Engineering Guide to Light Curtains: Selection, Installation, and Safety

Automation and Control Industrial Automation and Control
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This is the index for a series of articles describing safety-critical light curtains. It includes links for selection, mounting, and alignment. It also includes a teardown and description of the essential OSSD waveform used to communicate states with a safety relay. An example is the safety-rated deTec4 on the workbench as shown in Figure 1.

This article is part of the DigiKey Field Guide for Industrial Automation

Location: Select It → Safety Devices
Difficulty: :gear: Engineer — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 13 Mar 2026

Figure 1: Close-up of a SICK deTEC4 series light curtain.
Figure 1: Close-up of a SICK deTEC4 series light curtain.975×732 103 KB

Figure 1: Close-up of a SICK deTEC4 series light curtain.

Light Curtain Facts

Here are a few light curtain facts that I learned along the way:

  • Not all light curtains are safety devices. There are applications where a light curtain sensor is the optimal solution. For example, a light curtain may be more flexible than a retroreflective sensor, as they can detect objects of multiple heights that would otherwise be missed by a single beam.

  • The IR beam can be seen using a cell phone as shown in Figure 2. Note that the SICK SLG-2 is not a safety sensor. As a rule, safety light curtains are yellow.

  • DigiKey sells light curtains with beam count ranging from 4 to 240.

Figure 2: The infrared beams for this non-safety SICK light curtain are visible as purple dots to a cell phone.
Figure 2: The infrared beams for this non-safety SICK light curtain are visible as purple dots to a cell phone.975×732 70.8 KB

Figure 2: The infrared beams for this non-safety SICK light curtain are visible as purple dots to a cell phone.

List of Topics

Topics are organized into clusters. Please follow the links for a detailed description of each topic:

Help us to help you

If you would like to know more about light curtains, please add your question in the comments section. We may answer directly or move your question to the core of this guided learning series.

Best wishes,

APDahlen

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If this discussion was helpful, you may also want to explore:

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About This Author

Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.

Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.

Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.

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Good overview, especially the OSSD waveform section. One thing that tends to cause confusion once you move from understanding the OSSD signal to actually wiring the safety circuit is that an OSSD pair and a dual-channel E-stop are not really interchangeable inputs.

The E-stop is just two dry N.C. contacts that depend on the safety relay for test pulses and fault detection, while the light curtain OSSDs are actively driven and self-monitored outputs. On a relay with independent dual-channel inputs, I’d normally keep the E-stop on one input and the curtain OSSDs on the other rather than putting them in series.

I found this wiring-focused explanation that complements the OSSD side of the guide pretty well:

It also goes into reset and contactor feedback/EDM, which are usually the next questions once the curtain itself is selected and aligned.

Obviously the terminal numbers depend on the actual relay and curtain being used, but the distinction between dry-contact safety channels and OSSD outputs is worth keeping in mind.