Well, if you know what a Radar is and what it can do then you already have a good understanding of what makes Lidar tick. Let's just remind ourselves what Radar actually is and what it does.

Finding Aircraft and Ships

Radar is actually an acronym, it stands for RAdio Detection And Ranging, which is also a fair description of what it is and does. Most people are familiar with the Radar dishes used at airports, by ships and also many military applications. Most people know that they allow aircraft and ships to be detected, and their positions to be determined and monitored at very great distances and with great precision.

How does Radar work?

Well it's really very simple. The whole principle relies on sending a short pulse of radio waves towards the expected target and then looking for reflections of this pulse some time later.

The pulse transmitted is usually very short, this is important if the range of the target is to be accurately measured. Also, the pulse needs to be quite intense. The pulse is directed towards the aircraft, and travels at the speed of light towards it. Once it reaches the aircraft some of the pulse is reflected (metal objects reflect radio waves quite well). Actually only a small fraction of the energy is reflected and these small reflections are scattered in all directions. So, an even smaller part of this reflected energy will happen to travel in a direction directly back towards the Radar transmitter. This small reflection is collected by the Radar dish and the time since the original pulse was sent accurately measured. This time is directly proportional to the distance to the aircraft and so allows this distance to be precisely measured.

The direction of the aircraft is also known since it is arranged that the transmitted radio pulse is very directional (that is the job of the familiar Radar 'dish'). So, only targets that are in this specific direction that the dish happens to be pointing can be detected. This is also why the dish rotates for many Radars, this allows the instrument to measure a whole sequence of directions and so measure any aircraft (or ships or whatever) no matter what direction they are in.

This basic principle is exactly how a simple Lidar works. Lidar is also an acronym standing for LIght Detection And Ranging. The only difference is that instead of transmitting a short pulse of radio waves, a short pulse of light is transmitted. OK, so there are other differences too, instead of a large dish used as an antenna by a radar, a Lidar will normally use a modest-sized telescope to detect the bac-scattered light. A typical Lidar is not normally used to detect aircraft or ships, although it is possible.

So, what are the applications of a Lidar?

Gazing at Clouds

Looking at clouds is actually one of the first applications for Lidar. Clouds, since they are composed of very many small water droplets, scatter light very well. Thus they provide a strong back-scattered signal that a Lidar can easily detect). This is then how a simple backscatter Lidar works.

An obvious application is the measurement of the height of the cloud base, information that is of great importance for landing aircraft at airports for example. However there are many other applications. Such Lidars are also able to measure very precisely the density and distribution of particulates (dust and smoke for example) in the atmosphere. Applications for this type of Lidar are many. They include measuring dispersion around buildings and other structures, monitoring smoke emissions from fossil fuel sources such as power stations and incinerators, measuring the topography by taking measurements from an aircraft equipped with a suitable downward pointing Lidar. They may also be used to assess the particulate size distribution.

Why is dust so important?

Dust pollution is conventionally monitored by sampling devices measuring at a single point. These devices may attempt to differentiate between particles of different sizes (PM10 or PM2.5 for example with their distinct health hazard impacts). Whilst a simple backscatter Lidar is able to measure such particulates at very low concentrations, it is not able to provide any information on the size and size distribution of the particles.

However, it is possible to adapt a Lidar so that it can determine the distribution of particle sizes. This is done by making measurements with the Lidar using light of two or more widely separated wavelengths (or colours).

This relies on the fact that a given size of particle will scatter light of different wavelengths differently (scattering cross-section). Typically, the strongest scattering will occur when the particle is of a similar size as the wavelength of the incident laser light. If the wavelength is much bigger than the particle size, however, the scattering intensity will be much smaller.

In this example the blue (short wavelength) light is much more strongly scattered than the red (long wavelength) light. This indicates that the particles in the cloud are likely to be of small size. If instead, the red laser light was scattered more strongly, it would indicate the presence of particles of larger sizes. This principle can be extended by adding further distinct wavelengths, so that more accurate estimates of the particulate sizes and size distribution can be made.

There are other applications of a Lidar where more than one wavelength is used. For example, measuring gases distributions in the atmosphere.

Gas Monitoring Lidars - Raman

Lidars can also be used to measure the presence and distribution gases in the atmosphere. There are two ways that this can be done. Perhaps the simplest to understand is the Raman Lidar.

Although we have only described Lidars measuring the scattering from dust or other particles in the atmosphere, gas molecules themselves are also able to scatter light. This scattering is essentially from individual molecules; it is this scattering process (from sunlight) that makes the sky 'bright' in the daytime. The blue colour of the daytime sky was first explained by Lord Rayleigh, hence molecular scattering is often referred to as Rayleigh scattering.

Most of the light scattered by molecules in this way is largely unchanged by the scattering process, i.e. the colour (or wavelength) of the scattered light is unchanged. However a very small fraction of the light may also undergo a change in wavelength during the scattering process. This change arises because the scattering gas molecule absorbs some of the photon energy. More rarely it even adds to or subtracts from the photon energy, causing a change of wavelength. The amount of energy that a particular molecule might subtract or add in this way is a fundamental and unique property of each particular gas, thus leading to a unique spectral signature of the gas molecule.

This wavelength-shifted scattering is usually referred to as Raman scattering. The intensity of Raman scattering is typically 1000 times weaker than the direct (i.e. Rayleigh) scattering. Therefore some considerable care is needed to filter the weak Raman signal from the much stronger Rayleigh signal.

The weak intensity of the Raman signal (due to the low scattering cross-section) results in one of the main problems with such Lidars, they are generally not suitable for measuring very low concentrations of gases. One positive feature of this type of Lidar is, however, that a single instrument can potentially measure the concentrations of several distinct gases. It is only necessary to add an additional channel with suitable filter and detector corresponding to the extra molecular species to the telescope and the associated receiver detection system. The additional species can then be detected via its Raman-scattered signal at relatively little extra cost.

There is another type of gas monitoring Lidar that can achieve much higher sensitivity for gaseous molecules present at low concentrations than a Raman system, this is referred to as a DIAL.

Gas Monitoring Lidars - DIAL

DIAL is yet another acronym, it stands for DIfferential Absorption Lidar, this type of instrument is essentially a combination of a gas spectrometer and a simple backscatter Lidar.

To understand how a DIAL system works it is necessary to understand the principle of simple gas absorption spectrometry. Most gases are able to absorb light of certain colours. Very few actually absorb visible light. In the infrared region, however, most gases exhibit absorption properties. If light of different colours is transmitted through a sample of gas and this gas absorbs some of the colours then it is possible to use this absorption to measure the presence of the gas. Knowing the absorbed wavelengths allows particular gases to be uniquely identified, while the extent of the absorption provides a measure of the quantity of gas in the sample.

A simple spectrometer could be made using an ordinary light bulb as the source and a prism to disperse the spectrum as illustrated here. Generally, commercial spectrometers would employ other techniques, such as diffraction gratings to achieve the dispersion however the principle is the same.

Extending this spectrometry principle to a Lidar is relatively simple. In fact a similar principle to that described above for particle sizing is used. Two separate measurements are made, each at different wavelengths (or colours). This can be achieved either by using a tuneable laser or by using two separate lasers. One of the wavelengths is chosen to correspond to be strongly absorbed by the target gas, the other is chosen to be only weakly absorbed.

Written by Steve Sutton, Lidar Technologies Ltd. UK