ISO 8600-5:2020

INTERNATIONAL ISO
STANDARD 8600-5
Second edition
Optics and photonics — Medical
endoscopes and endotherapy
devices —
Part 5:
Determination of optical resolution of
rigid endoscopes with optics
Optique et photonique — Endoscopes médicaux et dispositifs
d'endothérapie —
Partie 5: Détermination de la résolution optique des endoscopes
optiques rigides
PROOF/ÉPREUVE
Reference number
ISO 8600-5:2020(E)
©
ISO 2020

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ISO 8600-5:2020(E)

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Published in Switzerland
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ISO 8600-5:2020(E)

Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Test method . 4
4.1 General . 4
4.2 Apparatus . 5
5 Test report . 6
Annex A (informative) Measurement procedure for Characteristic A, limiting resolution .7
Annex B (informative) Measurement procedure for Characteristic B, Contrast Transfer Function .8
Annex C (informative) Measurement procedure for Characteristic C, Modulation Transfer
Function . 9
Annex D (normative) Contrast transfer function (C ) and modulation transfer function (M ) .10
TF TF
Annex E (informative) Conversion of spatial frequencies .15
Annex F (informative) Example of relay optics .17
Annex G (informative) Measurement frequency .18
Bibliography .19
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ISO 8600-5:2020(E)

Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
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).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www .iso .org/ patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the
World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/
iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 172 Optics and photonics, Subcommittee
SC 5, Microscopes and endoscopes.
This second edition cancels and replaces the first edition (ISO 8600-5:2005), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— document has been restructured;
— Clause 2 added;
— Clause 3 revised and updated;
— quality characteristics “Contrast Transfer Function” and “Modulation Transfer Function” as
measurement methods are introduced;
— Measurement with limiting resolution moved to informative Annex A;
— informative Annex B added;
— informative Annex C added;
— normative Annex D added;
— informative Annex E added;
— informative Annex F added;
— informative Annex G added.
A list of all parts in the ISO 8600 series can be found on the ISO website.
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ISO 8600-5:2020(E)

