Centering Measurement of Infrared Lenses with OptiCentri...

Centering Measurement of Infrared Lenses with

OptiCentric®

and

OptiCentric®

Overview

Contents

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Page

Measurement Principle of OptiCentric® Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Principle of the Centering Error Measurement in Reflection and Transmission Mode . . . . . . . . . . 7

Principle of the Centering Error Measurement of Assembled Optics . . . . . . . . . . . . . . . . . . . . . . 9

Infrared Spectral Ranges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11

Differences to VIS from the Operator Side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Differences to VIS from a Technical Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12

Centration Measurement of all Variants of Single Lenses and Objective Assemblies . . . . . . . . .14

OptiCentric® Measuring Head . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Lens Rotation Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Stands with Travel Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Sample Holder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

Measurement Examples for the Most Common Infrared Lenses . . . . . . . . . . . . . . . . . . . . . . . . .17

Centration Testing of Single Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Centration Testing of Highly Sensitive Single Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

Centration Testing of infrared Meniscus Lenses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

Centration Testing of Aspherical Lens Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

Assembly of Optical Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22

Characterization of Complex Lens System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

OptiSurf® Low Coherence Interferometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

MultiLens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

AspheroCheck® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

OptiSpheric® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

OptiCentric® Principle Setup of the Instruments 13

Application 17

Accessories 25

Software 25

Technical Data 27

Order Information 28

OptiCentric® Extensions for Infrared Lens Measurement 24

Centration Measurement 4

Special Considerations for the Infrared Spectral Ranges 11

OptiCentric® for Infrared Lenses 3

OptiCentric®

3

Discover the Best Solutions for CentrationMeasurement and Assembly of Infrared Optical Systems

Whether zinc selenide or germanium, singlelenses or complex objectives, TRIOPTICS offersyou the measurement system that is best suit-ed to your application in the infrared. Opti-Centric® measurement systems are marketleaders with over 20 years of experience incentration testing and alignment of singlelenses and complex optical systems.

The OptiCentric® measurement system uses amodular design and is configured accordingto your application. Different air bearings, ro-tary units or distance sensors can be selecteddepending on the sample diameter and ap-plication.

The choice of measurement head is also par-ticularly relevant for the measurement of in-frared samples. TRIOPTICS provides measure-ment heads in VIS, SWIR, MWIR or LWIR. Themajority of centration measurements can becarried out with a more cost-effective visualmeasurement head. An infrared measure-ment head is only required for the measure-ment of optical systems or for measurements

in transmission.

A large range of additionalparameters besides centra-tion can be measured in allkinds of optics with the Opti-Centric® measurement sys-tems. All you need is to adda corresponding extensionmodule to the system.

We recommend the follow-ing options for the measure-ment of infrared lenses andoptics: MultiLens for themeasurement of objectives,AspheroCheck® for the mea-surement of aspheres, Opti -Spheric® for the measure-ment of typical lens para-meters such as effective fo-cal length or flange focallength. OptiSurf® for air gapsand distance measurement.In combination with Opti-Centric® called OptiCen-tric®3D.

Please refer to the Opti- Centric® brochure for addi-tional information about theOptiCentric® instruments.

OptiCentric® for Infrared Lenses

OptiCentric® 3D infrared measuring an infrared optic

Measurement of Centering Errors

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According to ISO 10110 a centering error of alens is represented by a mismatch of opticaland reference axis, respectivelythese are different in positionand/or direction.

Centering Errors of Lenses

Centering errors have a decisiveinfluence on the optical imagingquality of an imaging system. Acentering error is present if theaxis of symmetry of an opticalelement does not coincide witha given reference axis. The refer-ence axis typically is the axis ofsymmetry of a mount or cell. Thecentering error is given as an an-gle between the optical axis ofthis element and the referenceaxis. A centering error may alsobe expressed as the distance,for example, of a center of cur-vature to the reference axis. Fig.1 provides an overview of theopto-mechanical parametersOptiCentric® instruments are ca-pable to measure:

1. Translational displacement of a lens

2. Tilt of a lens

3. Surface tilt error of a spherical surface

4. Cementing error

5. Tilt of the aspherical axis

6. Air gaps and center thickness

OptiCentric® is able to precisely define all ofthese errors in accordance to ISO 10110-6.OptiCentric® features capabilities for measur-ing single lenses as well as for complete ob-jectives. The so-called MultiLens® procedure

enables the measurement of the centeringerror of all single surfaces of a fully assembledobjective without the need to dismantle it.

