pigmented colorants: dependence on media and time - Cornell ...

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However, while the photographs are not completely physically accurate depictions of the actual color of the samples, they do give the viewer a reasonable subjective comparison. An important observation occurs when viewing the variation in binding media over the visible spectrum. Typically, the specific pigment determines the general shape of the spectral curve. That is, the intrinsic chemical properties of the material from which the pigment was created determines the central tendency of the material’s reflectance, as well as the wavelengths that are primarily absorbed. Yet, the specific binding media determines the exact behavior of how light behaves when striking the material. Each material has its own characteristics that it imparts and one recognizes these differences in a work of art. A painting in one media will in- voke a different sensation than that created in a different media. This relationship holds for all of the pigments in the study. This observation is logical; as one of the criteria for a binding material is that it does not drastically affect the materials that are suspended within it. An artist expects a pigment to maintain a relatively specific hue, whether it is dispersed in oil or watercolor or another media. However, as seen with Lapis Lazuli, while the general color is similar between binding media, there is great variation in the resulting color between the different materials. A common way to compare colors is to transform them into a perceptually uniform color space and study the results. This allows one to quantify the magnitude of the difference between colors. In our case, this can be used to evaluate how much binding materials affect the color of a pigment. The basic computation is to first convert the spectral data into CIE tristimulus 169 values XY Z. This is done using the response matching functions, an illuminant
(such as CIE standards C, D50, orD65) and Equation 3.3. Then, XY Z values can be converted into a number of nearly uniform color spaces, such as Munsell, CIE L ∗ a ∗ b ∗ or CIE L ∗ u ∗ v ∗ . Converting XY Z values into the Munsell color space is typically done via a three-dimensional look-up table, as there is no easy way to mathematically transform XY Z values into the Munsell space. Since the Munsell color space is defined by physical samples, the reflectances of the samples which define the space can be measured and converted into XY Z tristimulus values. From this data, one searches the table (such as from [Lab05]) for the nearest XY Z values. The Mun- sell HV C (hue, value and chroma) coordinates are found by linearly interpolating between adjacent points. Recently, it is of note that there has been a promising attempt to model a general transformation between reflectance spectra and the Munsell color system with good correspondence between the calculated and actual coordinates [LJP06]. Converting XY Z values into the other two spaces is much easier numerically. Tristimulus values XY Z are converted to L ∗ a ∗ b ∗ using Equations 3.9 & 3.11. By design, the distance between any two points in the L ∗ a ∗ b ∗ color system is nearly proportional to the perceptional color difference. Thus, the Euclidean distance (given by Equation 3.12) between any two color coordinates determines approxi- mately the perceptual difference between the two colors. The Munsell color system is also a perceptually uniform space, though it is more difficult to define perceptual differences via a discrete number. Due to the cylindrical natural of the Munsell system, one has to take into account differences in arc length to compare different hues. 170 Fortunately, there is free software available from GretagMacbeth (the com-
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PIGMENTED COLORANTS: DEPENDENCE ON
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ABSTRACT We present a physically ba
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Acknowledgements This work would no
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Table of Contents 1 Introduction 1
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List of Tables 2.1 Average pigment
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3.15 Photoreceptor absorption .....
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B.15 Variation of spectral reflecta
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corrode the surfaces of materials.
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Figure 1.1: Detail of the Azor-Sado
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However, since mural artwork is typ
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at a very high quality (albeit limi
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heterogeneous sum of very different
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which linseed oil derives (the most
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oration of a rigidly painted layer
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ing will hit the ground and reflect
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tion of paint tubes in 1841 and the
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Figure 2.8: Natural pigments of ant
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ecome more common recently. The Ame
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A V = O(r2 ) O(r3 ) = 1 O(r) 24 (2.
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Table 2.2: Pigment specific gravity
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provide the underlying color to a p
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though the speed of light in air is
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In general, when light travels from
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materials while lit from behind. Li
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Table 2.4: Index of refraction η f
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to completely congeal. Hence, these
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ond together to form proteins. Exam
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to its original state by any means.
