How does a light curtain work?
The light curtain's transmitter contains LEDs (light emitting diodes) that send harmless, invisible pulses of infrared light to the receiver's PTs (photo-transistors).

The PTs in the receiver detect each light pulse and convert it to an electrical signal. The light curtain immediately signals the machine to stop when any light beam is interrupted.

How can I be sure my interface with the machine's stop circuits is control reliable?
Light curtains usually have two cross-checked captive contact (force guided) mechanical relays. Typically they are normally open (N/O) outputs and are connected in series with the machine's stop circuit. Some users prefer one normally open and one normally closed output (1NO/1NC). When using 1NO/1NC and connecting to a single emergency stop string, the N/C logic has to be inverted at the machine control. This inverting logic requires careful consideration because it must also be control reliable.

How important is safety distance?
Determining the correct safety distance is paramount to safety in the workplace. Safety distance applies to any safety device that electrically signals a stop, such as two-hand controls, electrically inter-locked barriers/gates, and presence-sensing devices. A light curtain must be mounted far enough away from the hazard so the machine will stop before the person gets too close to the hazard.

There are established formulas for calculating safety distance. OSHA 1910.217 states that the safety distance from the light curtain's sensing field to the pinch point must be no less than:

Ds = 63 inches/second x Ts

In this formula, Ds equals the minimum allowable safety distance in inches, Ts is the worst case stopping time, and 63 inches per second represents the established hand-speed constant.

By comparison, ANSI safety distance formulas are more stringent. One additional requirement is called the "depth penetration factor" based on your light curtain's object sensitivity. Object sensitivity is the smallest object a light curtain can detect anywhere in its light field.

The OSHA or ANSI formula should be used as the basis for determining proper safety distance in your application.

However, if you use a presence-sensing device as a perimeter guard where a person could walk into the light path you might want to use a more generous hand-speed constant.

What about height?
Once you've determined the correct safety distance, you should consider the light curtain height. Your light curtain must be installed so someone can't reach over, under, or around the light curtain and into the hazardous area of the machine without being detected. This requires a light curtain that is the proper height and at the proper safety distance, combined with ancillary physical guards.

What's the optimum angle of divergence and acceptance?
The angle of divergence and acceptance refers to the size of the cone of light emitted from the transmitter and how much of that light is detected by the receiver. It relates to the possibility that light reflected off some unintended surface (such as a shiny material or glossy surface on the machine or plant floor) could cause an "optical" short circuit. A small angle of divergence and acceptance means there will be a predictable plane of protection, with no holes in the coverage. If it is too small, it may be impossible to keep the transmitter and receiver properly aligned.

Industry and world standards have determined that +/- 2° is the optimum angle. This angle ensures safety combined with ease of alignment and tolerance to vibration.

Do light curtains inhibit productivity?
No, effective safeguarding should actually enhance productivity while reducing costs. Light curtains, for example, help to protect personnel without obstructing visibility or interfering with operation. The easy, productive way to work becomes the safe way. Insurance premiums may be reduced. With proper safeguarding, workers also feel more comfortable and confident, and are less afraid of being injured on the job.

What's fixed blanking?
Fixed blanking causes the light curtain to become blind to stationary fixtures such as a table or conveyor. Typically, there are two methods: external blanking and internal, or programmable, blanking. If you use fixed blanking and the obstruction is partial, safety distance will be affected because the light curtain becomes less sensitive.

What's the difference between external and internal fixed blanking?
With external blanking, physical blanking windows are installed on the receiver allowing stationary fixtures to protrude into the safety curtain's light field. This leaves no question or doubt as to where an obstruction is penetrating the sensing field.

Internal blanking is electronically programmed in the light curtain's control circuitry and therefore may not be visible. Some light curtains use small LEDs to indicate which beams have been internally blanked, while other light curtains don't provide any visible indication. Internal blanking could be hazardous if the beams being blanked are not obvious. Some light curtains with internal blanking claim to have the ability to detect the presence or absence of an obstruction. Unfortunately, no light curtain can determine whether the obstruction is all the way from the transmitter to receiver (such as a work table), or very narrow (such as a slim conveyor). Essentially this could lead to a hole in the overall protected area.

As always, the rule of thumb is to optimize safety. Regulations require that any type of blanking must be under supervisory control, the installation must remain intact, and blanking must be obvious to avoid an unsafe situation or misconception of coverage.

How does the floating blanking feature help productivity?
A floating blanking window allows a moving object to randomly obstruct the light curtain's sensing field as long as it blocks only one beam at a time. Some allow more than one beam to be interrupted. Any additional obstruction within the field will immediately activate a stop command. This is useful in applications where material must move within the sensing field in a random pattern. However, the floating window should be used only when material or parts take up no more than 1-1/8" within the sensing field. Larger objects would block more than one light beam at a time and therefore send a stop command. Note that the use of a floating blanking window will affect safety distance because it alters the sensitivity of the light curtain.

Why choose a rugged light curtain when my environment doesn't require it?
When selecting a light curtain, make certain it's rugged, reliable, easy-to-maintain, and meets strict industry standards.

A good light curtain should tolerate tough environments because you're counting on it for safety.

What does all this mean to me?
The best approach in selecting a control reliable light curtain is usually the simplest. Choose a light curtain that meets all the safety requirements and is easy to install and maintain. Be sure your light curtain is rugged enough to withstand rough use and still ensure safety under all conditions.

Look for a supplier with a proven track record in the light curtain industry. Find out what quality control and testing procedures they employ. Also make sure your supplier stands behind the light curtain after the sale.

These are just a few of the questions to consider. In the final analysis, making workplace safety your highest priority should ensure that you make the right choice.

For additional facts and information
Call 1-800-333-3282 to talk to our authorities on machine safety. They'll be happy to answer your questions or provide a detailed assessment of your requirements.

Our safety seminars reflect more than 25 years of hands-on experience, involving tens of thousands of light curtain installations. We've also had a prominent role in the development of related guidelines and safety standards. Our machine safety authorities are a very knowledgeable -- and uncommonly accessible -- source of practical advice on meeting current regulations and industry standards.