light back from far beyond the rated range. Pepperl+Fuchs engineers describe a diffuse sensor with a specified range of 15 inches detecting shiny metal 6 feet away. Through-beam and background suppression handle shiny objects well.

3. How close is the background? A diffuse sensor sees the conveyor frame or the wall just as happily as the box. As a rule of thumb, if the background is nearer than about twice the sensing distance, use background suppression or through-beam.

Type	Parts to buy	Lanbao rated distance	Best for	Weak spot
Through-beam	Emitter + receiver (2 units, 2 cables)	10 m (PSE-TM10)	Long gaps, fast counting, dusty areas, shiny objects	Needs a mounting point on both sides and alignment
Retro-reflective	Sensor + reflector (1 cable)	3 m with TD-09 reflector (PR18S-DM3)	Doorways, pallets, conveyor ends where only one side has power	Very shiny or clear objects can return the beam like the reflector
Diffuse	Sensor only	10 cm (PR18-BC10), 40 cm (PR18-BC40), 100 cm (PSE-BC100)	Matte objects at short range with open space behind	Range changes with colour and finish, sees close backgrounds
Background suppression	Sensor only	10 cm adjustable (PR18-YC10), 15 cm laser (PSE-YC15)	Objects right in front of a wall, rail or machine part	Shortest range, highest price

Why does a diffuse sensor trigger on the wall behind?

Because a diffuse sensor has no idea what it is looking at. It only measures how much light comes back, and a light-coloured wall a little beyond the target can return as much as the target itself, so the output stays on with no box in front. The same physics works the other way: a black rubber part absorbs most of the light and has to come much closer than a white one before the sensor sees it. Pepperl+Fuchs put the black to white difference at around 190 mm on a standard diffuse sensor, against only 10 mm on a background suppression model.

A background suppression sensor fixes this with a second receiver element and a small optical angle. It judges where the reflection comes from, not how strong it is, and switches off for anything beyond the set distance. Lanbao's PR18-YC10 specifies a white to black variation of 10% or less at its 10 cm range, which is why it detects a dark part in front of a light machine frame that a plain PR18-BC10 would miss or confuse. The laser PSE-YC15 goes further with a spot of 2 mm or less at 15 cm, for small parts and precise edges.

Two fitting rules save most call-outs. Angle a diffuse or background suppression sensor a few degrees off perpendicular when the target or background is glossy, so the mirror reflection misses the lens. And never aim one at a reflector, a window or a polished panel.

What does the Lanbao part number tell you?

The code on the box is the spec sheet in short form, and reading it avoids the two commonest wrong orders.

• PR18 is an M18 threaded cylinder in nickel-copper alloy; PR18S is the same size in PBT plastic; PSE is the square plastic housing.
• BC10 / BC40 / BC100 means diffuse at 10 cm, 40 cm or 100 cm. DM3 means retro-reflective at 3 m. YC10 / YC15 means background suppression at 10 or 15 cm. TM10 means through-beam at 10 m.
• D is a DC sensor for 10 to 30 V; A is an AC 2-wire sensor for 20 to 250 V that switches the load directly, with no PLC needed.
• N is NPN, P is PNP. O is normally open, C normally closed, R gives both outputs on separate wires.
• -E2 is an M12 connector instead of the 2 m cable, so you add a QE12 cordset.

So PR18-BC10DNR is a metal M18 diffuse sensor, 10 cm, DC, NPN, with NO and NC wires. And the through-beam emitter PSE-TM10DR has no N or P in its code because it has no output at all: the receiver (PSE-TM10DNBR or PSE-TM10DPBR) is the part that decides NPN or PNP.

Lanbao photoelectric sensors compared

All figures below are from Lanbao's own datasheets, linked on each product page.

