The Technology of Duotone Color Transformations in a Color Managed Workflow

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1 The Technology of Duotone Color Transformations in a Color Managed Workflow Stephen Herron, Xerox Corporation, Rochester, NY ABSTRACT Duotone refers to an image with various shades of a hue mapped in an vector or wedge through a color space. The colorant, the gradient curve, and the number of colorants used define the slice through the color space. The image is printed with two or more analogue colorants. The colorants may be custom formulated or selected from a named color system. Typically two colorants are placed on a substrate by a halftone procedure, and the visual result, the mixture of the two colorants, is a third color. A gamut map of the colorants requires an accurate model of the thrid color that results from halftoning and printing the two inks. Color management procedures convert this gamut model to a vector through a monitor RGB color space and then to CMYK for proofing. This paper describes such a color management procedure. Keywords: calotype, colorant, color profile, color space, vector, cyanotype, duotone, gamut, ink, pigment, primary color 1.0 INTRODUCTION A duotone is an image reproduced with two or more colorants. Images created with more than two colorants often consists of less saturated analogue equivalents of the primary colorants, e.g., mid-tone gray ink is the analogue equivalent of 50% screened black ink The categorizing function is that the coloration of the image is the product of a craft, not defined through a standard color-space description and not color managed. In this paper, for simplicity, duotone is used generically, i.e., for any image appearing to be represented in a single hue regardless of the actual number of colorants used. 1.1 History Duotone is an emulation of a photographic print process popular at the at beginning of the last century, e.g., calotype, cyanotype, and albumen. The resulting photographic print have tones of sepia, cyan, brown, and purple. The tonality is the result of the color of the metallic salt or crystals used in the photographic development process and density from the black of the carbon or oxidation process. The greater the exposure to light, the darker and more saturated the result. Saturation in this case refers to the purest hue of the metallic color. In this paper, chroma is expressed as the a* and b* coordinates of L*a*b* color space. The actual chroma may be very light in a colorimetric saturation scale and its gray component, (expressed as L*), is always lighter than the darkest density of the oxidation. The salt particles are evenly distributed on a paper substrate. Light darkens the salt particles in direct proportion to the light source s duration or intensity. Ultraviolet light is the significant source. 1 The salts are about 12 microns to 24 microns in diameter and arranged in a semi-random order. The result is similar to a photograph printed with frequencymodulated screening on an inkjet printer. The traditional duotone s smooth gradient consists of particles increasing in darkness and in number, and of various sizes. Figure 1: Example of a cyanotype separation (Cyan: left. Black: right)

2 1.2 Current duotone reproduction methods Three methods of producing a duotone are considered. 1. Production run on an offset press. The photograph is reproduced with two or three inks. The coloration of the photo is the result of the mixture of ink colorants and the halftone reproduction process. 2. Soft proofed on a computer monitor. Displayed as a vector through an RGB color space. 3. Hard proofed on a CMYK output device The result of a color-space transformation from monitor RGB On an inkjet printer using six or seven colorants Examples of contemporary duotones are: 1. Four or six shades of black colorant in Icefields quadtone inkjet printing system The Epson Stylus Photo 2200 inkjet printer which produces duotones with seven colorants. (The seven colorants chart three primary axes. 1. Gray to black, 2. Light cyan to saturated cyan, 3. Light magenta to saturated magenta, and 4. Yellow. (See Figure 2.) 3. Adobe s Photoshop s duotone mode displyed in RGB monitor color space. Figure 2: Example of a light magenta/magenta inkjet print (Light magenta: left. Magenta: right.) 1.3 Typical duotone color-managed workflow A typical workflow includes soft proofing on a monitor, hard proofing on an inkjet printer and production printing on an offset press. Normally the workflow moves from color selection at the scanner or monitor through prepress and proofing to the press. However in the case of duotones, the color selection begins at the end of the workflow with the selection of offset inks. These colorants are named colors such as those of the Pantone color system or custom mixed colorants The workflow procedure 1. Convert the analogue colorants to L*a*b* spatial coordinates with spectrophotometer measurements, or by selecting the Pantone colors using the Pantone L*a*b* color-space guideline. 2, 3, 4, 5, 6, 7 2. Use any of the well documented color mixture formulas to forecast the resultant vector of the mixture. 3. ICC profiles can be used to convert the L*a*b* color vector to a RGB working color space and then to the RGB monitor color space Conversion to CMYK for hard proofing is easily accommodated with ICC profiles The photo is production printed with the two named colorants and is not part of the coor management workflow.

