Lab color space
, The L * a * b * color space is a color space that covers the area of the perceptible colors. The color space is constructed based on the opponent colors theory. One of the main characteristics of the L * a * b * color model is its device independence, that is, the colors are defined independently of the mode of production and reproduction technology, how they present themselves. Show Playback Media and device farbverfälscht of this distortion is detected, not the underlying notional value. The color model is standardized in EN ISO 11664-4. The standard replaces in Germany in 2011 zurückgezogenene DIN 6174: " Colorimetric evaluation of color coordinates and color differences to the approximately uniform CIELAB color space ".
Introduction
The L * a * b * color system was developed in 1976 by the CIE from the CIE XYZ model to represent the perceptive color characterization better than in the CIE Tristimulusmodell of 1931.
In this color model to the same Euclidean distances perceptually equal color differences correspond to what approximated well succeed. This was achieved by a logarithmic transformation of the xy chromaticity plane of the CIE XYZ model.
The so standardized color space is equidistant and device independent. Any perceptible color in the color space is defined by the color coordinates with the coordinates { L *, a *, b *} is defined. In application of opponent colors theory here green and red are on the a * axis opposite. The b * axis corresponds to the complementary colors blue and yellow.
The L * axis is perpendicular to this plane, and outputs the brightness again. The L * axis can be referred to as the neutral gray axis, as they (= 100 L), the end points black ( L = 0) and white and the intermediate values are achromatic gray tones on that axis.
For these parameters, the Lab color values of the Hunter Lab system were enhanced. In analogy to the xy chromaticity plane can be an a * / b * diagram calculated. At the origin of coordinates beginning, the curves with increasing wavelength for black starting by the blue-green quadrant, and to finally end again in black.
On the basis of this color space, the CIE color difference formula has the 1994 and 2000 adjusted to practical requirements, while also material factors were added. The difference equations of CMC ( Color Measurement Committee) use the Lab coordinates. A requirement for further development of the CIE color model of 1931 was that the agreed differences here are the color coordinates depend on tolerances of the measured value.
Coordinate system
, The L * a * b * color space is described by a three-dimensional coordinate system. The a * axis describes the green or red component of a color, with negative values for green and positive values for red stand. The b * axis describes the blue or yellow share a color with negative values for blue and positive values for yellow stand. The scales of the a * axis and the b * axis include a speed range of -150 to 100, and -100 to 150, notwithstanding that there is no noticeable correlation for some values . Since the color perception was taken into account in the preparation of this color space, the resulting color solid is non-uniform in Cartesian coordinates. The L * axis describes lightness ( luminance) of the color values of 0 to 100,
Compared to the original XYZ space of the CIE chromaticity diagram, the green area is in a * b * system smaller, while the purple - blue - cyan region is larger.
A general idea of which is to give * -b * diagram the curves in a. While yellow and green values can reach up to 150, show red and blue numbers only to 100 basis of the diagram depicted are idealized rectangular spectra with slit widths of 10 nm to 100 nm and a reflectance of 100 %, the calculated tristimulus values and the L * a * b * color space were transformed. According to this investigation are maximum information.
Importance
The coordinates of the L * a * b * color map based on the physiological characteristics of human perception ( color perception), they are based only indirectly on physical color stimuli ( colorimetry ).
The advantage over the color spaces previously created was to improve the visual equidistance, ie, the color differences between two color points in the Lab system corresponded to the visually perceived color differences. Fundamental to the distortion of the CIE system was the work of MacAdam. If we let subjects ( in the historic trial, however, there were only MacAdam himself and his employees) assess various color representations of the xy system on their subjective distance, the tolerance ranges are not farbortunabhängigen circles, but the MacAdam ellipses. The size and location of these ( observed ) curves and their direction from the trial size ( the color space) and the currently selected central locus dependent. Since the color differences in L * a * b * system are still not perfectly reproduced by the CIE color difference formulas have been improved further developed over the years that take into account this very dependent on the color location and viewing conditions deviations better. The other color bodies are achieved through complex calculations that can be managed by progressive computer technology, better than distortion of the original CIE body.
Application of the L * a * b * color solid
The comprehensive L * a * b * color space includes, as well as XYZ, all potential colors, device independent, and thus permits lossless conversion of color information from a color system to another, from one device type to another.
- Photo editing software like Adobe Photoshop and SilverFast use to convert L * a * b * color system as a reference. From the Device RGB or RGB vorgenormten (s- RGB, Adobe RGB) to images from the RGB color space of the monitor can be converted into the required to print CMYK system.
- L * a * b * coordinates are used as an exchange format between different devices.
- L * a * b * is the internal color model of PostScript Level II
- The RAL design system uses the Lab - formalism, which is mandatory from the DIN 6174.
XYZ and RGB
The colored box includes all visible to the average human eye colors of the CIE standard colorimetric system -. Clearly, the three-dimensional space is represented, including the brightness of the color bodies by Rösch from 1928. With this so-called " color mountain" and further investigations he delivered basic scientific considerations for creating the XYZ color system. In the surface only two dimensions can be displayed, this chromaticity plane (xy plot) is shown here.
