Overview
ISO 6728:1983 - Photography - Camera lenses - Determination of ISO colour contribution index (ISO/CCI) specifies a standardized method to quantify how a camera lens (or other optical elements) shifts the overall colour of a photographic film image. The standard defines an ISO Standard camera lens spectral reference, prescribes measurement requirements (monochromator bandwidth, integrating sphere), and provides a calculation procedure to produce a three-number Colour Contribution Index (CCI) - typically reported as “ISOKCI 0/5/4” for the reference lens.
Key topics and technical requirements
- Scope & intent: Evaluates the spectral contribution of lenses to colour balance assuming an average daylight-type colour film exposed under photographic daylight (D55).
- Measurement fundamentals:
- Determine spectral transmittance of the lens across the film-sensitive wavelength range using a monochromator (10 nm bandwidth recommended) and an integrating sphere or equivalent.
- Use the ISO Standard lens spectral transmittance table and average film spectral sensitivity data (blue/green/red layers normalized to 100) as references.
- Weighted spectral sensitivity: Combine film sensitivity and D55 spectral power distribution to create weighted sensitivity curves for blue, green and red layers.
- Calculation method:
- Multiply lens transmittance by each weighted sensitivity curve to get blue, green and red photographic responses (RB, RG, RR).
- Convert responses to log10, subtract the smallest log value from each to normalize, and express results as a three-number CCI (values sometimes multiplied by 100 to remove decimals).
- Reporting & tolerances:
- The standard recommends aiming for the reference index 0/5/4; Annex B provides guidance on acceptable tolerances and recommends using a trilinear graph to assess compliance.
- Standards referenced: ISO 511, ISO 2239, ISO 3028 and CIE Publication No. 15 (Colorimetry).
Applications and users
Who benefits:
- Camera and lens manufacturers - set spectral-transmittance aims and quality targets to ensure consistent colour reproduction.
- Optical designers - evaluate how coatings, glass types and filters affect image colour.
- Film and photographic process developers - ensure film spectral sensitivity and process aims match typical lens behaviour.
- Test labs / QA teams - perform standardized CCI testing for compliance, product comparison and labelling.
Practical uses:
- Specifying lens colour performance for interchangeability with film and flash systems.
- Assessing the colour impact of spectrally selective coatings, filters, or added optical elements.
- Providing a repeatable metric (ISOKCI) for product datasheets and acceptance testing.
Related standards
- ISO 511 - Density measurements (terms/symbols)
- ISO 2239 - Light sources for sensitometric exposure (D55)
- ISO 3028 - Camera flash spectral distribution index
- CIE Publication No. 15 - Colorimetry
Keywords: ISO 6728:1983, ISO/CCI, colour contribution index, camera lenses, spectral transmittance, D55, weighted spectral sensitivity, photographic daylight, lens colour balance.
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