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Ocean colour: How to measure it
Splitting light into colours Recording colour intensity Pixels and image grids Making the measurements
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Measuring water clarity with a Secchi disk.  |
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The spectrometers on the yachts in the Volvo Ocean Race are examples of modern instruments used for ocean colour measurements. |
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Methods for measuring colour
There are many different ways of measuring the clarity and colour of water. Some of the older methods are so simple that you can do it yourself. One such method, a Secchi disk, and coloured filters, is still used today.
More accurate modern measurements are usually made with radiometers or spectrometers, which measure the intensity of electromagnetic radiation at different wavelengths. These are the instruments you will find onboard satellites, air-craft or ships that make ocean colour measurements. They give quantitative information - numbers you can use for calculations, for instance of chlorophyll in the water.
Colour photography and colour video are also useful modern aids, but qive qualitative information (an impression of colour intensity, which cannot easily be used in calculations).
All the methods have two things in common:
- They separate the light into different wavebands (colours).
- They record the intensity of the light in each of these wavebands.
Splitting light into colours
White light contains all the colours in the rainbow - that is light of many different wavelengths. For colour measurements we need to measure the different wavelengths separately. In modern instruments (and in colour photography or video) this is done in two fundamentally different ways:
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Left: A prism spreads the different wavelengths, separating the colours. Right: A red filter lets red light pass and removes other colours. |
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Either by using filters, which remove all the unwanted wavelengths, leaving only one colour.
Or by dispersing (spreading) photons of different wavelength (colour) in a similar way to what happens naturally in a rainbow. This can be done with a glass prism, or - more commonly - with a diffraction grating (see below).
Filters are used in colour photography and video to separate the light into red, green and blue - the three basic colours seen by the human eye. They are also used in radiometers that measure only a small number of colours. Spectrometers, which measure a larger number of colours, generally use gratings to spread the different wavelengths.
Recording colour intensity
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Diagram showing what happens inside a spectrometer. Above left: A reflection grating is a type of mirror with many narrow groves close together. Some of the light is reflected in the normal way (m=0), but light that falls in the grooves is diffracted - it changes direction (m=1). Light of longer wavelength have a greater diffraction angle, so the colours are separated. A little of the light is diffracted further away (m=2), similar to what happens when you get a double rainbow. Above right: Light spread by the grating, falls on a linear detector array. (Efforts are made to record only the m=1 light). Each element in the array absorbs photons in a single waveband (colour), and creates an electric signal (current or voltage). The signal increases when the intensity of the light increases. |
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Once the light has been separated into photon streams of different wavelength (colour), it falls onto a line of light detectors - a light detector array.
Each detector in the array changes the light signal from a particular waveband (colour) into an electric signal (voltage or current). The size of the voltage (or current) increases when the light intensity increases.
Seawater spectrum from light measured in 40 wave-bands, each 10nm wide, in the range 350 to 750 nm.
Pixels and image grids
Some instruments record only the colour of one single area. However, imaging spectrometers (or radiometers) record the voltage from several areas. The current from each area is represented by a pixel. The pixels are arranged in rows and columns; together they form an image.
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A digital image is a grid of pixels. A video image, or a true colour image from an imaging spectrometer is composed of measurements made of light at red, green and blue wavelenghts.
Top row: When each pixel represents a large area, details disappear. The measured light intensity in each pixel is an average from the whole area covered by that pixel. Bottom row: More details are visible when each pixel covers a smaller area. (The pixel size here is 1/10 of the row above, and the spatial resolution is 10 times higher.) |
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| Light from the water (water-leaving light) measured in winter and spring. | ||
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| Light from the sun and sky (incident light) measured at the same time. | ||
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From these sets of two measurements (up and down) we can calculate reflectance
- the proportion of the incident light that is reflected back up by the water.
As you see the difference in water colour from Decemeber to March is not as great as it seemed at first.
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Making the measurements
Measurements at sea level
Spectrometers that measure light in a single area are often used from ships, boats or platforms moored at sea. They can be used to measure light above the surface or they may be lowered into the water to record how the light changes with depth. At the same time another sensor measures the light from the sun and sky, allowing scientists to correct for variations in light level.
When working from research ships or platforms, scientists also take samples of the water. That way they can relate water colour to water content.
Measurements from aircraft
Many countries have aircraft that carry imaging spectrometers. These give better spatial resolution (smaller pixel size - typically 5-10m), but each image covers a much smaller area.
Aircraft are expensive to run, so they are generally only used when there is a special reason for making the measurements. The daily monitoring of the Earth's oceans is left to the satellite instruments.
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Left: Satellite image from west Wales. (NASA SeaWiFS / ORBIMAGE) |
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Measurements from satellites
A number of satellites now orbit the Earth, carrying imaging radiometers (or spectrometers), which produce daily ocean colour images from all over the world. (See our pages on ocean colour satellites for more information.)
Because the satellites are high above the surface of the Earth, the pixel size is large - typically 1km, but each image covers a large area.
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Last update: 28 February 2009 |
Contact: o4s@noc.soton.ac.uk |