A Technical Guide to Safety Light Screens: Selection, Design, and Implementation

Written By Jon Anna

Safety light screens are among the most widely used presence-sensing safeguarding devices in modern manufacturing. When properly selected and integrated into a machine’s control system, they allow employees to safely interact with equipment while maintaining productivity. When poorly designed or incorrectly installed, however, they can create a false sense of security and leave workers exposed to serious hazards.

This guide provides a technical overview of safety light screens, how they work, how they should be applied, and the engineering principles that determine whether they provide adequate protection.

What Is a Safety Light Screen?

A safety light screen is an electro-sensitive protective equipment (ESPE) device consisting of a transmitter and receiver positioned opposite one another. The transmitter projects multiple parallel infrared beams to the receiver, creating an invisible protective field.

When one or more beams are interrupted, the light screen sends a stop signal to the machine’s safety control system.

Unlike physical guards, light screens do not prevent access. Instead, they detect entry into a hazardous area and initiate a stop before a worker can reach the danger point.

Applicable Standards

Several standards govern the design and application of safety light screens.

In the United States, the primary references include:

  • OSHA 29 CFR 1910.212 – General Machine Guarding

  • OSHA 29 CFR 1910.217 – Mechanical Power Presses (where applicable)

  • ANSI B11.19 – Performance Requirements for Risk Reduction Measures

  • ANSI B11.0 – Safety of Machinery

  • ANSI/RIA R15.06 – Industrial Robots

  • NFPA 79 – Electrical Standard for Industrial Machinery

Internationally, the most frequently referenced standards include:

  • ISO 13855 – Positioning of safeguards

  • ISO 13849-1 – Safety-related parts of control systems

  • IEC 61496 – Electro-sensitive protective equipment

  • IEC 62061 – Functional safety of machinery

Designing a compliant system typically requires considering several of these standards together rather than relying on any single document.

How Safety Light Screens Work

Each transmitter contains dozens—or even hundreds—of infrared emitters.

The receiver continuously verifies that each beam is received.

During operation, the system performs internal diagnostics by:

  • Monitoring emitter timing

  • Verifying beam synchronization

  • Detecting cross-talk

  • Monitoring internal electronics

  • Detecting wiring faults

  • Confirming output integrity

Modern safety light screens use redundant safety outputs, commonly referred to as OSSD (Output Signal Switching Device) outputs.

Both outputs must remain in agreement. If either channel detects a fault, the system transitions to a safe state.

Protective Height

Protective height is often confused with overall device length.

The protective height represents only the active sensing area.

For example:

  • Overall housing = 1,400 mm

  • Protective height = 1,200 mm

Only the active sensing area provides safeguarding.

Openings above or below this area may require additional guarding.

Safety Distance

One of the most critical—and commonly misunderstood—aspects of light screen installation is safety distance.

The light screen must be positioned far enough from the hazard that the machine can completely stop before a person reaches the danger point.

This distance depends on:

  • Human approach speed

  • Machine stopping time

  • Light screen response time

  • Safety controller response

  • Output relay response

  • Resolution compensation factor

The general equation used by ISO 13855 is:

S = (K × T) + C

Where:

  • S = minimum safety distance

  • K = approach speed constant

  • T = total stopping time

  • C = additional distance based on resolution

A common mistake is calculating safety distance using only the machine stop time while ignoring the response times of the light screen, safety relay, contactors, or safety PLC.

Every component contributes to total stopping time.

Machine Stopping Time

Accurate stop-time measurement is essential.

Stop time should be measured using a calibrated stop-time meter rather than estimated.

Stopping time may increase over the life of a machine due to:

  • Brake wear

  • Hydraulic valve degradation

  • Pneumatic leakage

  • Increased inertia

  • Mechanical wear

Periodic verification ensures the original safety distance remains valid.

Detection Capability

Light screens may detect:

  • Fingers

  • Hands

  • Arms

  • Entire bodies

They do not detect:

  • Objects smaller than their resolution

  • Personnel approaching from outside the protective field

  • Hazards occurring after a person has entered and remained inside the safeguarded space

Because of this, secondary safeguards are often necessary.

Blanking Functions

Modern light screens often support blanking.

Fixed Blanking

Specific beams remain permanently ignored.

Applications include:

  • Conveyor supports

  • Structural members

  • Machine tooling

Only predefined beams may be blocked.

