ISO/TR 8612:1997
(Main)Ophthalmic optics and instruments — Tonometers
Ophthalmic optics and instruments — Tonometers
Instruments ophtalmiques — Tonomètres
General Information
Relations
Standards Content (Sample)
TECHNICAL
ISOrm
REPORT 8612
First edition
1997-06-l 5
Ophthalmic instruments - Tonometers
Instruments ophtalmiques - Tonom&tres
Reference number
ISWTR 8612: 1997(E)
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lSO/TR 8612:1997(E)
Page
Contents
1 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2 Classification of tonometers . . . . . . . . . . . . . . . . . .
3 General design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I imits of error . . . . . . . . . . . . . . . . .
4 Specific construction requirements and
5 Marking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Annexes
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
A Principles of tonometry
15
B Criteria for the clinical evaluation of different types of tonometers
C Verification of mechanical and mechanical-electrical impression
19
tonometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D Verification of mechanical-optical applanation tonometers
having a constant diameter applanation circle . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
E Verification of air-impulse tonometers
45
F Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0 IS0 1997
All rights reserved. Unless otherwise specified, no part of this publication may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case Postale 56 l CH-1211 Geneve 20 l Switzerland
central @ iso.ch
Internet
x.400 c=ch; a=4OOnet; p=iso; o=isocs; s=central
Printed in Switzerland
ii
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@ IS0 ISORR 8612: 1997(E)
Foreword
IS0 (the international Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
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. IS0
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
The main task of technical committees is to prepare International
Standards, but in exceptional circumstances a technical committee may
propose the publication of a Technical Report of one of the following types:
- type 1, when the required support cannot be obtained for the
publication of an International Standard, despite repeated eff otts;
- type 2, when the subject is still under technical development or where
for any other reason there is the future but not immediate possibility of
an agreement on an International Standard;
- type 3, when a technical committee has collected data of a different
kind from that which is normally published as an International Standard
(“state of the art”, for example).
Technical Reports of types 1 and 2 are subject to review within three years
of publication, to decide whether they can be transformed into International
Standards. Technical Reports of type 3 do not necessarily have to be
reviewed until the data they provide are considered to be no longer valid or
useful.
ISOTTR 8612, which is a Technical Report of type 1, was prepared by
Technical Committee ISOUC 172, Optics and optical instruments,
Subcommittee SC 7, Ophthalmic optics and instruments.
This document is being issued as a type 1 Technical Report providing a
classification of tonometers and a description of the different principles of
tonometry, as well as a compilation of specific construction requirements
and test methods for these different types of tonometers.
At the time of publication of ISOmR 8612, work on an International
Standard on fundamental requirements and test methods for tonometers
has already been started and it is intended to withdraw this Technical
Report when the International Standard is published.
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ISO/TR 8612:1997(E) @ IS0
Introduction
The tonometer is one of the most important measuring instruments for the
detection and diagnosis of glaucoma. The term “glaucoma” indicates
diseases of the eye which may show an increase of the intraocular
pressure (IOP) as one of the common symptoms and which may lead to
lesions of the optic nerves and thus to scotomata and even blindness. The
development of a glaucoma can follow an increase in IOP before
pathological changes occur in the fundus of the eye. Results obtained by
the use of tonometers give an indirect measurement of IOP and these
results represent a close approach to the actual IOP.
Requirements relating to tonometers without subclause 4.5 (see [l], [2],
[lo], [l l] and [13]) and the instructions for their verification without
annex E were accepted at the meeting of the Committee on Standardiz-
ation of Tonometers (ICST) of the International Council of Ophthalmology
held in Paris in 1974.
The assessment of the IOP is made more difficult because each of these
indirect methods, owing to the contact with the eye as a physical system,
effects a change of volume and thus a pressure change as a result of the
force exerted on the eye during the measurement procedure. Hence, the
user does not measure the IOP in an unstressed eye but rather an
increased pressure raised more or less by the measuring procedure.
The different principles of tonometry are described in annex A. Procedures
for verifying requirements laid down for tonometers are described in
annexes C to E.
