All of these tools are self-leveling. Most rely on a pendulum hung from gimbals, using gravity to bring the device to level in a second or two. The exception is the PLS HVL 100, which relies on a pair of servo motors; they take longer (about four seconds) to level the tool but are better able to maintain a steady line when foot traffic or power equipment is shaking the floor. Servo motors are typically found in rotary models; it's unusual to see them in a line laser. Lasers with servo motors are best for applications where the tool is left in the same location for a period of time. Pendulum models are better when you need to level or plumb something quickly and then move on.
A laser's internal mechanism is delicate and must be protected against shock damage. This is usually achieved by means of a manually activated lock, which prevents the pendulum from swinging around when the tool is transported. A couple of models don't have locks: Hilti's pendulum is protected by rubber padding inside the housing, and the PLS laser doesn't require a lock because its mechanism is held in position by a spring and the two servo motors.
Typically, a laser must be placed within 3 to 5 degrees of level for it to level itself. If the tool can't come to level, it will send out an audible beep, turn the beams off, or flash the beams. This function is important because otherwise you might accidentally use lines that aren't level or plumb.
The usual way to project sloped lines is by using the laser with the pendulum in the locked position. When operated this way, the tool will remind you that it's not level by flashing the beams or by flashing or illuminating an indicator lamp. Flashing beams are better because they're impossible to miss; you might not notice the indicator lamp. If the tool does rely on an indicator, it's a good idea to check every now and again to make sure that the laser is set the way it's supposed to be.
The beams shown in Spectra Precision's illustration of its 5.2XL model (top) are typical of the plumb and level lines produced by line lasers. Less typical are the points that project from the top, bottom, and back (second from top). Each laser reviewed for this story projects points (or crossed lines) up and down and can be used in place of a plumb bob. The Bosch GLL3-80 (second from bottom) and PLS HVL 100 (bottom) provide greater coverage than other tools – three 360-degree lines that project level, plumb, and square onto all surrounding surfaces.