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.
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INTERNATIONAL STANDARD ISO 8600-5:2020(E)
Optics and photonics — Medical endoscopes and
endotherapy devices —
Part 5:
Determination of optical resolution of rigid endoscopes
with optics
1 Scope
This document applies to rigid endoscopes designed for use in the practice of medicine. Endoscopes
having a fibre-optic or opto-electronic imaging system are excluded. It specifies a test method for
determining the optical resolution of endoscopes.
This document provides a measurement method for characterizing three aspects of the optical
resolution of a rigid endoscope. Characteristic A is used to provide a simple measurement of the limiting
resolution of the endoscope image. Characteristic B provides a measurement of low spatial frequency
resolution and characterizes the sharpness, or contrast, of the endoscope image. Characteristic C
provides a measurement of the spatial frequency response of the endoscope image.
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 12233:2017, Photography — Electronic still picture imaging — Resolution and spatial frequency
responses
ISO 15529:2010, Optics and photonics — Optical transfer function — Principles of measurement of
modulation transfer function (MTF) of sampled imaging systems
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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/
3.1
angular limiting resolution
smallest angle whose vertex is at the entrance pupil (can be approximated by the distal window surface
if the target distance from the distal window is significantly larger than the distance between the distal
window surface and the entrance pupil) of the endoscope at which a line pair (lp) at a given working
distance d can just be resolved with normal visual acuity, with the unit of degrees/lp
Note 1 to entry: Angular limiting resolution is calculated using the formula
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1
α =arctan
dr⋅ d
()
where r(d) is the limiting resolution.
3.2
camera
image detector used in the measurement, which is connected to the endoscope under test via
coupling optics
Note 1 to entry: As used in this standard, the camera includes an image detector (typically CCD or CMOS image
sensor), supporting electronics, and firmware/software used to obtain a digital sampling of the image formed by
the coupling optics.
3.3
contrast
ratio of the difference between the intensities of the brightest and the darkest regions of a bar test
target with square-wave modulation or its image divided by the sum of the intensities of the brightest
and the darkest regions of the target or its image, and subsequently multiplied by 100 to measure as a
percentage
Note 1 to entry: Contrast is given by
II−
maxmin
C()% = ×100
II+
maxmin
where I and I are the intensities of the brightest and darkest regions of the target or its image, respectively.
max min
3.4
contrast transfer function
C
TF
plot of contrast (3.3) transfer factor, C , as a function of spatial frequency (3.13), u
TF
Note 1 to entry: Contrast transfer function is given by
Cu()
out
Cu()=
TF
Cu()
in
where Cu is the output contrast (i.e. the image contrast) and Cu is the input contrast (i.e. the target
() ()
out in
contrast).
Note 2 to entry: If the target has high contrast so that Cu is close to one, Cu can be approximated by
() ()
TF
in
Cu . If C is normalized to 1 at zero frequency, constant Cu will get factored out in the normalization.
() ()
TF
out in
The C of an endoscope can be obtained by measuring a series of square-wave bar targets with different spatial
TF
frequencies. The low-frequency contrast of the target as imaged through the endoscope may be measured with
target patches of light and dark large enough that the intensity profile through the patch clearly reaches a
steady value.
Note 3 to entry: For the purposes of the test specified in Characteristic B of this document, the “targets” will be
the bar test targets specified in 4.2.5.2 and the measured results will be the C .
TF
3.5
limiting resolution
r(d)
maximum number of line pairs per mm (lp/mm) which can be resolved at a given working distance d of
the endoscope
Note 1 to entry: The limiting resolution is only applicable to Characteristic A.
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3.6
maximum image height
radius of a circle which circumscribes the image
Note 1 to entry: If the image is rectangular, the maximum image height is half of the diagonal.
Note 2 to entry: If the image is circular, the maximum image height is the radius of the image circle.
3.7
maximum object height
radius of a circle which circumscribes the portion of the object which can be imaged by the endoscope
Note 1 to entry: If the image is rectangular, the maximum object height is half of the distance between the object
points which map to the corners of the image rectangle.
Note 2 to entry: If the image is circular, the maximum object height is the radius of the object space circle which
maps to the image circle.
3.8
modulation
M
ratio of the difference between the intensities of the brightest and the darkest regions of a sinusoidal
test target or its image divided by the sum of the intensities of the brightest and the darkest regions of
the target or its image, and subsequently multiplied by 100 to measure as a percentage
Note 1 to entry: Modulation is given by
II−
maxmin
M % = ×100
()
II+
maxmin
where I and I are the intensities of the brightest and darkest regions of the image, respectively.
max min
3.9
modulation transfer function
M
TF
plot of the modulation (3.8) transfer factor, M , as a function of spatial frequency (3.13) u, for a sine-
TF
wave target
Note 1 to entry: M is a measure of the transfer of modulation from the object to the image.
TF
Note 2 to entry: Modulation transfer factor is given by
Mu()
out
Mu()=
TF
Mu
()
in
where Mu() is the output modulation (i.e. the image modulation) and Mu() is the input modulation (i.e.
out in
the target modulation).
Note 3 to entry: If the target has high contrast so that Mu() is close to one, Mu() can be approximated by
in TF
Mu() . On the other hand, M curves are always normalized to 1 at zero frequency, which make a constant
TF
out
Mu() negligible. The M of an endoscope can be obtained by measuring a series of sinusoidal targets with
TF
in
different spatial frequencies, or by other methods such as a slanted-edge target as discussed in 4.2.5.3.
3.10
off-axis limiting resolution
limiting resolution (3.5) at an image point at 70 % of the maximum image height (3.6)
Note 1 to entry: See Figure A.1.
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3.11
on-axis limiting resolution
limiting resolution (3.5) at the image centre
Note 1 to entry: See Figure A.1.
3.12
optical resolution
numerical measure of the image quality of an optical system
3.13
spatial frequency
u
measure of how often a structure (e.g., sine-wave or square-wave bars) on the target or image repeats
per unit distance or angle
Note 1 to entry: Spatial frequency has different units (e.g. lp/mm, lp/degree, lp/pixel), depending on whether the
target period is measured in distance or angle units.
Note 2 to entry: The narrower definition of spatial frequency only refers to the sinusoidal components of the
structure.
Note 3 to entry: The units of lp and cycles are often used interchangeably; in this standard, the unit of lp is used
when referring to a square-wave or bar target (limiting resolution and C measurement), and cycles are used
TF
when referring to sine wave components (M ) and to spatial frequencies in general.
TF
3.14
working distance
d
design distance defined by the manufacturer between the object and distal end of the endoscope
Note 1 to entry: For the purposes of the test specified in this document the “object” will be the resolution target
