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Chapter 3: Colorimetry How to measure or specify color? Color dictionary?
Colorimetry • Measurement of color and its faithful reproduction are important! Eye can only detect ~150 different colors. • Appearance of color is determined by: 1. Hue: Main color characterized by dominant wavelength l. D. Intensity 2. Saturation: Purity of color characterized by width of the spectral curve. 3. Brightness: Overall intensity characterized by height of the spectral curve. l. D l (nm)
Newton’s Color Wheel • Simple colorimetric system that summarizes results from additive color mixing. Original Modified to account for the purples
Newton’s Barocentric Color System Features: • Hues: 8 unequal segments. • Saturation: Distance from the center. Purest colors lie on the periphery, center is white. • Complementary colors lie diametrically opposite.
Review Using Newton’s system predict the results of the following mixtures of colored light: a) 2 Parts red and 1 part green. 3 parts unsaturated red-orange b) Equal amounts of red, yellow and indigo. White c) Equal amounts of yellow and blue Unsaturated green
Colorimetry • Two approaches in use today: Analyze a spectrum and assign numbers (Tristimulus Value) Two spectra with same tristimulus values have same color CIE System Use standard color samples Samples arranged in many different ways Munsell, Ostwald systems
Color Matching • Metamerism: Basis of color reproduction techniques. • Metamers are two (or more) colors that appear identical, but have different spectra. • Example: Metameric whites… Intensity B: 480 nm 400 Y: 580 nm 500 600 700 l (nm) 400 500 600 700 l (nm) http: //www. cs. rit. edu/~ncs/color/a_game. html http: //www. cs. brown. edu/exploratories/free. Software/repository/edu/ brown/cs/exploratories/applets/spectrum/metamers_guide. html
Color Matching (Contd. ) • Color matching by a set of 3 additive primaries: IR = Intensity of red primary of wavelength l. R IG = Intensity of green primary of wavelength l. G IB = Intensity of blue primary of wavelength l. B • Note: Choice of l. R, l. G, and l. B is arbitrary. • Let Ul be unit intensity of any wavelength l. • Question: Can we match any l in hue, saturation, and brightness by adding IR, IG, and IB? • Answer: Any color, except saturated ones! e. g. R + G = Unsaturated Y • However, Unsaturated Y = Saturated Y + Little B! http: //www. physics. utoledo. edu/~lsa/_color/21_chroma. htm
Color Matching (Contd. ) • Pure colors can be matched if one is allowed to subtract colors. • Example: Saturated Y = R + G - Little B • Therefore, for any wavelength l, Ul = IR + IG + IB • One of IR, IG, or IB is negative, depending on the choice of l. • Disadvantages: • For many spectra, one of the primaries is negative. • Choice of primaries is arbitrary.
Color Matching (Contd. ) • Calculate the relative intensities needed to match 500 nm. http: //users. starpower. net/jworthey/jimtalk 2004 nov. html • Answer: IR = -0. 12; IG = 0. 37 ; IB = 0. 2
The CIE System (1931) COMMISSION INTERNATIONALE DE L'ECLAIRAGE INTERNATIONAL COMMISSION ON ILLUMINATION • “Imaginary” primaries that can be derived from any system of real primaries. No negative numbers. • Green primary has a curve that matches the visual sensitivity of the human eye. Apparent brightness is represented. • Tristimulus values (X, Y, Z): Are the relative amounts of R, G, and B primaries needed to match a given spectrum. Value Y also represents the total brightness of light. • Metameric pairs have the same (X, Y, Z) values.
Tristimulus Value Color Matching Functions for the XYZ System Matches the visual sensitivity curve
Chromaticity Coordinates • Tristimulus values: (X, Y, Z) • Define coordinates: • Note: x + y + z =1 • Hue and saturation are specified by (x, y) and brightness is specified by the tristimulus value Y. • Complete description of a spectrum is given by: (x, y, Y) or (l. D, p, Y)
CIE Chromaticity Diagram Graphical representation of chromaticity coordinates (x, y). Spectrum Locus: All visible colors are contained within this horse-shoe shaped curve. Equal Energy White E(1/3, 1/3) Purple Line: Produced By mixing R & B.
