Why satellite images have the colors that they do
The value in obtaining multiple images at different wavelengths can be seen in the above figure. Here we show the individual images that were obtained by Band 2 (0.45 - 0.52 microns), Band 7 (0.76 - 0.90 microns) and Band 11 (8.5 - 14.0 microns). Careful inspection of these three images shows that the different parts of Honolulu have different brightnesses at different wavelengths.
For example, at points "a" in Band 2, we can see through the shallow water along the coastline to see the coral reef along Waikiki. At point "b" in Band 2, the rain forest on the Koolau Range is quite dark, but at the same place in Band 7, the vegetation appears quite bright. This is due to the reflective nature of chlorophyll in the leaves at this wavelength. Indeed, the relative brightness between the rain forest and the city is flipped as we look at the two images.
This change in contrast is also seen between Bands 7 and 11. If we look at area "c" in Band 11, we see some dark patches. These are clouds, which in the thermal infrared are cold, but at shorter wavelengths (Bands 2 and 7) are highly reflective due to the water droplets and so are bright. Band 11 is very sensitive to temperature, and we can also find things that are warm (i.e., bright) in this image that are dark in other bands. Areas "d", which include parts of the airport runways and the sides of Diamond Head facing the sun, are both warmer than the average scene and so are bright. It is interesting to try to compare these areas in Band 2 or 7, since they are obviously quite different.
Go on to Page 4, or return to Page 2, of the Remote Sensing Introduction.
Author: Pete Mouginis-Mark
Copyright by P. Mouginis-Mark
Curator: Lori Glaze