3 Figure 3: Typical duotone workflow 1.4 The color pipeline 1. The colorants are described in channels of grayscale or luminous values with TRC curves. 2. Each channel is reproduced with an analogue colorant. The colorant changes the luminous values of the grayscale channel from L* values to a vector in a chroma direction. (See Figure 4.) 3. The resulting vectors are described in L*a*b* coordinates. 4. The vector of the mixture of two or more colorants in L*a*b* color space is forecast with subtractive mixing formulas 2. 3 or lookup tables derived from the formulas. 5. Color management then follows ICC profile conventions. Figure 4: Grayscale data reproduced with a colorant changes the vector direction through color space

4 2.1 Single axis duotone 1 colorant of 1 to N shades 2.0 ANALYSIS OF A DUOTONE COLOR SPACE The duotone consists of several analogue colorants mapped along one vector. The result is increasing densities, or saturation, from the combination of an increase in colorant density and halftone dot density. (See Figure 5.) 2.2 Two axes duotone 2 colorants of 1 to N shades Figure 5: Example of a neutral-gray tritone gamut The duotone consists of two vectors. Each vector consists of several analogue colorants. The result is increasing densities, or saturation, from the combination of colorant density and halftone dot density. (See Figure 6.) Figure 6: Example of a duotone gamut

5 3.0 DUOTONE COLOR-SPACE TRANSFORMS A duotone may consist of two or more colorants mapped in any polar direction and separated by degrees of rotation. The combination of two or more colorants results in a third color. This third color is mapped as a plane through the L*a*b* color space. (See Figure 6.) The saturation point is defined as the colorant hue at 100% coverage. The saturation of a colorant limits the gamut s boundary. The luminous boundaries are defines as the white point of the substrate and the density of the black ink. Accurately determining the visual result of a mixture of two colorants is a complex process. Factors include reflectance of the substrate, ink spread-gain, optical gain, and halftone dot overlap. Generally two types of color mixing formulas are used. 1. Halftone modeling formulas such as the Neugebauer equations and the Yule-Nielsen model. 2. Subtractive models include the Beer-Bouguer law. 2 Gamma and Gaussian functions of the primary colorants, expressed as tetrahedral lookup tables of CIE L*a*b* coordinates with TRCs for density gain of each colorant, provide efficient but approximate results. Adobe Systems Photoshop Duotone mode provides transformations of the result of the named colorant mixture to RGB, L*a*b* and CMYK values. The following chart consists of Photoshop s L*a*b* values. The colorants are black and Pantone process cyan. The colorant mixture s L* channel is charted in a Gaussian relationship. The a*, b* values display gamma function relationships. Primary to Duotone Relationship Black % Black L* Black a* Black b* Cyan % Cyan L* Cyan a* Cyan b* Duo % Duo L* Duo a* Duo b* Figure 7: Vector display of Photoshop s duotone as charted above The selected Pantone Process Cyan colorant is well within any CMYK gamut. However, since the duotone colorant can be custom mixed, the gamut boundary may be outside RGB and CMYK gamuts. In this case gamut compressing methods or ICC profile device-independent L*a*b* to device-dependent CMYK transformations can be used.

6 3.1 Color-space adjustments for dot gain and white point Duotone color management is a device-dependent color-space to device-dependent color-space transformation. The input color-space is defined by the colorants and is mapped to tristimulous primaries (monitor RGB) or four primaries (proofer CMYK). The TRCs adjust each vector to control dot gain by lowering the density and saturation. Figure 8: ICC color-space transformation of duotone gamuts combined with TRC adjustments 4.0 CONCLUSION Historically, duotone colorations were the result of a combination of metallic chroma and black pigments. Today duotones are made from colorants selected from a name color system or custom analogue formulation that produce a third color. This third color is the result of personal craft and intent. The selection of colorants, the curves that control the gradients, and the manner in which the colorants overlap all contribute to the image s coloration. Good execution can sharpen detail, smooth gradients and increase the number of gray levels. 9 The combination of colorants, complex forecasting formulas, and TRCs results in a very large number of color vectors making accurate proofing very difficult. The difficulity is componded by lack of terminology.for example, there isn t a term for the custom colors used in a duotone, (primary color?), or the mixture visual coloration. 1. John Barnier, Coming into Focus, Chronicle Books, 2000 REFERENCES 2. Henry R. Kang, Color technology For Electronic Image Devices, SPIE 1997, pgs Roy S. Berns, Principles of Color Technology, John Wiley & Sons, Inc. 2000, pgs John A. C. Yule, Principles of Color Reproduction, GATF Press, 2000, pgs Gabriel Marcu and Kansei Iwata, A Model of Color Appearance of Printed Textile Materials, In Recent Progress in Color Science, IS&T, 1997, pgs Gabriel Marcu and Kansei Iwata, Computer Simulation of Printed Colors on Textile Materials, In Recent Progress in Color Science, IS&T, 1997, pgs Eric J. Stollnitz, Victor Ostromoukhov, David H. Salesin, Reproducing Images Using Custom Inks, Siggraph 98 Computer Graphics Proceedings, Edward J.Giorgianni, Thomas E. Madden, Digital Color Management, Addison-Wesley, Stephen Herron, The Quadtone Reproduction process: The Technology used for Inkjet Printing, In Proceedings of SPIE Color Imaging: Device Independent Color, Color Hardcopy, and Graphic Arts IV, January 1999, pgs Eric J. Stollnitz, Reproducing Color Images with Custom Inks. Ph.D. thesis, University of Washington, Joanna L. Power, Brad S. West, Eric J. Stollnitz, David H. Salesin, Reproducing Color Images as Duotones, In Proceedings of SIGGRAPH 96. ACM, New York, 1996 Richard Farber, Historic Photographic Processes, Allworth Press, 1998.