SIST EN ISO 14880-4:2006
(Main)Optics and photonics - Microlens arrays - Part 4: Test methods for geometrical properties (ISO 14880-4:2006)
Optics and photonics - Microlens arrays - Part 4: Test methods for geometrical properties (ISO 14880-4:2006)
ISO 14880-4:2006 specifies methods for testing geometrical properties of microlenses in microlens arrays. It is applicable to microlens arrays with very small lenses formed on one or more surfaces of a common substrate and to graded index microlenses.
Optik und Photonik - Mikrolinsenarrays - Teil 4: Prüfverfahren für geometrische Eigenschaften (ISO 14880-4:2006)
Dieser Teil der ISO 14880 Norm legt Verfahren zum Prüfen der geometrischen Eigenschaften von Mikrolinsen in Mikrolinsenarrays fest. Die Norm gilt für Mikrolinsenarrays mit sehr kleinen Linsen, die auf einer oder mehreren Oberflächen eines gemeinsamen Substrats angeordnet sind, und für Gradientenindex-Mikrolinsen.
Optique et photonique - Réseaux de microlentilles - Partie 4: Méthodes d'essai pour les propriétés géométriques (ISO 14880-4:2006)
L'ISO 14880-4:2006 spécifie des méthodes d'essai pour les propriétés géométriques des microlentilles dans les réseaux de microlentilles. Elle s'applique aux réseaux de microlentilles avec de très petites lentilles qui composent une ou plusieurs surfaces d'un substrat commun et aux microlentilles à gradient d'indice.
Optika in fotonska tehnologija - Vrste mikroleč - 4. del: Preskusne metode za geometrične lastnosti (ISO 14880-4:2006)
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
SIST EN ISO 14880-4:2006
01-julij-2006
2SWLNDLQIRWRQVNDWHKQRORJLMD9UVWHPLNUROHþGHO 3UHVNXVQHPHWRGH]D
JHRPHWULþQHODVWQRVWL ,62
Optics and photonics - Microlens arrays - Part 4: Test methods for geometrical properties
(ISO 14880-4:2006)
Optik und Photonik - Mikrolinsenarrays - Teil 4: Prüfverfahren für geometrische
Eigenschaften (ISO 14880-4:2006)
Optique et photonique - Réseaux de microlentilles - Partie 4: Méthodes d'essai pour les
propriétés géométriques (ISO 14880-4:2006)
Ta slovenski standard je istoveten z: EN ISO 14880-4:2006
ICS:
31.260 Optoelektronika, laserska Optoelectronics. Laser
oprema equipment
SIST EN ISO 14880-4:2006 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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EUROPEAN STANDARD
EN ISO 14880-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2006
ICS 31.260
English Version
Optics and photonics - Microlens arrays - Part 4: Test methods
for geometrical properties (ISO 14880-4:2006)
Optique et photonique - Réseaux de microlentilles - Partie Optik und Photonik - Mikrolinsenarrays - Teil 4:
4: Méthodes d'essai pour les propriétés géométriques (ISO Prüfverfahren für geometrische Eigenschaften (ISO 14880-
14880-4:2006) 4:2006)
This European Standard was approved by CEN on 3 May 2006.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Central Secretariat or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Central Secretariat has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania,
Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2006 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 14880-4:2006: E
worldwide for CEN national Members.
---------------------- Page: 2 ----------------------
EN ISO 14880-4:2006 (E)
Foreword
This document (EN ISO 14880-4:2006) has been prepared by Technical Committee ISO/TC 172
"Optics and optical instruments" in collaboration with Technical Committee CEN/TC 123 "Lasers
and photonics ", the secretariat of which is held by DIN.
This European Standard shall be given the status of a national standard, either by publication of
an identical text or by endorsement, at the latest by December 2006, and conflicting national
standards shall be withdrawn at the latest by December 2006.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of
the following countries are bound to implement this European Standard: Austria, Belgium,
Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary,
Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland,
Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Endorsement notice
The text of ISO 14880-4:2006 has been approved by CEN as EN ISO 14880-4:2006 without any
modifications.
2
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INTERNATIONAL ISO
STANDARD 14880-4
First edition
2006-06-01
Optics and photonics — Microlens
arrays —
Part 4:
Test methods for geometrical properties
Optique et photonique — Réseaux de microlentilles —
Partie 4: Méthodes d'essai pour les propriétés géométriques
Reference number
ISO 14880-4:2006(E)
©
ISO 2006
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ISO 14880-4:2006(E)
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ii © ISO 2006 – All rights reserved
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ISO 14880-4:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols. 1
4 Coordinate system. 3
5 Test methods. 4
5.1 Pitch and surface modulation depth measurement . 4
5.2 Physical thickness. 9
5.3 Radius of curvature . 9
5.4 Surface preparation of microlens array for measurement . 12
6 Procedure . 13
6.1 Measurement of pitch and surface modulation depth (lens sag) . 13
6.2 Measurement of physical thickness . 13
6.3 Measurement of radius of curvature. 13
7 Results and uncertainties . 13
8 Test report . 14
Annex A (normative) Measurement with a Fizeau interferometer system . 16
Annex B (informative) Uniformity of array spacing . 19
Bibliography . 22
© ISO 2006 – All rights reserved iii
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ISO 14880-4:2006(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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 14880-4 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 9,
Electro-optical systems.
