Unit
- Electromagnetic radiation: Visible light occupies only a narrow band of the electromagnetic spectrum, ranging from approximately 380 to 780 nanometers. Wavelengths shorter than 380 nm are ultraviolet; longer than 780 nm are infrared, both invisible to humans.
- Spectral power distribution: Every light source emits a unique combination of wavelengths. Sunlight has a continuous, smooth distribution, while fluorescent lamps produce spikes at specific wavelengths. This distribution determines how colors appear under that light.
- Color stimulus: When light reflects off an object, the object selectively absorbs and reflects certain wavelengths. The reflected light enters the eye and creates a color sensation. An object appears red because it reflects long wavelengths and absorbs others.
- Standard illuminants: The International Commission on Illumination (CIE) defines standard light sources like D65 (average daylight, 6500K) and A (tungsten filament, 2856K). These standards provide a consistent reference for color measurement.
- Metamerism: Two colors may match under one light source but appear different under another. This phenomenon occurs when the spectral distributions differ even though the visual appearance is the same. Metamerism challenges color correction consistency across viewing environments.
- Spectrophotometry: A spectrophotometer measures the intensity of reflected or transmitted light at each wavelength, producing a spectral curve. This curve is the most accurate fingerprint of a color, used for precise profiling and formulation.
- CIE 1931 color matching functions: Based on human vision experiments, these three functions represent how the average human eye responds to red, green, and blue primaries. They form the basis for all mathematical colorimetry.
- Tristimulus values: By multiplying a spectral distribution with the color matching functions and summing, we obtain three numbers (X, Y, Z) that uniquely describe a color. XYZ is the fundamental color space from which others derive.
- Chromatic adaptation: The human visual system adjusts to different light sources, a process called chromatic adaptation. Color correction software must simulate this adaptation using transforms like the von Kries or Bradford chromatic adaptation models to maintain consistent appearance when moving between D65 and D50 illuminants.
- Color temperature: Measured in Kelvin, color temperature describes the warmth or coolness of a light source. Low temperatures (2700K) appear warm orange, while high temperatures (6500K) appear cool blue. Standard D65 daylight at 6500K is the reference for most color critical work.
- Gamma correction: Displays apply a nonlinear power function to luminance output, known as gamma. Standard gamma values are 2.2 for sRGB and 1.8 for some Mac systems. Proper gamma encoding ensures linear perception of brightness and correct tonal reproduction across gradients.
- White balance: Accurate color correction requires neutralizing the overall color cast of an image by setting known neutral points. In camera raw processing, white balance adjustments shift the red, green, and blue channels relative to the illuminant's color temperature to achieve a neutral gray.
Design Principle
Principle: Spectral Control for Accurate Color Reproduction
Definition: The fidelity of color correction depends on controlling the spectral power distribution of light sources and measurement devices. Using broadband, continuous-spectrum light sources (like full-spectrum LED) minimizes metamerism and ensures consistent color matches across different media.
Application: In a print production environment, install 5000K D50 standard viewing booths with high CRI (Color Rendering Index above 90) for proof evaluation. Replace fluorescent tubes with full-spectrum LEDs that have a smooth spectral output. Calibrate all monitors to the same white point (D65) to align soft proofs with hard proofs under standard lighting.
Quick Revision
- Visible light spans 380-780 nm; spectral power distribution defines a light source color quality.
- Metamerism causes color matches to break under different illuminants.
- Spectrophotometers measure spectral curves for precise color characterization.
- CIE tristimulus values (XYZ) derive from human vision response functions.