One of the most ubiquitous items on the optical table is the beam block. It gets moved around, sometimes to block a stray reflection, at other times to block a “what if” beam. (What if I miss the mirror?) But despite the beam block’s omnipresence, I could find only two or three vendors who actually sell them. There are many commercial beam dumps — devices designed to terminate primary beams on the optical bench. These even come in two flavors, air- and water-cooled — one’s choice depending on the amount of power they are expected to dissipate. However, the beam blocks I am writing about are mostly homemade devices, intended to block stray, accidental or secondary beams and reflections.They usually are made of aluminum and often bent into an “L” shape, and they come in a variety of sizes. The larger ones — a foot or more on a side — are really perimeter guards and not intended to stop a particular beam. They are more like a fort — a protective wall around the optical setup (Figure 1). Other beam blocks are cards that obstruct the residual transmission through a beam-steering mirror or shrouds over an optical system that keep the laser light in and room light out.Figure 1. Large beam blocks can serve as a perimeter guard around an entire optical setup.Although they are humble devices, beam blocks are crucial elements from a laser safety perspective. Stray reflections are one of the great hazards in the laser lab. Lab personnel must always exercise diligence in finding and eliminating them before an injury occurs. Each time an optic is manipulated, the possibility of a stray reflection exists. It is true that the Fresnel reflection from an uncoated piece of glass is only 4 percent at normal incidence, but, unlike a 4 percent annual salary raise (which may indeed seem unnoticeable), 4 percent reflection from a laser beam easily can be enough to cause serious eye injury. When dealing with pulsed lasers, it takes nanojoules or less to cause an eye injury, so caution and blockage of stray beams are especially prudent.Mechanical stability of the beam blocks is an important consideration. A beam block placed on the optical table or on a desk is not an effective safety device. It should be either screwed down or magnetically held in place, or it should be weighted so that it cannot tip over easily (Figure 2). A neat trick is to label 7it in big letters, “beam block.” Brightly colored labels work very well. Then, if it gets knocked over or winds up on the floor, it is easy to see the problem.Figure 2. Magnetic mounts can hold beam blocks securely in place.In summary, beam blocks are important to your safety. Many a laser accident can be traced back to an unblocked beam and especially to an invisible beam. Beam blocks are your friends — use them.Meet the authorKen Barat is a laser safety officer at Lawrence Berkeley National Laboratory in California and has held the same position with the National Ignition Facility Directorate at Lawrence Livermore National Laboratory; e-mail: kbarat@lbl.gov.