Safety Light Curtains

Safety light curtains are a non-separating guard used for the protection of hazardous areas. They detect the passage of objects and direct the machine to stop the hazard. Because they do not provide a physical barrier, consideration must be made to place them a suitable distance from the hazard point to ensure the hazard is stopped in time. 

Safety Light Curtains (or AOPD) can be used, provided that:

• The dangerous movement can be stopped at all times and that it is ensured that the dangerous area can only be reached after the movement has come to a standstill.
• The stopping time for the machine and all safety components used are known.
• No objects (work pieces, liquids, etc.) can be ejected.
• The AOPD meet the requirements of Type 2 or Type 4 acc. to EN 61496.
• The dangerous area can only be reached by passing through the protected field of the AOPD.
• Reaching over, under or through the protected field is impossible.
• The start or restart command devices are fitted in such a way that the entire hazardous area is completely visible from the outside and it cannot be activated from within the hazardous area.
• The safety distance is calculated and constructively applied in accordance with EN ISO 13855

Finger safe and hand safe are terms that refer to the resolution of the Safety Light Curtain.

Resolution is the distance between two adjacent beams, and defines the object profile dimension that will be reliably detected passing through the array. EN ISO 13855 sets the biometric data for finger detection to 14 mm, for hand detection to 30 mm, for leg detection up to 70 mm and for body detection to over 70 mm. 

So, a light curtain with a 14 mm resolution will detect an average sized finger, thereby making it “finger safe”, while a 30 mm resolution will detect hands and is thus “hand safe”.

The following standards apply to Safety Light Curtains:

IEC 61496-1:2020 - Safety of machinery - Electro-sensitive protective equipment - Part 1: General requirements and tests 
IEC 61496-1:2020 specifies general requirements for the design, construction and testing of non-contact electro-sensitive protective equipment (ESPE) designed specifically to detect persons or part of a person as part of a safety-related system. Special attention is directed to functional and design requirements that ensure an appropriate safety-related performance is achieved.
Visit the IEC site for more information

IEC 61496-2:2020 -Safety of machinery - Electro-sensitive protective equipment - Part 2: Particular requirements for equipment using active opto-electronic protective devices (AOPDs)
IEC 61496-2:2020 specifies requirements for the design, construction and testing of electro-sensitive protective equipment (ESPE) designed specifically to detect persons as part of a safety-related system, employing active opto-electronic protective devices (AOPDs) for the sensing function. Special attention is directed to features which ensure that an appropriate safety-related performance is achieved.
Visit the IEC site for more information

ISO 13855:2010 - Safety of machinery — Positioning of safeguards with respect to the approach speeds of parts of the human body
ISO 13855:2010 establishes the positioning of safeguards with respect to the approach speeds of parts of the human body. It specifies parameters based on values for approach speeds of parts of the human body and provides a methodology to determine the minimum distances to a hazard zone from the detection zone or from actuating devices of safeguards.
Visit the ISO site for more information

There are two Types of safety light curtains, as defined in IEC 61496, determined by how they perform. 

Type 2 safety light curtains feature a periodic self-test, which may experience a failure in between test cycles. They can be used for lower risk situations, up to a SIL2 or PLc and typically feature a large sensing distance and are considered a lower cost solution.

Type 4 safety light curtains feature an active self-test that happens throughout the operation and have high tolerance to faults. They are used in high-risk applications and meet the requirements for the highest safety levels of SIL3 or PLe. 

A risk assessment based on ISO 12100 or similar should be performed to determine which Type is needed. Additionally, safety standards such as IEC 61508 and ISO 13849 which evaluates the safety functions of machines, can be used to define a SIL level or Performance Level (PL) which will also determine the required Type of the light curtain.

The difference is the number of beams and the general application for the devices.

Safety light curtains use light beams (non-visible infrared), so mirrors can be used to deflect the beams to form a L- or U-shaped protective field around the perimeter or section of a hazard or machine. 

Special considerations should be made when applying mirrors:

1. With every mirror used the beam strength and range is reduced by 10% or more, depending on brand and application. It is recommended to use no more than 2 mirrors. 

2. Mirrors should be mounted to sturdy pilons or similar posts and protected being bumped into. If the mirrors are rotated or moved, then the beams between the emitter and receiver will become misaligned.

Safety laser scanners and safety light curtains are both safety devices used in industrial settings to protect operators from hazards, but they operate differently and suit different applications.

In summary, the main difference lies in their coverage and application suitability. Laser scanners are ideal for complex, customizable zones and mobile applications, while light curtains are well-suited for protecting defined, linear areas that require frequent operator access. Both devices are integral to enhancing safety but serve distinct roles based on workspace configuration and operational needs.

Safety light curtains are a viable safeguarding option for point of operation or perimeter guarding. Choosing the right one is not terribly complicated, once a thorough risk assessment is performed.  It helps to define the Type, Resolution, Protection field, and Special operation modes needed for your application.