ISO 14880 consists of the following parts, under the general title Optics and photonics — Microlens arrays:
⎯ Part 1: Vocabulary
⎯ Part 2: Test methods for wavefront aberrations
⎯ Part 3: Test methods for optical properties other than wavefront aberrations
⎯ Part 4: Test methods for geometrical properties
iv © ISO 2006 – All rights reserved
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ISO 14880-4:2006(E)
Introduction
This part of ISO 14880 specifies methods for testing geometrical properties of microlens arrays. Examples of
applications for microlens arrays include three-dimensional displays, coupling optics associated with arrayed
light sources and photo-detectors, enhanced optics for liquid crystal displays, and optical parallel processor
elements.
The market in microlens arrays has generated a need for agreement on basic terminology and test methods.
Standard terminology and clear definitions are needed not only to promote applications but also to encourage
scientists and engineers to exchange ideas and new concepts based on common understanding.
This part of ISO 14880 contributes to the purpose of the series of ISO 14880 standards, which is to improve
the compatibility and interchangeability of lens arrays from different suppliers and to enhance development of
the technology using microlens arrays.
The measurement of physical characteristics of pitch and surface modulation depth can be made using a
stylus instrument and non-contact optical probe system. Physical thickness can be measured with a
micrometer. The measurement processes are described in the body of this part of ISO 14880.
© ISO 2006 – All rights reserved v
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INTERNATIONAL STANDARD ISO 14880-4:2006(E)
Optics and photonics — Microlens arrays —
Part 4:
Test methods for geometrical properties
1 Scope
This part of ISO 14880 specifies methods for testing geometrical properties of microlenses in microlens arrays.
It is applicable to microlens arrays with very small lenses formed on one or more surfaces of a common
substrate and to graded index microlenses.
2 Normative references
The following referenced documents are indispensable for the application 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 14880-1, Optics and photonics — Microlens arrays — Part 1. Vocabulary
3 Terms, definitions and symbols
For the purposes of this document, the terms, definitions given in ISO 14880-1 and the following apply.
NOTE 1 The symbols adopted for this part of ISO 14880 are chosen for clarity in this application to microlens arrays
but some may not be those commonly used for surface texture measurement.
NOTE 2 The parameters P , P and h are used in this part of ISO 14880 to describe geometrical parameters
x y
encountered in the measurement of surface texture. P , P are spacing parameters and are defined as the average value
x y
of the length of the mean line section containing a profile peak and adjacent valley. An amplitude parameter, h, is defined
as the average difference between peak of the lens profile and the rim. Figure 1 illustrates the geometrical properties of
microlens arrays which are to be measured.
3.1
pitch
P , P
x y
distance between the centres of adjacent lenses which may vary across and will vary with direction
See Figure 1.
NOTE 1 The pitch is expressed in millimetres.
[ISO 14880-1:2001, term 6.2.1.5]
NOTE 2 For a stylus instrument this will generally equate to the mean width of the profile elements calculated from the
roughness profile, RS (see 3.2.2 and 4.3.1 in ISO 4287:1997).
m
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ISO 14880-4:2006(E)
3.2
surface modulation depth
h
peak-to-valley variation of the surface height
See Figure 1.
NOTE 1 For a purely refractive microlens, this will be the same as the lens sag.
NOTE 2 The surface modulation depth is expressed in millimetres.
[ISO 14880-1:2001, term 6.2.1.8]
NOTE 3 For stylus instruments this will generally equate to Rz (see 4.1.3 in ISO 4287:1997).
3.3
physical thickness
T
c
maximum local thickness of the array
See Figure 1.
NOTE The physical thickness is expressed in millimetres.
[ISO 14880-1:2001; term 6.2.1.9]
3.4
radius of curvature
R
c
distance from the vertex of the microlens to the centre of curvature of the lens surface
See Figure 1.
NOTE 1 The radius of curvature is expressed in millimetres.
[ISO 14880-1:2001; term 6.1.4]
NOTE 2 For rotationally invariant microlenses or cylindrical microlenses.