All technically realizable color stimuli, both light colors and body colors are within the parabola -like solid. This area is called because of the shape and " shoe sole ", in English, this is referred to as " horseshoe " ( Horseshoe ).
With technical equipment ( computer monitor), not all these colors represent those emission sources correspond due to generation (Red-Green - Blue) RGB color spaces. Different display technologies such as TN (Twisted Nematic), IPS (In Plane Switching ) and VA ( Vertical Alignment) produce different results representation despite identical input values according to the L * a * b * color model. This is a known fact and is taken into account the effect on the choice of color for example on the internet that not too closely spaced colors are used when an illustration or a contrast to be clearly perceptible. Although a color difference in the L * a * b * color model is displayed sufficiently, it may be that it does not provide for the observer sufficiently discernible difference in practice. Therefore, one makes do with the use of so-called Web colors with sufficient Farbwertunterschiedlichkeit in the context of the recommendations of the W3C organization. There are several RGB color spaces as there are device-dependent differences because of different manufacture or operated phosphors. (Note: The outer portion of the field is saturated, as it can be displayed on the screen This limited device color space is called the gamut. ).
- The gray line includes the CMYK color space,
- The black one RGB color space in the form of sRGB ( monitor RGB ).
The XYZ color space is not perceptually uniform as the oldest standard color space. The numerically same color distance between two points with the same distance from the white point is not perceived in different areas of the color chart as equal. It can change the color based on the vector data is reached again, but for a tolerance education, as she is known for lengths, this color space is unsuitable, the distance is from the hue -dependent.
In RGB color spaces the desired distance is prescribed and limited by the electrical or lighting control variables.
, The L * a * b * space is now has the advantage of overcoming the disadvantage described above and still contains all of RGB color spaces. The corresponding transformations are precisely defined by mathematical operations.
Conversion from XYZ to Lab
Red value X and blue value Z are centered on the type of lighting. The normalization values are derived from the chromaticity of the white point of a black ( Planckian ) radiator in the XYZ color space that belongs to the selected illumination.
Thus Xn and Zn are set, the green ( and light reference ) value Y is due to its property with Yn = 1 predefined ( as soon as it comes to light sources ) and therefore Y is used directly. The un-normalized values Xn, Yn, Zn, and the reference illuminant can not be processed with normalized by the sum of the three color values variables (x, y, z) can be confused.
For example, for a light source with approximately 6500K temperature, the normalized reference values x = 0.31 y = 12:33, and z = 1 -xy, however, the un-normalized values Xn = x / y = 0.95, Y n = 1 and Zn = z / y = 1.09. Sometimes the XYZ values are scaled to the range 0 to 100, then according to Xn = 95, Yn and Zn = 100 = 109
In the American region is preferably selected D50 = 5000K (direct solar radiation ), Europe usually according to EN standard, the standard illuminant D65 = 6500K ( covered sky in color matching on the north window).
For small values
Is replaced by the following relation, the third root:
Where P stands for each X, Y, Z.
The factors 500 and 200 * to bring the resulting values for a * and b in the usual orders of magnitude, which * also match the maximum L of 100.
Conversion from Lab to LCh ( HLC)
The Lch color space with the coordinates (L *, C *, h ° ) corresponds to the Lab color space, but here are the Cartesian coordinates a * and b * in the form of polar coordinates C * and h ° indicated (angle to the a axis, distance from the origin ). Through this conversion a Lab color is much more vivid. H can be used as respective base color ( Hue ) are interpreted and C as the color saturation (chroma, chroma ).
From L * a * b * can be saturation C * and hue h ° calculated as follows:
The spellings LCh, LCH and HLC are common. In the latter, the usual order comes in the color picker (hue - saturation - brightness ) for expression. HLC is not to be confused with the HSB / HSL color space, which exists in many computer programs. Although this can also be interpreted as " hue, saturation, brightness ", but runs less perceptually because they result from a conversion of the technical RGB definition.
Conversion from RGB to Lab
In contrast to the RGB coordinate * a * b * coordinates, the luminance is separated from the color information in the color point specification in L. RGB images are changed in the brightness, as to change the individual components that make up the ink. * Images on L * a * b color information in a stay * - untouched and b * channel., The L * a * b * color space having a larger color gamut than the spaces defined on the device characteristics of RGB or CMYK coordinates and includes the color gamut of each complete.
The L * a * b * color system describes all perceivable colors. With technical devices can be detected depending on the substrates and device type only some of these colors. Thus, in the L * a * b * measurement space is kept free memory for colors that read no scanner, no monitor display and can output a printer. In practice this does not lead to increase of image files.
A transformation from the RGB color space to the L * a * b * color space is carried out in each case via the XYZ coordinates. The conversion formulas from RGB to XYZ (monitor red R in X: virtual Red, monitor - green G in Y: virtual green, monitor - blue B in Z: virtual blue) are ( by way of example is the sRGB presented ):