False-color

False-color

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False-color

A false-color image is an image that depicts a subject in colors that differ from those a faithful full-color photograph would show.

A true-color image of a subject is an image that appears to the human eye just like the original subject would: a green tree appears green in the image, a red apple red, a blue sky blue, etc. When applied to black-and-white images, true-color means that the perceived lightness of a subject is preserved in its depiction. Absolute true-color is impossible to achieve due to the differences between the chemistries of the display media and an eye.

In a false-color image this close correspondence between subject color and image color is violated. That can happen in many ways. For example, a photographic negative could be called a false-color image, since it shows the complementary colors of its subject. However, the term "false-color" is typically used to describe images whose colors represent measured intensities outside the visible portion of the electromagnetic spectrum (or outside the electromagnetic spectrum altogether).

For example infrared film captures a small region of the near infrared spectrum (wavelengths from about 700 nm to 900 nm). Black-and-white infrared film maps the average intensity in this portion of the spectrum to a "false" gray value. Typical color infrared film is sensitive in the visible and near infrared range; it is usually used, in conjunction with a blue-cut filter, for shifting the near infrared spectrum into the visible spectrum (ca. 400 nm to 700 nm). Loosely speaking, this process effectively subtracts ca. 150 nm from the subject wavelength: a green subject is depicted as blue; a red subject is depicted as green; and an infrared subject is depicted as red (blue subjects cannot be depicted; they appear black).

More generally, false-color images can be generated by digital image processing techniques to represent up to three independent measurements over a two-dimensional map or image. For example, a satellite might capture average intensities in short ranges of the ultraviolet spectrum and the near infrared spectrum. These measurements can be combined with altitude information in such a way that blue represents ultraviolet, green represents altitude, and red represents infrared. Then a bright yellow pixel indicates a high-altitude object that emits or reflects infrared light but not ultraviolet light; a magenta pixel indicates a low-altitude object that emits or reflects both infrared and ultraviolet light; etc. Because human visual perception is limited to three independent coordinates (Combination of Red, Green and Blue) at most three measurements can be depicted in such a way.

A pseudo-color image is derived from a greyscale image by mapping each pixel value to a color according to a table or function. A familiar example is the encoding of altitude using hypsometric tints in physical relief maps, where negative values (below sea level) are usually represented by shades of blue, and positive values by greens and browns. Although pseudo-coloring does not increase the information contents of the original image, it can make some details more visible, by increasing the distance in color space between successive gray levels. Pseudo-color images differ from false-color images in that they are made from only one original gray-scale image, rather than two or three.

False-color and pseudo-color images are frequently used for viewing satellite images, such as from weather satellites, the Hubble Space Telescope, and the Cassini-Huygens space probe's images of the rings of Saturn. Infrared cameras used for thermal imaging often show their image in false colors.