CIE Chromaticity Diagram Features • Colors on the spectrum locus are the most saturated. • Colors closer to the center are unsaturated. D S E • Complementary colors lie on opposite sides of E. • Dominant wavelength l. D of a source S is found by intersection of the line joining E, S, and the spectrum locus. • Purity (or saturation) is given by:
Review Problem Tristimulus values of a spectrum are: X = 11, Y = 7, Z = 4 • Find: a) Color coordinates b) Color region (0. 5, 0. 32) Pink b) Dominant wavelength c) Purity 650 nm About 41% d) Complementary wavelength 494 nm
Review Problem • Estimate the complementary wavelength of each of the following: • 460 nm 570 nm • 600 nm 488 nm • 520 nm Mixture of red (700 nm) and violet (400 nm)
Colored Surface Analysis • Reflected color depends on: • Incident light spectrum • Reflection characteristics of the surface • Note: A surface could have different chromaticities depending on the light source. • Standard sources adopted by the CIE: Source Color Coordinates Color Temperature A (Incandescent Lamp) (0. 43, 0. 4) 2854 K B (Noon Sunlight) (0. 35, 0. 35) 4870 K C (Average Daylight) (0. 3, 0. 32) 6700 K
Colored Surface Analysis (Contd. ) • Dominant wavelength and purity are found by replacing E by the source type (A, B, or C). • Brightness is specified by “Y”. (Note: Same (x, y) pair can have different Y!) • Yw : When a standard source illuminates a white surface. • Ys : When the same source illuminates a given surface.
Review Problem Following are the tristimulus values when a CIE source C, illuminates a White surface: (35, 30) Colored surface: (30, 28, 10) What are the chromaticity coordinates? What is the dominant wavelength? What is the purity? 583 nm ~ 60% What is the reflected brightness? (0. 44, 0. 41) 80%
The CIE System • Reduces a spectrum to a set of numbers: (X, Y, Z) or (x, y, Y) or (l. D, p, Y) • Visual sensation can be different even if the set of numbers is the same! • Under a given set of conditions, two different spectra with identical tristimulus values will appear identical. • CIE system can only be used for color matching.
Application of the CIE System What is all this Color Management stuff anyway? The problem with color is that different devices make it in different ways. Monitors produce light by exciting red-, blue- and green-emitting phosphors by bombarding them with an electron beam. This is called additive color. Printers use subtractive color. Light scattered from the paper makes it look white. Ink on the paper subtracts or absorbs certain colors and the remaining ones bounce off the paper back at you. For example, if the ink subtracts cyan and magenta, yellow is what you see. It also means that if you change the lighting white will look more yellowish or more bluish and if you print on pink paper getting any white is impossible. The end result of all this is that the range of color devices such as a monitor, or a printer, can display is not the same. The official name for this range is the color gamut of a device.
The Munsell Color Notation System (1915) • Color classification is based on three perceivable qualities: 1. Hue: Vertical planes, each hue subdivided into segments. 2. Value: Brightness on a scale of 0 (Black) to 10 (White). 3. Chroma: Saturation on a scale of 0 (neutral) to 12 (saturated). Notation example: 5 R 3 / 14 Is the color designation of red in the US flag http: //www. learn. londonmet. ac. uk/packages/synthlight/visuals/chapter 1/munsell. html
The Ostwald Color System (1917) • Variables for a given hue are colored pigment (C), white pigment (W), and black pigment (B) • C, W, B form the corners of a triangle and colors within are additive mixtures. B + W + C = 1 1. Isotones: Colors parallel to WC. Fixed B content. 2. Isotints: Colors parallel to BC. Fixed W content. 3. Isochromes: Colors parallel to BW. Fixed C content.
Comparison Munsell System • Specific notation replaces loosely stated or unrelated color terms. • Room for newly discovered colors. Ostwald System • Useful for mixing paints and pigments. • Need to add a new page for newly discovered colors. • Advantage: Color grading and specification is easily done by direct perceptual comparison. • Disadvantage: Color chips fade in time.