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While it has been used since the 19
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organic binder found in more conven
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3.1.1 Visible Light Spectrum Light
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Figure 3.2: Hue, brightness and sat
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measures how much light is reflecte
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Figure 3.6: Additive color mixing o
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Figure 3.9: The physical amount of
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The vascular tunic lies on the wall
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Figure 3.11: Cross section of the r
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Not all areas have the same sensiti
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identify and distinguish between va
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that range between zero and full in
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spectrum. This insures that the res
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Figure 3.19: Graphical representati
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X x = (X + Y + Z) Y y = (X + Y + Z)
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color into the appropriate RGB colo
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components in the image (the smalle
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haviors of subtractive mixing and i
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of perceptually equal sizes. Unfort
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enabling one to transform between t
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andwidth as converted signals are s
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Figure 3.28: Retinal ganglion recep
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example shown in Figure 3.28. Under
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Figure 3.31: The work of Josef Albe
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sensations for each trial and picke
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Chapter 4 Previous Work There is no
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different location. Subsurface scat
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describes the scattering and absorp
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only accounts for a small percentag
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σ ′ t = σa + σ ′ s and α
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104 Figure 4.4: A model of an alaba
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effects or are too computationally
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108 Figure 4.6: Results using Kubel
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specifying pigments works adequatel
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color representation, it is not fea
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time. A metallic surface patina is
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the system consisted of wetness and
Page 131 and 132: or darkening enables the museum per
Page 133 and 134: in front of a displayed work. Felle
Page 135 and 136: light exposure. This effect is espe
Page 137 and 138: over time. Perceptually, these hues
Page 139 and 140: The rate of change of the concentra
Page 141 and 142: over white grounds) suffer the grea
Page 143 and 144: protects the deeper remaining color
Page 145 and 146: light source. This method is used w
Page 147 and 148: chemical smog and have been identif
Page 149 and 150: Chapter 5 Preparation & Measurement
Page 151 and 152: tending transparent pigments used i
Page 153 and 154: 140 Figure 5.1: Spectral reflectanc
Page 155 and 156: the requirements of the work at han
Page 157 and 158: 144 Figure 5.2: Left: Magnified vie
Page 159 and 160: Table 5.2: Relevant data regarding
Page 161 and 162: historically one of the best grades
Page 163 and 164: created far too strong of a binder,
Page 165 and 166: too brittle and cracking. While wor
Page 167 and 168: 154 Figure 5.4: Reflectance values
Page 169 and 170: 5.1.5 Painting After the layers of
Page 171 and 172: The gray card reflected 18% of the
Page 173 and 174: 160 Figure 5.8: The optical setup f
Page 175 and 176: Note that harmonics reflect at the
Page 177 and 178: Note that the calibration factors f
Page 179 and 180: Chapter 6 Experimental Results 6.1
Page 181: 168 Figure 6.1: Lapis Lazuli in dif
Page 185 and 186: 172 Table 6.2: Perceptual differenc
Page 187 and 188: 174 Figure 6.3: Munsell plots of La
Page 189 and 190: and j has a saturation of red rsat,
Page 191 and 192: 178 Figure 6.4: Variation of spectr
Page 193 and 194: visible spectrum are increasing at
Page 195 and 196: 182 Figure 6.5: Munsell plots of La
Page 197 and 198: trying to match color ca from the p
Page 199 and 200: as it ages. This is in direct contr
Page 201 and 202: teractive viewer. In our applicatio
Page 203 and 204: 190 Figure 7.1: Our simulated canva
Page 205 and 206: where n is the normal at the curren
Page 207 and 208: In summary, our implementation prov
Page 209 and 210: Chapter 8 Conclusion We have studie
Page 211 and 212: inks are beginning to find their wa
Page 213 and 214: Appendix A Derivation of K-M theory
Page 215 and 216: ∆i − =(K + S) idx 202 ∆j −
Page 217 and 218: Equation A.8 is now: Rearranging yi
Page 219 and 220: and if the paint is infinitely thic
Page 221 and 222: One concatenates the layers into a
Page 223 and 224: 210 Figure B.1: Cold Glauconite in
Page 225 and 226: Table B.1: Color conversions from t
Page 227 and 228: 214 Figure B.4: Munsell plots of Co
Page 229 and 230: 216 Figure B.5: Burnt Sienna in dif
Page 231 and 232: Table B.5: Color conversions from t
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220 Figure B.8: Munsell plots of La
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222 Figure B.9: Red Ochre Tint in d
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Table B.9: Color conversions from t
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226 Figure B.12: Munsell plots of C
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228 Figure B.13: Variation of spect
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230 Figure B.15: Variation of spect
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Table B.13: Color conversions of Bu
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234 Figure B.18: Munsell plots of C
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236 Figure B.20: Munsell plots of R
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References [Alb71] Josef Albers. In
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240 [GLL + 04] Michael Goesele, Hen
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242 [KM31] Paul Kubelka and Franz M
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[Rat72] Floyd Ratliff. Contour and
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