Floating Blanking

A limited number of adjacent beams may be interrupted anywhere within the field.

Typical applications include:

  • Moving material

  • Pallets

  • Continuous product flow

Floating blanking reduces detection capability and must be evaluated carefully during the risk assessment.

Muting

Muting temporarily suspends the protective function to allow materials to pass through the light screen automatically.

Common examples include:

  • Pallet conveyors

  • Automated storage systems

  • Packaging equipment

Muting requires carefully designed sensor logic.

Typical requirements include:

  • Multiple muting sensors

  • Limited muting duration

  • Visual muting indicators

  • Automatic reset of protective function

Muting should never allow personnel to enter hazardous areas undetected.

Override

Override differs from muting.

Override intentionally bypasses the protective function to recover from abnormal conditions, such as clearing a jam.

Override should:

  • Require deliberate operator action

  • Operate only at reduced speed where appropriate

  • Be time limited

  • Require additional safeguards

Permanent override functions are unacceptable.

Control Reliability

Safety light screens are only one component of the safety function.

The overall system typically includes:

  • Light screen

  • Safety PLC or safety relay

  • Safety contactors

  • Emergency stop circuit

  • Machine brake

  • Feedback monitoring

The complete system must achieve the required Performance Level (PL) or Safety Integrity Level (SIL) identified during the machine risk assessment.

Installing a Category 4 light screen on a Category B control system does not result in a Category 4 safety function.

The weakest component determines the overall system capability.

Common Installation Errors

The majority of light screen deficiencies identified during machine safeguarding assessments involve installation rather than equipment failure.

Common issues include:

  • Light screen positioned too close to the hazard

  • Inadequate stopping distance calculations

  • Incorrect resolution selection

  • Reach-over hazards

  • Reach-around hazards

  • Reach-under hazards

  • Reflective surfaces causing beam deflection

  • Improper muting logic

  • Reset buttons located inside the hazard zone

  • Automatic restart after beam restoration

  • Unmonitored external device failures

Many of these deficiencies can allow an operator to access hazardous motion before the machine reaches a safe state.

Environmental Considerations

Modern light screens perform reliably in harsh industrial environments, but several factors should still be evaluated:

  • Oil mist

  • Welding spatter

  • Coolant overspray

  • Vibration

  • Shock loading

  • Direct sunlight

  • Dust accumulation

  • Temperature extremes

  • Electromagnetic interference

Routine cleaning and inspection should be incorporated into preventive maintenance programs.

Inspection and Functional Testing

Safety light screens should be verified:

  • At installation

  • After maintenance

  • Following machine modifications

  • After relocation

  • At defined inspection intervals

Verification should include:

  • Beam interruption testing

  • Stop-time verification

  • Indicator functionality

  • OSSD output verification

  • Reset operation

  • Muting function (if applicable)

  • Alignment checks

  • Inspection of cables and connectors

Documentation should be retained as part of the machine’s safety records.

Advantages

Properly implemented safety light screens provide several benefits:

  • Improved operator productivity

  • Faster material loading

  • Reduced ergonomic strain

  • No physical gates to open

  • Continuous hazard monitoring

  • Integration with modern safety PLCs

  • Flexible machine access

For many automated manufacturing operations, they offer the best balance between safety and operational efficiency.

Limitations

Safety light screens are not appropriate for every application.

They should generally not be used where:

  • The hazard cannot stop quickly enough.

  • Flying chips, sparks, or debris require a physical barrier.

  • Ejected parts present a projectile hazard.

  • Workers can remain inside the hazard zone undetected.

  • Bypassing the protective field is reasonably foreseeable.

In these situations, fixed guards, interlocked gates, pressure-sensitive devices, or alternative safeguarding methods may provide more effective risk reduction.

Final Thoughts

Safety light screens are sophisticated safety systems—not simply “electronic guards.” Their effectiveness depends on careful engineering, including hazard analysis, stop-time measurement, appropriate resolution selection, safety distance calculations, and proper integration into the machine’s safety control system.

Organizations that treat light screens as engineered safeguards rather than off-the-shelf components are far more likely to achieve both regulatory compliance and meaningful risk reduction. A thorough risk assessment, adherence to applicable standards, and periodic verification are essential to ensuring that a light screen performs its intended safety function throughout the life of the machine.

Jon Anna
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