The CLidar is a scientific instrument used for measuring particulates (
aerosols
An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...
) in the lower atmosphere. CLidar stands for camera
lidar
Lidar (, also LIDAR, or LiDAR; sometimes LADAR) is a method for determining ranges (variable distance) by targeting an object or a surface with a laser and measuring the time for the reflected light to return to the receiver. It can also be ...
, which in turn is a
portmanteau
A portmanteau word, or portmanteau (, ) is a blend of words[light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...](_blank)
" and "
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
". It is a form of
remote sensing
Remote sensing is the acquisition of information about an object or phenomenon without making physical contact with the object, in contrast to in situ or on-site observation. The term is applied especially to acquiring information about Earth ...
and used for
atmospheric physics
Within the atmospheric sciences, atmospheric physics is the application of physics to the study of the atmosphere. Atmospheric physicists attempt to model Earth's atmosphere and the atmospheres of the other planets using fluid flow equations, che ...
.
Description
In this technique a very wide-angle lens images light scattered from a laser beam onto a CCD (
Charge-coupled device
A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
) camera. The camera is positioned hundreds of meters away from the (usually vertically-pointed) laser beam. The geometry of the CLidar is shown in the figure. It is important in the analysis that the optics, the wide-angle lens in this case, accurately maps equal angles onto an equal number of pixels throughout the 100 degree field-of-view.
image:CLidarDiagram2008.jpg
Example
In the second figure, an image from the CCD camera is shown which is analyzed by adding up the individual pixels at each altitude. The camera was 122 meters from the vertically pointed, circularly-polarized laser beam. The beam is brighter near the ground due to near-ground aerosols. A bright spot due to a cloud can be seen near the top of the beam. The beam was positioned diagonally on the CCD array to use the space more effectively. A lighthouse and power pole can also be seen in the image.
Differences between Lidar and CLidar
The CLidar technique has the advantage over the lidar technique of being able to measure all the way to the ground. This difference derives from their distinct geometric configurations, where in the lidar technique the receiving telescope and sourcing optics are monostatic (axially aligned) while the receiving and outputting optics for the CLidar technique are bistatic (non-zero perpendicular distance between receiving and outputting optics). The signal strength is also much more constant than a lidar signal which can change by many orders of magnitude. It has very high altitude resolution in the lower atmosphere. The instrument components are typically simpler than those in the lidar also.
Disadvantages include poor altitude resolution in the upper atmosphere, difficulty designing optics that gathers substantial amounts of light, and a loss in noise rejection (
signal-to-noise ratio
Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power, often expressed in deci ...
).
References
{{reflist
Sources
* Barnes, J. E., S. Bronner, R. Beck, and N. C. Parikh, Boundary layer scattering measurements with a CCD camera lidar, Applied Optics, 42, 2647-2652, 2003.
* Barnes, John E., N. C. Parikh Sharma and Trevor B. Kaplan, Atmospheric aerosol profiling with a
bistatic imaging
Bistatic radar is a radar system comprising a transmitter and receiver that are separated by a distance comparable to the expected target distance. Conversely, a conventional radar in which the transmitter and receiver are co-located is called ...
lidar system, Applied Optics, 46, 2922-2929, May, 2007.
Meteorological instrumentation and equipment
Lidar