It is recognized that each tonometer type may employ different parameters
and/or correlations in order to assess IOP indirectly. Furthermore, within a
given type, variations in specific design are anticipated. It follows that each
type and/or design of a tonometer which fulfil the clinical criteria (annex B)
may require a test method unique to the design of the instrument and set of
physical criteria; by necessity, test method and requirements are clearly
specific to the design of the instrument. This Technical Report is not,
however, intended to preclude other types or designs not covered therein.
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TECHNICAL REPORT 0 IS0 lSO/TR 8612:1997(E)
Ophthalmic instruments - Tonometers
1 Scope
This Technical Report specifies requirements and test methods for instruments designed to determine intraocular
pressure (IOP). It lays down criteria for the clinical evaluation of different tonometer types (annex B), test methods
for tonometers (annexes C to E) and physical criteria (clause 4) which, when satisfied by the instrument under test,
verify that its measurement calibration meets the clinically relevant criteria.
This Technical Report is applicable to impression tonometers of the mechanical and mechanical-electrical type, to
applanation tonometers of the mechanical-optical and mechanical-electrical type as well as air-impulse tonometers.
2 Classification of tonometers
Mechanical and mechanical-electrical impression tonometers
2.1
Instruments which measure the deformation of the cornea resulting from the application of the tonometer itself or the
tonometer measuring head, i.e. the sinking-in of the plunger. The deformation is indicated by a digital or analog
display or by a recording device.
2.2 Mechanical-optical applanation tonometers
Instruments which measure either the force necessary to flatten the cornea with a pressure body to a diameter
between 2,5 mm and 4,0 mm depending on the type of instrument, or the diameter of the applanation circle at a
known measuring force, indicated in scale divisions by a digital display or by a recording device.
2.3 Electromechanical applanation tonometers
Instruments which, by means of a measuring head, measure the force necessary to flatten the cornea to the
diameter specified by the tonometer type. The force or the pressure is indicated in scale divisions by a digital display
or by a recording device.
2.4 Air-impulse tonometers
Instruments employing a brief air impulse of increasing force which either measure the time or measure the
instrument plenum chamber pressure necessary to deform the cornea to a consistent configuration. The time or the
pressure is indicated by a digital display or by a recording device.
Air-impulse tonometers do not mechanically touch the cornea during the measuring procedure.
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ISO/TR 8612:1997(E)
3 General design
3.1 The mechanical parts of the tonometers shall be composed of material with sufficient mechanical resistance
and invariability.
3.2 The electrical parts of the tonometers shall be composed of material with sufficient mechanical resistance and
electrical invariability.
3.3 The surfaces of the tonometer that are intended to come into contact with the cornea shall be composed of
rust-free and acid-resistant steel or of material which is inert to biological tissue.
4 Specific construction requirements and limits of error
4.1 Mechanical impression tonometers
4.1 .l Effective load
The effective load of the lever-pointer-plunger system when the tonometer is in a vertical position shall be:
- (55 + 0,15) g when indicating scale division “5”;
- (5,5 k 0,20) g when indicating scale division “10”.
4.1.2 Mass and additional loads
The mass of the tonometer, without handle, shall be (16,5 + 0,5) g.
The additional loads shall be
- with inscription 7,5: (2,0 + 0,02) g;
-
with inscription 10,O: (4,5 + 0,02) g;
- with inscription 15,0: (9,5 + 0,02) g.
4.1.3 Friction between plunger and plunger sleeve
When the tonometer is moved slowly and uniformly from the horizontal position into the vertical position with the
plunger on the upper stop, the plunger shall begin to slide into the footplate hole before the angle of the tonometer
axis to the horizontal exceeds 25O. During this manoeuvre the lever shall not touch the plunger,
NOTE - A drawing of footplate and plunger is given in figure C.5.
The tonometer shall be able to slide easily in its handle (see C.2.5).
Dimensions of footplate and plunger
4.1.4
The dimensions of the footplate shall be as given in table 1.
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ISOnR 8612: 1997( IE)
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- Dimensions of footplate
Table 1
Dimensions in millimetres
Feature Dimension
Diameter IO,1 &0,2
I
15,0 + 0,25
Radius of curvature of the spherical front surface
I
9,0 min.
