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 Return to Top Why can't I "tune-up" my monitor by eye? Color perception by the human eye is subjective and is different from person to person. The Spyder2 is like an “electronic eye” that scientifically measures color and adjusts it to the industry standard. Therefore, wherever color is critical, a calibrated monitor is essential for the best color experience possible. Return to Top Who uses monitor calibration? Professional photographers as a cornerstone of their digital darkroom - Serious photographers that are passionate about their images - Graphic designers and creative professionals who need accurate color for their clients - Web masters who need to trust what they see on screen - Scrapbookers who want their cherished memories to look their best - PC Gamers who want a much richer gaming experience (and are tired of getting killed in the dark by clogged shadows) Return to Top What is color? Even in the darkest night, our world is filled with light. Objects absorb most of the light that reaches them, but the portion that is reflected determines how we perceive the object’s color. The reflected light reaches the photoreceptor cells in our eyes where it produces stimuli that are sent via optic nerves to our brain. Our brain interprets the stimuli and we “see” the object’s color. All animals, including humans, experience color differently. This is partly because of the physical differences between individuals’ eyes—no two eyes are identical. Color perception and interpretation also varies from person to person. A person can perceive colors differently at different times depending on their mental state and according to the mood they’re in. Many people are able to perceive color independent of light. In one form of the phenomenon known as synesthesia (where real information from one sense organ produces phantom perception in another sense) colors are “heard” as musical notes. Harmonies and music are linked with certain colors, with high notes most often associated with lighter colors and low notes with darker colors. Return to Top How is color defined? Using words to describe sensory perceptions is challenging. With sound we use words like “loud” and “soft.” Color is more complex. We use three values to define a color. The first is the hue, which is sometimes referred to as “shade.” The second value is chroma. And the third is lightness. Every color that we can see can be described using these three parameters. This is the fundamental basis of colorimetry. We diagram color by arranging a hue circle that runs clockwise from yellow through orange, red, violet, blue, bluish green and green back to yellow again. A color shade may appear lighter or darker according to its lightness value. Chroma relates to color saturation. If the chroma of a color is reduced, the color will be closer to gray and will appear less brilliant. Achromatic colors have zero saturation. Black, white and all of the intermediate shades of gray are achromatic colors, as determined by their lightness. Return to Top How can a color description be standardized? It’s possible to standardize human interpretation of color if we consider three variables. The variables are the light source, the object and the observer. Color is not a physical characteristic of objects, it is a sensory perception. Color depends on light; with no light there is no color. The visible spectrum, the light that human beings can perceive, is the portion with wavelengths between roughly 400nm (nanometers) and 700nm, and can be described in words as the color spectrum between violet and red. When light falls on a colored object, some of the light is absorbed and the rest is reflected. For example, when light reaches a red apple, the red parts of the visible spectrum are reflected, primarily, and the rest is absorbed and converted to heat. Colorimetry uses the percentage of reflected incident light (%R) within the visible wavelength range (400~700nm) to describe an object’s color. Ultraviolet radiation (UV) lies at the short end of the range, in the 350-400nm band, just below visible light, while near infrared (NIR) radiation can be found at the other end in the 700-1300nm range. Special applications such as whiteness indices and camouflage color analysis take these extremes into consideration. By analyzing this data and plotting it in a reflection curve, every colored object can be defined and each has a unique “fingerprint.” The International Commission on Illumination (CIE) has standardized the spectral power distributions for a variety of light sources. Two of the most important—and most commonly encountered—light sources are natural daylight (CIE Illuminant D65) and the incandescent (household) light bulb (CIE Illuminant A). To create a reliable standard, it is necessary to also standardize the human eye and the individual interpretation of color in the brain. When light enters the eye it falls on light-sensitive cells on the retina. Rod-shaped cells enable night vision but do not provide color data to the brain. Cone-shaped photoreceptors respond to high levels of light and provide the color stimuli needed for color vision. Animals that lack cone-shaped cells are thought to be colorblind. There are three types of cone-shaped cells, and each is sensitive to a different portion of the visible spectrum. One group of cells is sensitive to the blue portion of the spectrum, the short-wavelength area. Another type is sensitive in the medium-wavelength, green area. The third type is sensitive to the long-wavelength, red area. Stimuli from all three types of photoreceptors are combined in the brain and lead to the perception of color. The CIE standardized three color-matching functions for people observing tiny objects using only the fovea, the most acute portion of the retina (the two-degree Standard Observer) and three color-matching functions for people observing