Why Resolution and Range Drive the Specification

Light curtains are not commodity sensors. Resolution (beam spacing) and protective range determine whether a curtain detects a hand, an arm, or a full body, and at what distance it remains reliable. For OEMs and integrators, these two parameters cascade into mounting geometry, machine stopping time, and compliance with ISO 13855. Getting them wrong means either nuisance trips or an unsafe gap. This guide covers what to specify, what to check before ordering, and how to source from Chinese factories without sacrificing safety.

Resolution: Match Detection Capability to the Hazard

Resolution is the minimum object size the curtain reliably detects, typically defined by beam spacing plus lens diameter. Common classes:

• 14 mm (finger/hand detection): Used for point-of-operation guarding where operators can reach into the hazard zone. Requires the curtain to be mounted close to the hazard and the safety distance calculated with the 14 mm resolution.
• 30 mm (hand detection): A common compromise for access guarding where full hand entry is possible but finger-level detection is not required.
• 50–70 mm (body/arm detection): For perimeter guarding, robot cells, and areas where only larger body parts can enter. Allows greater mounting distance and fewer beams, reducing cost.

Do not overspecify resolution. A 14 mm curtain costs more per beam and forces tighter mounting, which can be impractical on large machines. Conversely, using 50 mm resolution at a point of operation is a safety violation. Confirm the required resolution from your risk assessment (ISO 13849 / ISO 13855) before requesting quotes.

Protective Range: More Than a Distance Number

Protective range is the maximum distance between emitter and receiver over which the curtain maintains its specified detection capability. It is not a simple optical limit; it depends on beam intensity, receiver sensitivity, and ambient light rejection. Key considerations:

• Range vs. resolution trade-off: Many curtains achieve their longest range only at coarser resolutions. A 14 mm curtain may be limited to 4–6 m, while a 30 mm unit may reach 10–15 m. Check the datasheet for the specific resolution/range combination.
• Optical alignment tolerance: Longer ranges demand tighter alignment and stiffer mounting. Vibration and thermal expansion on large frames can push a long-range curtain out of alignment, causing nuisance stops.
• Environmental derating: Dust, mist, and weld spatter reduce effective range. In dirty environments, specify a curtain with higher excess gain or choose a shorter range with margin.

What to Specify Before Ordering

Provide suppliers with a complete specification to avoid back-and-forth:

• Resolution and protective range at that resolution.
• Number of beams and protected height (beam count × spacing).
• Safety category and PL/SIL rating required (e.g., Type 4, PL e, SIL 3).
• Output type: OSSD (PNP/NPN), relay, or safety bus interface.
• Response time (critical for safety distance calculation).
• Housing size, mounting brackets, and connector type (M12, flying leads).
• IP rating and operating temperature range.
• Certifications: CE, TÜV, UL, and functional safety certificates.

Ask for the response time in writing. It is often omitted from marketing sheets but is essential for ISO 13855 calculations.

Cost Considerations

Light curtain cost scales with resolution, range, beam count, and safety rating. Practical levers:

• Resolution: Moving from 14 mm to 30 mm can reduce cost per protected height by 20–40% at Chinese factories, because fewer, higher-power beams are needed.
• Range: Longer range units use higher-power emitters and more sensitive receivers, adding cost. If your application allows, specify the shortest range that covers the span with 20–30% margin.
• Beam count: Protected height drives beam count. Avoid custom heights if a standard length fits; standard lengths are stocked and cheaper.
• Certification: Type 4 / PL e units with third-party certificates carry a premium. For lower-risk applications, Type 2 may be acceptable and cheaper, but verify against your risk assessment.

Sourcing Tips from Chinese Factories

• Request the safety certificate, not just a CE declaration. Functional safety requires a notified body certificate (TÜV, UL). Ask for the certificate number and verify it.
• Demand a sample for bench testing. Test response time, alignment tolerance, and immunity to ambient light and weld flash. A sample that trips on sunlight is a red flag.
• Check the OSSD output specification. Confirm short-circuit protection, output current, and compatibility with your safety relay or controller.
• Clarify the warranty and failure policy. Safety components should have at least 18–24 months warranty and a documented failure analysis process.
• Ask for the MTTFd and PFHd data. These are needed for your ISO 13849 validation. Reputable factories provide them.
• Consider the total installed cost. Cheap curtains with poor alignment tolerance can cost more in commissioning and downtime than a slightly pricier unit with better optics.

Bottom Line

Specify resolution from your risk assessment, not from a catalog default. Choose the shortest protective range that covers your span with margin, and confirm that the resolution/range combination is certified. When sourcing from Chinese factories, insist on third-party functional safety certificates, response time data, and a test sample. The cheapest curtain is the one that installs without nuisance trips and passes validation the first time.