Outside diameter of the spherical front surface, dmin
I I
3,3 max.
Either Diameter of the recess or counterbore on the front surface up to a minimum height h of
I,5 mm, &
0,2 max.
Radius of the inside edge curvature, r
3,7 max.
Or Diameter of the circle at the transition between footplate curvature and edge curvature of
the recess of counterbore (central area), ds
The dimensions of the plunger shall be as given in table 2.
Table 2 - Dimensions of plunger
Dimensions in millimetres
Dimension
Feature
I I
Diameter at the front surface up to a minimum height of I,5 mm 3,0 + 0,03
I I I
Radius of curvature of the spherical front surface
0,25 + 0,015
Radius of the edge curvature
I I I
Extension of the plunger below the spherical footplate, hmax 3,0 max.
I I
4.1.5 Surface condition
The front surfaces of the footplate and of the plunger shall be smooth when felt with a finger and, when examined by
unmagnified corrected vision under direct illumination, shall be free from surface imperfections that would damage
the eye. The outside surface of the front surface of the footplate and the inside edge at the recess of counterbore
shall be rounded.
4.1.6 Scale
The scale may be arranged parallel or inclined to the axis of the plunger. The scale shall be divided into at least 15
equal scale divisions (-1 to 15 or 0 to 15 respectively); the distance between two adjacent lines shall be equal to a
plunger displacement of 0,05 mm, so that the displacement of the plunger shall correspond to the values given in
table 3 depending on the number of scale divisions. The scale shall show integers only.
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lSO/TR 86123 997(E) @ IS0
Table 3 - Plunger displacement
Values in millimetres
Plunger
Scale division
displacement
values
from to
-1 5 0,30 +0,01
-1 10 0,55 If: 0,02
-1 15 0,80 +0,03
-1 18 0,95 +0,05
0 5 0,25 kO,Ol
0 10 0,5orto,o2
0 15 0,75 +0,03
0 18 0,90 +0,05
The division of the scale shall consist of lines. The lines shall be straight, of equal width and directed towards the
axis of the pointer. No line shall be wider than l/4 of the distance between two lines nor more than 0,25 mm.
4.1.7 Pointer
In the area of the scale, the pointer shall not be wider than the smallest width of a line in the area of the scale. If the
pointer moves over the scale, it shall overlap the shortest lines by at least one-third; the tip shall not extend beyond
the scale lines. The distance between pointer and the plane of the scale shall not be greater than I,0 mm in the area
of the scale.
4.1.8 Plunger
At some point between scale indications 5 and 10, the plunger axis and the lower surface of the lever shall form a
right angle at the point of contact.
4.1.9 Scale indication
When the instrument is tested on a test block with a radius of curvature of 14,75 mm, the pointer shall indicate
-1,O + - 0,2 on the scale; when tested on a test block with a radius of curvature of 16 mm, it shall indicate 0 k 0,2 on
the scale.
With the tonometer in position on the test block, the scale reading shall not vary by more than 0,4 scale divisions
when the plunger is turned or moved laterally or when the lever is moved laterally.
4.1 .lO Verticality of tonometer
When the tonometer is picked up without restraint at the holding point of the handle, the axis shall naturally assume
a vertical position.
4.1 .ll Recording device
If a recording device is used, the limits of error for the record shall be the same as for the scale readings.
4.2 Mechanical-electrical impression tonometers
4.2.1 Effective load
The effective load of the plunger measured with the instrument ready for use shall be (5,5 + 0,l) g within the
measuring range and with the measuring head in a vertical position.
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ISO/TR 8612: 1997(E)
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4.2.2 Mass and additional loads
The mass of the tonometer measuring head, without handle, shall be (16,5 + 0,5) g.
The additional loads shall be:
with inscription 75: (2,0 + 0,02) g;
-
- with inscription 10,O: (4,5 & 0,02) g;
with inscription 15,O: (9,5 + 0,02) g.
-
4.2.3 Friction between plunger and plunger sleeve
After the tonometer measuring head is moved slowly and uniformly from the horizontal position into the vertical
position after the plunger is moved 4 mm, the plunger shall begin to slide into the footplate hole before the angle of
the tonometer axis to the horizontal exceeds 25’.
