Basics of Color – Comprehensive Lecture Notes Course & Reference Overview Course: TEX 209-0723 – Textile Dyeing and Printing Lecture Topic: Basics of Color Instructor: Shohag Chandra Das (email: shohagdasbutex@gmail.com) Copyright © Shohag C. Das, 2025 Key reference texts “Basic Principles of Textile Coloration” – Arthur D. Broadbent (2nd Ed., 2001, Vol. 1–2) “Principles of Instrumental Analysis” – Douglas A. Skoog What Is Color? By-product of the spectrum of light as it is reflected or absorbed , captured by the eye, interpreted by the brain. Visual perceptual property corresponding to everyday categories (red, yellow, blue, black, etc.). Physically derives from the spectral power distribution (light energy vs. wavelength) interacting with retinal receptors. Figure (slide): diagram showing incident light → object → reflected wavelengths → eye/brain processing. Practical implication: controlling reflected/absorbed wavelengths → controlling color appearance in textiles. Fundamental Attributes of Color Three psychological/visual dimensions that fully describe a color sensation. Hue “Name” of the color; similarity to or mixture of basic sensations red, yellow, green, blue. Saturation / Chroma Intensity or purity of hue. High saturation ⇒ vivid; low saturation ⇒ dull/greyish. Lightness / Value (Brightness) Relative light-dark position. Add white → tint (lighter). Add black → shade (darker). Figures: attribute wheels and Munsell-style charts (virtualartacademy.com, musabi.ac) shown to students. Color Models & Systems (Why We Quantify Color) Purpose: Convert visual perceptions into numerical data for communication, recipe prediction, QC, and reproduction. CIE (Commission Internationale de l'Éclairage) provides internationally accepted standards. Common models used in textile/color science: CIE XYZ (1931): Theoretical perceptual foundation. CIE Lab (1976): Work-horse for measurement, recipe modification, QC. CIE LCH (cylindrical Lab): Intuitive for shade matching; separates chroma & hue. Munsell System: Hue-Value-Chroma chips; design & naming. CMYK: Printing, screen & dye-sublimation workflows. CIE XYZ Theory Based on research from the 1920s using a Standard Observer (average human with normal color vision) & Standard Illuminants . 1931 definition produced the universal CIE XYZ color space. Physiological underpinning: three retinal cone types roughly sensitive to long-(X), medium-(Y), short-(Z) wavelength primaries (often associated with R,G,B). Tristimulus values X , Y , Z X, Y, Z X , Y , Z Chromaticity coordinates x = X X + Y + Z , y = Y X + Y + Z x = \frac{X}{X+Y+Z},\; y = \frac{Y}{X+Y+Z} x = X + Y + Z X , y = X + Y + Z Y z = 1 − x − y z = 1 - x - y z = 1 − x − y Plotting x , y x,y x , y horseshoe-shaped diagram : Border curve = pure spectral colors (monochromatic). Straight line between border points = all colors producible by mixing the two endpoints. Entire interior = real visible colors. Triangles connecting RGB primaries illustrate device gamuts. Reflective thought questions from slide: Why horseshoe (spectral locus) rather than simple geometric shapes? → Because spectral wavelength‐to‐chromaticity mapping is non-linear. Why letters X, Y, Z? → To emphasize these are imaginary primaries distinct from physical R, G, B used in early experiments. CIE Lab (CIELAB, 1976) Introduced to give a perceptually uniform space so Euclidean distance ≈ visual difference. Coordinates: L ∗ L^* L ∗ a ∗ a^* a ∗ b ∗ b^* b ∗ Color difference formula Δ E a b ∗ = ( Δ L < e m > ) 2 + ( Δ a < / e m > ) 2 + ( Δ b ∗ ) 2 \Delta E_{ab}^* = \sqrt{(\Delta L^<em>)^2 + (\Delta a^</em>)^2 + (\Delta b^*)^2} Δ E ab ∗ = ( Δ L < e m > ) 2 + ( Δ a < / e m > ) 2 + ( Δ b ∗ ) 2 Figures: dermatology image & 3-D Lab plot illustrate axis orientation and uniform spacing. CIE LCH (CIELCH°) Cylindrical transform of Lab for intuitive handling. L = L ∗ L = L^* L = L ∗ Chroma C ∗ = ( a < e m > ) 2 + ( b < / e m > ) 2 C^* = \sqrt{(a^<em>)^2 + (b^</em>)^2} C ∗ = ( a < e m > ) 2 + ( b < / e m > ) 2 Hue angle H ∘ = tan − 1 ( b < e m > a < / e m > ) H^\circ = \tan^{-1}\left(\frac{b^<em>}{a^</em>}\right) H ∘ = tan − 1 ( a < / e m > b < e m > ) Benefits: easier visualization of changes in saturation vs. directional hue shifts; frequently used in shade sorting & recipe correction. Figure: side-by-side Lab vs. LCH cylinder. Color Consistency vs. Inconstancy Color consistency: Ability of a color to appear identical under different illumination, angles, substrates, or batches. Essential for brand standards & customer acceptance. Color inconstancy: Single sample’s tendency to change appearance under varying lights. Quantified by Δ E \Delta E Δ E Definition: Two samples match under one viewing condition but mismatch under another. Illustrated by a fabric pair matching in D65 (daylight) but mismatching under sodium arc . Types: Illuminant metamerism – change light source. Geometric metamerism – change viewing angle/orientation. Observer metamerism – two observers perceive differently (individual spectral sensitivities). Instrumental metamerism – different instruments/conditions yield different coordinates. Causes: instrument calibration, geometry, or type. Spectrophotometer Basics Instrument measuring spectral absorbance or reflectance of a sample; cornerstone of quantitative color control. Invented (Beckman DU) 1940 by Arnold J. Beckman (National Technologies Laboratory). Primary use in color labs: measure textile reflectance; also analytical chemistry (solution concentration). Working Principle – Beer–Lambert Law A = ε C L A = \varepsilon C L A = εC L A A A ε \varepsilon ε C C C L L L Spectrophotometer passes monochromatic light, compares incident vs. transmitted intensity → calculates A A A Core Components Light Source Deuterium lamp (UV 200–400 nm) Tungsten-halogen lamp (Visible 400–700 nm) Dual-lamp switching in UV-Vis instruments. Monochromator / Wavelength Selector Prisms or diffraction gratings + slits isolate desired λ \lambda λ Sample Holder (Cuvette) Path length typically 1 cm. Material: quartz (UV), glass/plastic (visible). Detector Photodiode array: fast, broad spectrum. Photomultiplier tube (PMT): ultra-sensitive, amplifies weak signals. Signal Processor & Display Converts photocurrent → absorbance or %T; outputs graph or numeric data. Instrument Configurations Single-Beam: Measures blank and sample sequentially; simpler, cheaper; susceptible to source drift. Double-Beam: Splits beam into reference & sample paths; simultaneous measurement improves stability.Typical Output UV-Vis Spectrum: plot of A A A λ \lambda λ Figure from ACS Omega illustrates concentration-dependent absorbance bands. Practical & Ethical Implications Accurate color communication prevents costly re-dyeing, waste, and customer dissatisfaction. Understanding metamerism leads to specifying proper lightboxes (D65, TL84, A) in QC. Instrument maintenance & inter-lab agreement (instrumental metamerism) vital for global supply chains. Standardized color spaces (CIE) form the legal backbone for product specifications & contracts. Connections & Recap Builds upon earlier coursework in light theory & human vision (physics, physiology). Foundation for advanced dyeing recipes, computer color matching (future lectures). Spectrophotometry bridges textile coloration with analytical chemistry principles (Beer–Lambert) highlighted in Skoog’s reference text. Key Equations & Numerical Facts (Quick Sheet) Beer–Lambert Law: A = ε C L A = \varepsilon C L A = εC L Chromaticity: x = X / ( X + Y + Z ) , y = Y / ( X + Y + Z ) x = X/(X+Y+Z),\; y = Y/(X+Y+Z) x = X / ( X + Y + Z ) , y = Y / ( X + Y + Z ) Color difference: Δ E a b ∗ = ( Δ L < e m > ) 2 + ( Δ a < / e m > ) 2 + ( Δ b ∗ ) 2 \Delta E_{ab}^* = \sqrt{(\Delta L^<em>)^2 + (\Delta a^</em>)^2 + (\Delta b^*)^2} Δ E ab ∗ = ( Δ L < e m > ) 2 + ( Δ a < / e m > ) 2 + ( Δ b ∗ ) 2 LCH transforms: C ∗ = ( a < e m > ) 2 + ( b < / e m > ) 2 , H ∘ = tan − 1 ( b < e m > / a < / e m > ) C^* = \sqrt{(a^<em>)^2 + (b^</em>)^2},\; H^\circ = \tan^{-1}(b^<em>/a^</em>) C ∗ = ( a < e m > ) 2 + ( b < / e m > ) 2 , H ∘ = tan − 1 ( b < e m > / a < / e m > ) End-of-Lecture Notes "Thank you! Have a nice day!" (Slide 23) Next steps: review chromaticity diagrams interactively (YouTube link provided), perform lab exercise on spectrophotometer calibration.