LIDAR

From open-encyclopedia.com - the free encyclopedia.

Lidar stands for light detection and ranging. Lidars transmit laser pulses and detect the backscattered signal. Very similar to radar (radiowave detection and ranging), the range to the scattering object is determined with the time delay between transmission and detection. The acronym LADAR (LAser Detection And Ranging) for elastic backscatter lidar systems is mainly used in military context. The term laser radar is also in use but somewhat misleading as laser light and not radiowaves are used.

Contents

1 General description

General description

The primary difference of lidar to radar is that much smaller electromagnetic wavelengths are used. In general one can image a feature (or object) only about the same size as the wavelength, or larger. Thus lidar is very sensitive to aerosols and cloud particles and has many applications in atmospheric research and meteorology.

An object needs to produce a dielectric discontinuity in order to reflect the transmitted wave. At radar (microwave radio) frequencies a metallic object produces a significant reflection. However non-metallic objects, such as rain and rocks produce weaker reflections and some materials may produce no detectable reflection at all, meaning some objects or features are effectively invisible at radar frequencies.

Lasers provide one solution to these problem. The beam densities and coherency are excellent. Moreover the wavelengths are much smaller than can be achieved with radio systems, and range from about 10 micrometers to the UV (ca. 250 nm). With these sorts of wavelengths a lidar systems offer much higher resolution than radar. The wavelengths are ideal for making measurements of smoke and other airborne particles (aerosols), clouds, and of air molecules. A laser typically has a very narrow beam which allows to map atmospheric features with very high resolution compared with radar. Another advantage of lidar is that many chemical substances interact more strongly at visible wavelengths than at microwaves. Suitable combinations of lasers can allow for remote mapping of atmospheric contents by looking for wavelength-dependent changes in the intensity of the returned signal. Lidar has been used mostly for atmospheric research and meteorology. More recently a number of map-making and surveying applications have been developed. Another newer use is to map the eye during LASIK eye surgery, in order to allow the main cutting beam to follow any movements of the eye.

In Geology and Seismology a combination of Aircraft based LIDAR and GPS have evolved into an awesome tool for detecting faults and measuring uplift. The output of the two technologies can produce extremely accurate elevation models for terrain that can even measure ground elevation through trees. This combination was used most famously to find the location of the Seattle Fault. This combination is also being used to measure uplift at Mt. St. Helens by using data from before and after the 2004 uplift.

One situation where lidar has notable non-scientific application is for vehicle speed measurement. The technology for this application is small enough to be mounted in a hand held camera "gun" and permits a particular vehicle's speed to be determined from a stream of traffic. The equivalent radar based systems are often not able to isolate particular vehicles from the traffic stream and are generally too large to be hand held.

Military applications are not yet in place, but a considerable amount of research is underway in their use for imaging. Their higher resolution makes them particularly good for collecting enough detail to identify targets, such as tanks. Here the name LADAR is more common.

Laser imaging systems can be divided into scanning systems and non-scanning systems. The scanning system can again be divided into sub-groups by the way the laser beam is scanned across the object. Beam-scanners scan a narrow beam, typically in lines on top of each other, therefore this type of system is called a Laser Line Scanner (LLS). Fan-beam scanners scan a fan-shape beam across the object.

3-D imaging is done with both scanning and non-scanning systems. "3-D gated viewing laser radar" is a non-scanning laser radar system that applies the so-called gated viewing technique. The gated viewing technique applies a pulsed laser and a fast gated camera. There are ongoing military research in Sweden and Denmark with 3-D gated viewing imaging at several kilometers range with a range resolution and accuracy less than ten centimeters.

Design

There are 3 basic components to a lidar:

First is the laser. 600-800 nm lasers are most common for non-scientific application. They are cheap and can be found with sufficient power but they are not eye-safe. Eye-safety is often a requirement for military apps. 1550 nm lasers are eye-safe but not common and are difficult to get with good power output. You must choose your laser repetition rate (which sets how fast you can take pixels) and pulse length (which sets your range resolution).

Second is your scanner and optics. How fast you can take images (your Hz) is determined by how fast you can scan it in. You have several options to scan your azimuth and elevation. You can use two oscillating plane mirrors, a combination with a polygon mirror, a dual axis scanner, or some other option. You need to determine your angular resolution and set your optics to achieve the range you want (also governed by receiver sensitivity and laser power). A hole mirror or a beam splitter are options to get a return signal.

Thirdly, a receiver and receiver electronics are required. Receivers are made out of several materials. Two common ones are Si and InGaAs. They are made in either PIN or Avalanche photodiode configurations. The sensitivity of your receiver is another parameter that has to be balanced in your LIDAR design.

In general there are two types of lidar systems, "high energy" systems and micropulse lidar systems. Micropulse systems have developed as a result of the ever increasing amount of computer power available combined with advances in laser technology. They use considerably less energy in the "beam", typically on the order of one watt, and are often "eye-safe" meaning they can be used without safety precautions. High-power systems are common in atmospheric research, where they are widely used for measuring many atmospheric parameters: the height, layering and densities of clouds, cloud particle properties (extinction coefficient, backscatter coefficient, depolarization), temperature, pressure, wind, humidity, trace gas concentration (ozone, methane, nitrous oxide, etc.).

External links

Atmospheric-Optics Laboratory: Lidar Basics - at Dalhousie University, Halifax, Canada
Atmospheric Physics: LIDAR (at University of Wales)
Autonomous LADARVision System (for LASIK)
National Science Digital Library - MPL Quicklook Page
DOE Atmospheric Radiation Measurement MPL Instrument Description
www.lidar.com

de:Lidar