L

idar is an acronym for LIght Detection And Ranging, a technique similar in principle to Sonar and Radar to determine ranging, or the measurement of distance by time domain reflections.  Lidar uses a (usually pulsed) beam source while measuring the round-trip time and intensity of reflected energy. When there is no 'solid' target, there is still a small portion of the beam that is reflected by atmospheric constituents along the line of sight (backscatter.)  This measurement technique can infer several characteristics of the target and also of the intervening atmosphere.

The processed return signal or echo can be used to develop 2 and 3 dimensional plots, and has numerous applied uses: (See also Bibliography References, below)

CCNY Laboratory Lidar System Description:

Laser 1: 

Q-switched Coherent 40-100 Nd-YAG with variable output power up to 400 mJ.  This system has three channels with wavelengths at 1064 nm, 532 nm and 355 nm.  Repetition rate of 50 Hz with 6 ns pulse duration; 0.7 mrad beam divergence.  Beam is on axis to the telescope, directed by three dichroic mirrors.  (See image below.)

Laser 2: 

Q-switched Spectra-Physics Quanta Ray Pro230 Nd-YAG with variable output power up to 475 mJ @ 532 nm, 950 mJ @ 1064 nm and 300 mJ @ 355 nm. This system currently has five channels with wavelengths at 1064 nm, 532 nm, 355 nm, including a Raman channel at 407 nm for water with a second Raman channel at 387 for nitrogen.   Repetition rate of 30 Hz with 1-2 ns pulse duration @ 532 nm; <0.5 mrad beam divergence.  Beam is on axis to the telescope, directed by three dichroic mirrors. A fifth channel is currently being implemented.

Telescope: 

20 inch Newtonian Reflector, F3.5

Detectors: 

APD (silicon enhanced avalanche photodiode) for the 1064 nm (infrared) channel.  PMT (Hammamatsu photomultiplier tubes)  for the 532 nm (green), 386 nm (Raman) and 355 nm (UV) channels.

Digitizing System: 

Lidar Transient Recorder TR 40-160 (LICEL) with 12-bit, 40 MHz A/D converter for signals between 10 MHz and 200 MHz, 64-level fast discriminator for signals in the high frequency domain above 200 MHz.

Range Resolution: 

500 m to 15 km

Data Aquisition: 

Aquisition system is configured by using the TR 40-160 Transient Recorder modules for all channels in a five-channel rack comprising power supplies and interface ports to a PC computer equipped with a National Instruments digital I/O card, DIO-32F.  Each channel can be configured and controlled separately by the host computer. Typically, return signals from 3000 pulses are averaged, then background subtracted for each time segment.

Radar Interface: 

A vertical radar emitter and antenna continually search for the presence of aircraft and provide a failsafe interlock for the beam while the laser is in operation.  On detection of aircraft, the laser beam is automatically disabled and lidar data aquisition halted.  When the airspace is again clear, the system can be reset by the operator after observer confirmation and data collection will automatically resume.

Laboratory Laser:

Lidar Telescope

A

irborne particulate matter has been a major interest to atmospheric scientists. Knowledge of aerosol optical properties becomes importance because of studies correlating airborne particulate matter with adverse health effects: an increase in mortality and respiratory problems; pulmonary function decreases with increase in ambient particle mass concentrations. The small aerosol component, PM , is of most concern to human health because it can be easily inhaled deep into the lungs. Along with health issues, aerosol distributions have significant implications for natural environmental aesthetics and climatic change conditions. Visual range compared to a clean atmosphere is typically 50-67 % in the western U.S. and 20 % in eastern U.S. Combustion products from transportation and power sources produce most of the nucleation centers which grow to aerosol sizes that change the visibility and radiative flux at the Earth's surface by optical scattering.

Unlike radar, where the backscatter echo is due mainly to the target, the lidar backscatter echo Optical Bench intensity depends not only on the backscatter target, but also on the attenuation of the intervening  gasses and aerosols. The molecular atmosphere (e.g. O2, N2, Ar) contributes to backscatter, as well does other small molecular gasses (e.g. the oxides of nitrogen and carbon dioxide.) Therefore, in order to isolate the aerosol contribution, suitable preprocessing of the lidar signal must be performed to eliminate these molecular components. Furthermore, since the aerosol signal depends on both the backscatter and extinction properties of the aerosol, assumptions on the ratio of the extinction to backscatter ratio must be made based on aerosol climatology. Finally, since absolute radiometric calibration is very difficult, it is important to calibrate the backscatter signal. This calibration is almost always performed by probing the atmosphere at sufficiently high altitudes where the signal is dominated by the molecular component which can be calibrated given supplemental temperature and pressure profiles. Further reference information and details can be found in the links given below.

CCNY Backscatter Lidar Image Library:

A library display tool by date, (1064, 532 and 355 nm). The most recent image set is displayed on loading. This is a public database.

CCNY Backscatter Lidar Image Search Tool:

A visual index display tool containing prior images of backscatter data at 1064 nm. The most recent image is displayed on loading. This is a public database.

CCNY Lidar Data Archive:

An archive of recent raw data. These files are compressed using RAR (WinRAR) and may be uncompressed using the same tool available for all major computing platforms. If you are unable to uncompress these data on your computer, a tool is available for downloading here. This is a password protected database.

Recent CCNY Lidar Images:

You may click on these links to view samples of recent CCNY Backscatter Lidar images taken at the wavelengths of 355 nm,  532 nm, and 1064 nm. From these data and correlation with others, the Aerosol Optical Depth (AOD) is determined, viewed here. (Click again to hide.)

References, Presentations and Papers:

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    CCNY Lidar Presentations and Papers

    References

Bibliography by Topic:

You may click on topics below to expand list. (Click again to hide.) Teaching references are provided for educational use to students and instructional staff at CCNY only.

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    Aerosol

    Polarization Lidar

    Doppler Lidar

    Planetary Boundary Layer (PBL)

    Raman Lidar