The traffic light is a universally recognized device (except perhaps by the severely color-blind). Green means proceed through the intersection — it is safe; red means stop — danger; and yellow means (no, it doesn’t mean hurry up and beat the red) proceed with caution. The laser warning light is the faithful sidekick of the laser laboratory. We’re not talking about the small LED emission indicator on a laser housing — which is close to useless in our opinion — but about the illuminated sign outside the laboratory, which shows the status of the laser.The American National Standard for Safe Use of Lasers requires the use of warning signs in controlled areas where lasers are used. An acceptable form of a warning device is a lighted sign, posted outside the entrance of each affected room. Single, double (left) and triple (right) lights typically are available. Courtesy of Rockwell Laser Industries Inc.The ANSI Z136.1 — 2007, American National Standard for Safe Use of Lasers, requires that a Class 3B or Class 4 indoor laser controlled area be established by your laser safety officer. The area must include appropriate warning signs, posted so as to identify it as such. An “activation warning device” also is recommended for Class 3B and required for Class 4 laser systems, so that personnel may know when the laser is operating. According to Section 4.3.9.4.2 of this standard, one acceptable form of an activation warning device is a lighted sign, posted outside each entrance to the room. As with the traffic light, these warning lights come in several varieties.• Tri-light: Green means it’s safe, OK to enter; yellow means caution, the laser is energized but the beam contained; and red means danger, keep out, beam exposure possible.• Dual light: One light for safe or caution and another for danger.• Single light: One light, meaning the laser is on. This is the most commonly used laser warning indicator, excluding the passive, ANSI-style warning sign on the door. Unlike the red traffic light, the message is ambiguous. It means some hazard is present, or might be present or used to be present. A red light might mean that a laser is on behind the door, but how do you know whether the beam is contained? It might mean that the laser power supply is on but that the laser is not lasing. It also might mean that there is a darkroom inside and that you’ll expose film if you open the door. Making sure the sign is seenThe placement of a laser warning sign is important. Can it be clearly seen as you approach the door? Many lights are placed too high above the door, out of the line of sight. A related consideration is whether to place additional signs inside the laboratory, to keep personnel inside aware of the laser’s status. If so, the sign(s) should be clearly visible throughout the laboratory, even when you are wearing protective eyewear. As with a posted sign, a light that is always on can become invisible over time; ask any researcher whose laser has been shut down suddenly by the door interlock. Flashing lights are harder to miss, but you don’t want them to be so noticeable that they become a disturbance. The light does not have to be visible a kilometer away, just at the door.In a brightly lit hallway, the illuminated sign can be washed out to the point that it’s hard to tell whether it is on or off. On the other hand, if the hallway is too dark, the illuminated sign stands out, but other warning signs cannot be read.It also is important to decide what will activate the warning light. It might be turned on with a switch inside the laboratory; however, this type is not the most reliable because of the human element. A more reliable option is a warning light assembly that is interfaced to the laser controller or power supply.Bulb life is an important consideration in warning signs. Is it safe to enter the laboratory, or is the bulb burned out? Fluorescent lights have a longer life than incandescents, and lifetime really isn’t a problem with LEDs. Low-voltage LED technology can offer between 50,000 and 100,000 h of life, as opposed to 1000 h for a typical incandescent light-bulb. In addition, using LEDs saves electricity, generates little or no heat and reduces electrical hazards. Expect to pay a bit more if you choose the low-voltage LED sign — but in the long term, the benefits definitely outweigh the cost. LED-based warning signs come in two basic styles. One is based on programmable logic control, which allows a wide variety of display messages and options, but it is costly. The other is the simple sign, which is inexpensive but has limited display options and is not designed as a safety device.From a safety perspective, the laser warning lights should be fail-safe — if the light goes out, the laser shuts down. Unfortunately, most warning lights are not manufactured or installed that way. (One word of warning: With a fail-safe system, laser users have spent hours trying to figure out why their laser is not working, only to discover that a dead bulb in the warning sign has caused the system to shut down.) Another way to ensure that all is safe is by using a two- or three-color sign. In this way, the green light is a certain indicator that it is safe to enter the room.We have found a few commercial laser warning signs that are approved by Underwriters’ Laboratories or similar institutions. Some can be constructed from Underwriters’ Laboratories-approved exit signs with the cover plate changed. If your institution requires such testing, you will need to seek these products.Meet the authorWilliam Ertle is president of Rockwell Laser Industries in Cincinnati; e-mail: billertle@rli.com. Ken Barat is the laser safety officer at Lawrence Berkeley National Laboratory in Berkeley, Calif.; e-mail: kbarat@lbl.gov.