specified in 4.2.5.
4 Test method
4.1 General
Three characteristics of endoscope resolution are given in the method below. At least one characteristic
should be measured. Table 1 may be used as a guide to determine the appropriate characteristic. See
Annex A, B and C for acceptable test methods. Other methods of measuring the characteristics are
allowed; if an alternative method is used, it shall be described in the test report.
Table 1 — Characteristics and measuring
Characteristic Where Appropriate
A single-valued subjective metric for rapid manufacturing testing, quality
assurance (e.g. in end of line endoscope assembly testing), or for providing
A – Limiting Resolution
a simple metric that can be easily understood by end users. See Annex A for
information on the measurement procedure.
A multi-valued objective metric for device validation and verification. It is less
scientifically sophisticated than M , but it is mathematically simple, can be
TF
B – Contrast Transfer Function easily understood by end users, and can be measured with a square-wave tar-
(C ) get that can be easily produced. Multiple target images are needed to obtain
TF
C measurements at different spatial frequencies. See Annex B for informa-
TF
tion on the measurement procedure.
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Table 1 (continued)
Characteristic Where Appropriate
A multi-valued objective metric where all values can be obtained from a sin-
gle target; most used in engineering (e.g. design verification), diagnostic, and
C – Modulation Transfer Func-
image evaluation purposes and serves as an umbrella function to derive other
tion (M )
TF
metrics (e.g. sharpness, acutance). It is widely used in scientific fields. See
Annex C for information on the measurement procedure.
NOTE Annex E gives advice on the conversion of spatial frequencies. Annex G shows examples for relay optics.
4.2 Apparatus
4.2.1 Optical bench/optical rail, with mounting apparatus for endoscope and resolution target.
4.2.2 Coupling optics, to magnify a portion of the endoscope’s field of view and image it onto the
camera. Examples of coupling optics designed to meet the requirements of endoscope resolution
measurement are discussed in Annex F. The coupling optics shall not change the measured limiting
resolution significantly (Characteristic A only). The entrance pupil diameter of the coupling optics shall
be larger than the exit pupil of the endoscope; the coupling optics shall not cut off rays of the endoscope.
On-axis and off-axis measurement shall be made at the same focus.
NOTE The coupling optics still can have a significant influence on the result, because of coherent coupling
between the coupling optics and the endoscope.
4.2.3 Camera, with monitor and means of digitally recording the relative intensity of each pixel. As
described in D.4, the response of the system shall be maintained within the linear operating range of the
camera (e.g. any sharpening or image enhancement is turned off, and gamma is set to 1,0).
NOTE If the camera cannot be set within the linear operating range, the intensity of a target image can be
converted to linear intensity based on the opto-electric conversion function (OECF) of the camera. The camera
OECF can be measured based on the ISO 14524.
4.2.4 Light source, white light, unless the endoscope is specifically designed for a specific wavelength,
in which case this specific wavelength should be used.
NOTE It is acceptable to use a magnified image of a part of the endoscope’s field of view containing the target,
projected onto the test camera, to reduce the resolution requirements of the camera. Examples of coupling optics
designed to meet the requirements of endoscope resolution measurement are discussed in Annex F.
4.2.5 Resolution target
4.2.5.1 Characteristic A: Resolution target, having adequately graduated black and white test
patterns arranged at least in two directions, tangential and sagittal. Note that, for inclined direction of
view endoscopes (endoscopes having a non-zero direction of view), tangential and sagittal are defined
with respect to the direction of view. A resolution target consists exclusively of on-axis and off-axis test
patterns arranged as in Figure A.1. Resolution test patterns consist of two transmittance or reflective
values (Figure A.2). Alternatively, a single test pattern may be used if it is moved perpendicular to the
optical axis to obtain off-axis measurements. It shall be ensured that the endoscope’s resolution is
determined and not the resolution of the target (see D.5).
4.2.5.2 Characteristic B: C target, having a black and white pattern that consists exclusively of
TF
two transmittance or reflectance values (Figure A.2). The surface of a reflectance target should be a
Lambertian surface. The resolution target should contain on-axis and off-axis test patterns arranged as
in Figure A.1. Alternatively, a single test pattern may be used if it is moved perpendicular to the optical
axis to obtain off-axis measurements. It shall be ensured that the endoscope’s resolution is determined
and not the resolution of the target (see D.5).
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4.2.5.3 Characteristic C: M target, having slanted edge, slit, or pinhole meeting the recommendations
TF
for edge roughness and contrast and corrected if necessary, as described in D.5. Either a crossed target or
a means for rotating the target to two perpendicular orientations shall be provided.
5 Test report
Any test report resulting from following the test method described in Clause 4 shall contain at least the
following details:
a) a reference to this document, i.e. ISO 8600-5:—;
b) laboratory or company carrying out the test;
c) name of the testing engineer;
d) place and date of test;
e) standard used (e.g. ISO 15529, ISO 12233).
f) type of endoscope, manufacturer, model and serial numbers;
g) test characteristic used (A, B or C), including a reference to the clause which explains how the
results were calculated;
NOTE Results will be different for each method.
h) description of alternative test method, if used, or deviations from the procedure
i) any unusual features observed
j) test targets used and, if off-axis target orientation is not according to Figure A.1, the target
orientation used;
k) coupling optics used;
l) distance between target and distal end of endoscope;
m) light source used and light source spectral curve;
n) camera used and camera spectral response;
o) value of on-axis limiting resolution and angular limiting resolution (Characteristic A only) or on-
axis C (Characteristic B only) or M (Characteristic C only). If Characteristic B (C ) is reported,
TF TF TF
the measurement shall include at least the spatial frequency of 40 lp/maximum object height. If
Characteristic C (M ) is reported, the measurement shall include at least the spatial frequency of
TF
40 cycles/maximum image height ;
p) single values and averaged value of off-axis limiting resolution and angular resolution
(Characteristic A only) or off-axis C (Characteristic B only) or M (Characteristic C only). If
TF TF
Characteristic B (C ) is reported, the measurement shall include at least the spatial frequency of
TF
40 lp / maximum object height. If Characteristic C (M ) is reported, the measurement shall include
TF
at least the spatial frequency of 40 cycles / maximum image height;
q) Criterion used for assessing best focus (e.g. maximize on-axis value, maximize average of on-axis
and off-axis values, maximize lowest off-axis value, etc.);
r) if Characteristic C has been used: description of test target, equipment and procedure. If an edge-
based target is used, the contrast shall be reported;
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Annex A
(informative)

