Why satellite images have different colors
In Virtually Hawaii, we show you a large number of images taken from aircraft and spacecraft that have unusual colors compared to the ones we can see with our eyes. Often, we are asked "why is one part of the image red and the other blue?". This is because we have chosen to use display three different wavelengths on our computer screen where the surface is highly reflective (bright) at these wavelengths. These colors are the result of using instruments that study different parts of the spectrum to the part that our eyes can see. While we do not want to give you a complete course in physics, we thought that you might like to have a bit of the background to these remote sensing images.
First, we need to know that a spacecraft (Landsat, SPOT, or the SIR-C radar) does not "see" in color. Every image is obtained in black and white at a precise wavelength (usually between 0.4 to 12.0 microns). These electronic cameras only collect information in black and white, but they can obtain many images at the same time in different parts of the spectrum.
If we look at the diagram below of the spectrum, we see several broad regions that include the ultraviolet (wavelengths between 0.3 - 0.4 microns), visible (0.4 to 0.7 microns), near-infrared (0.7 to 1.2 microns), the solar reflected infrared (1.2 to 3.2 microns), the mid-infrared (3.2 to 15 microns) and the far infrared (longer than 15.0 microns).
Notice that the vertical axis (called % transmission) shows where our atmosphere allows a lot of the Sun's rays to reach the ground. It is easy to see why our eyes work in the visible part of the spectrum, because it is here that almost 100% of the Sun's energy reaches the surface. In contrast, due to water vapor in the atmosphere, almost no energy (%0 transmission) reaches the ground at 1.4 and 1.9 microns.
Our images of Honolulu are all obtained in between 0.4 and 12.0 microns as a set of 12 individual black and white images. All 12 images are obtained at exactly the same time, and each is a computer image where we can assign an 8-bit binary number between 0 (black) and 255 (white). In this diagram, we show six of these images (or "bands"), along with the wavelength of the band. We show bands 2, 4, 6, 8, 10, and 12 just to cover the whole range, and to show that not all bands have the same wavelength range (the width of the color bar beneath the spectrum). See how Band 12 is very wide (about 4 microns) while band 4 is very narrow (about 0.05 microns).
Go on to Page 2 of this Remote Sensing Tutorial to see how we get the images back to Earth from the spacecraft.
Author: Pete Mouginis-Mark
Copyright by P. Mouginis-Mark
Curator: Lori Glaze