SpruceRoots Maps - July 2004

There's more in light from the sun than meets the eye. There are the colours that most humans can see in a rainbow, a photograph, or the view out the window. But beyond the ends of the spectrum we can see there are ultraviolet and infra red rays, some of which can be seen by fish, insects and other animals. And beyond those there are many other invisible rays at shorter and longer wavelengths, such as x-rays and radio waves. The earth's atmosphere filters out most of the shorter waves like ultraviolet, and allows most visible light and the longer rays like infra-red to enter.

The light that does reach the earth's surface is either absorbed or reflected by whatever it strikes. Rocks, water and plants are each made of different molecules, so they absorb and reflect light in different ways. A leaf in summer takes in the red, orange, blue and violet parts of the spectrum; what we 'see' are the shades of green reflected by chlorophyl molecules inside the leaf.

We can't see the invisible parts of the spectrum, but ultraviolet and infra-red rays are also absorbed and reflected in different ways by different things. Water absorbs the longer rays like infra-red and reflects blue and ultra-violet. Plants absorb the shorter wavelengths, using them as 'fuel' for photosynthesis (meaning to make something with light), and reflect back most of the infra-reds.

Although we can't see the invisible rays there are instruments that do. As the Landsat 5 satellite sweeps in orbit around the earth, it collects a continuous stream of information about the light reflected from the surface. Using special filters, it records what it sees in three bands of visible light and five bands of infra-red. In fact, it takes millions of 'pictures' at a time. Each scene is actually made up of thousands upon thousands of rows of tiny pixels, each covering a 30-metre square of the ground. Within in each square, it measures the light reflected at each of the seven wavelengths.

Most of the satellite pictures we see are made by computers using different combinations of the data. There are the 'real colour' images of cities and landscapes, which use the three visible bands to simulate what we would see from space if we were looking through a high-power telescope. Then there are 'false colour' images, which are made by changing the infra-red information into colours so we can visualize how the invisible rays are reflected. The information itself is not false but the choice of colours is made by the person operating the computer.

People often study false colour images made from red and infra-red information. Because plants reflect the most infra-red light - and different plants reflect it in different ways - changes in vegetation that are impossible to see at visible wavelengths are clearly pronounced.

This image of the Islands is a false colour image, made by combining two bands of infra-red and one band of red light from Landsat 5 satellite. These were sorted by a computer into 250 combinations (or vegetation classes), in which each 30-metre pixel is given a colour to portray what it contains. In the original satellite image, viewed at full size on a computer screen, you can see kelp beds and beaches at the ocean's edge, forests containing different ages and kinds of trees, rivers, lakes, and muskeg, and high alpine meadows. You can also see the roads, airport runways, townsites and other marks of industry on Haida Gwaii.