Optical Transmission: What is it, what it means, and how it is used in LISST instruments
[Sequoia, July 12, 2012]
Optical transmission is a measure of what proportion of light is transmitted through a turbid medium. Light may be attenuated due to absorption in the medium, or it may be scattered out of the beam. With LISST instruments, we are normally concerned only with the latter, i.e. optical transmission due to light scattered out of the beam by particles.
In the figure above, the use of a focusing lens and a pinhole ensures that only the laser beam gets to the transmission sensor, not light scattered out of the beam [a small amount of forward scattered light also does get through]. The first obvious question is: what does optical transmission measure?
Consider a particle illuminated by a beam of light. It intercepts a part of the beam and removes that light from the remaining beam that continues to propagate. The amount of light that is removed is equal to the energy falling on the cross-section of the ‘particle’, i.e. area. In other words, the transmission of light depends on the area of a ‘particle’. The larger the particle surface area, the smaller the transmission. The same applies to a large number of small particles. The transmission now depends on the total area of particles, or the area concentration. That is, optical transmission measures particle area concentration. It specifically does not measure volume concentration or mass concentration. Optical transmission does not represent volume concentration or mass concentration of particles.
We note that when particle diameters are smaller than the wavelength of light, the light removed from the beam is not proportional to the area. In that case, a different efficiency applies, which depends on size. The efficiency is defined as ratio of light removed from the beam divided by the amount of light falling on the geometrical area of the particle. See texts for further information.
So, why is optical transmission important?
The first reason is historical. In oceanography, transmission sensors were used long before better sensors came along (e.g. laser diffraction). But even more importantly, it is a simple measure that has some comparative value. It is a ‘gross measure’ of particles in water, even if not of volume concentration.
Second, in applications such as laser diffraction (LISST technology) the transmission measurement is essential to de-attenuate the signal. To explain this idea, consider the figure again. The laser beam arrives at a volume located some distance x from its original entry point. At this point, passing through the water, it arrives somewhat attenuated. The light scattered by particles at x then suffers further attenuation. Thus, the attenuation along the path is the measure of how the measured signal was weakened due to attenuation by the time it reached the detectors. One needs this measurement to de-attenuate the measured signal.
There is another value in the measurement of optical transmission. Because it is a simple, single measure of particle content, it serves as a cross check on some occasions. For example, if optical transmission is available from two LISST instruments measuring the exact same water, their values must match. This is easier to test than the full size distribution which requires processing the raw measurements.
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