Measurement procedure for Characteristic A, limiting resolution
a) Mount the endoscope on the optical bench.
b) Attach the target to a diffusive plate and mount it on the optical bench at the working distance d.
Align the target at an angle perpendicular to the endoscope’s direction of view. Adjust the suitable
test pattern on the target to the image points to be tested (see Figure A.1). The target should be
uniformly illuminated from behind with a white LED backlight or similar spatially-uniform white
light source. Front illumination is permissible only if the target surface is a Lambertian surface.
c) Attach the camera and coupling optics to the endoscope. Adjust the focus to maximize resolution.
d) Display the target image on the monitor or print it with a printer.
e) Determine the on-axis (point ‘A’ in Figure A.1) resolution of the endoscope in lp/mm in object space.
The closest spacing resolution target group which can just be perceived shall be recorded.
f) Determine the off-axis resolution of the endoscope in lp/mm in object space. For this the image is
to be divided into four congruent quadrants, each of them containing one image point to be tested
(points ‘B1’ through ‘B4’ in Figure A.1). The limiting resolution is determined for all image points
‘B1’ through ‘B4’. At each off-axis image point the resolution target group at all pattern directions
which can just be perceived is recorded. The results at all off-axis image points are averaged
and reported independently. If the resolution target patterns were arranged to measure sagittal
and tangential resolution separately, then these measurements are independently averaged and
reported.
g) Calculate the angular limiting resolution for both on-axis and off-axis image points.
Figure A.1 — Image points to be tested for optical resolution (B1, B2, B3 and B4 are at 70 % of
the maximum image height from the image centre A)
Figure A.2 — Typical limiting resolution and C test targets
TF
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Annex B
(informative)

