Scintillation is what makes stars twinkle. It is what causes mirage in deserts or on hot surfaces. It is what causes shimmer underwater in the vicinity of hot vents in oceans. These effects result from temperature fluctuations in air or water. The same effect can occur in a LISST optics cell if not properly controlled .
The importance of scintillation in LISST instruments first became evident from field data using a LISST-StreamSide. The instrument had been placed in a shed on a river bank, which would heat up and cool down daily. Cool river water was pumped through the instrument. Large diurnal variations in temperature of the instrument were found to correlate with strange behavior in the data. In the laboratory at Sequoia, we confirmed that variations in the data resulted from the temperature difference between the instrument and sample water. The cure was to allow water to flow through the LISST longer, bringing the optics to the same temperature as the water.
Scintillation can be explained simply as follows. Light bends when entering a region of different refractive index. When temperature turbulence exists, as in the boundary layer over the glass windows of laser diffraction instruments, the laser beam bends multiple times across temperature interfaces. This multiple refraction creates scattering. Scattering appears first on the inner rings of the LISST detectors, which is interpreted as large particles. The phenomenon of this ‘beam-spread’ was modeled and has been published [Light scattering on oceanic turbulence; Darek J. Bogucki, Julian A. Domaradzki, Robert E. Ecke, and C. Randal Truman, APPLIED OPTICS Vol. 43, No. 3020 October 2004; pp 5662-68].
In Figure 1, we show the measured intensity of scattered light into the smallest 6 angles while filtered water of a temperature different from that of the optics passed through a LISST-Infinite. Note the large increase in signals at start, and the gradual decay. This transient is the duration of scintillation. These signals caused by scintillation are interpreted as particles. In Figure 2, we show the corresponding equivalent concentration (EC). It can be seen that it takes about 400 seconds for the EC error due to scintillation to fall below 1 mg/L. This is why, all our instruments with internal cells must be brought to thermal equilibrium by allowing water to flow for ~400 seconds for both, backgrounds or samples.
Published: 24 Jul 2014