Why do some satellite images have strange colors?
Most colour digital images are made up of three individual grayscale images; one for the Red, one for the Green and another for the Blue component of the image.
Diagram showing a colour image separated into its three colour components (Red, Green and Blue).
Some satellites can record images in portions of the electromagnetic spectrum that are invisible to the human eye. This makes them very useful for observing patterns that we might otherwise not be able to notice.
The Landsat 7 satellite is capable of recording 8 different images of the same area simultaneously. Each of these images (called Bands) records information from the scene through a different filter. These Bands are actually grayscale images and show relative intensities as brightness values; 0 (low intensity) to 255 (high intensity).
Landsat 7 is able to record Blue, Green, Red, Near Infrared, Mid Infrared, Thermal and a special band called the Panchromatic band which is a combination of the Green, Red and Near Infrared portions of the spectrum. One grayscale image is created for each band.
All 8 Bands recorded by the Landsat 7 satellite’s Enhanced Thematic Mapper (ETM+) sensor.
(Honolulu Harbour, Oahu, Hawaii, USA)
Because each band is recording different information about objects in the scene, we can use any one of these bands as the Red, Green or Blue component of a colour image to help us view different characteristics about objects in the scene. This gives us 336 possible band combinations to choose from. Because we may be looking at "invisible" information (to the naked eye) the colours will look strange and often nothing like what we would normally see. Scientists can use these different colour patterns to identify specific characteristics about objects in a scene.
This is a diagram showing the creation of a “Normal” composite image where the Red band is creating the Red component, the Green band is creating the Green component and the Blue band is creating the Blue component of the image. The resulting image is then enhanced to reveal more detail.
This is a diagram showing the creation of a traditional “False Colour Near Infrared" composite image where the Near Infrared band is creating the Red component, the Red band is creating the Green component and the Green band is creating the Blue component of the image. The resulting image is then enhanced to reveal more detail.
This is a diagram showing the creation of a custom “False Colour” composite image where the Mid Infrared 2 band is creating the Red component, the Mid Infrared 1 band is creating the Green component and the Near Infrared band is creating the Blue component of the image. The resulting image is then enhanced to reveal more detail.
Images on this site were compiled by Paul Illsley.
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