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False-color

 
Sci-Tech Dictionary: false color
(¦föls ¦kəl·ər)

(optics) Color assigned to frequency bands that are normally invisible to the human eye (such as infrared radiation) in an image in order to enhance contrasts or to display those colors.


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Wordsmith Words: false colors
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(fawls KUL-uhrs)

noun
Deceptive actions.

Etymology
When ships approached each other at sea, sailors would look to the flag to determine whether the other vessel was from a friendly or enemy nation. They'd often try to confuse the other by flying a false flag until they were close enough to attack

-Guest Wordsmith John (Ol' Chumbucket) Baur (chumbucketATtalklikeapirate.com)

Usage
"It will be sailing under false colors, says Richard Kogan, an economist at the Center on Budget and Policy Priorities. Tax cuts discussed by the Bush administration won't give much of a boost to the sluggish economy in 2003, he says." — David R. Francis; Assessing the Form, Impact, of Looming Tax Package; The Christian Science Monitor (Boston, Massachusetts); Dec 16, 2002.


Idioms: false colors
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Pretense, misrepresentation, or hypocrisy; deceptive statements or actions. For example, She's sailing under false colors--she claims to be a Republican, but endorses Democratic legislation. This term alludes to the practice of pirate ships sailing under false colors--that is, running a particular flag specifically to lure another vessel close enough to be captured. [Late 1600s]


Wikipedia: False-color
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A "false color" image showing the Chesapeake Bay and the city of Baltimore.
This "true color" image shows the same region as the "false color" image above, but uses true colors, e.g., the plants are shown in green.

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.

Contents

Pseudo-color

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. Pseudo-coloring can make some details more visible, by increasing the distance in color space between successive gray levels. Pseudo-coloring can be used to store the results of image elaboration; that is, changing the colors in order to ease understanding the image[1]. Alternatively, depending on the table or function used, pseudo-coloring may increase the information contents of the original image, for example adding geographic information, combining information obtained from infra-red or ultra-violet light, or MRI scans[2].

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.[citation needed]

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.

See also

References

  1. ^ C J Setchell; N W Campbell (July 1999). "Using Colour Gabor Texture Features for Scene Understanding". 7th. International Conference on Image Processing and its Applications. University of Bristol. http://www.cs.bris.ac.uk/Publications/pub_master.jsp?id=1000367. Retrieved 2009-03-18. 
  2. ^ Leonid I. Dimitrov (1995). "Pseudo-colored visualization of EEG-activities on the human cortex using MRI-based volume rendering and Delaunay interpolation". Institute of Information Processing, Austrian Academy of Sciences. http://www.viskom.oeaw.ac.at/~leon/docs/paper.ps. Retrieved 2009-03-18. 

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Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "False-color" Read more