The spec must cover the protected height, the calculated safety distance per ISO 13855, the restart interlock mode, the output configuration, and the enclosure rating; omitting any one of these fields forces the supplier to assume values and inflate the quote with engineering hours [S3][S4].
What a Safety Light Curtain Is, and Why the Warehouse Application Changes the Spec
A safety light curtain is an active opto-electronic protective device (AOPD / ESPE) formed by a transmitter stick that emits an array of infrared beams and a receiver stick that detects beam interruption, with the receiver driving output signal switching devices (OSSDs) into the machine safety circuit [S1][S7]. The core system block is the same in a press shop and in a conveyor gap, but the warehouse use case adds muting for pallets and totes, blanking for fixed racking, and cascaded sticks for tall pallet-flow aisles, all of which have to be on the RFQ line [S2][S4].
Selection starts with the hazard category: per ANSI B11.TR3 risk assessment logic, automated conveyors, AS/RS shuttles, and pallet-stacker cells typically sit in the higher risk categories, which translates into a Type 4 device per IEC 61496 and at least PLe / SIL 3 per ISO 13849-1 / IEC 62061 [S2][S4]. Specifying anything below Type 4 in these zones is a compliance risk and will be flagged by any integrator quoting to OSHA 1910.212 general machinery guarding obligations [S3][S5].
RFQ Line 1, Identification and Standards Compliance
Lead the RFQ with the formal type designation and the certification stack: Type 4 / PLe per IEC 61496-1/-2, ISO 13849-1 PLe Cat 4, IEC 62061 SIL 3, cULus listed, and CE marked to the Machinery Directive; warehouse OEMs also routinely require TUV certification on the same declaration line [S1][S2][S4].
Add the documentation requirement: failure rates (PFHd, MTTFd, DC) on the vendor's datasheet, the SISTEMA library file or equivalent calculation artefact, and a Declaration of Conformity referencing the Machinery Directive 2006/42/EC; without this paperwork, the device cannot be integrated into the safety file by the system integrator [S2][S3].
RFQ Line 2, Protected Height, Beam Pitch, and Resolution
State the protected height in millimetres as a single number (typical conveyor and palletiser RFQs run 300-1800 mm), and pair it with the beam pitch or the optical resolution: 14 mm pitch for finger detection, 20-30 mm for hand detection, 40 mm for whole-body presence around pallet-flow aisles [S1][S3][S4]. The beam pitch drives the detection capability and the calculated safety distance together; suppliers will refuse to quote if these two numbers are not on the line [S3].
Add the protected field length and the number of beams as derived values; for a standard stick, height equals (number of beams minus one) times pitch, and the RFQ should state the orientation (vertical, horizontal, or L-shaped) so the supplier can price the brackets and the cascading cable [S1][S4].
RFQ Line 3, Safety Distance per ISO 13855 and Approach Geometry
The safety distance S from hazard to detection point follows ISO 13855 and is calculated as S = K x T + C, where K is the approach speed (1600 mm/s for hands, 2000 mm/s for whole body), T is the total stop time, and C is the intrusion-distance constant based on resolution; the calculation must accompany the RFQ as a stated value, not a request to the supplier [S3][S4].
Quote the mounting distance you need (typical warehouse conveyors land at 500-1500 mm from the pinch point) and confirm the resolution chosen keeps the calculated C term inside the available mounting envelope; if S exceeds the available floor space, the spec has to change to a higher resolution or a horizontal placement, and that decision has to be made before the RFQ goes out [S3].
RFQ Line 4, Outputs, EDM, Restart Mode, and Cascading
Specify two OSSD outputs (PNP, short-circuit monitored) plus an EDM input that checks the state of the downstream contactors, because EDM adds the 99% diagnostic coverage expected from a Cat 4 / PLe safety function [S2][S7]. State the restart interlock explicitly: automatic restart for guarded conveyor tunnels, manual restart for palletiser cells where the operator must re-enter to clear a jam [S2][S7].
For tall AS/RS aisles or multi-segment cells, add a cascading line so multiple sticks daisy-chain into a single safety controller; without cascading on the RFQ, suppliers will quote each stick as an independent pair and the system integrator will face wiring rework during commissioning [S1][S4]. Also list the integration interface: safety relay, safety PLC (e.g. CIP Safety, PROFIsafe, FSoE), or a dedicated safety controller, so the OSSD sink is unambiguous [S1][S2].
RFQ Line 5, Muting, Blanking, and the Warehouse Material-Flow Requirements
Muting is non-optional on conveyor gaps where totes and pallets must pass while personnel must still be detected; the RFQ must state whether muting is initiated by override switch, photoelectric mute sensors, or RFID/ID-tag read, and whether the mute is total (all beams) or partial (a defined window) [S2][S4].
Blanking comes next: floating blank (object can move inside the field) and fixed blank (object is permanently at a known position) for fixed racking and support tables that penetrate the curtain without triggering a stop [S2]. Both functions are firmware options on most modern sticks; not putting them on the RFQ line locks the buyer into the default firmware image, and firmware upgrades after the fact cost more than the function itself [S2].
