Facsimile and static presentation processing – Static presentation processing – Attribute control
Reexamination Certificate
1998-11-25
2002-11-12
Wallerson, Mark (Department: 2622)
Facsimile and static presentation processing
Static presentation processing
Attribute control
C358S001100, C358S001500
Reexamination Certificate
active
06480299
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to color management methods, apparatus and systems.
2. Description of the Related Art
While color management has many goals, in the digital color printing industry, the accurate and the characterization of a color printer's gamut and the subsequent image color matching has become a necessity. The main problem is that characterizing color printers usually involves measuring a large number of color patches that have been printed by the printer in question. In addition, the information that is obtained by measuring these color patches is only valid for that particular instance of the printer, thus creating the need for a quick printer re-characterization method. Printer characterization systems that use a small number of color patch measurements for ICC profile generation exist, but they fail to give accurate results. A traditional approach to generating ICC profiles and to performing color matching involves linear interpolation schemes that require large profile lookup tables.
An additional important consideration to this art is any accuracy/cost tradeoffs that can be made between the accuracy of a printed color and the cost of the ink that produces that printed color. For example, it is known that a slight color error between a signal-requested color and an ink-produced color can be tolerated because the human eye is incapable of noticing such a slight color error. Any ink cost optimization scheme that uses this inability of the human eye to notice slight color errors would be a valuable contribution to the art.
The nature of color as it relates to the electromagnetic spectrum is shown in FIG.
1
. Light is a form of electromagnetic energy or electromagnetic waves. Visible light is a form of electromagnetic energy, very much like ultraviolet, infrared, and radio waves. Electromagnetic waves are characterized by their wavelength, which can vary from kilometers to about a millionth of a millimeter. Light waves have wavelengths of about half a millionth of a millimeter, and the unit used for measuring visible light is the nanometer (nm. 10
−9
m).
FIG. 1
uses a logarithmic scale to illustrate a wide range of wavelengths. Color can be defined as the perceptual results of electromagnetic waves in the visible region of the spectrum, having wavelengths in the region of about 400 nm to 700 nm incident upon the eyes of a human. That is, color exists only in the human brain.
Humans can see colored objects due to the three different types of cones that are present with the retina of the human eye. These cones are referred to as S, M, and L (for short, medium and long wavelengths), and have their respective maximum sensitivities at 445 nm (violet), 535 nm (green) and 570 nm (yellow).
Distribution of these three types of cones is not uniform over the eye's retinal surface, and the number of S cones is much less than the number of the other two types of cones. More specifically, the cone ratios are 1 to 20 to 40, respectively, for the S, M and L cones.
A white surface, such as a piece of white paper, usually has the same visual appearance under both tungsten light and fluorescent light. This phenomenon is termed color constancy, and is due to the ability of the human brain to compensate for changes in both the level and the color of lighting. Sometimes extreme changes in the color appearance of an illumination source may cause detectable changes in the visual appearance of colors. Nevertheless, color constancy is an important feature of human vision.
Three basic perceptual attributes that characterize a color stimulus are: Brightness: Attribute of a color by which it seems to exhibit more or less light: Hue: Attribute of a color that causes the color to be perceived as being other than black, white, or gray; and Saturation: Attribute of a color by which it appears to be pure and containing no white or gray.
The term lightness is typically used to describe the brightness of a color relative to that of an equally illuminated white background. If illumination intensity changes, then both the color and the white background change equally, thereby maintaining the same lightness. In addition, colorfulness can be judged in proportion to the brightness of white, and is termed chroma. Lightness and chroma are, therefore, defined as: Lightness: The brightness (light/dark) of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting; and Chroma: The colorfulness (strong/weak) of an area judged in proportion to the brightness of a similarly illuminated area that appears to be white or highly transmitting.
Colorfulness can also be judged relative to the brightness of the same area, instead of to the brightness of a white area. This again is saturation. These definitions only apply to related colors; that is, to colors that are viewed together under the same conditions. Unrelated colors, on the other hand, are colors that are isolated from each other.
In the early years of color science, colored surfaces were compared by placing them side by side under a standard light source. This methodology was used since little was known about the human eye and its functions. Today, color science has a good understanding of the eye's cone response, and of how the cones interact to produce a sensation of color within the human brain. A software or hardware model of color must take into account a knowledge of the human mechanism of color vision.
Since humans have the three types of cones S, M, and L, and since photopic vision and color vision are a function of three variables, it is expected that an apparatus/method/system for the evaluation of color from spectral power data would require three different weighting functions.
An internationally-accepted method for evaluating color is called trichromic color matching or three color matching.
FIG. 2
shows a basic experimental arrangement for trichromatic color matching. In
FIG. 2
, the target color to be matched is projected on the lower half of the eye's field of view, while, on the other half of the filed of view, a mixture of three lights or stimuli is projected. The three projected light stimuli are usually 700 nm (red), 546.1 nm (yellowish green) and 435.8 nm (bluish violet). Color matching is then achieved by changing the intensities of the red, green, and blue source, until the target color is matched.
Many objects appear to be colored by absorbing and/or reflecting portions of the light that is incident on them, rather than by emitting light themselves. It is, therefore, clear that both the quality and the type of light affects a human viewer's perception of a reflective color. Humans are exposed to several light sources that have very diverse spectral characteristics. For example, incandescent light bulbs (light is emitted by a heated filament often made of tungsten) tend to distribute the majority of their energy to the longer wavelengths. This characteristic makes incandescent light appear more yellow than natural day light, which light is fairly neutral.
Any illuminant source, therefore, can be defined as a special kind of light source that is defined in terms of its spectral characteristics. Since most sources of illumination produce light by heating an object, these sources can be characterized by specifying the temperature of a black body radiator that appears to have the same hue.
Color temperature T
c
(in units of Kelvin, K) is defined as the temperature at which a Plankian black body radiates light that has the same chromaticity as that of a given light source. This definition is only applied to light sources that are very similar to Plankian radiators. The Correlated Color Temperature T
cp
is defined as the Plankian radiator temperature that most closely approximates the hue of the light source in question. This second definition applies to sources that do not exactly resemble a Plankian radiator.
The CIE standard illuuminant A is used to simulate incandescent lighting and it i
Drakopoulos Paul Apostolos
Subbarayan Ganesh
Crowe & Dunlevy
University Technology Corporation
Wallerson Mark
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