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ISO 14880-4:2006(E)
Key
1 substrate
T physical thickness
c
R radius of curvature
c
P , P pitch
x y
h surface modulation depth (lens sag)
Figure 1 — Geometrical parameters of microlens arrays
4 Coordinate system
To measure the geometrical properties of a microlens array, a Cartesian coordinate system is used, as shown
in Figure 2. In a right-handed Cartesian set, the x- and y-axis lie in the substrate plane and the x-axis provides
the direction of trace. The z-axis is the outward direction from the material to the surrounding medium.
Key
1 substrate
2 microlens
3 light pass
Figure 2 — Microlens array with a Cartesian coordinate system
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ISO 14880-4:2006(E)
5 Test methods
5.1 Pitch and surface modulation depth measurement
5.1.1 Use of stylus instrument
5.1.1.1 Principle
The basic principle using a stylus instrument is to obtain a profile of the surface of the array. Care shall be
taken to ensure that the profile passes through the centre of each lens and that the stylus remains in contact
with the surface throughout the measurement process. This enables the pitch and surface modulation depth to
be determined.
5.1.1.2 Set-up and preparation
The measurement of the geometrical characteristics of a microlens array is similar in principle to the
measurement of any surface using a stylus instrument. A typical stylus instrument consists of a stylus that
physically contacts the surface and a transducer to convert its vertical movement into an electrical signal.
Other components can be seen in Figure 3 and include the following: a pick-up, driven by a motor and
gearbox, which draws the stylus over the surface at a constant speed; an electronic amplifier to boost the
signal from the stylus transducer to a useful level; a device for recording the amplified signal or a computer
that automates the data collection.
The part of the stylus in contact with the surface of the array is usually a diamond tip with a carefully
manufactured profile. Owing to their finite shape, some styli on some arrays may not penetrate into valleys
and will give a distorted or filtered measurement of the surface. The effect of the stylus forces can have a
significant influence on the measurement results. Too high a force can cause damage to the surface of the
array. Too low a force and the stylus will not stay reliably in contact with the surface.
The stylus instrument shall be used in an environment that is as free as possible from dust, vibration and
direct sunlight in a location where the ambient temperature is maintained in the range 20 °C ± 5 °C (with a
condensation-free humidity below 70 % relative humidity). Remove any gross contamination from the surface
of the instrument preferably by blowing the surface with filtered air. Any oil or grease may be removed using a
suitable solvent.
Due consideration shall be given for testing under more adverse conditions.
4 © ISO 2006 – All rights reserved
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ISO 14880-4:2006(E)
Key
1 base
2 fixture
3 microlens under test
4 stylus
5 probe (pick-up)
6 measurement loop
7 column
8 drive unit
Figure 3 — Elements of a typical stylus instrument
The electrical unit on the stylus instrument shall be switched on at least one hour before any measurements
take place. This will allow time for the instrument to stabilize (the manufacturer’s instructions will normally
specify a minimum stabilization time for a given instrument). Calibration of the instrument is essential prior to
measurement. Before calibration of the instrument takes place the stylus should be checked for signs of wear
or damage. A damaged stylus tip can lead to serious errors.
After measurement of the calibration artefact the indicated value shall be compared with the value attached to
the test object. If the measured value differs from the value that is shown on the calibration certificate then re-
calibration is required.
© ISO 2006 – All rights reserved 5
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ISO 14880-4:2006(E)
5.1.1.3 Stylus size and shape
It is important that the dimension and shape of the stylus are chosen appropriately as this can affect the
accuracy of the traced profile in a number of ways. On arrays with deep, narrow valleys the stylus may not be
able to penetrate fully to the bottom because either the tip radius or the flank angle of the stylus is too large. In
such cases, the value of the surface modulation depth will be smaller than the true value. The ideal stylus
shape is a cone with a spherical tip. This usually has a cone angle of either 60° or 90° with a typical tip radius
of 1 µm, 2 µm, 5 µm or 10 µm.
5.1.2 Use of confocal microscope
5.1.2.1 Principle
The confocal principle can be used for the measurement of surface topography. Depth is discriminated by
moving the surface of the object through focus and measuring the reflected intensity using a detector and
confocal pinhole. When the object point lies at the focus, the maximum intensity is detected whereas the
signal is reduced when the object point is displaced from the focus. The principle has been established in the
scanning confocal microscope. By scanning an imaged light spot over the object an area is measured point by
point.
5.1.2.2 Set-up and preparation
The principle of the confocal microscope has been developed by generating an array of light spots on the
object using a multiple pinhole mask (Nipkow disc) which allows for parallel data acquisition of multiple object
points. The Nipkow disc can be replaced by a microlens array in order to improve the light efficiency, as
shown in Figure 4.