RFQ Line 6, Environment, Enclosure, and Mounting
Warehouse duty puts the sticks in fork-truck aisles, dock doors, and freezer zones, so the RFQ must call out the IP rating (IP65 minimum, IP67/IP69K for washdown or cold-storage), the operating temperature range (typical -10 to +55 C; -25 C or lower for cold storage), and the housing material (aluminium housing, PMMA or polycarbonate optics) [S1][S5][S6].
Bracketry, laser alignment aids, and cascading cables must be priced on the same line, because installation cost is the variable that swings total installed cost more than the stick price itself; the Wintriss guidelines explicitly recommend asking the vendor for installation labour as a separate line, since bracket and alignment differences can double the install hours between equivalent devices [S1][S4].
RFQ Line 7, Response Time, Range, and Diagnostics
Quote the response time in milliseconds (modern Type 4 sticks run 8-30 ms depending on height) and the operating range in metres (typical 0.5-12 m, long-range up to 50 m) so the safety distance calculation can be sanity-checked; the calculation has to use the worst-case response time, not the typical figure from the brochure [S1][S3][S8].
State the diagnostic interface (LED status, IO-Link, or safety bus diagnostic), the expected MTBF, and the PFHd figure, because the ISO 13849-1 PLe budget for the whole safety function has to be summed across the light curtain plus the contactors it drives [S2][S8]. Beam coding to suppress mutual interference between adjacent curtains in the same aisle is a real option, not a marketing line, and should be on the RFQ if more than one pair is in line of sight of another [S2][S8].
Comparison: Type 2 vs Type 4 vs Cascade for Warehouse Use
For warehouse conveyor and AS/RS duty the comparison comes down to three options: Type 2 (PL c, SIL 1) for low-risk open aisles with no automatic motion, Type 4 (PL e, SIL 3) for any powered conveyor, robot cell, or shuttle, and Type 4 cascaded for tall pallet-flow or multi-segment cells [S1][S2][S4]. On response time Type 4 sticks win (sub-15 ms typical vs 20-30 ms for Type 2 in the same form factor), on muting and blanking features Type 4 is the only option because Type 2 firmware usually lacks the override switches and partial-mute windows, and on diagnostics Type 4 with EDM gives the 99% DC figure that the safety controller calculation needs [S2][S4][S8].
Cost ranking puts Type 2 lowest per stick, Type 4 single-sticks next, and cascaded Type 4 highest, but the labour and bracket differences between an unmanaged Type 2 and a managed Type 4 narrow the gap once the safety distance calculation, muting sensors, and EDM wiring are added on the integrator side [S1][S4].
Common RFQ Mistakes That Force a Requote
Three errors repeat on warehouse RFQs: leaving the safety distance off the spec (forces the supplier to assume T and K, which fails the ISO 13855 check), specifying Type 2 in a robot-cell or palletiser application (fails the risk assessment per ANSI B11.TR3), and omitting the muting/blanking spec (locks the buyer out of a feature they almost always need) [S2][S3][S4].
A fourth is failing to declare the operating temperature; cold-storage and outdoor dock doors need -25 C or -30 C rated optics, and a standard -10 C stick in those zones faults repeatedly, which the supplier's warranty will not cover [S1][S5]. A fifth is asking for "OSSD" without specifying the sink (safety relay vs safety PLC) and the EDM loop, which leaves the supplier to guess the wiring approach and adds hours during commissioning [S2][S7].
Sourcing, Standards, and Selection Triggers
The standards that should appear on the warehouse RFQ are IEC 61496-1/-2 (AOPD type), ISO 13849-1 (PL and category), ISO 13855 (safety distance), IEC 62061 (SIL), and OSHA 1910.212 / 1910.147 for the US regulatory hook, with EN ISO 13849 and the Machinery Directive 2006/42/EC for European sites [S2][S3][S4][S5]. A useful sanity-check test is the third-party certification mark: TUV, cULus, and CSA on the same nameplate confirms the device has been audited to all three regimes, not just self-declared CE [S1][S4].
For a deeper comparison of how light curtains sit next to other warehouse guards and interlocks, the warehouse helmet and PPE spec workflow and the safety relay selection for conveyor and robot-cell duty walk through adjacent RFQ lines on the same site visit. The conceptual background on the safety function architecture is covered in the machine safety and fire safety reference pages, which explain the PL / SIL vocabulary the supplier will see in the SISTEMA file. For the structural side of the same warehouse build, the door, window, and curtain wall and glass curtain wall references show how dock-door guarding is specced alongside the light curtain system.
Trackable signals: the IFA / DGUV list of approved light-curtain AOPD modules, SISTEMA library updates from major vendors (SICK, Omron, Keyence, Banner, Pilz, Rockwell), and any revision of ISO 13855 affecting warehouse reach distances are the next nodes to watch. A new RFQ for a conveyor or AS/RS cell that turns up a Type 2 spec in a powered-cell application is the clearest indicator that the risk assessment on the existing line is incomplete and has to be reworked before procurement proceeds.