ISO 9336-3:2020

INTERNATIONAL ISO
STANDARD 9336-3
Second edition
2020-01
Optics and photonics — Optical
transfer function — Application —
Part 3:
Telescopes
Optique et instruments d'optique — Fonction de transfert optique —
Application —
Partie 3: Télescopes
Reference number
©
ISO 2020
© ISO 2020
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ii © ISO 2020 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 General description of test specimen types and the relevance of OTF tests .1
5 Test arrangement . 2
5.1 General . 2
5.2 Arrangement of optical bench . 2
5.3 Collimators . 3
5.4 Spectral response. 3
5.5 Spatial frequency range . 3
5.6 Azimuths. 4
5.7 Preparing the test specimen. 4
5.8 Auxiliary equipment . 5
6 Normalization of OTF values . 5
7 Test condition . 5
8 Specification of the imaging state . 5
8.1 Test specimen . 5
8.2 Measuring equipment . 6
8.3 Measurement . 6
9 Presentation . 8
10 Accuracy of equipment. 8
Annex A (informative) MTF Test methods using detector arrays . 9
Annex B (informative) Deriving an objective image quality criterion from the MTF .13
Bibliography .17
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
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described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
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iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee
SC 4, Telescopic systems.
This second edition cancels and replaces the first edition (ISO 9336-3:1994), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— update of the document based on the latest technical developments;
— Annex A regarding tests on components and sub-assemblies using azimuth scanning systems
removed, due to lack of practical relevance;
— two new Annexes added regarding test methods using detector arrays and deriving an objective
image quality criterion from the MTF.
A list of all parts in the ISO 9336 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 2020 – All rights reserved

Introduction
Methods of assessing the imaging quality of telescopic systems can be found in ISO 14490-7. The
methods described in this document are basically subjective, relying as they do on the judgement
of the observer and the quality of his vision. The technique of measuring the “limit of resolution” is
relatively easy and quick to perform and provides a single figure of merit for each orientation of the
test pattern. However, being a subjective measurement, it can be open to significant variations in its
results. Measuring the optical transfer function (OTF), or more usually just its modulus, the modulation
transfer function (MTF), provides a completely objective means of evaluating imaging quality that can
be compared directly with the theoretical assessment done by the optical system designer.
Integration of the system MTF over a certain domain of spatial frequencies and normalised to the
diffraction limited MTF will provide a single figure of merit that is a reasonable representation of the
system performance without relying on any subjective assessment. When the spatial frequency domain
is selected in accordance with the properties of the detector system the method can be applied to
telescopic systems operating with any detector type, thus not limiting the method to visual observation.
This is of importance as in state-of-the-art telescopes the same optical path can be used for visual
observation as well as for wavelengths outside the visual range (using appropriate detector systems).
As a special case, an “objective limit of resolution”, providing a single figure of merit, can be derived
from a measurement of MTF by using the latter in combination with a “contrast sensitivity” curve for
the eye and a measurement of MTF may also be used as the basis for several other image quality criteria
(see Annex B).
INTERNATIONAL STANDARD ISO 9336-3:2020(E)
Optics and photonics — Optical transfer function —
Application —
Part 3:
Telescopes
1 Scope
This document specifies a method of testing telescopes in terms of imaging states aimed at making
valid optical transfer function (OTF) measurements.
This document includes two annexes (Annex A and B) that provide information on the more recent
techniques for measuring optical transfer function and methods of deriving image quality criteria from
such measurements.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 9334, Optics and photonics — Optical transfer function — Definitions and mathematical relationships
ISO 9335, Optics and photonics — Optical transfer function — Principles and procedures of measurement
ISO 14132-1, Optics and photonics — Vocabulary for telescopic systems — Part 1: General terms and
alphabetical indexes of terms in ISO 14132
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 9334 and ISO 14132-1 apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 General description of test specimen types and the relevance of OTF tests
The specimens considered are telescopic observational instruments with direct view used for viewing
remote objects and include many instruments such as telescopes, binoculars, telescopic sights or
spotting scopes.
Ideally, instruments would be best with no astigmatism and no field curvature coupled with good
chromatic correction but frequently compromises as mentioned above shall be tolerated.
Many optical systems include roof prisms to give a compact instrument. However, the image produced
by such systems is basically made up of two superimposed images and the accuracy with which they
match will depend on the accuracy with which the roof edge has been constructed. In such cases the
orientation of the roof edge shall be noted (see 5.5).
In use, the eye is coherently coupled to the telescope, so it may be contended that the only valid test
would be one that included the eye: reference is made to the case of cascaded optical systems in the
introduction to ISO 9334. However, in observer tests using telescopes, improved performance has been
obtained with instruments with better measured OTF performance in a variety of tests, including
contrast sensitivity using sinusoidal grating targets, which confirms the value of OTF tests.
OTF tests also enable performance to be compared with that computed by the telescope designer and
provide effective quality assurance tests of production specimens.
When considering the details of tests, some features of the eye need to be borne in mind, especially its
ability to accommodate for varying object distances and to adjust the working aperture, varying the
iris size, according to the ambient illumination. Thus firstly, unlike the photographic lens testing case,
refocusing for off-axis tests is necessary. Secondly, the working aperture of the telescope, i.e. the exit
pupil diameter, will need to m
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