Measurement procedure for Characteristic B, Contrast Transfer
Function
a) Attach the coupling optics to the camera. Mount the endoscope to the camera/coupling optics on
the optical bench.
b) Attach a bar target from the set of bar targets for the testing of the endoscope to a diffuse plate
and mount it on the optical bench at the working distance of the endoscope. Align the bar target
at an angle perpendicular to the endoscope's direction of view. Evenly illuminate the bar target
from behind with a white LED backlight or similar spatially-uniform white light source. Front
illumination is permissible only if the target surface is a Lambertian surface.
c) Adjust the focus to maximize resolution.
d) Digitally capture the camera image. If individual measurements are to be made with a small target
moved to the correct field position, the target is moved in a plane perpendicular to the endoscope's
direction of view and camera images are digitally captured for each off-axis field.
e) Calculate and record the on-axis (point ‘A’ in Figure A.1) contrast of the endoscope system (C ).
on-axis
f) Calculate and record the off-axis (points ‘B1’ through ‘B4’ in Figure A.1) contrast of the endoscope
system (C ). For this the image is to be divided into four congruent quadrants, each of them
off-axis
containing one image region to be tested. The C are calculated and recorded for all off-axis
off-axis
image points ‘B1’ through ‘B4’. The results at all off-axis image points are averaged and reported
independently. If the resolution target patterns were arranged to measure sagittal and tangential
resolution separately, then these measurements are independently averaged and reported.
g) Check that the combination of the camera and endoscope being tested meets the requirements for
endoscope optical cut-off frequency described in D.3.
h) Repeat above steps for each bar target to measure C and C values at desired spatial
on-axis off-axis
frequencies.
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Annex C
(informative)

Measurement procedure for Characteristic C, Modulation Transfer
Function
a) Attach the coupling optics to the camera. Mount the endoscope to the camera/coupling optics on
the optical bench.
b) Attach the target to a diffuse plate and mount it on the optical bench at the working distance of the
endoscope. Align the target at an angle perpendicular to the endoscope's direction of view. Evenly
illuminate the target from behind with a white LED backlight or similar spatially-uniform white
light source. Front illumination is permissible only if the target surface is a Lambertian surface.
c) Adjust the focus to maximize resolution.
d) Adjust the system to bring the on-axis point (point ‘A’ in Figure A.1) onto the measurement area of
the camera, and digitally capture the camera image.
e) Calculate and record the on-axis M of the endoscope (M ).
TF TF,on-axis
f) Adjust the system to bring each of the off-axis points (points ‘B1’ through ‘B4’ in Figure A.1) onto
the measurement area of the camera, and digitally capture the camera image for each of the points.
g) Calculate and record the off-axis M of the endoscope (M ), for each of the off-axis points.
TF TF,off-axis
The M is calculated and recorded for all off-axis image points ‘B1’ through ‘B4’.
TF,off
...