6 © ISO 2006 – All rights reserved
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ISO 14880-4:2006(E)
Key
1 light source
2 beam splitter
3 rotating microlens disc (analogue to a Nipkow disc)
4 objective
5 sample
6 imaging lens
7 pinhole
8 detector
Figure 4 — Confocal microscope measurement System A
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ISO 14880-4:2006(E)
Figure 5 shows another configuration of the confocal microscope using a microlens array and a pinhole array.
It will enhance the optical radiation collection efficiency and improve the scanning speed, which depends on
the pinhole array with parallel scanning. An example of a measurement is shown in Figure 6.
Key
1 light source
2 rotating microlens disc
3 beam splitter
4 pinhole array (Nipkow disc)
5 objective
6 sample
7 imaging lens
8 charge-coupled device (CCD) camera
Figure 5 — Confocal microscope measurement System B
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ISO 14880-4:2006(E)
Dimensions in micrometres
Figure 6 — Example of the microlens array surface structure
using a confocal microscope measurement system
5.2 Physical thickness
5.2.1 Principle
The two anvils of the micrometer contact the top and bottom surfaces of the array to measure the physical
thickness of the array at a given point. The anvils of the micrometer may be parallel or spherical.
5.2.2 Set-up and preparation
The micrometer shall be calibrated using a gauge block or a similar procedure prior to the measurement.
Thoroughly wipe the spindle and the measuring faces of the micrometer using clean lint free paper or cloth.
Leave the instrument and the microlens array long enough to adjust to room temperature (at least 1 h). Care
shall be taken to ensure that the micrometer is not subject to sudden temperature changes, direct sunlight,
radiant heat or air currents that may cause significant temperature variations.
5.3 Radius of curvature
5.3.1 Principle
The basic principle is to locate, by optical means as shown in Figure 7, the vertex of the microlens under test.
The displacement necessary to locate the focal position or the centre of curvature of the surface is then
measured. This enables the radius of curvature R to be determined respectively. Care shall be taken to avoid
c
incorrect settings b) and c) as shown in Figure 7.
NOTE It is only possible to locate the centre of curvature of the test surface using this method if the lens surface is
spherical. Otherwise light is not retroreflected to form a confocal image. If it is suspected that the test surface is not
spherical, the shape can be estimated from a zonal analysis using interferometry.
© ISO 2006 – All rights reserved 9
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ISO 14880-4:2006(E)
a) Example of correct setting
b) Example of incorrect setting
c) Example of incorrect setting
Key
1 substrate surface
2 optical probe
R radius of curvature
c
Figure 7 — Location of the centre of curvature and the spherical lens surface with an optical probe
10 © ISO 2006 – All rights reserved
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ISO 14880-4:2006(E)
5.3.2 Measurement arrangement and test equipment
5.3.2.1 General
The testing of microlenses is similar in principle to testing larger lenses. In many cases however, the
measurement of very small lenses presents practical problems, which make it difficult to use standard
equipment. In general, two optical techniques can be used. One is based on microscopy, the other is based
on interferometry.
a) The first technique uses a microscope fitted with a displacement transducer, suitable light source, test
object, microscope video camera, monitor and image analyser (line intensity scan). This microscope is
used to locate, by focusing, the vertex of the microlens. The radius of curvature is deduced from a
measurement of the displacement necessary to reposition the microscope and locate the centre of
curvature of the surface as in Figure 8.
A focusing aid in the microscope such as a split-field focusing graticule enables the featureless vertex of
a microlens to be more readily located when viewing with reflected light. The position of the centre of
curvature may be located when the microscope is focused close to the centre because a confocal image
is formed by retroreflection of the rays at near-normal incidence on the lens surface. Tests may be
performed in white light or monochromatic illumination.
b) The second technique uses interferometry to generate interference patterns that indicate the location of
the test surface or the centre of curvature. The test interferometer may be one of several types, for
example a Fizeau, a lateral shearing or Twyman-Green. These are more fully described in ISO 14880-2
and ISO/TR 14999-1. One advantage of interferometry is that for strongly aberrated lenses, the variation
in radius of curvature with aperture radius can be readily deduced from the interference patterns.
Interferometry is sensitive to small variations in optical path lengths and it is usually necessary to mount
the interferometer on an anti-vibration table and to minimize air-borne disturbances.
Clauses 5 to 8 concentrate on the microscope technique while an interferometric technique is described in
Annex A.
Key
1 source to illuminate lens surface
2 microscope objective
3 microlens under test
4 beam splitter
5 CCD-camera
6 lateral (x, y) adjustment to centre microlens
7 axial (z) adjustment of microscope to locate lens vertex and centre of curvature
Figure 8 — Microscope used to measure the radius of curvature of the microlens surface
© ISO 2006 – All rights reserved 11
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ISO 14880-4:2006(E)
5.3.2.2 Test system
5.3.2.2.1 Microscope
A microscope fitted with a focusing aid such as a split-image rangefinder is used to enable focus settings to be
made on featureless surfaces and to enable the centre of curvature to be located by confocal imaging. The
displacement of the test surface relative to the microscope objective is measured with a calibrated
displacement transducer.