Model	Type	Rated distance	Light source	Response	Housing	Price
PR18-BC10DNR / DPR	Diffuse	10 cm, fixed	Infrared 880 nm	< 8.2 ms	M18 x 53.5 mm metal, IP67	$25.61
PR18-BC40DNR / DPR	Diffuse	40 cm, adjustable	Infrared 880 nm	< 8.2 ms	M18 x 53.5 mm metal, IP67	$28.19
PSE-BC100DNB / DPB	Diffuse	100 cm, push-button set	Infrared 860 nm	≤ 1 ms	Square PC+ABS, IP67	$28.19
PR18S-DM3DNR / DPR	Retro-reflective	3 m with TD-09	Infrared 880 nm	< 8.2 ms	M18 x 53.5 mm PBT, IP67	$28.29
PSE-TM10DR + DPBR	Through-beam	10 m	Red 630 nm	≤ 1 ms	Square PC+ABS, IP67	$11.28 + $16.98
PR18-YC10DPR-E2	Background suppression	10 cm, adjustable	Red 680 nm	< 2 ms	M18 x 86.6 mm metal, IP67, M12 plug	$37.34
PSE-YC15DPRL	Laser background suppression	15 cm, spot ≤ 2 mm	Red laser 650 nm, Class 1	≤ 0.5 ms	Square PC+ABS, IP67	$50.48
PR18-BC10ATO / ATC	Diffuse, AC 2-wire	10 cm, fixed	Infrared 880 nm	< 50 ms	M18 x 70 mm metal, IP67	$28.19

Two things stand out. The square PSE models respond in 1 ms against 8.2 ms for the M18 cylinders, so for counting small fast parts the PSE-TM10 pair is the better buy even though it costs about the same. And the AC 2-wire PR18-BC10ATO responds in 50 ms, fine for a conveyor gate or a door, too slow for counting.

When is a photoelectric sensor the wrong choice?

Metal parts at a few millimetres. An inductive proximity sensor is cheaper, needs no reflector, and does not care about dust, light or colour. A Lanbao LR12XBF02 costs $12.38 against $25.61 for the cheapest photoelectric model. Our guide on flush versus non-flush proximity sensors covers how to size one.

Clear bottles, film or glass. A standard retro-reflective or diffuse sensor can see straight through them, or see the gloss instead of the object. That job needs a sensor specified for transparent objects, which is not in our current range, so ask before ordering.

Liquid level through a tank wall. Use a capacitive sensor such as the CR18XSCN12Y, which detects through plastic.

Direct sunlight on the receiver. The PSE datasheets state an anti-sunlight figure of 10,000 lux and 3,000 lux for incandescent light; the PR18 datasheets publish no ambient light figure at all. Outdoors, mount the receiver so the sun can never sit behind the object, or use through-beam with the receiver facing away from the sky.

Fast counting on an AC 2-wire sensor. At 50 ms response the AC models suit gates and doors, not product counting. Use a DC model into a PLC input instead.

Frequently asked questions

What is the difference between diffuse and retro-reflective sensors?

A diffuse sensor reads light scattered back by the object itself, so it works without anything opposite but only at short range and only as well as the object reflects. A retro-reflective sensor reads light returned by a reflector and detects the object when the beam is blocked, so it reaches further (3 m on the PR18S-DM3) and does not depend on the object's colour. The reflector is bought separately.

Which photoelectric sensor is best for shiny objects?

Through-beam, because the object only has to block the beam. Background suppression is the next best when nothing can be mounted opposite. Plain diffuse is the worst choice, since a glossy surface can return light from well beyond the rated range.

Do I need NPN or PNP?

Match the input of whatever the sensor drives: PNP (sourcing) for most European-style PLC inputs, NPN (sinking) for many Asian-market controllers. The Lanbao R versions give both normally open and normally closed on separate wires, so you only need to get N or P right. In a through-beam pair only the receiver has an output, so only the receiver carries an N or P code.

What reflector does a retro-reflective sensor need?

The one the datasheet names. Lanbao rate the PR18S-DM3 at 3 m using the TD-09 reflector. A smaller or lower-grade reflector shortens the usable range, so do not substitute without testing.

Can a diffuse sensor be adjusted to ignore the background?

Only a little. Turning the range down on a PR18-BC40 helps when the background is far away, but a diffuse sensor still cannot tell a near dark object from a far light one. If the background is within about twice the sensing distance, use a background suppression model such as the PR18-YC10.