Accuracy With the laser set to level or grade, at a given distance, how far from perfection will the laser beam be. Usually expressed as plus or minus a fraction of an inch at 100 feet. (Or ±Xmm at 30m) Accuracy may also be expressed in terms of arc seconds. 10 Arc Seconds translates to ±1/16" at 100'. (±1.6mm at 30M.) Accuracy decreases with distance from the laser. If the accuracy of a laser is ±1/16" per 100 feet, it will be ±1/8" at 200 feet. Arc Second A circle can be divided into 360 degrees. Each degree is divided into 60 arc minutes, and each arc minute is divided into 60 arc seconds.  Automatic Leveling Automatic Leveling and Self Leveling are used synonymously. The major types of self leveling systems are servo motor and compensated. In the servo motor system, electronic level sensors send information to a processor, which activates motors that drive the unit to level. A compensator is a gravity based pendulum mechanism. Servo systems have a much greater self leveling range than compensated systems. In a compensated system, the user turns leveling screws to center a level vial. Once the unit is rough leveled, the compensator maintains the beam at its specified accuracy. When using a servo system, the user mounts the laser on a tripod, turns on the power, and waits for the unit to drive itself to level. Compensator Gravity type mechanism, usually some type of pendulum system to insure that laser beam is level. Compensators may be wire or bearing hung.  Deadband Usually applied to a receiver. This is the distance that a receiver can be moved and still give an on grade reading. If the deadband is too fine, it becomes difficult to get an on grade reading, if the deadband is too wide, it becomes difficult to maintain accuracy. Grade Capability The range of slope that can be dialed into the laser. Expressed as percent of slope. Laser Diode Solid state device that produces laser light. Light can be visible or invisible. Construction lasers use one of three wavelengths. They are infra-red (invisible, 780nm), red (650 or 635nm), and green (532nm).  Laser Acronym for Light Amplification by Stimulated Emission of Radiation Manual Leveling The laser is kept level through the use of glass level vials. The user must check these vials periodically, because the level bubbles may move during the day, in response to vibrations and changes in temperature. Operating Range The maximum distance from the laser where the detector will still function. Usually given in terms of an operating radius or diameter. Penta Mirror or Penta Prism Device that reflects a light beam outward at a 90º angle in relation to the incoming beam Rotational Coverage The circle of working coverage around the laser. Most lasers offer a full 360º circle of coverage. Self Leveling The laser is leveled either with a compensator, or with an electronic servo motor system. If the laser is disturbed or goes out of level, the mechanism will make a correction. If the mechanism cannot bring the laser back to level, it shuts the laser off. Self Leveling Range Expressed as ± x number of degrees or arc minutes. This figure denotes how far the laser can be tilted, while still maintaining its ability to self level.