ISO 14490-10:2021

Optics and photonics - Test methods for telescopic systems - Part 10: Test methods for axial colour performance

This document specifies the test method for the measurement of the axial colour performance which includes axial chromatic aberration and spherical aberration of telescopic systems and observational telescopic instruments.

Optique et photonique — Méthodes d'essai pour systèmes télescopiques — Partie 10: Méthodes d'essai pour la performance de couleur axiale

General Information

• Status: Published
• Publication Date: 09-Feb-2021
• ICS: 37.020 - Optical equipment
• Technical Committee: ISO/TC 172/SC 4 - Telescopic systems
• Drafting Committee: ISO/TC 172/SC 4/WG 2 - Telescopic devices
• Current Stage: 6060 - International Standard published
• Start Date: 10-Feb-2021
• Due Date: 23-Sep-2021
• Completion Date: 10-Feb-2021

Overview

ISO 14490-10:2021 specifies standardized test methods for measuring the axial colour performance of telescopic systems and observational telescopic instruments. Axial colour performance covers the combined effects of axial chromatic aberration (wavelength-dependent focal shift) and spherical aberration (monochromatic focus variation). The standard defines a reproducible procedure to quantify colour-dependent focal shifts so manufacturers, test labs and optical engineers can assess and compare imaging colour performance.

Key topics and requirements

• Measurement principle: Evaluate the best-focus position on-axis for three spectral bands (green e = 546 nm, blue F’ = 480 nm, red C’ = 644 nm) to determine axial focal shift relative to green.
• Test arrangement: Uses a collimator with a back-lit negative test target at the focal plane, a bright light source, spectral filters (narrow-band, FWHM < 20 nm), and a dioptric tester or image analyser for focus read‑out.
• Instrument requirements:
  • Collimator and dioptric tester must have very low axial chromatic aberration (the document gives recommended limits and advises reflective collimators, e.g., parabolic mirrors).
  • Dioptric tester magnification around 6× is noted to improve dioptre precision.
• Procedure highlights:
  • Perform all measurements on the optical axis (target centered).
  • Set dioptric tester to 0 m and compensate examiner’s eye at green before measuring.
  • Refocus for blue and red illumination; record axial chromatic focal shifts (reported in metres or dioptres).
  • Annex A describes a method to characterise the dioptric tester itself using an image analyser.
• Uncertainty and limits: The fundamental precision limit is governed by the Rayleigh depth of focus; measurement uncertainty depends on exit pupil diameter and residual aberrations of the specimen.
• Reporting: Results and test conditions (per ISO 14490-1 Clause 13) must be included in the test report, with results presented in tables/plots and test-target details.

Applications and users

ISO 14490-10:2021 is directly applicable to:

• Optical designers and manufacturers of telescopes, spotting scopes and observational instruments
• Quality assurance and optical testing laboratories
• R&D teams evaluating chromatic and spherical aberration trade-offs
• Certification bodies checking compliance and comparative performance claims

Practical uses include bench testing during design validation, production acceptance testing, failure analysis, and comparative benchmarking of axial colour behaviour across instruments.

Related standards

• ISO 14490-1:2005 - general test methods for telescopic systems
• ISO 14132-1 - vocabulary for telescopic systems
• ISO 9336-3 - MTF testing for telescopes (recommended companion measurement)
• ISO 15795, ISO 10934, ISO 19012-2 - related optics / chromatic correction references

Keywords: ISO 14490-10:2021, axial colour performance, axial chromatic aberration, spherical aberration, telescopic systems, dioptric tester, collimator, test methods.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 14490-10
First edition
2021-02
Optics and photonics — Test methods
for telescopic systems —
Part 10:
Test methods for axial colour
performance
Optique et photonique — Méthodes d'essai pour systèmes
télescopiques —
Partie 10: Méthodes d'essai pour la performance de couleur axiale
Reference number
©
ISO 2021
© ISO 2021
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Published in Switzerland
ii © ISO 2021 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Requirements . 1
4.1 General . 1
4.2 Test arrangement . 1
4.3 Preparation and carrying out of measurements . 2
4.4 Determination of results . 2
4.5 Uncertainty and fundamental limit of the measurement . 3
4.6 Test report . 3
Annex A (informative) Measurement method for axial colour performance of the dioptric tester .4
Bibliography . 6
Foreword
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This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 4, Telescopic systems.
A list of all parts in the ISO 14490 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2021 – All rights reserved

Introduction
ISO 14490-7 mentions several characteristics to determine image quality of telescopic systems besides
the limit of resolution evaluation. One unmentioned characteristic in ISO 14490-7 is the “axial colour
performance” which may be noted by the user as a coloured halo around objects or even a hue of
objects in the centre of the field of view. Typically, the axial colour performance also affects the colour
performance in the entire field of view.
The axial colour performance of a telescopic system is mainly determined by two major intrinsic
contributions. These are spherical aberration and axial chromatic aberration. According to ISO 10934
axial chromatic aberration is defined as the aberration of a lens, by which light of different wavelengths
is focused at different points along the optical axis. The axial chromatic aberration originates from the
intrinsic difference of the refractive index of glass as a function of the incident wavelength of light,
i.e. the dispersion. For a singlet (positive) lens the axial chromatic aberration yields different focal
lengths for different wavelengths, which may also be called chromatic focal shift. Multi-lens groups or
assemblies are designed to reduce and compensate this focal shift to go below the intrinsic dispersion
of singlet systems. The footprint of the axial chromatic correction of lenses is partially classified by
terms like “achromatic” or “apochromatic” lenses.
Axial chromatic aberration originates from the dispersion of the lens material. In contrast, spherical
aberration is related to the geometry of a lens and is classified as a monochromatic aberration. Spherical
aberration causes rays in the image space to intersect the optical axis before or after the image point
formed by the paraxial rays (see also ISO 10934). As a consequence the “best focus” is not well defined
even for a monochromatic evaluation of a system. From that it is obvious that the measurement of a
pure axial chromatic aberration may be influenced by spherical aberration.
This document thus describes the measurement of the joint effect of these two major contributions
since in practical use an observer will not be able to separate these two effects. However, for deeper
analysis in the laboratory the two effects may be analysed separately.
In the case of afocal systems, such as telescopes, the axial chromatic aberration as well as the spherical
aberration of the objective lens is imaged to infinity by the eyepiece (looked at by the user) and can be
measured in dioptres. The measurement of the axial colour performance as described in this document
may be combined with a monochromatic evaluation of the modulation transfer function (MTF, see
ISO 9336-3) to obtain an overall figure for the imaging performance of a telescopic system.
INTERNATIONAL STANDARD ISO 14490-10:2021(E)
Optics and photonics — Test methods for telescopic
systems —
Part 10:
Test methods for axial colour performance
1 Scope
This document specifies the test method for the measurement of the axial colour performance which
includes axial chromatic aberration and spherical ab
...