A safety light curtain guards an opening into a machine by sending infrared beams between an emitter and a receiver. When a hand or finger blocks a beam, the curtain’s safety outputs switch off and the hazard stops. A curtain that is too coarse, mounted too close, or installed with gaps lets a person reach the moving parts before the machine halts. Resolution and safety distance are the two numbers that make a light curtain actually protect, and they have to be calculated for the specific machine.
What resolution means
Resolution, also called detection capability, is the smallest object the curtain reliably detects. It is set by the spacing between beams and the diameter of the beams. A finger-detection curtain, around 14 mm resolution, catches a finger and suits point-of-operation guarding close to the hazard. A hand-detection curtain, around 30 mm, detects a hand or wrist and is common on larger openings. A body or access curtain, 40 mm or coarser, detects a torso or leg at a perimeter fence opening. The finer the resolution, the shorter the allowed safety distance can be, because the curtain detects the intrusion earlier.
Why resolution changes the distance
A coarse curtain does not react until the arm has pushed well past the beams, so the machine must be farther away to give time to stop. A fine curtain trips as soon as a finger enters, allowing a closer mount. This link is built into the safety-distance standard, which adds a depth factor based on resolution. Choosing 30 mm instead of 14 mm to save money can force the curtain a good distance back from the machine, sometimes farther than the layout allows. Resolution and floor space trade against each other.
The safety distance formula
The minimum mounting distance is based on how fast a person can reach the hazard, how long the machine takes to stop, and the curtain’s resolution. A common form multiplies a reach speed by the total stopping time, then adds a constant tied to resolution. The stopping time includes both the curtain’s response and the machine’s actual stop time, measured rather than guessed. If you mount the curtain closer than this distance, a hand can reach the hazard before the machine stops. Measure the stop time with a timer or the drive’s safe-stop function and recalculate after any change to brakes or valves.
Mounting height
A horizontal approach, reaching toward the machine, needs the curtain mounted at the correct height above the floor and with the protected field covering the reach zone. Standards give mounting heights for hand and finger curtains so a person cannot crawl under or reach over the field. If the curtain sits too high, someone can pass a hand underneath; too low and it may nuisance-trip on legs. For a vertical curtain across an opening, the field should cover the full opening with no gaps at top or bottom. Check the reach-over zone above the curtain as well.
Gaps and bypass
A light curtain only guards the plane it covers. If a person can reach around the side, duck under, or step over the field and still touch the hazard, the curtain gives false confidence. Use fixed guards to close any opening not covered, and mount the curtain so the distance from every reachable gap to the hazard still meets the formula. On a press with a wide frame, this often means side shields in addition to the front curtain. Walk the approach as an operator would and look for every path to the hazard.
Type 2 vs Type 4 and safety ratings
Light curtains carry safety integrity ratings. Type 4 curtains, suited to high-risk hazards, use redundant channels and fault detection and reach the highest performance levels, while Type 2 curtains, for lower-risk applications, are simpler and cheaper. Match the rating to the risk assessment: a press or robot cell that can cause serious injury generally calls for Type 4 with a high PL or SIL. Don’t install a low-rated curtain on a severe hazard because it was cheaper; the rating must cover the risk.
Muting and blanking
Muting temporarily suspends the curtain so material can pass on a conveyor while still protecting people, using sensors that distinguish a pallet from a person. Fixed or floating blanking lets certain beams stay blocked, for example for a fixture, but it effectively worsens resolution and must be applied with care. Any muting must not let a person pass unnoticed, and the safety controller should enforce the muting sequence and timing. These functions add flexibility but are easy to misuse into a permanent bypass.
Integration and stop performance
The curtain’s outputs must feed a safety relay or safety PLC that removes power through contactors or a safe stop, with the contactors monitored for welded contacts. The chain is only as fast as its slowest element, so include relay and valve or brake times in the stop calculation. After wiring, test that blocking each beam stops the machine and that a faulted channel is detected. A curtain wired into a normal PLC input with no safety logic does not provide protection even if it stops the axis in normal use.
Environment
Dust, coolant mist, welding flash, and bright light can interfere with optical beams. Use curtains rated for the environment, add lens shields or enclosures, and align them so vibration does not cause nuisance trips. A curtain that repeatedly faults gets bypassed by annoyed operators, which defeats it. Keep the lenses clean and choose the right enclosure rating, because reliability is part of safety.
Common mistakes
Mounting closer than the safety distance, using hand resolution where finger is needed, leaving reach-around or crawl-under gaps, underrating the curtain for the risk, misusing muting into a bypass, wiring to non-safety logic, and using guessed rather than measured stop times are recurring errors. Do the risk assessment, measure the stop, and set resolution and distance together.
Horizontal versus vertical mounting
Most light curtains mount vertically across a vertical opening, but some machines guard a floor area with a horizontal curtain laid flat, detecting a person stepping into the zone. Horizontal guarding uses a different safety-distance calculation based on height above the floor and detection capability, and the field must cover the reach zone without leaving a path around it. A horizontal curtain mounted too high can be stepped over; too low and it nuisance-trips. Choose the orientation from the layout, and remember that floor-mounted fields are more exposed to damage and contamination from carts and swarf.
Cascading and corner protection
Large openings or multi-sided robot cells sometimes need more than one curtain. Cascading lets a master and slave curtain protect adjacent sides without separate safety relays, covering corners that a single straight field cannot. When cascading, confirm the response time of the full chain, because each added segment can lengthen it, and recalculate the safety distance for the slowest path. At corners, avoid leaving a wedge-shaped gap between the two fields where an arm can slip through. Plan the protected boundary as a closed loop around the hazard rather than separate guarded faces.
Periodic testing and proof tests
A safety curtain needs more than a correct install. Verify it on commissioning by blocking beams at several heights and confirming the machine stops and will not restart until reset. Then set a periodic proof-test schedule based on the safety manual, often a functional check at defined intervals, and keep records. Test the restart interlock so the machine does not self-start when the field clears, and check the muting lamps and sensors. Optical surfaces age and alignment drifts, so a curtain that passed at install may develop dead beams later. Routine testing is part of the safety claim, not optional paperwork.
Choose vertical or horizontal mounting from the layout; horizontal floor fields need their own distance and height checks and are more exposed to damage. For multi-sided cells, cascade curtains to cover corners, confirm the full chain’s response time, and avoid wedge gaps at joins. After install, block beams at several heights, confirm the machine stops and needs a manual restart, and set a periodic proof-test schedule with records. Test muting lamps and sensors. Optical surfaces age and alignment drifts, so a curtain that passed at install can develop dead beams later. Routine testing is part of the safety claim.
Choose vertical or horizontal mounting from the layout. For multi-sided cells, cascade curtains, confirm the chain’s response time, and avoid corner gaps. After install, block beams at several heights, confirm a manual restart is needed, and set a periodic proof-test schedule with records.
Bottom line
Pick resolution by the body part at risk, 14 mm for fingers, 30 mm for hands, coarser for body access, and calculate the safety distance from measured stop time plus the resolution factor. Mount at the right height, close every reach-around gap, and match the Type and PL to the hazard. Use muting carefully and keep lenses clean. The right curtain stops the machine before anyone reaches the danger.
