There is a decision to be made when deciding between these 2 forms of instrument when considering a monitoring program.
Firstly both Ceilometers and MPL ( Micro pulsed Lidar) are instruments based on the LIDAR principle. MPL uses photon counting detectors, while Ceilometers use avalanche photodiodes in analog mode. Beyond that all sampling and data conversion are digital.
Ceilometers are significantly lower in cost than MPL This is mainly due to the laser technology used. The lasers in Ceilometers can last up to 6-8 years and the laser replacement cost is relatively low.
Most Ceilometers use a low cost solid-state pulsed laser diode, while the MPL uses a frequency doubled NdYag laser.
Most Ceilometers emit in the range 905 to 920 nm, which is not visible and the launch pulses are reduced in energy so that the sensor is eye safe. The MPL emits at 532 nm and is able to launch higher energy pulses meaning the range is greater, or conversely the ability to detect low levels of scattering ( eg MIE ) at low levels is enhanced. Potential users need to look at the differences between scattering properties of the target at the 2 wavelengths.
Ceilometers do not have dual polarisation channels and the discrimination capabilities of MPL may make them more suitable for the planned studies
Ceilometers are designed to be operated long term in a wide range of environments and are more suited to remote deployment where there is less maintenance available.
Ceilometers are widely used for PBL studies. Because of increased range/sensitivity MPL is favoured in high cloud studies, but high range ceilometers are available and might be selected if a multi sensor long term study is being undertaken.
There are very few Ceilometer manufacturers in the world. Ceilometers have advanced technical requirements and the cost of development is high. In the production phase ceilometers are assembled from many special optical and electronic parts. During testing they require specialised pulsed laser power meters, spectrophotometers and advanced electronic test equipment, together with a cloudy climate to enable regular testing and continuous product improvement.
Sensor Range Class
- 12500 ft range, ( 3800m) an example of which was the original CT12K. This range is now encompassed by the 25,000 ft range sensors.
- 25,000 ft range ( 7600m) These are the main sensors on the market since in the main application there is little operational need to go beyond even 12500 ft. These sensors also find application in Planetary Boundary Layer ( PBL) studies.
- 50,000 ft range (15,200m) Special instruments that find more application in volcanic ash warning in aviation and upper atmosphere studies in atmospheric science. Although in theory only requiring a modest increase in signal to noise ratio, the cloud species above 20,000 ft are most often comprised of ice crystals and have much lower volume back-scatter coefficients than water cloud so the reliable detection of thin layers of cirrus cloud becomes very difficult while maintaining the laser eye safety mandate. Ceilometers in this range generally achieve the necessary signal to noise ratio improvement by a range of techniques including increasing laser pulse energy ( while still remaining eye safe ), using a different laser wavelength, and or reducing the telescope field of view and laser beam divergence.
25,000 ft range for Aviation and PBL studies
CL31 : http://www.vaisala.com/en/products/ceilometers/Pages/default.aspx
8200-CHS : http://www.ceilometers.com/
CS135 : https://www.campbellsci.com.au/cs135
50000 ft Range for Atmospheric Science
CL51 : http://www.vaisala.com/en/products/ceilometers/Pages/default.aspx