Table of Contents
What Do We Perceive as “Color”?
Color is not a property of an object alone but the result of an interaction between:
- electromagnetic radiation (light),
- matter (which selectively absorbs, reflects, or transmits light),
- and the human visual system (eye and brain).
When we say “a dye is red,” we mean:
- it absorbs certain wavelengths of visible light,
- and reflects or transmits the remaining wavelengths,
- and our visual system interprets the remaining light as “red.”
The Visible Part of the Electromagnetic Spectrum
Light is electromagnetic radiation characterized by its wavelength $\lambda$ (distance between wave crests) or frequency $\nu$ (number of waves per second).
Visible light occupies only a small range of the electromagnetic spectrum:
- Approx. $400\ \text{nm}$ to $700\ \text{nm}$ in wavelength ($1\ \text{nm} = 10^{-9}\ \text{m}$)
- $\approx 400\ \text{nm}$: violet
- $\approx 450\ \text{nm}$: blue
- $\approx 500\ \text{nm}$: green
- $\approx 580\ \text{nm}$: yellow
- $\approx 600\ \text{nm}$: orange
- $\approx 650\ \text{nm}$: red
Outside this range:
- UV (< 400 nm) and IR (> 700 nm) are invisible to the human eye but can still interact with matter.
The energy $E$ of a photon is related to its frequency and wavelength:
$$
E = h \nu = \frac{h c}{\lambda}
$$
where
- $h$ = Planck’s constant,
- $c$ = speed of light.
Shorter wavelength $\Rightarrow$ higher frequency $\Rightarrow$ higher energy photons.
Light Sources and Spectral Composition
The color of the light source itself influences what colors we perceive from objects and dyes.
- White light (e.g., sunlight, many artificial light sources) contains a broad, continuous range of visible wavelengths.
- “White” objects appear white only under light that contains all visible wavelengths.
- Under a monochromatic light source (almost a single wavelength), many colored objects appear black or very dark if they cannot reflect that wavelength.
In color work (and in dye applications), the illumination conditions are always relevant, because:
- The same dyed fabric can look different under daylight, warm indoor light, or fluorescent light.
- This phenomenon is called metamerism: two samples that match under one light source may no longer match under another because their reflectance spectra differ.
Interaction of Light with Matter: Basis of Color
When light hits an object, several processes can occur:
- Absorption: Certain wavelengths are taken up by the material.
- Reflection: Light is bounced off the surface (specular or diffuse).
- Transmission: Light passes through (partly or fully).
- Scattering: Light is redirected in many directions (especially in turbid or particulate media).
The spectral distribution of reflected or transmitted light (which wavelengths remain after absorption) determines the perceived color.
Selective Absorption and the Color We See
Dyes are substances designed to selectively absorb light in particular regions of the visible spectrum.
Example:
- A dye that strongly absorbs light around $580\ \text{nm}$ (yellow) but reflects other wavelengths may appear blue or violet because the remaining light has more contribution from shorter wavelengths.
The observed (subtractive) color is approximately the complementary color of the absorbed light.
Additive and Subtractive Color Mixing
There are two key ways colors combine: by adding light or by removing light.
Additive Color Mixing (Light)
This describes mixing colored light sources. This is relevant for:
- Screens, projectors, stage lighting.
Primary additive colors:
- Red (R)
- Green (G)
- Blue (B)
Rules:
- Red + Green $\rightarrow$ Yellow
- Green + Blue $\rightarrow$ Cyan
- Blue + Red $\rightarrow$ Magenta
- Red + Green + Blue (in suitable intensities) $\rightarrow$ White
Additive mixing is about adding more light; the more you mix, the nearer you get to white.
Subtractive Color Mixing (Dyes, Pigments, Inks)
This describes mixing colorants (dyes or pigments) that absorb light. It is relevant for:
- Textiles, paints, inks, printing.
Primary subtractive colors (in an idealized model):
- Cyan (C)
- Magenta (M)
- Yellow (Y)
These selectively absorb components of white light:
- Cyan: absorbs red, transmits blue + green.
- Magenta: absorbs green, transmits red + blue.
- Yellow: absorbs blue, transmits red + green.
Idealized combinations:
- Cyan + Yellow $\rightarrow$ Green (red absorbed, blue partly absorbed; mainly green remains)
- Cyan + Magenta $\rightarrow$ Blue
- Magenta + Yellow $\rightarrow$ Red
- Cyan + Magenta + Yellow (sufficiently concentrated) $\rightarrow$ Black (nearly all visible light absorbed)
In practice (especially in printing), a separate black component K is added (CMYK) due to non-ideal behavior of real pigments.
Subtractive mixing is about removing light from white light; the more you mix, the nearer you get to black (or a dark, dull color).
Primary, Secondary, and Complementary Colors
Depending on whether we consider light (additive) or colorants (subtractive), the primary colors differ, but the basic idea of complementary colors is the same.
Complementary Colors
Two colors are complementary if they:
- together yield white (additive system),
- or together yield a neutral gray/black (subtractive system),
- and visually they appear as “opposites”.
Approximate complementary pairs in the visible spectrum:
- Red ↔ Cyan (green + blue)
- Green ↔ Magenta (red + blue)
- Blue ↔ Yellow (red + green)
For a dye:
- The color of the light it absorbs most is roughly complementary to the color it appears.
