Raw satellite images have not normally been corrected for brightness and colour. The sensor's gain will be set to deal with the wide range of surface reflectance values encountered across the globe, from bright deserts and ice caps to dark rain forests and lava fields. Adjustments are usually needed to maximise contrast within a single image and to compensate for the particular wavelength range of a sensor's colour channels (above left).
Colour adjustments are also needed for natural colour photography taken from the Space Shuttle or International Space Station (right). Colour photographic film is slightly more sensitive to blue light. Even after this colour bias has been corrected, a photograph from orbit will not show the exact colours of the Earth's surface features due to atmospheric contamination. The atmosphere scatters light of shorter wavelengths, causing a further blueing of the image. A camera lens also causes brightess to fall off toward the corners of the image (optical vignetting), a problem that is particularly apparent in Earth observation photos with relatively flat colour variation across the image.