Color Spaces FAQ
A color space is a method by which we can specify, create
and visualise color. As human's, we may define a color by its attributes of
brightness, hue and colorfulness. A computer will define a color in terms of
the excitations of red, green and blue phosphors on the CRT faceplate. A
printing press defines a color in terms of the reflectance and absorbance of
cyan, magenta, yellow and black inks on the paper.
If we imagine that each of the three attributes used to
describe a color are axes in a three dimensional space then this defines a
color space. The colors that we can percieve can be represented by the CIE
system, other color spaces are subsets of this perceptual space. For instance
RGB color space, as used by television displays, can be visualised as a cube
with red, green and blue axes. This cube lies within our perceptual space,
since the RGB space is smaller and represents less colors than we can see. CMY
space would be represented by a second cube, with a different orientation and a
different position within the perceptual space.
The summer sky--is it vivid blue? Or is it ultramarine blue,
cerulean blue, bonnie blue, sevres, bradley's blue, methyl blue, rood's blue,
or spectrum? The human eye can discern millions of colors, and the human
imagination has come up with thousands of names for them.
The Inter-Society Color Council (ISCC) and the the United
States Department of Commerce's National Bureau of Standards (NBS) (now called
the National Institute of Standards and Technology) created a method, called
the ISCC-NBS Method of Designating Colors, as a flexible, yet comprehensive
color naming system. The ISCC-NBS Method corresponds to the Universal Color
Language, Level 3 and identifies 267 color blocks, each of which identifies a
range of colors (COLOR Universal Language and Dictionary of Names.
The Pros and Cons of Federal Standard 595b
The Federal Standard colour system provide a means of
comparing colours visually. It has its origin in the US Military complex and is
still used there as prime source of colour reference.
The Catalog column in the table below links to PDFs
displaying triangles, each filled with a color and its sRGB coordinates and
name below it. Because the sRGB coding has limitations, those colors not inside
the sRGB gamut are marked with an X. The width of the X varies as the number of
sRGB primaries not within bounds. When a swatch (other than white) is marked
with a X, you can't be sure you are seeing the intended color.
Color Space Dimension Reduction describes the mathematics
used to produce these usable color catalogs (unlike the one to right).
Universal Color Language, Level 3 Color Names
This page shows color swatches defined by hexadecimal code
and named according to the Universal Color Language (UCL) described in COLOR
Universal Language and Dictionary of Names (listed in Links). Specifically,
this page shows colors named according to the UCL Level 3, where color
descriptions consist of 267 color blocks named by a method devised by the
Inter-Society Color Council (ISCC) and the the United States Department of
Commerce's National Bureau of Standards (NBS) (now called the National
Institute of Standards and Technology). This method is called the ISCC-NBS
Method of Designating Colors.
The ISCC-NBS Method identifies 267 blocks of colors. Each
block is given a color name devised using a set of adjectives and suffixes.
Each color block defines a range of colors--not a single color--that have the
same name. This range of color in each block is an acknowledged disadvantage,
but it is pointed out (COLOR, p. 4) that a set of 267 color names is analogous
to calendar dates for chronological events. This method is suitable for "a
variety of scientific and industrial applications" (COLOR, p. A-13). The
UCL at levels above 3 defines finer divisions of colors with no names, rising
to approximately five million divisions in UCL Level 6. COLOR also lists color
names from various sources for each UCL Level 3 color block (pp. 37-82) and a
Dictionary of Color Names which lists thousands of color names and their
corresponding UCL Level 3 block(s) (pp. 85-158).
NBS/ISCC Color System
Original and Improved 267 Color Centroids
Here is the complete set of color terms. Note that the force
of tradition argued for using "pink" and "brown" instead of
the more-logical "pale red" and "dark orange".
Language of colors with color dictionary: http://www.anthus.com/Colors/NBS.html
original by David A. Mundie Munsell converted to CIE XYZ and
then converted to Mac QuickDraw RGB: http://www.anthus.com/Colors/Cent.html The
Macintosh results appear to have colors red-shifted. "Web safe"
colors do not do that.
