Why would I want to measure transmissivity?
A transmissometer measures the percentage of light transmitted along a fixed path.  Eureka integrates 3 party transmissometers, which are mounted externally to the Manta multiprobe.  
Light transmission is a key factor in the ecology of water systems.  Visible radiation, or light, from the sun provides the energy necessary for ocean currents and wind-driven waves. Conversion of some of that energy into heat helps form the thin layer of warm water near the ocean's surface that supports most marine life. 
Visible wavelengths of light are captured by chlorophyll-bearing marine plants, which then make their own food through the process of photosynthesis. The organic molecules created by this process are an important energy source for many small organisms that are the base of the food chain. All life in the world’s waters is ultimately dependent upon the light and the process of photosynthesis that it initiates. 
 
How is transmissivity measured?
Scientists have developed several different methods and instruments to measure light transmission in water. The simplest measurement method involves the use of the Secchi disk, a white plate about 30 centimeters (12 inches) in diameter. It is fastened horizontally to a rope marked in meters. The disk is then lowered into the sea, lake, or other waterbody. The depth at which the disk is lost to sight is noted using the rope markings. This provides a rough estimate of the depth of light penetration.
A more sophisticated device for measuring light transmission is the nephelometer (like Eureka’s turbidity sensor), which measures the scattering of incident (incoming) light by particles in the water. The optical backscatter meter and light scattering meter work in a similar fashion by projecting a light beam into the water. A detector on the instrument measures the amount of light that is scattered back into it.
A transmissometer measures light attenuation, or the sum of scattering and absorption of light in water. It projects a beam of light of a known wavelength over a known distance, and the data may be used to calculate the percentage of light that is transmitted. Transmissometers measure the light lost across a known distance. The maximum signal for a transmissometer occurs when there is no target between the light source and the receptor.  The value read from the Transmissometer is recorded by the Manta multiprobe as % Transmission.
 
What should I know about transmissivity measurements in the field?
Eureka’s MantaPlus sondes may be coupled with 3rd party transmissometers, such as the C-Star by Sea-Bird Scientific.  The transmissometer is mounted externally to the sonde.  The C-Star incorporates a novel monolithic housing with a highly integrated opto-electronic design to provide a compact solution for underwater measurements of beam transmittance.  The optical surfaces of transmissometers must be cleaned periodically to ensure proper operation.  Fouling can block light and cause skewed readings.  Transmissometers with various pathlengths are available, for example 10 and 25 cm.