Type 2 vs Type 4 Safety Light Curtains: Which Should You Choose?

What is the difference between Type 2 and Type 4?

The important difference is how the protective device is designed to maintain its safety function when faults occur. Type is not a measure of beam spacing, sensing range or housing quality. Two curtains can detect the same test piece in normal operation yet have different permitted safety applications.

These levels are capability limits, not automatic results for the installed machine. The selected product must have the relevant evidence, and the complete safety function must meet its required performance. The Type-to-level relationship is summarized in Pilz’s IEC 61496 guidance.

IEC standard parts, device Type and control-system Category are different

IEC 61496-1 covers general requirements for electro-sensitive protective equipment (ESPE). IEC 61496-2 adds requirements for active opto-electronic protective devices (AOPDs), including conventional safety light curtains. A statement of compliance with Part 2 does not, by itself, identify the product as Type 4.

Type 2 and Type 4 are device classifications. Categories under ISO 13849-1 describe safety-related control architectures and fault behavior; PL describes achieved performance. The similar numbering does not make “Type 4” and “Category 4” interchangeable labels for a complete installation.

Which Type does your safety requirement allow?

Start with the machine risk assessment, the defined safety function and any applicable machine-specific requirements. Establish the required performance level (PLr) or required safety integrity level (SIL) before selecting the curtain.

• For PLr c or SIL 1: Type 2 may be a candidate if the application rules permit it and its testing and integration requirements can be met. Type 4 can also be considered.
• For PLr d or SIL 2: Type 2 is not sufficient. Type 4 is a candidate; a suitably rated Type 3 light curtain may also be considered where permitted.
• For PLr e or SIL 3: select a suitably rated Type 4 device, then verify the complete function.

Type 3 is not a typo: the 2020 edition of IEC 61496-2 introduced this intermediate class for AOPDs. Older comparisons that discuss only Types 2 and 4 can miss this option. Availability, exact-model evidence and application acceptance still matter.

Do not select from the machine name alone. “Packaging line,” “robot cell” and “press” do not describe every access point, hazard or stopping function. A machine-specific standard may require a particular safeguard or higher capability even when a simplified selection chart appears to permit less.

First confirm that a light curtain is an appropriate safeguarding method at all. Neither Type provides a physical barrier against ejected material, and neither solves a machine that cannot reach the required safe state before the hazard is reached. Those conditions require a different or additional protective measure.

How should you read SILCL on an older datasheet?

SILCL means safety integrity level claim limit and was used for subsystem capability. IEC 62061:2021 changed this terminology to the subsystem’s maximum SIL. Keep that capability separate from the required SIL of the safety function; do not ask for a “required SILCL” as though it were the complete machine requirement.

Why can a Type 2 curtain have self-tests and two safety outputs?

Because those features alone do not establish Type 4 performance. What matters is the complete design, the faults covered, their detection and the required response—not simply whether the brochure says “self-monitoring.”

For Type 2 equipment, the specified test arrangements are part of the safety design. Do not substitute a universal interval, such as “every 500 ms,” for the manufacturer’s instructions and the application requirements. Internal diagnostics, an externally initiated system test and a scheduled installation checkout serve different purposes.

Type 4 products use stronger fault-handling measures. For example, Banner’s SLC4 manual explains how redundancy and continuous monitoring work together: a detected component failure causes a stop signal and lockout. This does not mean every possible fault in the machine is detected instantly, or that installation checks are unnecessary.

A beam-interruption test demonstrates detection under the test conditions. It does not demonstrate every internal fault requirement. Follow the exact manual’s commissioning and periodic-check procedures, performed by the appropriately qualified person under controlled conditions; do not create live faults or use a person entering the hazard as a test method.

Does a Type 4 light curtain make the machine PL e?

No. The curtain is the sensing part of a larger safety function. The safety logic, final control elements, interconnections and fault measures must also support the intended performance.

A correctly functioning curtain can command a stop while a defective or incorrectly integrated downstream element prevents the intended safe state. Conversely, correctly selected components can still leave an accessible hazard if the curtain is positioned too close or a person can reach around it.

What Type does not prove about the installation

Type does not establish the correct protective height, prevent access over or under the field, or prove the actual machine stopping time. It also does not decide what happens if someone passes through the curtain and remains inside after the beams become clear.

Separation distance must use the applicable positioning method, the approach geometry, detection capability and complete stopping response. That response includes more than the curtain’s stated reaction time. ISO 13855:2024 addresses safeguard positioning; IEC 61496 product compliance alone does not determine where to mount the device.

For the inputs needed before deciding a mounting position, use the separate safety light curtain installation-distance guide. A Type 4 substitution does not make the old distance automatically valid.

Can you replace Type 2 with Type 4—or the other way around?

Neither direction is automatically a drop-in replacement. Moving to Type 4 may satisfy a higher device-capability requirement, but the electrical interface, detection field, response time and operating functions still need to match the safety design. Moving to Type 2 is unacceptable where the required level or application rules exclude it.

If the documented requirement instead permitted Type 2, cost could be part of the choice. Compare the complete installed solution: required test facilities, external control functions, configuration work, maintenance and validation—not just the price of the two bars. Do not reduce an existing safety capability simply to match a lower quote.

What should you verify before approving the model?

Ask for a traceable link between the quoted emitter/receiver pair, its safety rating and the configuration you intend to use. A family brochure saying “IEC 61496” is not enough to identify the supplied variant’s Type.

Read claims within their scope. A document that omits a suffix leaves coverage unresolved; it does not, by itself, show that the variant failed testing. Ask the supplier for the missing connection between the quoted product and its evidence before accepting the substitution.

The practical choice: first establish the required safety performance, then select an eligible Type, then verify the exact device and complete installation. Type 4 provides a higher capability ceiling; neither Type replaces a correctly designed and validated safeguarding system.