John Foster reconverted supplied Munsell values via Munsell
software downloaded from www.gretagmacbeth.com directly to RGB, and tried to
resolve some duplicates Some of these don't even look right, because some of
the bright colors are on the dark fringes with less chroma and are not centered
and high up on the hue curves. Many of the original Munsell values (noted) are
outside the RGB gamut, and have been adjusted to the closest brightest RGB
value by changing chroma until 0 or 255 is reached in one out of bounds RGB
component. Guesses were made in a few cases (noted) where the color was still illogical
compared to the name.
The Munsell colour model was initially proposed by Albert H.
Munsell in 1898 [Birren, 1969]. Munsell values themselves were described in
1905 (sphere), published as a color atlas in 1915 (slices of a bulging sphere),
and redefined in 1929 as the Munsell Book of Color. It was later revised by the
Optical Society of America in 1943 to more closely approximate Munsell’s desire
for a functional and perceptually balanced colour system. It was further renotated in the 1970s. This set of colors may be more divergent than originally intended
in 1955-1976; blue and purple even more so, since the date of the original
Munsell centroid values is unknown.
Color Conversion Formulas
As we receive regular inquiries about the math involved in
color conversion, we decided to publish the formulas used by our Color
Each conversion formula is written as a "neutral
programming function", easy to be translate in any specific computer
language (C, Java, Basic, Pascal, PHP, Perl etc.).
If you are searching for more generic information about
color, on the Net there are several good sites devoted to color science,
physics, psychology, physiology and technology.
The BNF Syntax Of CNS
T. Berk, L. Brownston, and A. Kaufman,
A New Color-Naming System for Graphics Languages
IEEE Computer Graphics and Applications, Vol. 2, No. 3 (May,
1982), pages 37-44.
<color name> ::= <achromatic name> |
<achromatic name> ::= [<lightness>] gray |
black | white
<chromatic name> ::= <lightness>
<saturation> <hue> | [<saturation>] [<lightness>]
<lightness> ::= very dark | dark | medium | light |
<saturation> ::= grayish | moderate | strong |
<hue> ::= <generic hue> | <halfway hue>
| <quarterway hue>
<generic hue> ::= red | orange | brown | yellow |
green | blue | purple
<halfway hue> ::= <generic hue> - <generic
<quarterway hue> ::= <ish form> <generic
<ish form> ::= reddish | orangish | brownish |
yellowish | greenish | bluish | purplish
The NBS/ISCC Color System
The Universal Language of Color
At the heart of the NBS/ISCC system is a standardized set of
color terms. The color space is sliced into fifteen hues such as
"yellow", "greenish yellow", "yellowish green",
and so forth. Within each slice, degrees of saturation and brightness are
specified by modifiers such as "vivid", "dark", and
For example, here is the complete set of modifiers for the
hue "purple". The color terms in this hue are "vivid
purple", "dark purple", "light purplish gray", and so
The reference colors of the standardized language are called
centroid colors. Because of non-linearities in our visual apparatus and
irregularities in our natural-language system of color names, not every hue has
the full complement of modifiers. There are in fact only 267 centroid colors.
That is a good practical number, small enough to be easily learned but large
enough to make the distinctions needed for many applications.
The Dictionary of Color Terms
Armed with the standardized language of color, the
researchers at the National Bureau of Standards reviewed a number of color
atlases and mapped their names onto the centroids. The result is a fascinating
dictionary of color terms.
Ever wondered exactly what color London Fog was? Celestial
yellow? Rembrandt's Madder? Ever needed a poetic name for a particular color?
The dictionary of color terms is the place to go.
Each of the letters in the table below will bring up the
list of color terms that start with it. Each such term is followed by the numbers
of the centroid colors that have been assigned to it by one color atlas or
another. Each number is linked to the full name and definition of the centroid.
Frequently-Asked Questions about Color
The ColorFAQ clarifies aspects of colour specification and
image coding that are important to computer graphics, image processing, video,
and the transfer of digital images to print.
