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Optical backscatter meter (OBS or Light scattering sensor (LSS)



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Optical backscatter meter (OBS or Light scattering sensor (LSS)

Bài gửi by tieuminh2510 on Tue Jun 24, 2008 3:22 pm

Transmissometers measure the transmission of light of a given wavelength over a known distance in seawater. Different types of particles and dissolved organic matter in the water absorb and reflect light of different wavelengths. We choose wavelengths for our instruments based on which particles we are studying. The wavelength we use the most is about 660 nanometers (red).

Surprisingly and fortunately, the amount of dissolved salt in the water does not affect the transmission of light through it. Transmission through particle-free fresh water and salt water is the same.

Changes in the transmission of light through water are primarily related to changes in the abundance and type of particles present. Most variations in transmission come from particles less than 20 microns in diameter. Large particles and aggregates larger than 500 microns in diameter are not abundant in the ocean. Only a few exist in 1000 milliliters of water, so they rarely appear in the small sensing volume of the transmissometer (45 milliliters). When they do, they appear as singular large values which we remove from our data.

Transmissometer [18K]
The amount of attenuation of light (c) equals the sum of light scattering (b) and absorption (a).
It's as simple as a+b=c.
Measuring these values is not so simple.
Most attenuation results from light scattering. Phytoplankton, however, contain little packets of chlorophyll in their cells that act as sponges for light, allowing them to absorb energy and use it in photosynthesis. Thus, living phytoplankton absorb more light than other particles.
With an instrument that can measure light absorption and attenuation (the a-c meter), we can start to distinguish living phytoplankton from dead plankton and other particles without taking water samples.

Optical backscatter meter (OBS or Light scattering sensor (LSS)

Light scattering is difficult to measure completely because light scatters in all directions and the angle of scattering depends on the particles' sizes. Furthermore, as soon as light bounces off of one particle it can bounce off of another. We cannot distinguish whether we are measuring primary, secondary, tertiary, or greater scattering.

Nevertheless, small scattering meters have been made that are useful-especially in turbid water-as they provide a crude measure of the particle abundance in the water. Scattering meters project a beam of light into the water while a detector next to the light source measures the amount of light scattered back into it.

The optical backscatter meter (OBS) built by Downing Associates and the light scattering sensor (LSS) built by SeaTech use an infrared light source and a detector for this purpose. To determine the amount of particulate matter in the water, these instruments (like transmissometers) have to be calibrated by filtering particles from a known volume of water and weighing them very precisely.

Optical backscatter meter (OBS) or Light scattering sensor (LSS) [11K]

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