Note that the area of the lens surface sampled for the radius measurement is limited by the NA (numerical
aperture) of the microscope objective.
5.3.2.2.2 Light source
A light source emitting radiation either in a large band of wavelengths, such as an incandescent source, or at
a specific wavelength is suitable for this test.
5.3.2.2.3 Image display
If the image generated by the microscope is relayed by a video camera to a TV display, an electronic intensity
display can be used to assist in locating the position of best focus. The resolution of the imaging system shall
be sufficient to enable the best focus image to be identified.
5.3.2.2.4 Standard spherical surface
A spherical surface of known radius of curvature shall be used as a reference artefact to verify the
performance of the measurement system. A typical value for the departure from sphericity shall be less than
λ/2 root mean square deviation. The radius of the artefact should be similar to the radius of the test object.
5.3.2.3 Preparation
For consistent results the test equipment shall be maintained in a temperature-controlled environment,
preferably 20 °C, and not exposed to vibration.
5.4 Surface preparation of microlens array for measurement
For consistent results the test equipment shall be maintained in a temperature-controlled environment,
preferably at 20 °C and not exposed to vibration.
The optical surfaces to be tested shall be clean. Uncoated glass surfaces may be safely cleaned with alcohol
and cotton wool. The cotton wool should be soaked in a very small amount of solvent before touching the
surface and wiped only once across the optical surface before being discarded. This minimizes the chances of
scratching the surface. Dust may be removed using a clean camel-hair brush or filtered compressed air.
Coated optical surfaces such as antireflection surfaces should be treated with great care and not cleaned
unless absolutely necessary. They may be dusted using filtered compressed air.
Guidance should be sought on the correct use of solvents and cleaning materials.
12 © ISO 2006 – All rights reserved
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ISO 14880-4:2006(E)
6 Procedure
6.1 Measurement of pitch and surface modulation depth (lens sag)
6.1.1 Preliminary measurements
[7] [8]
Standard instrument calibration procedures shall be carried out . An axis of the lens array shall be aligned
to the lateral (x, y) scan direction of the measuring stylus and within the vertical working range of the
instrument. A preliminary measurement should be made on the array in order to assess the surface profile.
Adjustments in position and level of the array s
...
EUROPEAN STANDARD
DRAFT
prEN ISO 14880-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
January 2005
ICS
English version
Microlens arrays - Part 4: Test methods for geometrical
properties (ISO/DIS 14880-4:2005)
Réseau de microlentilles - Partie 4: Méthodes d'essai pour Mikrolinsenarrays - Teil 4: Prüfverfahren für geometrische
les propriétés géométriques (ISO/DIS 14880-4:2005) Eigenschaften (ISO/DIS 14880-4:2005)
This draft European Standard is submitted to CEN members for parallel enquiry. It has been drawn up by the Technical Committee
CEN/TC 123.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations which
stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other language
made by translation under the responsibility of a CEN member into its own language and notified to the Management Centre has the same
status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France,
Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia,
Slovenia, Spain, Sweden, Switzerland and United Kingdom.
Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without notice and
shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36 B-1050 Brussels
© 2005 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN ISO 14880-4:2005: E
worldwide for CEN national Members.
---------------------- Page: 1 ----------------------
prEN ISO 14880-4:2005 (E)
Foreword
This document (prEN ISO 14880-4:2005) has been prepared by Technical Committee ISO/TC
172 "Optics and optical instruments" in collaboration with Technical Committee CEN/TC 123
"Lasers and laser-related equipment", the secretariat of which is held by DIN.
This document is currently submitted to the parallel Enquiry.
Endorsement notice
The text of ISO 14880-4:2005 has been approved by CEN as prEN ISO 14880-4:2005 without
any modifications.
