If a body of water takes up for a major portion of the measurement path, using a scintillometer is not recommended.
A scintillometer is an instrument that can measure the ‘amount’ of scintillations by emitting a beam of light over a horizontal path. The scintillations ‘seen’ by the instrument can be expressed as the structure parameter of the refractive index of air (Cn2), which is a representation of the ‘turbulent strength’ of the atmosphere.
The scintillations are mostly a result of temperature and water vapour fluxes from the earth surface. Scintillometers use an infra red light source and are designed to be primarily sensitive to scintillations from temperature fluxes.
Over most surfaces over land, the temperature related scintillations are most dominant and the those from water vapour are less significant. This means that Scintillometer will yield reliable Cn2 values. In addition, if the so called Bowen ratio is known, additional correction can be applied to improve the measurement even further.
However, over open water the scintillations from water vapour are dominant over temperature fluctuations. As a result, a scintillometer operating at infra-red wavelengths will underestimate values for Cn2. Although in principle the measurement could also be corrected using the fore mentioned Bowen ratio, this ratio is in many cases not accurately known over open water.
In case the path length has been set incorrectly during the installation of an LAS scintillometer, WINLAS can correct the Cn2 data.
In order to do so please use the following procedure.
After entering the relevant info in the parameters section, enter the path length setting set with the potentiometer of the LAS receiver in meters and enter the correct path length in the input field below.
Select ‘Run…’ in the WINLAS file menu.
WINLAS will now process the Scintillometer Cn2 data using a correction algorithm for the actual path length.
WINLAS corrects the path length in the following way:
Using the following equation to derive the intensity fluctuation data from the recorded Cn2 values calculated by the LAS using the incorrect path length setting.
(Wang et al., 1978)
Aperture diameter ~ 15cm
Initial path length
Variance of log intensity
The equation is re-written to yield the variance of the intensity fluctuations:
And finally re-calculate Cn2 with the correct path length:
End of FAQ.
A scintillometer measures the path weighted structure parameter of air, Cn2, using an optical transmitter and receiver.
In certain cases of relatively high Cn2 values the signal can become saturated depending on the diameter of the lens, wavelength and path length.
The so called saturation limit for Cn2 can be derived using the following formula (Ochs and Hill 1982)
Cn2 < 0.18.D5/3.L-8/3.λ2/6
the diameter of the scintillometer
[0.15m or 0.3 m]
the path length
the emitted wavelength
In the calculation tool below, you can calculate the saturation limit for the LAS and X-LAS scintillometer as a function of Path length.
Once the LAS has been installed and properly aligned the Path Length dial knob at the receiver control panel must be set for the correct distance between the transmitter and the receiver. The Path Length dial knob has 10 turns maximum with a vernier counter and a locking mechanism.
These graduations are NOT in units of distance! The precise path length must first be converted to a dial knob setting (Pot) using the following relationship for the LAS. The equations below can be used to find the correct Potentiometer setting as a function of pathlength for the LAS and X-LAS.
In addition you can use the calculation tool below to calculate the correct potentiometer setting for the (X)LAS.
The LAS and X-LAS scintillometer can be connected to the CR10x and CR1000 data loggers from Campbell Scientific.
Configuration examples for these data loggers can be viewed on this page, here:
Campbell CR10x Data Logger
A (X)LAS scintillometer can be connected to a Campbell CR10x data logger using a 2:1 voltage divider like VDIV2.1 from Campbell. The reason for this is the fact that the LAS has an output of 0..-5V and the standard input range of the CR10x is ± 2.5V.
The procedure is as follows:
Collect data using the data logger and check for normal operation of the Scintillometer data collection.
Campbell CR1000 Data logger
The connection procedure for the CR1000 is similar to the CR10x. The same connection to the terminals for differential measurement of the LAS signals can be used. However, the input range of this data logger is ± 5V so a voltage divider is not required.
A example configuration for the CR1000 in LoggerNet format can be found here: LAS_CR1000.CR1
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