The tonometer shall be able to slide easily in its handle.
4.2.4 Dimensions of footplate and plunger
The dimensions of the footplate shall be as given in table 1. The dimensions of the plunger shall be as given in
table 2.
4.2.5 Surface condition
The front surfaces of the footplate and of the plunger shall be smooth when felt with a finger and, when examined by
unmagnified corrected vision under direct illumination, shall be free from surface imperfections that would damage
the eye. The outside edge of the front surface of the footplate and the inside edge at the recess or counterbore shall
be rounded.
4.2.6 Scale
The scale shall be divided into at least 15 equal scale divisions (-1 to 15 or 0 to 15, respectively); the distance
between two adjacent lines shall be equal to a plunger displacement of 0,05 mm, so that the displacement of the
plunger shall correspond to the values given in table 3, depending on the number of scale divisions. The scale shall
show integers only.
If there is a digital read-out in a digital display, the last digit shall be changed by one unit for a displacement of the
plunger of not more than 0,005 mm.
The interval between the dividing lines shall be not less than 4 mm and may be subdivided. The division of the scale
shall consist of lines. The lines shall be straight, of equal width and directed towards the axis of the pointer. No line
shall be wider than % of the distance between two lines, nor more than 0,25 mm. The scale shall show integers only.
4.2.7 Pointer
In the area of the scale, the pointer shall be no wider than the smallest width of a line in the area of the scale. If the
pointer moves over the scale, it shall overlap the shortest lines by at least one-third; the tip shall not extend beyond
the scale lines.
4.2.8 Testing on test block
When the tonometer measuring head is placed on a test block having a curvature radius of 14,75 mm or 16,OO mm,
the scale reading shall be -I,0 or 0,O respectively, with a tolerance of + 0,2 of a scale interval or k 0,2 as indicated
by the final digits.
When the tonometer measuring head is placed on the attached test block marked “15”, the scale reading shall not
exceed 15 with a tolerance of + 0,2 of a scale interval or + 0,2 as indicated by the final digits.
5
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ISO/TR 8612: 1997(E) @ IS0
With the tonometer measuring head in position on the test block, the scale reading shall not vary by more than
0,2 scale divisions or + 0,2 as indicated by the final digits (i.e. 0,Ol mm displacement of the plunger) when the
plunger is turned or moved laterally.
4.2.9 Effect of variations in temperature or voltage on scale readings
4.2.9.1 At temperatures between 15 “C and 30 “C at constant operating voltage, the scale reading within the
measuring range shall not vary by more than + 0,2 of a scale interval or + 0,2 as indicated by the final digits
(0,Ol mm displacement of the plunger).
4.2.9.2 During a fluctuation in operating voltage of + 10 % at 20 “C, the scale reading shall not vary by more than
+, 0,2 of a scale interval or + 0,2 as indicated by the final digits (0,Ol mm displacement of the plunger).
4.2.10 Recording device
If a recording device is used, the limits of error for the record shall be the same as for the scale readings.
. .
If the tonometer is provided with a digital display, with each digit indicated seven linear segments, it shall be
bY
possible to check the correct functioning of every segment.
4.3 Mechanical-optical applanation tonometers
4.3.1 Diameter of applanation circle
Mechanical-optical applanation tonometers which measure the force needed to obtain a given area of applanation
shall have a constant value between 2,5 mm and 4,0 mm for the diameter of the applanation circle.
The tolerance for the diameter of the applanation circle shall be & 0,02 mm.
4.3.2 Surface of pressure body
The front surface of the pressure body shall be smooth when felt with a finger and, when examined by unmagnified
corrected vision under direct illumination, shall be free from surface imperfections that would damage the eye and
shall have a diameter of at least 6,0 mm.
4.3.3 Measuring force
The measuring force shall be continuously adjustable within the minimum range from 0 to 49,0 mN1), without the
range being altered and without the use of additional weights. The measured value of the force shall be clearly
legible on a linearly divided scale.
The change in force required to move the pressure body in the opposite direction (reverse span) at the point of
transition shall not exceed + 0,29 mN.