Type
There are two Types of AOPD, defined by IEC 61496. 
Type 2 features a periodic self-test and are used for lower risk situations, up to a SIL2 or PLc and typically feature a large sensing distance. They are considered a lower cost solution.
Type 4 features an active self-test that happens throughout the operation. Since they have high tolerance to faults, they are used in high-risk applications and meet the requirements for the highest safety levels of SIL3 or PLe. 
The risk assessment will determine the safety integrity level (SIL) or Performance Level (PL) you need to achieve.

Resolution
The resolution is the distance between consecutive beams, or the size of the object that will be detected passing through the protection field. This size, typically noted in millimeters, determines if the device can be used for point of operation applications or for perimeter guarding. The resolution can affect how close the protective field needs to be to the hazard.

Protection field
The protection field is the plane in which the infrared beams travel from emitter to receiver which will detect an object passing through. It should be of sufficient height and width (range) that operators cannot reach over, reach under, or go around to reach the hazard.

Special functions and operation modes
There are many possible special functions and operation modes available. A large hazard area may need a double acknowledgement reset. If materials need to pass through the protection field yet still prevent personnel from crossing, then blanking or muting may be necessary.  

For more detailed information, please see our technical article: Selection Guidelines for Safety Light Curtains

Blanking and muting are two distinct safety functions in light curtain systems used in industrial settings to manage safety without halting productivity unnecessarily. Both are essential in applications where certain objects or materials must pass through the light curtain without triggering a safety response.

Blanking allows specific light beams in a safety curtain to be intentionally deactivated or ignored, enabling objects of a predetermined size or shape to pass through without interrupting the machine. There are two types of blanking: fixed and floating. Fixed blanking permanently deactivates specific beams, typically for an obstruction in the field that does not move such as a conveyor belt. Floating blanking ignores obstruction of one or more beams to permit movement within a specific area, allowing objects to pass through shifting areas. Floating blanking is used where material entering the protection field may move over time, like wire spooling onto a reel. Blanking requires careful configuration to ensure that only items or parts enter, as it allows gaps in the protected area and floating blanking can effectively change the resolution. 

Muting is a temporary bypass function where the entire light curtain deactivates momentarily to let objects pass through without triggering a shutdown. This function uses additional sensors that detect the object and initiate muting for a brief period, after which the light curtain resumes its full safety function. Muting is commonly used in automated systems where materials need to enter or exit a hazardous area safely, such as in palletizing operations or robotic loading and unloading zones. By using muting, operators ensure that only designated items, rather than unauthorized personnel, pass through.

In summary, blanking is used when selective beams need deactivation, while muting provides a full, temporary bypass. Both functions enhance productivity by allowing controlled, safe material flow through safety barriers, reducing unnecessary downtime in automated industrial processes.

Schmersal offers several options for status indicators in our Safety Light Curtains

The operating modes of an AOPD must be defined according to the risk analysis of a machine. 

Automatic / Protective mode
The protective mode switches the AOPD outputs to an ON state (protection field not interrupted), without external release of a switching device. This mode of operation creates an automatic machine restart if the protection field is not interrupted and should only be selected with the restart interlock of the machine.

Restart interlock (manual reset)
The restart interlock (manual reset) prevents an automatic enabling of the outputs (OSSD's ON state) after switch-on of the operating voltage or an interruption of the protection field. The system switches the outputs only to an ON state when an external command device generates an enabling signal at the restart input (receiver).

Restart interlock with double acknowledgement/reset
In applications with access monitoring, a complete overview of the hazardous areas is often not possible; despite that, a reset of the command device for the restart interlock outside of the hazardous area by third parties is enabled at all times. This hazardous situation of an unexpected start-up can be avoided by means of a double reset, i.e. integration of one command device inside and one outside the hazardous area.

Cyclic Mode
Cyclic Mode, also known as Presence Sensing Device Initiation (PSDI) allows the safety light curtain to be used as a machine control device. The programming of the machine expects the protection field to be interrupted between machine cycles, to place or remove materials. The machine will initiate the next cycle once the beams are restored. This function should not override the primary function of the curtain - if the protection field is interrupted during the machine cycle, the hazardous motion is stopped, and the machine is put into an interlock state until the safety is reset.

Setting mode
Before commissioning an AOPD, the best possible alignment of the sensors should be determined. The set-up mode visualises the set-up quality during the installation of the sensors. Visualisation is via a 7-segment display, a status display or optionally via a smartphone with the "SLC Assist" app. 

Regular inspections

A regular visual inspection and functional test, including the following steps, should be performed:

1. The components do not have any visible damages.
2. The optics cover is not scratched or soiled. Clean with a clean, soft cloth with low pressure.
3. Hazardous machinery parts can only be accessed by passing through the detection zone of the AOPD.
4. The staff remains within the detection area, when works are conducted on hazardous machinery parts.
5. The safety distance of the application exceeds the mathematically calculated one
    

Half-yearly inspection

The following items must be checked every six months or if a machine setting is changed.

1. Machine stops or does not inhibit any safety function.
2. No machine modification or connection change, which affects the safety system, has taken place.
3. The outputs of the AOPD are correctly connected to the machine.
4. The total response time of the machine does not exceed the response time calculated during the first commissioning.
5. The cables, the connectors, the caps and the mounting angles are in perfect condition.