ISO 20859:2005

Overview

ISO 20859:2005 - "Cinematography - Spectral response of photographic audio reproducers for analog dye sound tracks on 35 mm film" - defines the required spectral behaviour of a photographic audio reproducer taken as a unit (light source, any interposed optical filtering, and receptor). The standard covers reproduction of analog 35 mm sound tracks including cyan dye-only, conventional, and high-magenta formats, and is intended to ensure reliable, high-quality playback across projector and reader designs.

Key topics and technical requirements

• Scope: specifies the combined spectral response of light source + filter + receptor for photographic audio reproducers intended for 35 mm analog dye sound tracks.
• Peak response: the combined unit shall have a peak (maximum) response at 660 nm with an allowed tolerance of +20 / −30 nm.
• Out‑of‑band rejection: the integrated response to wavelengths greater than 800 nm must be less than 5% of the total integrated response measured from 400 nm to 800 nm.
• Formats covered: applies to cyan dye-only tracks (no silver redevelopment) and to conventional and high-magenta tracks (with silver redevelopment).
• Design implications: the unit may include optical filtering, and is intended to support a transition from infrared-sensitive, tungsten-based reproducers (used for silver tracks) to red-sensitive reproducers using LEDs or lasers for dye tracks.

Practical applications and users

Who uses ISO 20859:2005:

• Film projector and audio reproducer manufacturers designing light sources, filters, and sensors for 35 mm playback.
• Film laboratories and post-production houses establishing release-print processes for dye and high-magenta soundtracks.
• Cinema operators and technicians ensuring playback compatibility and sound quality during conversion from silver-based to dye-only prints.
• Archive and preservation specialists specifying playback equipment to minimise damage and ensure faithful reproduction.
• Standards bodies and test labs validating conformance of reproducers.

Benefits and practical outcomes:

• Ensures optimal modulation of cyan dye sound tracks by matching reproducer spectral response to dye absorption, reducing cross‑modulation distortion and preserving high-frequency response and SNR.
• Facilitates environmental improvements by enabling dye-only workflows (eliminating silver redevelopment and associated effluent).
• Encourages use of longer‑life solid-state red sources (LEDs/lasers) for improved reliability.

Related standards

Prepared by ISO/TC 36 (Cinematography). For broader context, consult other ISO cinematography and optical measurement publications and national implementation guidance produced by ISO member bodies.