Knowing some basics about light transmission through water samples will enhance your understanding of how OBS sensors operate and help you select the best system/instrument for an application. It will also help you recognize technical and operational limitations that can reduce the value of your data and challenge the success of your monitoring program. Light transfer through a water sample is affected in complex ways by water molecules, material dissolved in the water, and scattering by suspended particles. All light-transfer processes depend wavelength indicated by the λ symbol in the following discussion.

Light is an ensemble of photons that are absorbed and scattered by water, suspended particles, and dissolved matter as they travel through a sample. The absorption coefficient, a(λ), is a measure of the conversion of radiant energy to heat and chemical energy. It is numerically equal to the fraction of energy absorbed from a light beam per unit of distance traveled in an absorbing medium. The accompanying graph of a(λ) shows that the absorption coefficient of pure, particle-free, water ranges from 0.03 - 0.06 cm^-1 in the 760-1000 nm band. OBS sensors operate in the near infrared spectral band (780 & 860 nm) to limit their response to direct and reflected sunlight, the NIR content of which is strongly absorbed by water.

Light scattering changes the direction of photon transport, “dispersing” them as they penetrate a sample, without changing their wavelength. The scattering coefficient, b(λ), is equal to the fraction of energy dispersed from a light beam per unit of distance traveled in a scattering medium, in cm^-1. For example, water with b(λ) = 1 cm^-1 will scatter 63% of the energy out of a light beam over a distance of 1 cm whereas another sample with b(λ) = 0.1 cm^-1 will scatter the same proportion of energy in 10 cm. Both absorption and scattering reduce the light energy in a beam as it travels through a sample and larger values indicate stronger effects.

The attenuation coefficient, c(λ), is a measure of the light loss from the combined effects of scattering and absorption over a unit length of travel in an attenuating medium. The unit of c(λ) are cm^-1. An easy way to remember the relationship among these properties is to recall that a(λ) + b(λ) = c(λ). It is also important to remember than a(λ), b(λ), and c(λ) are inherent optical properties (IOP) that do not depend on how a sample is illuminated. They are the same during the day and night and when a sample is measured with turbidity meter A or sediment meter B.

Absorption and scattering processes are illustrated by the overhead projection of petri dishes containing inky water and skim milk. The projected images of both dishes will be gray because their contents attenuate light, however, the milky sample does so mainly by scattering whereas the inky one mainly absorbs light.

We elaborate a bit on light scattering because of its central importance in understanding how OBS' respond in the environment. The angular distribution of light intensity scattered from a beam by a water sample is called the volume scattering function, VSF. The angle between this beam and scattered light rays is the scattering angle. Forward-scattered radiation occupies the hemisphere surrounding the incident beam and oriented away from the source and backscattered radiation fills the opposite hemisphere. The graph shows VSFs computed from Mie theory for air bubbles, mineral grains, and biological material as well as the forward- (0 – 90°) and back-scattering (> 90°) VSF regions.

The VSF for bubbles is strongly peaked in the forward direction relative to the other materials and biological material backscatters 10 to 50% less light than the other particles. Like the material properties of water samples, the variety of VSFs is huge. Their shape depends, among other things, on particle size and refractive index. The size factor, x = p D λ^-1, where D is particle diameter, measures the important relative effects of the operating spectrum and the size of the scattering particles on the response of a meter to a particular sample. For example, the VSF for 100 µm spherical particles when illuminated by red light (650 nm) light would be similar to one caused by illuminating 131 µm particles with 850 nm light, other things being equal. Most suspended particles in streams, lakes, and the ocean are larger than the wavelength of a meters’ illumination system, and consequently they scatter about half the incident light energy into a 10-degree forward-directed cone and less than 2.5 percent of it in the backward direction. Because of the many combinations of particle size, shape, and color, similar turbidity readings can be obtained from samples containing physically distinct particles. Effects of sediment properties and sample-handling procedures on the values indicated by our meters are discussed in the answers to other FAQs (size, color, absorption effects).