2
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DRAFT INTERNATIONAL STANDARD ISO/DIS 14880-4
ISO/TC 172/SC 9 Secretariat: DIN
Voting begins on: Voting terminates on:
2005-01-13 2005-06-13
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION • МЕЖДУНАРОДНАЯ ОРГАНИЗАЦИЯ ПО СТАНДАРТИЗАЦИИ • ORGANISATION INTERNATIONALE DE NORMALISATION
Microlens arrays —
Part 4:
Test methods for geometrical properties
Réseau de microlentilles —
Partie 4: Méthodes d'essai pour les propriétés géométriques
ICS 31.260
ISO/CEN PARALLEL ENQUIRY
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ISO/DIS 14880-4
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ISO/DIS 14880-4
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions. 1
4 Coordinate system. 2
5 Test methods. 3
5.1 Pitch and surface modulation depth measurement . 3
5.1.1 Use of stylus instrument. 3
5.1.2 Use of confocal microscope. 5
5.2 Physical thickness. 8
5.2.1 Principle. 8
5.2.2 Set-up and preparation. 8
5.3 Radius of curvature . 8
5.3.1 Principle. 8
5.3.2 Measurement arrangement and test equipment. 10
5.4 Surface preparation of microlens array for measurement . 11
6 Procedure . 12
6.1 Measurement of pitch and surface modulation depth (lens sag) . 12
6.1.1 Preliminary measurements. 12
6.1.2 Making measurements and interpreting the results . 12
6.2 Measurement of physical thickness . 12
6.3 Measurement of radius of curvature. 12
7 Results and uncertainties . 13
8 Test report . 13
Annex A (normative) Measurement with a Fizeau interferometer system . 15
A.1 Measurement arrangement and test equipment. 15
A.2 Measurement of radius of curvature. 17
Annex B (informative) Uniformity of array spacing . 18
B.1 Uniformity of array geometry . 18
B.2 Theory . 18
B.3 Equipment . 20
B.4 Procedure . 20
Bibliography . 21
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ISO/DIS 14880-4
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 14880-4 was prepared by Technical Committee ISO/TC 172, Optics and photonics, Subcommittee SC 9,
Electro-optical systems.
ISO 14880 consists of the following parts, under the general title Microlens arrays:
Part 1: Vocabulary
Part 2: Test methods for wavefront aberrations
Part 3: Test methods for optical properties other than wavefront aberrations
Part 4: Test methods for geometrical properties
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ISO/DIS 14880-4
Introduction
This standard specifies methods for testing geometrical properties of microlens arrays. Examples of
applications for microlens arrays include three-dimensional displays, coupling optics associated with arrayed
light sources and photo-detectors, enhanced optics for liquid crystal displays, and optical parallel processor
elements.
The market in microlens arrays has generated a need for agreement on basic terminology and test methods.
Standard terminology and clear definitions are needed not only to promote applications but also to encourage
scientists and engineers to exchange ideas and new concepts based on common understanding.
Part 4 contributes to the purpose of ISO 14880, which is to improve the compatibility and interchangeability of
lens arrays from different suppliers and to enhance development of the technology using microlens arrays.
The measurement of physical characteristics of pitch and surface modulation depth can be made using a
stylus instrument and non-contact optical probe system. Physical thickness can be measured with a
micrometer. The measurement processes are described in the main body.
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Microlens arrays — Part 4: Test methods for geometrical
properties
1 Scope
This standard specifies methods for testing geometrical properties of microlenses in microlens arrays. It
applies to microlens arrays with very small lenses formed on one or more surfaces of a common substrate
and to graded index microlenses.
2 Normative references
The following referenced documents are indispensable for the application 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 14880–1, Microlens array – Vocabulary
ISO 14880–2, Microlens arrays – Part 2: Test methods for wavefront aberrations
ISO 3274, Geometrical Product Specifications(GPS) — Surface texture: Profile method — Nominal
characteristics of contact (stylus) instruments
ISO 4287, Geometrical Product Specifications(GPS) — Surface texture: Profile method — Terms, definitions
and surface texture parameters
ISO 4288, Geometrical Product Specifications(GPS) — Surface texture: Profile method — Rules and
procedures for the assessment of surface texture
ISO 5436-1, Geometrical Product Specifications (GPS) — Surface texture: Profile method; Measurement
standards — Part 1: Material measures
ISO/TR 14999-1*), Optics and photonics — Interferometric measurement of optical elements and optical
systems — Part 1: Definitions and fundamental relationships
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14880-1 and the following apply.
Table 1 lists the terms, symbols, units and definitions of the geometrical properties, which are used in this
standard. The symbols adopted for this standard are chosen for clarity in this application to microlens arrays
but some may not be those commonly used for surface texture measurement.
*) to be published
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ISO/DIS 14880-4
Table 1 — Terms, symbols, units and definitions
Term Symbol Unit Definition
Pitch P , P mm The distance between the centres of adjacent lenses. For a
X Y
stylus instrument this will generally equate to RS .
m
Surface modulation h mm Peak to valley deviation of the surface height. For stylus
depth [lens sag] instruments this will generally equate to R .
z
T
Physical thickness mm The maximum local thickness of the array.
C
Radius of curvature R mm The distance from the vertex of the microlens to the centre of
C
curvature of the lens surface, for rotationally invariant
microlenses or cylindrical microlenses.
The terms P , P and h are terms used here to sescribe geometrical parameter encountered in the
x y
measurement of surface texture. P , P are spacing parameters and are defined as the average value of the
x y
length of the mean line section containing a profile peak and adjacent valley. An amplitude parameter, h, is
defined as the average difference between peak of the lens profile and the rim. Figure 1 illustrates the
geometrical properties of microlens arrays which are to be measured.
