LIDAR

LIDAR stands for LIght Detection And Ranging. It is also commonly referred to as LADAR, for LAser Detection And Ranging. It is rather like radar but involves the use of light, typically a laser, rather than radio waves.

Table of contents

1 General description

1.1 Design

2 See also:

2.2 Applications
2.3 External links

General description

The primary advantage of a LIDAR system over radar is that smaller wavelengths provide higher resolution. In general one can image a feature (or object) only about the same size as the wavelength, or larger. For centimeter-sized radar waves this means that objects smaller than about the size of a drinking cup are difficult to see. Moving to shorter wavelengths in the radio spectrum is possible, but it becomes increasingly difficult to generate enough power for a useful system.

The object also 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 much 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. Although they are more complex than the systems needed to generate radio waves, the beam densities and coherency are excellent. Moreover the wavelengths are much smaller than can be achieved with conventional radio systems, anywhere from what the best radio systems generate, to far smaller wavelengths in the common blue-green laser. Some, mostly experimental in nature, have wavelengths into the UV and X-ray ranges.

With these sorts of wavelengths a LIDAR system offers thousands of times the resolution of radar systems. The wavelength is so small that LIDAR systems are often used for making measurements of smoke and other airborne particles, and in fact the molecules of the air themselves.

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 changes in the spectrum of the returned signal.

In more general terms though, LIDAR is difficult to use as a general purpose detection system like radar. There are two reasons for this.

One is that the increase in resolution also implies an increase in the total amount of data collected, which then must be processed. While this is not a hinderance for special duties like mapping the atmosphere in a narrow beam, or looking as specific objects, the amount of processing needed to scan the sky for aircraft and such is well beyond our current capabilities.

In addition a laser typically has a very narrow beam that is not easily spread out. In a radar system one can easily create a wide beam that is used for searching, and then narrow it down for accuracy. This is not easy to do with a laser. Likewise the radar "beam" can be moved around electronically, whereas this too is beyond our current capabilities with lasers.

For both of these reasons, LIDAR has been used mostly for scientific and meteorology uses. More recently a number of map-making and surveying applications have surfaced, as the cost of the computer power needed to process the massive amount of detail falls. 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.

One situation where LIDAR has notable practical 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. This has particular use in road policing and traffic monitoring.

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 LARAD is more common.

Design

There are 3 basic components to a LIDAR system:

First is the Laser. You have to choose your wavelength. 600-800 nm lasers are very common and cheap (relatively) and can be found with sufficient power but they are not eye-safe. Eye-safe is often a requirement for military apps. 1550nm lasers are eye-safe but are not common and are difficult to get with good power output. You must choose your laser rep 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 oscilating 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.
Third a receiver and receiver electronics is required. Receivers are made out of several materials. Two common ones are Si and InGaAs. They are made in either PIN or APD configurations. The sensitivity of your receiver is another parameter that has to be balanced in your LADAR design.

In general there are two types of LIDAR systems, older "high energy" systems and newer micropulse lidar systems. Micropulse systems have developed as a result of the ever increasing amount of computer power available, allowing for the practical extraction of more information from smaller signals. 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. Monopulse systems are common in the meteorologial field, where they are widely used for measuring the height, layering and densities of clouds, and older systems using large high power lasers are generally disappearing.