False color

For operations under false military colors, see False flag.

A mosaic constructed from a series of 53 images taken through three spectral filters by Galileo's imaging system as it flew over the northern regions of the Moon in December 1992.

A false-color image from the Meteor M2-2 satellite's imager MSU-MR. The image was received by an amateur radio station and is derived from the HRPT data.

False colors and pseudo colors respectively refers to a group of color rendering methods used to display images in colors which were recorded in the visible or non-visible parts of the electromagnetic spectrum. A false-color image is an image that depicts an object in colors that differ from those a photograph (a true-color image) would show. In this image, colors have been assigned to three different wavelengths that human eyes cannot normally see.

In addition, variants of false colors such as pseudocolors, density slicing, and choropleths are used for information visualization of either data gathered by a single grayscale channel or data not depicting parts of the electromagnetic spectrum (e.g. elevation in relief maps or tissue types in magnetic resonance imaging).

Types of color renderings

True color

The concept behind true color can help in understanding false color. An image is called a true-color image when it offers a natural color rendition, or when it comes close to it. This means that the colors of an object in an image appear to a human observer the same way as if this same observer were to directly view the object: A green tree appears green in the image, a red apple red, a blue sky blue, and so on.

Two Landsat satellite images showing the same region:
Chesapeake Bay and the city of Baltimore

This true-color image shows the area in actual colors, e.g., the vegetation appears in green. It covers the full visible spectrum using the red, green and blue / green spectral bands of the satellite mapped to the RGB color space of the image.

The same area as a false-color image using the near infrared, red and green spectral bands mapped to RGB – this image shows vegetation in a red tone, as vegetation reflects most light in the near infrared.

Burns Cliff inside of Endurance crater on Mars. The color is approximate true color because, instead of the red spectral band, infrared was used. The result is a metameric failure in the color of the sky, which is slightly green in the image – had a human observer been present, then that person would have perceived the actual sky color to have a bit more orange in it. The Opportunity rover which captured this image does have a red filter, but it is often not used, due to the higher scientific value of images captured using the infrared band and the constraints of data transmission.

Absolute true-color rendering is impossible. There are three major sources of color error (metameric failure):

• Different spectral sensitivities of the human eye and of an image capture device (e.g. a camera).
• Different spectral emissions / reflections of the object and of the image render process (e.g. a printer or monitor).
• Differences in spectral irradiance in the case of reflective images (e.g. photo prints) or reflective objects – see color rendering index (CRI) for details.

The result of a metameric failure would be for example an image of a green tree which shows a different shade of green than the tree itself, a different shade of red for a red apple, a different shade of blue for the blue sky, and so on. Color management (e.g. with ICC profiles) can be used to mitigate this problem within the physical constraints.

Approximate true-color images gathered by spacecraft are an example where images have a certain amount of metameric failure, as the spectral bands of a spacecraft's camera are chosen to gather information on the physical properties of the object under investigation, and are not chosen to capture true-color images.

This approximate true-color panorama shows the impact crater Endurance on Mars. It was taken by the panoramic camera on the Opportunity rover and is a composite of a total of 258 images taken in the 480, 530 and 750 nanometer spectral bands (blue / green, green and near infrared).

False color

A traditional false-color satellite image of Las Vegas. Grass-covered land (e.g. a golf course) appears in red.

In contrast to a true-color image, a false-color image sacrifices natural color rendition in order to ease the detection of features that are not readily discernible otherwise – for example the use of near infrared for the detection of vegetation in satellite images. While a false-color image can be created using solely the visual spectrum (e.g. to accentuate color differences), typically some or all data used is from electromagnetic radiation (EM) outside the visual spectrum (e.g. infrared, ultraviolet or X-ray). The choice of spectral bands is governed by the physical properties of the object under investigation.

As the human eye uses three spectral bands (see trichromacy for details), three spectral bands are commonly combined into a false-color image. At least two spectral bands are needed for a false-color encoding, and it is possible to combine more bands into the three visual RGB bands – with the eye's ability to discern three channels being the limiting factor. In contrast, a "color" image made from one spectral band, or an image made from data consisting of non-EM data (e.g. elevation, temperature, tissue type) is a pseudocolor image (see below).

For true color, the RGB channels (red "R", green "G" and blue "B") from the camera are mapped to the corresponding RGB channels of the image, yielding a "RGB→RGB" mapping. For false color this relationship is changed. The simplest false-color encoding is to take an RGB image in the visible spectrum, but map it differently, e.g. "GBR→RGB". For traditional false-color satellite images of Earth a "NRG→RGB" mapping is used, with "N" being the near-infrared spectral band (and the blue spectral band being unused) – this yields the typical "vegetation in red" false-color images.

False color is used (among others) for satellite and space images: Examples are remote sensing satellites (e.g. Landsat, see example above), space telescopes (e.g. the Hubble Space Telescope) or space probes (e.g. Cassini-Huygens). Some spacecraft, with rovers (e.g. the Mars Science Laboratory Curiosity) being the most prominent examples, have the ability to capture approximate true-color images as well. Weather satellites produce, in contrast to the spacecraft mentioned previously, grayscale images from the visible or infrared spectrum.

Eagle Nebula is false-color, as can be inferred from the pink stars. Three pictures were taken by the Hubble Space Telescope, then first picking up light in the frequency of sulfur ions (arbitrarily assigned to the color red), the second hydrogen (green), the third oxygen ions (blue). The actual color of the nebula is unknown, but if one viewed it at a distance making the 1-light-year-long "pillars" similarly visible, is probably a nearly uniform brownish grey to human eyes.