4.3.4 Scale
Lines shall be used as graduations on the measuring scale. The lines shall be straight, of equal width and shall be
engraved or otherwise permanently marked. No line shall be wider than l/4 of the distance between two lines.
1) 1 mN = IO-3 N; 1 N is that force which, when applied to a body having a mass of 1 kg, gives it an acceleration of 1 m.so2.
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ISO/TR 8612: 1997(E)
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One scale unit shall represent either 0,98 mN or 1,96 mN. The main scale graduations shall be numbered. The
width of the reference mark shall not be greater than the smallest width of the graduation lines on the measuring
scale.
4.3.5 Tolerance for measurement of force
for the measured value of the force
When the pressure body is adjusted to the verification position, the tolerance
within the measuring range shall be + 1,5 % of the nominal value, but not less than + 0,49 mN, over a temperature
range from 15 “C to 30 “C.
4.3.6 Recording device
If a recording device is used, the limits of error shall be the same as those specified in 4.3.5.
4.3.7 Tonometer used as limit gauge
If the tonometer is to be used as a limit gauge only, the diameter of the applanation circle and the measuring force
shall be constant. Tolerances specified in 4.3.1 and 4.3.5 shall be applied.
4.4 Mechanical-electrical applanation tonometers
4.4.1 Diameter of applanation circle
required in applanation, the diameter of the applanation circle
4.4.1 .l For tonometers which measure the force
shall have a constant value not less than 2,5 mm.
The tolerance for the diameter of the applanation circle shall be * 0,02 mm.
4.4.1.2 For tonometers which measure the diameter of the applanation circle, the force shall be constant. The
diameter of the applanation surface shall be measured and the tolerance specified in 4.4.1 .l shall be applied.
In determining the pressure (force/area), the diameter of the applanation surface area can be equal to the diameter
of the front surface of the measuring head.
4.4.2 Surface of pressure body
The front surface of the pressure body shall be smooth when felt with a finger and, when examined by unmagnified
corrected vision under direct illumination, shall be free from surface imperfections that would damage the eye and
shall have a diameter of at least 6,0 mm.
4.4.3 Measuring force
The measuring force shall be continuously adjustable within the minimum range from 0 to 49,0 mN, without the
range being altered and without the use of additional weights The measured value of the force shall be indicated on
a linear scale or typed in scale divisions using a recorder, or indicated digitally.
The determination of the force applied to the front surface of the pressure body within the circle of applanation shall
also be permitted to be made from a concentric circular plane the diameter of which is equal to or smaller than the
radius of the circle of applanation using a mechanical-electrical measuring device.
The change of force required to move the pressure body in the opposite direction (reverse span) at the point of
transition shall not exceed + 0,29 mN.
4.4.4 Scale
If the force applied to flatten the cornea is recorded on a scale, lines shall be used as graduations on the measuring
scale. The lines shall be straight, of equal width and shall be engraved or otherwise permanently marked. No line
shall be wider than 1/4 of the distance between two lines, nor more than 0,25 mm in width.
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ISO/“!-R 8612: 1997(E)
One scale unit shall represent either 0,98 mN or I,96 mN. The main scale graduations shall be numbered. The
width of the reference mark shall not be greater than the smallest width of the graduation lines on the measuring
scale.
4.4.5 Pointer
If the force applied to flatten the cornea is recorded on a scale, the pointer shall not be wider than the smallest width
of a line in the area of the scale. If the pointer moves over the scale, it shall overlap the shortest lines by at least
one-third; the tip shall not extend beyond the scale lines
4.4.6 Tolerance for measurement of force
When the pressure body is adjusted to the verification position of its range of movement, the tolerance for the
measured value of the force within the measuring range shall be +, 1,5 % of the nominal value, but not less than
+ 0,49 mN, over a temperature range from 15 “C to 30 “C.
4.4.7 Recording device
If a recording device is used, the limits of error shall be the same as those specified in 4.4.6.
4.5 Air-impulse tonometers
4.5.1 Alignment
Each type of air-impulse tonometer shall include mechanical and optical or opto-electronic means for achieving
alignment relative to the patient’s cornea, in three dimensions. The alignment shall be observed by means of an
optical or a camera/monitor system.