larger objects (the 10-degree Standard Observer). Tristimulus values are calculated by combining a) the color-matching functions with b) the CIE spectral power distribution of the illuminant and c) the spectral reflectance curve of the object. The tristimulus value is expressed as a set of three numbers that describe the perceived color of the object. Return to Top Why do we need color spaces? Color space is a model that objectively describes a color as a set of precisely defined variables that can be communicated and reproduced. Why do we need color spaces? Imagine that you are going to order custom seat covers for your SUV and the manufacturer asks you to provide a precise description of the color over the telephone. Do you think you’ll be happy with the outcome? Probably not. We need some mechanism to objectively and unambiguously classify and describe every color. Over the years, a number of attempts have been made to construct a color space that was both easy to interpret and had equal intervals in all color areas. Here’s the rundown on the most important systems in colorimetry in use at this time. Early on, the CIE X, Y and Z tristimulus values were used to describe color. X corresponded to the observer's red stimulus, Y to the green stimulus and Z to the blue stimulus. Y also corresponded to the perception of lightness. The tristimulus values were converted into CIE x, y and Y values and the color displayed on a CIE x, y chromaticity diagram to make it possible to display the hue and chroma of a color without considering its lightness. The CIELAB color space is an improvement over the CIE X, Y and Z color space. Three color values L* (lightness), a* (red-green axis), b* (yellow-blue axis) or L* (lightness), C* (chroma/saturation) and h (hue/shade) are calculated from the CIE tristimulus values. In this color space, the lightness (L*) value ranges between 0 = black and 100 = white. Positive +a* values represent red hues and negative -a* values green hues. Similarly, positive +b* values represent yellow hues and negative -a* values blue hues. The chroma (C*) is 0 for a purely achromatic color and increases as the color becomes more brilliant. Hue (h) is a circle from 0° yellow through 90° red, 180° blue, 270° green and back to yellow. In this color space, colors can be defined as lightness (L*), chroma (C*) and hue (h) values or as lightness (L*), +a* and +b* values. Return to Top What is metamerism? Many objects appear to be one color when viewed under a given light source (daylight, for example) but another color when viewed under a different light source. This is common and happens with nearly all colored objects for a variety of reasons. Some people incorrectly refer to this phenomenon as metamerism. True metamerism occurs when two different objects are perceived to have exactly the same color when viewed under one illuminant, such as daylight, but perceived to have different colors when viewed under a different light source. This is normally an undesirable effect. Imagine if you used touch-up paint that exhibits metamerism to conceal the scratches on your refrigerator. The paint job would look perfect in daylight, but in the evening in the light from an electric bulb the patched areas would appear to be a completely different color. If you have ever experimented with a so-called “black light” you may have witnessed a similar effect. In fact, UV light sources are sometimes used to detect forgeries because of the metameristic nature of certain inks. This effect can be understood by considering the standard color values X, Y and Z, which represent a color perception. The color of the paint and the color of the refrigerator are identical under the same illumination if their XYZ values for this type of lighting are identical. This is the case when we have samples with identical spectral reflectance curves. However, metameric samples have different spectral reflectance curves. The two curves will have the same X, Y and Z values and have the same color under one illuminant, but different X, Y and Z values and different colors under a different illuminant. Return to Top What are color differences? Nearly every industry involved in consumer products—from bakeries to dressmakers—must maintain consistency in the color of their products. Deviations from the original or standard must be as slight as possible so as to go undetected by the human eye. It’s virtually impossible to match colors 100%, even in the same sample, and products often exhibit minimal color variations from batch to batch. Color differences of any amount can be measured and recorded using colorimetry. To quantify the color difference between two samples, the color coordinates of the standard and the match are entered in a color space. The distance between the two points as entered represents the color difference of the two samples. The CIELAB color system and color differences are used most frequently. The plotted variables are as follows: DE* represents the total color difference, which is made up of the differences in chroma, DC*, (more brilliant/duller), hue, DH*, (greener, yellower, bluer, redder) and lightness, DL* (lighter or darker). The total color difference can also be made up of the difference in lightness, DL*, a*, Da*, and b*, Db*. Return to Top What are tolerances? No two items possess 100% identical color values. And human perception of color is too subjective to accurately discern the differences between color samples. Manufacturers who produce products in specific colors must satisfy their customers’ quality requirements for color accuracy, and there must be a completely objective way to do it. Fortunately, there is. Colorimetry can be used to stipulate the maximum allowable color differences that are deemed acceptable. This is known as color tolerances. Color tolerances can be established using all color systems, including the CIE xyY system, but the CIELAB system