ISO 6728:1983

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.