4.5.2 Indication of measurements
The display shall be digital.
In the case of segment digital display, it shall be possible to check the correct functioning of every segment of each
. .
drgrt .
The measuring range shall extend from “12” to at least “50”.
4.5.3 Verification of display
For the verification using apparatus such as that described in annex E, the correlation between the digital display
and the measured value of force in the optimum alignment of the system, the air impulse on a face 2,5 mm in
diameter for the named types shall be as given in table 4.
Table 4 - Correlation between digital display and measured value of the force of air impulse
for Non-Contact Tonometer II (NCT II) and XPERT Non-Contact Tonometer (XPERT NCT)
Force of air Digital Force of air Digital Force of air Digital Force of air
Digital
display impulse display impulse display impulse display impulse
mN reading
reading mN reading mN reading mN
21 31
990 16,0 41 23,0
22 32 16,7 23,7
12 997 42
195
IO,4 33
13 23 17,4 43 24,4
293
24 II,1 34 18,l 44 25,l
14
397
15 25 II,8 35 18,8 45 25,8
496
12,5 36
16 26 19,5 46 26,5
594
27 13,2 37 20,2 47 27,2
17
691
18 28 13,9 38 20,9 48 27,9
699
14,6 39
19 29 21,6 49 28,6
797
15,3 40 22,3 50 29,3
20 30
893
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a IS0 ISO/TR 8612:1997(E)
Other tables of correlation between digital display and measured value of the force of air impulse apply to other air-
impulse tonometers. In these cases the manufacturer is required to publish this table with a detailed description of
the verification procedure and apparatus.
4.5.4 Effect of variation in temperature or voltage on display
For digital display in the temperature range from 15 “C to 30 “C at constant operating voltage, the maximum
permissible errors are:
- between digital readings “12” and “30”: + 1 as indicated by the final digit;
- between digital display readings “31” and “50”: + 2 as indicated by the final digits.
During a fluctuation in the operating voltage of +, IO % at 20 “C, the maximum permissible error shall be +, 1 as
indicated by the final digits.
5 Marking
. Each tonometer shall be marked with the following information:
51
name of manufacturer or trademark;
a)
b) country of manufacture;
c) serial number.
5.2 On each applanation tonometer with constant diameter of the applanation circle and variable measuring force,
the diameter of the applanation circle shall be indicated, unless its value is 3,06 mm.
5.3 Each instrument and its necessary accessories, except for the pressure body of the mechanical-optical
applanation tonometers and the measuring head of the mechanical-electrical impression tonometers and
mechanical-electrical applanation tonometers, shall be marked with an individual serial number. Each tonometer
measuring head with its associated plunger shall be identically numbered. All other parts of tonometers shall not be
marked.
5.4 Each removable objective/tube unit shall bear the same number as the air impulse tonometer to which it
relates.
5.5 On each air-impulse tonometer, the measurement range shall be indicated as given in the following example:
“Measurement range from 12 to 50 on digital display”.
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%SO/TR $612:1997(E)
Annex A
(informative)
Principles of tonometry
A.1 General
The following principles will prove clinically useful in obtaining a measurement which is an acceptable correlation
with the IOP.
a) impression principle
A plunger deforms the cornea to obtain a state of quasi-equilibrium between external and internal pressure and then
the size of the “indentation” is measured.
b) Applanation principles
A plane surface is pressed against the cornea so as to determine the force necessary to flatten a defined area
(Goldmann principle) or to determine the size of a flattened area subjected to a defined force (Maklakoff principle).
Both principles are based on the Imbett-Fick law, which states that if a plane surface is applied with force F to a thin
spherical membrane within which a pressure pt exists at equilibrium the following equation is valid:
F
=-
. . .
(1)
Pt
A
where
A is the area of the applied surface;
pt is the pressure increase due to the measuring procedure (see figure A.1).
1 Pressure body 2 Membrane
Figure A.1 - General principle of tonometry
A.2 Impression tonometry
Using an impression tonometer, Friedenwald investiga
...
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