Fig. 1: Typical centering errors

➀

➂

➃

➄ ➅

➁

Fig. 2: Schematic diagram of the surface tilt error

Axis perpendicular tothe upper surface

Centre of curvature

C1

χ

σ L

Centration Testing


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The Centering Error of a Spherical Surface

The centering error of a spherical surface isdefined by the distance "L" of its center ofcurvature "C1" to a reference axis. The sur-face tilt error Sigma (σ) may also be used.The following correlation applies (R = radius ofthe surface under test):

σ = arcsin (L/R)

χD = 3438 σ [arc minutes]χD = angle of the centering error in arcmin-utes

It is also possible to provide the measured sur-face tilt error as eccentricity S at the lensedge if required. (Fig. 5)

S = D✕tan(σ)D = lens diameter

The Centering Error of a Lens

The optical axis of a single lens (see figure 3,left side) is the line connecting the centers ofcurvature of the two spherical surfaces. Thecentering error is now defined using the angleσ and the distance L to a given referenceaxis.

The centering error of a single lens can alsobe represented relative to the edge of a lens.In this case, the reference axis is given by theaxis from one of the centers of curvature tothe center of the diameter of the circumfer-ence of the lens. Measuring the relative dis-tance of the other center of curvature with re-spect to this axis gives the so-called surfacetilt error or wedge error of the lens (Fig. 3,right).

Fig. 3: Schematic diagram of the centering error of a lens

Center of Diameter withrespect to the edge

Reference axis

C1

C2

σ

σ

Reference axis

Optical axis of theindividual lens

C1

L

C2

σ

Once the shift and angle of the asphericalsurface have been determined, this data canbe used to calculate the following parame-ters:

• Orientation of the asphere with respect to the primary reference axis of the measurement system (corresponding to the axis of rotation)

• Orientation of the asphere with respect to the optical axis of other optical surfaces

• In case of a double-sided asphere: relativeorientation of the two aspherical surfaces

Centering Errors of Optical Surfaces withinan Assembled Objective

For the analysis of multi-surface optical as-semblies, the following measurement para-meters are of particular interest:

• Orientation of lenses/groups of lenses with respect to each other

• Orientation of lenses/groups of lenses with respect to a best-fit optical axis of a super-ordinated reference (e.g., the overall opti-cal axis)

• Orientation of lenses/groups with respect to an outer mechanical reference


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Centering Errors of Aspherical Lenses

In contrast to spherical surfaces, the symme-try of an aspherical surface is represented byan axis and not by a single point. Hence, therelative orientation of this axis of symmetryand the reference axis defines the centrationstatus of an aspherical surface. For the mea-surement, the following two values must bedetermined for an aspherical surface, eachwith their x and y components:

• The lateral position of the paraxial center ofcurvature from the reference axis

• The angle between the axis of symmetry and the reference axis

The lateral position corresponds to the classi-cal centering error of aspherical surfaces,and is measured in the same way using anelectronic autocollimator.

For measurement of the angle of asphericallenses, an additional distance sensor is re-quired that measures the run-out of the outeredge of the aspherical surface. Here, thehigh-resolution non-contact TRIOPTICS Asphe-roCheck® distance sensor is the instrument ofchoice.

Fig. 4: Main parameters of an aspherical lens

asphere asphere

angle between reference and axis of the asphere

shift

reference axisparaxialsphere

center of curvature of theparaaxial part of the lens

a a

b b

symmetry axis ofthe asphere

Measurement of centering errors

7

Measurement Principle of OptiCentric®

Instruments

The following sections describe the measure-ment principle behind the TRIOPTICS OptiCen-tric® instruments.The measurement of a centering error primar-ily involves defining a point of symmetry in re-lation to a reference axis. Thiscan, for example, be the cen-ter of curvature of a lens sur-face or the location of a focuspoint of a lens in relation to areference axis.When the center of curvature ofa lens surface is used for themeasurement of centering er-rors, the measuring procedureis known as Measurement in Re-flection. Similarly the measuringprocedure using the focal pointof a lens is known as Measure-ment in Transmission.

Principle of the Centering ErrorMeasurement in Reflectionand Transmission Mode

The basic procedure to identifythe centering errors of a lensimplies the rotation of the lensaround a precise reference ax-is. This precise reference axis isdecisive for the accuracy of the measurement. OptiCentric®product line provides differentinstrument modules and acces-sories featuring a high accura-cy reference axis for the mea-surement.