is used most often. There is one disadvantage to the traditional systems, however: visually these do not have equidistant spacing. In the CIELAB system, for example, you could establish that a maximum color difference of dE*=1 is acceptable. But the absolute value of dE*=1 varies depending on the color. When assessing pairs of colored samples with a measured color difference of dE*=1 you may find that dE*=1 is an acceptable color difference with brilliant yellow or green tones, but with achromatic, gray colors dE*=1 represents another, unacceptable color. This is because the same mathematical difference of 1 does not correspond to our visual perception. To avoid having to establish different color tolerances in the CIELAB system for every color, several new tolerance formulas have been developed. These formulas correct for the non-equidistant spacing of the CIELAB system. Today the most common published tolerance formulas are the CMC and the CIE-94 formulas. However, recent transformations such as CIEDE2000 or DIN99 are becoming more popular. Return to Top What are color mixtures? There are two types of color mixtures: additive and subtractive. In a nutshell, additive color mixtures are used when mixing light; subtractive color mixtures are used when mixing pigments or ink. In 1860, James Clerk Maxwell discovered that every (and all) color can be produced by mixing together different quantities of red, green and blue (R, G, B) colored light. When the quantities of R, G and B light are exactly the same, white light is produced. Additive color mixtures are the basis of television technology. Dots of phosphor that emit red, green and blue light are embedded in the shadow mask of the screen. The individual dots are so small that they cannot be seen separately. Therefore, an additive color mixture is produced and that is the color that we see. The rules change when applied to pigments and dyes, however. When the three primary colors cyan, magenta and yellow (C, M, Y) are mixed in equal quantities, black (K) is produced. This is called a subtractive color mixing. This happens because dyes and pigments absorb some of the incident light and subtract it from the reflected light. Adding colorants make the material darker. Subtractive color mixing is used in the commercial printing, textile, plastics, paint, paper and glass industries, and in the photographic color reproduction process. Return to Top What is whiteness? As all schoolchildren learn, white is a color. In colorimetric terms it is an achromatic color. Perfect white does not exist in nature, but as a theoretical quantity this “ideal white” would reflect all incident light in the 400~700nm range. Barium sulphate (BaSO4), which was once used as the standard benchmark for perfect white, delivers an average reflection of only about 98% of the visible spectrum. We use the term “whiteness” to describe how close to perfect white a material is. Whiteness is one indicator of quality for certain white materials such as inkjet paper and certain textiles. The whiteness can come very close to perfect white if the material is bleached to the point that nearly all light-absorbing color pigments are destroyed. However, if the white needs to be “bright,” chemical brighteners are used. Optical brighteners are also found in common household items like laundry detergent and toothpaste. They have a unique property that causes them to absorb radiation in the ultraviolet (UV) range (i.e., below 400nm—a range that is not visible to humans) and then release it again in the area of the spectrum that is visible. Materials that have been treated with optical brighteners can actually have a reflection value greater than 100% and can be described as “bright” white. Return to Top What is measurement geometry? How the color of any item is perceived depends on a) the angle of the incident light that is illuminating the item and b) the angle from which the item is observed. If either value changes, the perceived color of the sample also changes. Therefore, this information must be considered and defined in the measurement techniques as well as in the spectrophotometers used. The lighting variable is described as either diffused lighting (d) produced by a flash lamp illuminating an integrating sphere, or lighting directed at a specific angle (either 0° or 45°). The reflection is always measured at a specific angle of observation, either 0°, 8° or 45°. Different measurement geometries have been developed for specific applications. In the paper industry, for instance, a d/0° measurement geometry is used. In the textile and plastics industry, d/8° is used. The graphic arts industry uses 0/45° or 45/0°. Return to Top What influences instrumental color measurement? Modern spectrophotometers are very precise, reliable instruments that produce highly accurate results. Under identical conditions, identical samples will always yield the same results even through repeated trials. Measuring instruments are not prone to the errors and inaccuracies often associated with human observers. Despite their extreme accuracy, they are still mechanical instruments and can therefore be influenced by certain environmental factors. Instrumental color measurement can be adversely affected by temperature fluctuations or extremes, by humidity and by inconsistent or improper sample preparation. To assure controlled test conditions, temperature and humidity should be monitored and regulated. The sample preparation process should be scripted and care should be taken to avoid measuring errors. Additionally, samples should be conditioned before measurement so that temperature and humidity remain constant during the measurement procedure. A temperature difference of 10°C can change the color of a sample sufficiently to cause completely different color values for the same sample. —Jon Sienkiewicz Return to Top
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