Laser range-finder
From Biocrawler, the free encyclopedia.
A laser range-finder, or LIDAR (LIght Detection And Ranging), is a device which uses a laser beam in order to determine the distance to an opaque object. It works by sending a laser pulse in a narrow beam towards the object and measuring how long it takes for the pulse to bounce off the target and return to the sender. The pulse may be coded in order to reduce the chance that the range-finder can be jammed and it is possible to use Doppler effect techniques to judge whether the object is moving towards or away from the range-finder, and if so how fast.
The accuracy of a LIDAR instrument is determined by the brevity of the laser pulse and the speed of the receiver. A LIDAR that uses very short (sharp) laser pulses and has a very fast detector can range on object to within a few centimeters.
In order to make laser-range finders and laser-guided weapons less useful against military targets, various military arms may have developed laser-absorbing paint for their vehicles. Regardless, some objects don't reflect laser light very well and using a laser range-finder on them is difficult.
Despite the beam being narrow, it eventually spreads over long distances. This can make it difficult to get an accurate reading of an object, say, beneath some trees or behind bushes. Some of the laser light might reflect off leaves or branches which are closer than the object, giving an early return and a reading which is too low. This has to be taken into account.
Some LIDAR instruments are able to determine multiple returns (as above). These instruments use waveform-resolving detectors, which means they detect the amount of light returned over a certain (very short) time. The waveform from a laser pulse that hit a tree and then the ground would have two peaks. The first peak would be the distance to the tree, and the second would be the distance to the ground.
The ability for aircraft-mounted LIDAR instruments to see "through" dense canopies and other semi-reflective surface (such as the ocean) provide many applications for airborne LIDAR instruments such as:
- Creating "bare earth" topographic maps - removing all trees
- Creating vegetation thickness maps
- Measuring topography under the ocean
- Forest fire hazard
- Overwash threat in barrier islands