Key
1 substrate
T physical thickness
C
R radius of curvature
C
P ,P pitch
X Y
h surface modulation depth (lens sag)
Figure 1 — Geometrical properties of microlens arrays
4 Coordinate system
To measure the geometrical properties of a microlens array, a Cartesian coordinate system is used, as shown
in Figure 2. In a right-handed Cartesian set, the x-axis provides the direction of trace, the y-axis lies nominally
on the real surface, and the z-axis is the outward direction from the material to the surrounding medium.
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ISO/DIS 14880-4
Key
1 Substrate
2 Microlens
Figure 2 — Microlens array with a Cartesian coordinate system
5 Test methods
5.1 Pitch and surface modulation depth measurement
5.1.1 Use of stylus instrument
5.1.1.1 Principle
The basic principle using a stylus instrument is to obtain a profile of the surface of the array. Care shall be
taken to ensure that the profile passes through the centre of each lens and that the stylus remains in contact
with the surface throughout the measurement process. This enables the pitch and surface modulation depth to
be determined.
5.1.1.2 Set-up and preparation
The measurement of the geometrical characteristics of a microlens array is similar in principle to the
measurement of any surface using a stylus instrument. A typical stylus instrument consists of a stylus that
physically contacts the surface and a transducer to convert its vertical movement into an electrical signal.
Other components can be seen in Figure 4 and include: a pickup, driven by a motor and gearbox, which
draws the stylus over the surface at a constant speed; an electronic amplifier to boost the signal from the
stylus transducer to a useful level; a device, also driven at a constant speed, for recording the amplified signal
or a computer that automates the data collection.
The part of the stylus in contact with the surface of the array is usually a diamond tip with a carefully
manufactured profile. Owing to their finite shape, some styli on some arrays may not penetrate into valleys
and will give a distorted or filtered measurement of the surface. The effect of the stylus forces can have a
significant influence on the measurement results. Too high a force can cause damage to the surface of the
array. Too low a force and the stylus will not stay reliably in contact with the surface.
The stylus instrument shall be used in an environment that is as free as possible from dust, vibration and
0 0
direct sunlight in a location where the ambient temperature is maintained in the range 20 C ± 5 C (with a
condensation-free humidity below 70 % relative humidity). Remove any gross contamination from the surface
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ISO/DIS 14880-4
of the instrument preferably by blowing the surface with filtered air. Any oil or grease may be removed using a
suitable solvent.
Due consideration shall be given for testing under more adverse conditions.
Key
1 base
2fixture
3workpiece
4stylus
5 probe (pick-up)
6 measurment loop
7 column
8drive unit
Figure 3 — Elements of a typical stylus instrument.
The electrical unit on the stylus instrument shall be switched on at least one hour before any measurements
take place - this will allow time for the instrument to stabilise (the manufacturer’s instructions will normally
specify a minimum stabilisation time for a given instrument). Calibration of the instrument is essential prior to
measurement. Before calibration of the instrument takes place the stylus should be checked for signs of wear
or damage. A damaged stylus tip can lead to serious errors.
After measurement of the calibration artefact the indicated value shall be compared with the value attached to
the test object. If the measured value differs from the value that is shown on the calibration certificate then re-
calibration is required.
5.1.1.3 Stylus size and shape
It is important that the dimension and shape of the stylus are chosen appropriately as this can affect the
accuracy of the traced profile in a number of ways. On arrays with deep, narrow valleys the stylus may not be
able to penetrate fully to the bottom because either the tip radius or the flank angle of the stylus is too large. In
such cases the value of the surface modulation depth will be smaller than the true value. The ideal stylus
shape is a cone with a spherical tip. This usually has a cone angle of either 60º or 90º with a typical tip radius
of 1, 2, 5 or 10 µm.
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5.1.2 Use of confocal microscope
5.1.2.1 Principle
The confocal principle can be used for the measurement of surface topography. Depth is discriminated by
moving the surface of the object through focus and measuring the reflected intensity using a detector and
confocal pinhole. When the object point lies at the focus, the maximum intensity is detected whereas the
signal is reduced when the object point is displaced from the focus. The principle has been established in the
scanning confocal microscope. By scanning an imaged light spot over the object an area is measured point by
point.
5.1.2.2 Set-up and preparation
The principle of the confocal microscope has been developed by generating an array of light spots on the
object using a multiple pinhole mask (Nipkow disc) which allows for parallel data acquisition of multiple object
points. The Nipkow disc can be replaced by a microlens array in order to improve the light efficiency, as
shown in Figure 4.
