Achromatic color

Achromatic colors are white, black, and gray. Black and gray differ from white only in their relative reflection of incident light. The purples are nonspectral chromatic colors. All other colors are chromatic for example, brown is a yellow of low lightness and low saturation. It has a dominant wavelength in the yellow or orange range. 

Wallach, H. 1948. Brightness constancy and the nature of achromatic colors. J. Exper. Psychol. 38(3), 310-324. 

Given an initial pltysical match, a difference in color can be introduced by either of two procedures, which are often carried out in combination. In the first instance, the radiance of one part of a homogeneous field is altered without any change in its relative spectral distribution. This produces an achromatic color difference. In the second case, the relative spectral distribution of one field is changed such that, for all possible relative radiances of the two fields, no match is possible. This is called a chromatic color difference. 

The least expensive (and most common) objectives are the achromatic objectives that are designed to limit the effects of chromatic and spherical aberration. Achromatic objectives are corrected to bring two wavelengths of light (typically red and blue) into focus in the same plane. The limited correction of achromatic objectives leads to problems with color microscopy and photomicrography. 

Apochromat Microscope objective that has better color correction than the much more common achromat objectives corrected chromatically for two wavelengths of light (red and blue). Apochromatic objectives are corrected chromatically for three colors (red, green and blue) and spherically for two colors, which practically eliminates chromatic aberration. 

Al] photographic lenses require to be achromatized, or so constructed as to refract the different colors equally, in such a way that the yellow and violet foci may ooincido because the yellow rays produce the visible image, while all the raya oi the spectrum wliich lie between the green and the extreme lavender produce more or less chemical effect upon the sensitive photographic tablet. The oommon view-lens is an achromatic meniscus, and it may be achromatized in two different ways, one of which consists in placing a double... 

Undertone of Near-White Samples and Gray Undertones. The undertone of an almost uncolored sample is the small amount of color by which the color of a sample differs from ideal white or achromatic material. It is described by hue and chroma. The distance and direction of the CIELAB color position of the test sample (a , bf) from the achromatic position (0, 0) are used to characterize the hue. The relative undertone is expressed by the distance and direction between the CIELAB color position of the test sample (a, b%) and that of the reference pigment (ag, b ). In both cases, the distance is expressed by a figure and the direction by a color name. For standards, see Table 1 ( Hue relative of near white specimens ). Apparatus spec-... 

The addition of 3 % of Pigment Blue 61 already results in a significant step towards blue/red and shows almost identical incremental changes with further additions (see color locations 11,12, and 13). In addition, the bronze effect occurs which intensifies with increasing distance from the achromatic point. With Pigment Blue 15 3 in numerically equivalent increments, the hue of the black ink moves towards blue/green in the opposite direction from the achromatic point and with a negative shift (color locations 14-16). 

Figure 2.7 Three color opponent mechanisms. The first is an achromatic channel. The second compares the red and green channels. The third compares yellow and blue. The connection to the blue cones in the achromatic, as well as the red-green channel, is drawn with a dotted line as the role of the blue cones is unclear. (Cone illustration from from LifeART Collection Images 1989-2001 by Lippincott Williams Wilkins, used by permission from SmartDraw.com.)...

In principle, the algorithms for color constancy, which are described in the following chapters, can also be applied to the rotated coordinates (Figure 2.20). Land notes that, in practice, the transformation may not be that simple if the algorithm is nonlinear, i.e. contains a thresholding operation. We see later, e.g. in Section 6.6, Section 7.5, or Section 11.2, that some of the algorithms for color constancy are also based on a rotated coordinate system where the gray vector, i.e. the achromatic axis, plays a central role. 

If the reflectance of the sample is equivalent to the reflectance of the background, then all input pixels will be equivalent. If we divide the pixel color by the maximum, then we again obtain an output value of [1, 1, l]r. Therefore, the output will be achromatic. 

Again, the color of the output pixel is independent of the illuminant. Note that both the sample and the background are achromatic. Therefore, the sample will appear to be achromatic because all color channels will have the same value. Hence, the white patch retinex algorithm is not in agreement with the results obtained by Helson. 

If this map is applied to the color measured by the sensor in the center of the image, we obtain the same result as we obtained for the white patch retinex algorithm. The output color will be achromatic for all three cases. 

The two-dimensional gamut-constraint algorithm first projects all colors c onto the plane at b = 1. Because both the sample and the background are achromatic, the results will be independent of the reflectance R of the sample. 

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