Example:
- A solution appears green.
→ It typically absorbs red light most strongly. - A substance appears yellow.
→ It often absorbs predominantly blue light.
This relationship between absorption maxima (in a spectrum) and perceived color is fundamental for understanding dye behavior.
Brightness, Saturation, and Hue
Color perception can be broken into three basic psychological/visual attributes:
- Hue
- What we usually call “color tone”: red, green, blue, etc.
- Related mainly to the dominant wavelength of light.
- Brightness (Lightness, Value)
- How light or dark the color appears.
- Depends on the total intensity of the reflected or transmitted light and on surroundings.
- Saturation (Chroma)
- How “pure” or intense the color appears (vivid vs. washed out or grayish).
- High saturation: clear, intense colors.
- Low saturation: pastel, grayish, or dull colors.
- Physically related to how narrow or broad the spectrum is and how much white/gray is mixed in.
In dyes:
- A highly absorbing, well-defined absorption band, leaving only a narrower region of the spectrum, often yields a highly saturated color.
- Scattering (e.g. in pigments or rough surfaces) and additional broad absorption can reduce saturation, making colors look more muted.
Color of Transparent vs. Opaque Materials
The way we see color depends on whether the material is mainly transmitting or reflecting light.
Transparent or Translucent (Solutions, Thin Films)
For a clear dye solution:
- Light passes through the solution.
- Certain wavelengths are absorbed.
- The transmitted light has a modified spectrum → perceived color.
Example:
- A blue copper sulfate solution absorbs more in the red/orange region, letting blue light pass.
Here, transmission spectra are most relevant.
Opaque (Paints, Dyed Fibers, Pigments)
For an opaque colored surface:
- Light is mainly reflected and scattered from the surface and internal interfaces.
- Absorption occurs within the material for certain wavelengths.
- The reflected light spectrum determines the observed color.
Here, diffuse reflectance spectra are important.
Dependence of Observed Color on Concentration and Thickness
For transparent dyed systems (e.g. solutions, thin polymer films), the observed color depends on:
- Concentration of the dye,
- Thickness of the layer the light passes through.
Qualitatively:
- If dye concentration or path length increases:
- more light is absorbed,
- color often becomes more intense up to a point,
- eventually, transmitted light may be strongly attenuated and appear almost black or very dark.
This behavior is described by a quantitative law relating absorption to concentration and path length, which is often used in analytical chemistry and will be treated in detail elsewhere.
For opaque systems (like heavily loaded paints), increased pigment/dye content typically:
- increases color strength up to a saturation point,
- beyond which further addition may change other properties (e.g. gloss, hiding power) more than the perceived color.
Geometric and Surface Effects on Color
The same material can look different depending on:
- Viewing angle,
- Surface texture (smooth vs. rough, glossy vs. matte),
- Scattering within the material.
Examples:
- A glossy surface shows strong specular reflection, can appear more saturated and “deep,” and may show highlights.
- A matte surface scatters light diffusely, often appearing lighter and less saturated.
- Effects like pearlescence, metallic shine, or iridescence arise from structured surfaces or layered materials that cause wavelength-dependent reflection and interference.
While such special effects are not typical for simple molecular dyes alone, they are important in modern color applications (e.g. automotive coatings, decorative pigments).
Color Vision and Human Perception (Basic Principles)
The human eye contains three main types of cone cells, each sensitive to different regions of the visible spectrum (commonly described as S, M, L for short-, medium-, and long-wavelength sensitive cones). Color perception arises from:
- the relative stimulation of these three types of cones,
- and subsequent processing by the brain.
Consequences:
- Different spectral distributions can lead to the same perceived color (metamerism).
- Some wavelengths may appear brighter or more intense due to the eye’s varying sensitivity across the spectrum (peak sensitivity is in the green region for typical daylight-adapted vision).
- Color defects (color blindness) arise when one or more cone types are missing or altered.
Understanding that human color perception is trichromatic (based on three receptor types) is crucial for:
- choosing and formulating dyes and pigments,
- reproducing colors reliably (e.g., in textiles and printing),
- interpreting color measurements from instruments that mimic the response of human vision.
Quantitative Description of Color (Overview)
In practice, colors are not described only verbally (e.g. “dark blue”) but also measured and specified. While the details are treated in dedicated analytical and technical contexts, a few basic ideas are useful:
- Spectral data: measurement of reflectance or transmittance $R(\lambda)$ or $T(\lambda)$ as a function of wavelength.
- Color coordinates: numerical values in a standardized color space (e.g. CIE systems) derived from spectral data and standard observer curves.
- Color difference: a numerical measure of how different two colors appear, used to judge whether a dyed fabric or paint matches a target.
Such quantitative methods are indispensable in industrial dyeing and color formulation but build conceptually on the fundamentals:
- selective absorption,
- the visible spectrum,
- and the trichromatic nature of human vision.
These fundamentals of color form the physical and perceptual basis for understanding how dyes operate, how their molecular structure influences which wavelengths they absorb, and how specific dye systems are selected and applied in practice. Subsequent chapters on natural and synthetic dyes and dyeing processes will build on these concepts.