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How the Rainbow Sensor Works
A layman’s explanation

How Seawater affects Sunlight
As every underwater photographer knows, seawater attenuates different colors of light at different rates. Water attenuates red light most quickly such that more than 95 percent of the red light has been absorbed at 10 meters of depth. Those fantastic reds one sees in underwater are the result of artificial light from camera strobes or an underwater flashlight, not sunlight.

However, what most people don’t know is that seawater contains particles and dissolved matter in addition to the water itself that affect how the light is attenuated. If seawater has high concentrations of organic components it attenuates blue light faster than does pure water. The effect of this is very clean seawater appears to be deep blue and seawater with a lot of organic components may appear greener. This is the characteristic of light in seawater that the Rainbow Sensor uses.

Construction of the Rainbow Sensor
The Rainbow Sensor is really three pairs of sensors, a blue pair, a green pair and a white pair. Standard Hobo Pendant sensors that look like an elongated matchbook are modified to create a Rainbow Sensor.

Hobo Pendant sensors measure white light that spans all the colors. However a special filter that only allows light in a specific wavelength (color) to pass through is inserted into the Pendant’s case. A blue sensor is modified with a filter that only passes blue light while a green sensor only passes green light. The third sensor is unmodified and thus passes all wavelengths (white light).

How the Rainbow Sensor Measures Water Clarity
Each blue pair and each green pair work in conjunction. The sensors at 12 meters measure blue and green wavelengths at that depth while the sensors at 20 meters measure blue and green at 20 meters depth.

Obviously, in the 8 meters between 12 meters depth and 20 meters depth the light has attenuated (become dimmer). The difference between the light levels at 12 meters and 20 meters is the attenuation that has occurred in 8 meter water column between the sensors.

Using some fancy scientific formula this reading is converted to numbers (attenuation coefficients) called Kblue12-20 and Kgreen 12-20. How the number is arrived at is not important except to scientists. Simply understand that Kblue12-20 is the attenuation (light loss) in the blue wavelength between 12 and 20 meters and Kgreen 12-20 is the attenuation in the green wavelength over the same span.

Kblue is essentially the scientific measure of water clarity. Clarity is somewhat different from what divers call visibility. Lower Kblue values indicate higher water clarity, and higher values indicate lower clarity Visibility is affected by lots of things including the amount of sunlight on the water. However, water clarity, is an important component of diver visibility. Generally speaking when the Kblue is low diver visibility will increase. From a diver perspective water clarity is a more accurate measurement of what is traditionally called visibility.

How the Rainbow Sensor measures the organic content of Seawater
There are many things that affect water clarity include mechanical things like suspended particles that will increase the Kblue. However, one thing that does affect the Kblue is organic components such as dissolved organic material and chlorophyll. Therefore water quality cannot be directly related to the presence of organic material because the attenuation may be due to inorganic material.

However, in the green range, light attenuation (increased Kgreen) is generally not affected by organic material but is affected by inorganic material. Therefore if the blue attenuation (Kblue) increases but the green attenuation (Kgreen) does not the difference is the result of organic material in the water.

Again, without going through the fancy scientific calculations, that is how the Organic Index (OI) is calculated. An increase of Kblue relative to Kgreen means increased organic content (OI)

How The Rainbow Sensor Works
Benefits for dive guests
How To fund a Rainbow Sensor
Rainbow Sensor Poster
Rainbow Sensor Blog
Rainbow Sensor Assembly
Constructing a rainbow Sensor Mooring
Rainbow Sensor Array Configurations
Site startup Instructions
Correlation Experiment