Overview
ISO 22028-4:2023 defines the eciRGB (2008) extended colour‑gamut, output‑referred RGB colour image encoding developed by the European Colour Initiative. The standard specifies how digital still images encoded in eciRGB (2008) can be stored, manipulated, transmitted, displayed and printed, and it provides two precision levels: 8 bits/channel (24‑bit) and 16 bits/channel. eciRGB is optimized to offer a larger gamut than sRGB while avoiding some quantization drawbacks of other wide‑gamut encodings.
Key topics and requirements
- Colour encoding definition: formal specification of the eciRGB (2008) colour space including primaries, white point and colour component transfer function.
- Tone curve: encoding uses a tone response designed to be linear relative to the L* axis in the CIELAB (1976) colour space for improved perceptual behavior.
- Precision levels: normative support for both 8‑bit/channel and 16‑bit/channel encodings to balance file size and tonal precision.
- Reference viewing environment: requirements for ambient illumination, display surround, viewing distance and glare control consistent with ISO viewing standards (e.g., ISO 3664).
- Reference display characteristics: expectations for display white point, luminance, black point and contrast ratio for accurate soft‑proofing and review.
- Encoding/decoding pathways: procedures for encoding 24‑bit eciRGB and for decoding to absolute XYZ (D50) tristimulus values for colour management and print conversion.
- Implementation guidance: informative annexes cover ICC profile considerations, practical viewing tolerances and comparisons of primaries to other gamuts.
Practical applications and users
eciRGB (2008) is intended for professional imaging workflows that need an output‑referenced, extended gamut RGB format:
- Photographers and photo labs: preserve saturated colours (logos, prints, scanned originals) without clipping or compressing to sRGB.
- Prepress and printing professionals: reliable interchange format for converting RGB images into print colour spaces (offset, digital press).
- Graphic designers and agencies: consistent asset storage and exchange across editing, proofing and production systems.
- Software developers and colour management integrators: implement encoding/decoding, ICC profiles and gamut mapping based on the ISO specification.
Using eciRGB helps maintain colour fidelity across capture, editing and print, minimizing gamut clipping and quantization artifacts while remaining compatible with standard colour management chains.
Related standards
- ISO 22028‑1: Architecture and requirements for extended colour encodings
- ISO 3664: Viewing conditions for graphic technology and photography
- ISO/CIE 11664‑1 and CIE Colorimetry publications for observer and tristimulus definitions
Keywords: ISO 22028-4:2023, eciRGB (2008), extended colour-gamut, RGB colour image encoding, European Colour Initiative, 8-bit, 16-bit, ICC profile, colour management.
