ISO 9336-3:2020

Overview

ISO 9336-3:2020 - "Optics and photonics - Optical transfer function - Application - Part 3: Telescopes" specifies a standardized method for testing telescopic instruments by measuring the optical transfer function (OTF), typically reported as the modulation transfer function (MTF). The standard describes test arrangements, imaging states and normalization procedures needed to obtain valid, objective measurements of imaging quality for telescopes, binoculars, spotting scopes and telescopic sights. Two informative annexes cover modern measurement techniques using detector arrays and methods for deriving image quality criteria from MTF data.

Key topics and technical requirements

• Scope and purpose: Defines methods to produce reproducible OTF/MTF measurements that can be compared to theoretical design predictions and used for quality assurance.
• Test arrangements: Bench layout for object generator, collimator and image analyser; rotation about entrance/exit pupils for off‑axis tests; refocusing procedures to account for field curvature and astigmatism.
• Collimators and optics: Recommendations for well‑corrected achromats, reflective or catadioptric collimators; image analyser collimator focal‑length guidance (example: ~100 mm) and considerations for microscope objectives and micrometer focus travel.
• Spectral response: Match the test spectral band to the observer’s vision or detector sensitivity; prefer quasi‑monochromatic tests at dominant wavelengths and use filters to control spectral response.
• Spatial frequency and azimuth: Define spatial frequency ranges and azimuth scanning relevant to the instrument under test; orientation of roof prisms and roof edge effects must be noted.
• Normalization and imaging states: Procedures to normalize OTF values to diffraction‑limited MTF and to specify evaluation pupil sizes that represent typical observer pupil diameters.
• Accuracy, presentation and reporting: Requirements for measurement procedure, equipment accuracy and presentation of results to allow objective comparison.
• Annexes: Annex A - MTF test methods using detector arrays; Annex B - deriving objective image quality criteria (e.g., objective limit of resolution).

Applications and users

ISO 9336-3 is used by:

• Optical engineers and designers for validating and comparing measured MTF to predicted performance.
• Manufacturers and production QA labs for acceptance testing and process control of telescopic systems.
• Metrology and calibration laboratories testing visual and non‑visual (e.g., infrared) telescope systems.
• Research groups, military/defence contractors and observatory instrument teams requiring objective image‑quality metrics.

Practical benefits include objective, repeatable imaging performance metrics, compatibility with a range of detector types, and the ability to derive single‑value criteria (e.g., objective limit of resolution) for specification and acceptance.

Related standards

• ISO 9334 - OTF definitions and relationships
• ISO 9335 - OTF measurement principles and procedures
• ISO 14132-1 - Vocabulary for telescopic systems
• ISO 14490-7 - Subjective imaging quality assessment methods (referenced for context)