Safety Light Curtains: One Way to Safeguard Your Machines

Posted by Technical Communications Team · January 09, 2017

Safety light curtains (also known as light guards or light screens) are optical sensing devices. They offer flexibility, quick and frequent access to machinery, and better visibility. Also, they’re easier and more cost-efficient to maintain than mechanical barriers. If you’re shopping for a quality safety system, we’re here to answer some of your burning questions.

How do safety light curtains work?

Light curtains come in pairs. They consist of a transmitter and receiver that communicate via invisible light beams. These light beams create a protective field. If breached, they’ll send a stop signal to the machine safety controls. The light beams also pulse at a specific frequency. This way they don’t interfere with other light sources and vice versa.

What is the difference between Type 2 and Type 4 safety light curtains?

Type 2 light curtains are generally a more economical choice. They don’t have as many features as type 4 light curtains. Opt for Type 2 safety light curtains for hand and body protection where there is a low risk factor. Type 2 safety light curtains are also a great option for supplemental safeguarding. Installing a supplemental safeguard will prevent the reset of a primary safeguard.

Type 4 light curtains are a bigger investment. But, they do offer more safety and control. Choose type 4 safety light curtains for moderate to high risk environments. “Type 4 light screens achieve high levels of fault tolerance through redundancy and monitoring. They meet OSHA regulations and ANSI standards for control reliability” (Carlson).

To identify whether a Type 2 or Type 4 light curtain is needed, the engineer or designer must conduct a complete risk assessment.

What are the advantages of safety light curtains over mechanical barriers?

Light curtains use light beams to detect when a person or object crosses into a dangerous area. Easy access to machines can increase efficiency if machine operators need frequent access. Safety light curtains also increase visibility of the machinery. In addition, Minimal wear and tear makes them more cost-efficient than mechanical barriers.

How do I know which resolution I need?

The resolution is the distance between two adjacent light beams and beam diameter. A light curtain with a high resolution can detect fingers. A low resolution safety light curtain is ideal for detecting arms or a whole person. Mid-range resolutions are suitable for detecting hands.

How do I calculate the appropriate separation distance for a safety light curtain?

The appropriate separation distance is unique to every circumstance. Factors to consider include: light curtain resolution, response time, machinery, and operator speed. The separation distance should allow enough time for the machine to stop before the operator comes in contact with the hazard. Below is a formula according to ANSI standards for calculating safety distance.

ANSI Safety Distance Formula

D_s=K × (T_s+ T_c+ T_r+ T_spm) + D_pf

K= Hand speed constant (63 inches/second)

T_s= Stop time of machine (measured with stop time measurement device)

T_c= Response time of the control system

T_r= Response time of the safety light curtain (provided by supplier)

T_spm= Additional stop time to compensate for normal stop time variations

D_pf= Depth penetration factor

What does “automatic reset” and “manual reset” mean?

Automatic reset and manual reset are optional functions. They modify the Output Signal Switching Devices (OSSDs) on a safety light curtain. OSSDs send stop signals to the machine controls.

An automatic reset function is useful when an operator needs to repeatedly reach or step inside the guarded area. When the operator crosses the guarded area, the OSSDs send a stop signal to the machine’s safety controls. Once the operator retreats, the light curtain will resume working as usual.

With manual reset, the operator must press a reset button when the guarded area is clear. This is useful with large protective areas. The operator can pass through the light curtain and be inside the protected area. When he exits the area, he resets the safety light curtain.

What is the difference between a blanking and muting function?

Blanking and muting are optional safety light curtain functions. The blanking function ignores moving or stationary objects of a certain size. This is perfect for situations where part of a machine crosses into the protective field.

Muting allows objects or a person to pass through a guarded area, but only during the non-hazardous part of the machine cycle. During this phase of the cycle the entire light curtain is temporarily disabled. The muting function also has the capability to only allow objects to pass through, not people.

Are there any precautions I should take during installation?

Yes. When placing a light curtain near a reflective surface, it’s possible that one or more of the light beams will reflect back. When this happens, the light curtain will not detect when an object passes through. This is more likely to occur with Type 2 light curtains as they have a larger Effective Aperture Angle. Thus, be sure to place light curtains at an appropriate safety distance from reflective surfaces.

What is the difference between safety light curtains and light grids?

A light curtain consists of several beams positioned close together. They detect the presence of fingers, a hand, or an entire person. On the contrary, light grids have only 1 to 4 light beams. These are useful for guarding large areas whereas light curtains are better for short spaces.

We admit light curtains can be a hefty investment. However, the advantages over mechanical barriers are worth giving light curtains some consideration. We have recently added Leuze’s entire line of safety light curtains to our webstore. Type 2 and Type 4 options are available in various resolutions to meet your needs.

Still unsure as to which type of light curtain is right for your application? Feel free to contact our technical sales specialist today!