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ISO/DIS 14880-4
Key
1 light source
2 beam splitter
3 rotating microlens disc (Nipkow disc)
4 objective
5sample
6 imaging lens
7 pinhole
8 detector
Figure 4 — Confocal microscope measurement system A
Figure 5 shows another configuration of the confocal microscope using a microlens array and a pinhole array.
It will enhance the optical radiation collection efficiency and improve the scanning speed, which depends on
the pinhole array with parallel scanning. An example of a measurement is shown in Figure 6.
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Key
1 light source
2 rotating microlens disc
3 beam splitter
4 pinhole array (Nipkow disc)
5 objective
6sample
7 imaging lens
8 CCD-camera
Figure 5 — Confocal microscope measurement system B
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Dimensions in µm
Figure 6 — Example of the microlens array surface structure using Confocal microscope
measurement system
5.2 Physical thickness
5.2.1 Principle
The two anvils of the micrometer contact the top and bottom surfaces of the array to measure the physical
thickness of the array at a given point. The anvils of the micrometer may be parallel or spherical.
5.2.2 Set-up and preparation
The micrometer shall be calibrated using a gauge block or a similar procedure prior to the measurement.
Thoroughly wipe the spindle and the measuring faces of the micrometer using clean lint free paper or cloth.
Leave the instrument and the microlens array long enough to adjust to room temperature (at least 1 hour).
Care shall be taken to ensure that the micrometer is not subject to sudden temperature changes, direct
sunlight, radiant heat or air currents that may cause significant temperature variations.
5.3 Radius of curvature
5.3.1 Principle
The basic principle is to locate, by optical means as shown in Figure 7, the vertex of the microlens under test.
The displacement necessary to locate the focal position or the centre of curvature of the surface is then
measured. This enables the radius of curvature R to be determined respectively. Care shall be taken to avoid
incorrect settings as shown in Figure 7.
NOTE It is only possible to locate the centre of curvature of the test surface using this method if the lens surface is
spherical. Otherwise light is not retroreflected to form a confocal image. If it is supected that the test surface is not
spherical the shape can be estimated from a zonal analysis using interferometry.
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ISO/DIS 14880-4
(a)
(b)
(c)
Key
1 substrate surface
2 optical probe
Figure 7 — (a) Locating the centre of curvature and the spherical lens surface with an optical probe.
(b) and (c) Examples of incorrect settings.
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5.3.2 Measurement arrangement and test equipment
5.3.2.1 General
The testing of microlenses is similar in principle to testing larger lenses. In many cases however, the
measurement of very small lenses presents practical problems, which make it difficult to use standard
equipment. In general, two optical techniques can be used. One is based on microscopy, the other is based
on interferometry.
a) The first technique uses a microscope to locate, by focusing, the vertex of the microlens. The radius of
curvature is deduced from a measurement of the displacement necessary to reposition the microscope and
locate the centre of curvature of the surface as in Figure 8.
A focusing aid in the microscope such as a split-field focusing graticule enables the featureless vertex of a
microlens to be more readily located when viewing with reflected light. The position of the centre of curvature
may be located when the microscope is focused close to the centre because a confocal image is formed by
retroreflection of the rays at near-normal incidence on the lens surface. Tests may be performed in white light
or monochromatic illumination.
b) The second technique uses interferometry to generate interference patterns that indicate the location of the
test surface or the centre of curvature. The test interferometer may be one of several types, for example a
Fizeau, a lateral shearing or Twymann-Green. These are more fully described in ISO 14880-2 and ISO/TR
14999-1. One advantage of interferometry is that for strongly aberrated lenses, the variation in radius of
curvature with aperture radius can be readily deduced from the interference patterns. Interferometry is
sensitive to small variations in optical path lengths and it is usually necessary to mount the interferometer on
an anti-vibration table and to minimise air-borne disturbances.
The body of this document concentrates on the microscope technique while an interferometric technique is
described in Annex A.
Key
1 source to illuminate lens surface
2 microscope objective
3 microlens test objective
4 beam splitter
5 CCD-camera
6 lateral (x, y) adjustment to centre microlens
7 axial (z) adjustment of microscope to locate lens vertex and centre of curvature
Figure 8 — A microscope used to measure the radius of curvature of the microlens surface.
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5.3.2.2 Test system
The test system consists of a microscope fitted with a displacement transducer, suitable light source, test
object, microscope video camera, monitor and image analyser (line intensity scan).
5.3.2.2.1 Microscope
A microscope fitted with a focusing aid such as a split-image rangefinder is used to enable focus settings to be
made on featureless surfaces and to enable the centre of curvature to be located by confocal imaging. The
displacement of the test surface relative to the microscope objective is measured with a calibrated
displacement transducer.
Note that the area of lens surface sampled for the radius measurement is limited by the NA (numerical
aperture) of the microscope objective.
5.3.2.2.2 Light source
A light source emitting radiation eithe
...
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