A laser rangefinder is a device which uses laser energy for determining the distance from the device to a place or object. Its principle of operation is analogous to radar: a pulse, or a series of pulses, of energy are sent out, and the device measures the round-trip time for them to return. Half that time, divided by the speed of light, is the range.
In keeping with the radar (Radio Detection and ranging) ancestry, laser rangefinders sometimes are called lidar for "light detection and ranging". There are a wide range of applications, both civilian and military, for laser rangefinders, from things as benign as warning that your car is about to back into the garage wall, to knowing the precise position of a target to be hit with a ton of explosives.
For both technical and operational reasons, laser rangefinders usually use invisible infrared light. Inexpensive and efficient solid-state lasers, and laser detectors, are widely available. In a combat situation, the user of the rangefinder may want the target to be unaware of the presence of the rangefinder or its user. The device may combine the rangefinder function with the laser designator function.
Some military targets have optoelectronics that can detect when laser energy is hitting them, even if the beam, in the infrared light spectrum for most military devices, is invisible to the human eye. They may try to jam the sensor looking for the laser rangefinder energy, perhaps by turning on infrared light or firing flares. Modern laser designators do not send a simple continuous wave, or steady set of pulses, at the target, but send a complex, changing, and unique pulse pattern. The rangefinder will look for its own pulse pattern, and ignore all others.
A common U.S. Army system that combines designation and rangefinding, the Lightweight Laser Designator Rangefinder (LLDR), is eye-safe. Laser rangefinders are widely used in civilian surveying and construction, field research, weather forecasting, and a host of other applications.
Infrared light, although not necessarily the coherent infrared light of a laser, are commonly used in autofocus cameras. The first commercial rangefinders for such cameras used ultrasound, but that proved impractical for consumer use. Imagine a tourist driving through a park, seeing a bear, and wanting to take a picture of it. While she may think she is focusing on the bear, an ultrasonic rangefinder will bounce back from the windshield and leave the bear out of focus. Infrared light will go through the glass and focus properly. Warning: if one has an old ultrasonic-focus camera, do not get out of the car into the bear's territory to focus on his portrait.
Aeiral LIDAR was first used in the 1960s in an attempt to develop new methods of detecting submarines. Over time, the possibility for LIDAR to be used to map the topography of a large area.
While the beams are usually invisible to the human eye, infrared laser energy can damage the eye. A widely ratified international agreement forbids the use of weapons whose "sole combat function or as one of their combat functions, is to cause permanent blindness to unenhanced vision."  The protocol does not apply to blinding as a collateral effect of other laser-based systems, such as weather instrumentation, rangefinders, etc., but it is a widely accepted principle that unless the device is a true laser weapon intended to kill, it is unwise to give even an incidental blinding capability. After all, the beam could accidentally hit onto friendly forces or civilians.
While some non-weapon systems have lasers that are not eye-safe, they will be replaced with eye-safe systems as soon as practical. Exceptions can be made when, for example, the rangefinder is on an automated factory assembly line that will shut down if a human being moves into the area where the beam is in use.
The Remote Miniature Weather Station (RMWS), from System Innovations, is an air-droppable unmanned weather station with a lightweight, expendable and modular system comprised of two components; a meteorological (MET) sensor and a ceilometer (cloud ceiling height) with limited MET. The basic MET system is surface-based and measures wind speed and direction, horizontal visibility, surface atmospheric pressure, air temperature and relative humidity. The ceilometer sensor determines cloud height and discreet cloud layers. The system provides near-real-time data capable of 24-hour operation for 60 days. The RMWS can also go in with US Air Force Special Operations combat weathermen  The man-portable version, brought in by combat weathermen, has an additional function, s remote miniature ceilometer. Designed to measure multiple layer cloud ceiling heights and then send that data via satellite communications link to an operator display, the system uses a Neodinum YAG (NdYAG), 4 megawatt non-eye safe laser. According to one weatherman, "We have to watch that one,” he said. “Leaving it out there basically we’re worried about civilian populace going out there and playing with it—firing the laser and there goes somebody’s eye. There are two different units [to RMWS]. One has the laser and one doesn’t. The basic difference is the one with the laser is going to give you cloud height."
He cautioned that the unit was not without its operational restrictions or limitations: the primary one being eye safety. As an example, one ancillary system to the RMWS is the remote miniature ceilometer. Designed to measure multiple layer cloud ceiling heights and then send that data via satellite communications link to an operator display, the system uses a Neodinum YAG (NdYAG), 4 megawatt non-eye safe laser. If the non-eye-safe laser is the only available ceiling measurement, the responsible commander must decide either not to use it, or to emplace it in a location where no humans will come near the beam. While this particular device is normally air-dropped, a reasonably safe alternative might be to nail it high in a tree.
- Crutchley, Simon and Crow, Peter (2009). The Light Fantastic: Using airborne lidar in archaeological survey. Swindon: English Heritage. p. 3.
- , Protocol IV on Blinding Laser Weapons, Convention on Certain Conventional Weapons
- Scott R. Gourley (Jul 23, 2003). Storm Warnings: A look at U.S. Air Force special operations combat weathermen. Retrieved on 2007-10-22.