I discuss the mapping from physical spectral power
distributions to perceived colour. I outline the CIE system, CIE XYZ, xyY,
L*u*v* and L*a*b* color systems, linear and nonlinear R'G'B', Y'CBCR, Y'PBPR,
Y'UV, Y'IQ and PhotoYCC, including the standards established by SMPTE, EBU and ITU-R
(formerly CCIR). I explain why HLS (HSL) and HSI are useless for the
specification of accurate color. I give a brief introduction to the CMY system
used in photography and CMYK system used in printing.
The Color FAQ is distributed in several formats, listed in
the table below. If you would like to view this document in typographic
quality, or if you would like to print it, I recommend that you obtain the
Acrobat PDF version.
Charles Poynton - Color links
This is a collection of links to resources concerning colour
technology, color image coding and accurate colour reproduction.
A comparison of RGB and CMYK color models.A color model is
an abstract mathematical model describing the way colors can be represented as
tuples of numbers, typically as three or four values or color components (e.g.
RGB and CMYK are color models). However, a color model with no associated
mapping function to an absolute color space is a more or less arbitrary color
system with little connection to the requirements of any given application.
Adding a certain mapping function between the color model
and a certain reference color space results in a definite "footprint"
within the reference color space. This "footprint" is known as a gamut,
and, in combination with the color model, defines a new color space. For
example, Adobe RGB and sRGB are two different absolute color spaces, both based
on the RGB model.
Frequently asked questions about Colour Physics
This FAQ concerns the measurement and control of coloured
surfaces such as plastics, textiles, surface coatings etc. It is intended for
practitioners rather than theoreticians. Those requiring a more theoretical
introduction to colour science or information about digital colour image
reproduction might prefer to start by consulting the Poynton Colour FAQ. If
your question is not in the FAQ you may wish to consider version 3.0 of the FAQ
which is available as a PDF file. You may wish to post your question on the
Colourware Color Forum. For more serious study, why not visit our bookstore for
our special collection of books on colour science.
How is light scattered?
When light strikes particles it may be scattered. When the
scattering particles are extremely small (to the order of 1000 nm) the light is
scattered according to a simple law proposed by Rayleigh: short wavelengths are
scattered more than long wavelengths. For larger particles (to the order of
4000 nm and larger) the amount of scattering is according to Fresnel's
equations: the amount of scattering depends upon the difference between the
refractive index of the particle and of the medium in which it is dispersed and
this difference is wavelength dependent.
If light is scattered evenly in all directions this is
called isotropic scattering but it is rarely the case. The absorption and
scattering properties of partices are complex and a number of theories exist to
describe them including the Kubelka-Munk theory.
Lab color space
Lab is the abbreviated name of two different color spaces.
The best known is CIELAB (strictly CIE 1976 L*a*b*) and the other is Hunter Lab
(strictly, Hunter L, a, b). Lab is an informal abbreviation, and without
further checking should not be assumed to be one or the other. The color spaces
are related in intention and purpose, but are different.
Both spaces are derived from the "master" space
CIE 1931 XYZ color space. However, CIELAB is calculated using cube roots, and
Hunter Lab is calculated using square roots. Except where data must be
compared with existing Hunter L,a,b values, it is recommended that CIELAB be
used for new applications.
The intention of both spaces is to produce a color space
that is more perceptually linear than other color spaces. Perceptually linear
means that a change of the same amount in a color value should produce a change
of about the same visual importance. When storing colors in limited precision
values, this can improve the reproduction of tones. Both are also absolute
color spaces, so they define colors exactly, unlike (for example) RGB or CMYK
which do not exactly define color, only a mixing recipe for light or ink
Metamerism is the situation where two color samples with
different spectral power distributions appear to be the same color when viewed
side by side. A spectral power distribution describes the proportion of total
light emitted, transmitted or reflected by a color sample at every visible
wavelength; it precisely defines the light from any physical stimulus. However,
the human eye contains only three color receptors (cones), which means all
colors are reduced to three sensory quantities, called the tristimulus values.
Metamerism occurs because each type of cone responds to the
cumulative energy from a broad range of wavelengths, so that different
combinations of light across all wavelengths can produce an equivalent receptor
response and the same tristimulus values or color sensation. Two spectrally
different color samples that visually match are metamers.