The rotation of the center ofcurvature can be followed liveon the monitor. The live reticuleimage shows the exact positionof the center of curvature in thex-y plane, whereby the centerof the circular path representsthe reference axis.

The radius of the circle is proportional to thesize of the centering errors. The result of themeasurement can be given as radius of therun out circle (in µm) or when measuring in re-flection as tilt of the surface and when mea-suring in transmission as tilt of the lens axis (inarcsec).

OptiCentric® infrared measuring an objective lens


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For the measurement an autocollimator is fo-cused either in the center of curvature of thesurface (Reflection Mode) or in the focalplane of the lens (Transmission Mode).

For the measurement in transmission a colli-mator is additionally needed, its parallelbeam emerging from the collimator is fo-cused in the focal plane of the sample to be

measured. The images reflected fromthe lens surfaces (Reflection method)as well as the images projected intothe focus of the lens (TransmissionMethod) are observed through theeye¬piece of an autocollimator, of atelescope, or of a microscope. Theautocollimator is equipped with aCCD Camera and the entire mea-surement process is software con-trolled. When a centration error is pre-sent, the observed image describesa circle while the sample is rotatedaround a reference axis.

Comparison between Reflectionand Transmission Mode

The reflection and the transmissionvalues are different and may becompared only to a limited extent. Asimple relationship between the twomeasurements for the centering errorof a single lens (without a mount) isgiven by:

T = (n - 1) × RR: Surface tilt error of top surface (Result of measurement in reflectionmode)

T: Angle deviation in transmissionmode n: Refractive index of glass

Measurements in reflection and trans-mission essentially provide differentresults. The reflection measurementwill provide exact information on thecentering error of single surfaces,while the transmission measurementprovides an “overall error” which is acombined result of the centering er-rors of all the single surfaces.

Fig.5: Measurement in Reflection Mode

illumination withreticule

CCD camera

spherical surface under investigation

autocollimatorfocussing lens

axis of rotation

Fig. 6: Measurement in Transmission

CCD camera

autocollimator

focussing lens

focal plane of the sample

samplecollimator

lens rotation device

lens rotation device

illumination withreticule


9

In transmission mode it is not possible to de-termine which one of the surfaces of a lens isproducing the centering error. In some cases,a lens tested in transmission mode can revealno centering error, although the lens is tilted inthe mount.

The images reflected from the lens surfaces,however represent an undoubted criterion forthe surface tilt and the individual centering er-rors. The reflection method is the only totaland true absolute method for the measure-ment of centering errors. Especially for non-transparent lenses (such as infrared materialswhen measured with a VIS measurementhead) the reflection method is preferably asmost of the measurements can be done witha cost effective visual measurement head.

However, the transmission method with somemechanical constraints gives in many casessatisfactory results. Thus, for a time-economicevaluation of the overall centration status thetransmission method is the method of choice.For a good economy of the optical manu-facturing both methods should be consid-ered. OptiCentric® allows both methods to beused and offers for measurements in Trans-mission infrared measurement heads.

Principle of the Centering Error Measure-ment of Assembled Optics

For the characterization of assembled opticalsystems the centration of each optical sur-faces is analyzed. Here, the measurement ofouter surfaces is straightforward centrationtesting in reflection as described before (seeFig. 7a). When testing inner optical surfaceswithin an assembled system the principalconcept stays the same as in the reflectionmode. However, focusing into its geometricalcenter of curvature will not generate an auto-collimation image because of refraction atpreceding surfaces. In order to provide an al-most perpendicular incidence of the mea-surement light coming from the autocollima-tor onto the surface under test, the refractionof the optics that were traveled through toreach the surface need to be considered.The axial position of the center of curvature ofsuch surfaces can easily be calculated if thebasic design parameters of all surfaces areknown: the radius of curvature, refractive in-dex and the lens thickness. Futhermore, theseparameters have to be taken into account forcalculating the lateral positions of the corre-sponding centers of curvature (Fig. 7b).

In the measurement the surfaces are succes-sively analyzed for the deviation of each cen-

ter of curvature with respect tothe reference axis. In the present-ed measurement system, the ro-tation axis of the air bearing rep-resents the reference of eachmeasurement. The obtained da-ta are processed in order to pro-vide the shift and tilt of an indi-vidual lens or group of lenses.The centering errors can also bereferred to a different axis, e.g.,the best-fit axis through all cen-ters of curvature. Alternatively, itcan be the optical axis of a sin-gle element of a defined groupof lenses.

S1

a)

S2

C1

C2

b)

S1

S2

C2

C2́

C1

7a) Measurement of the upper surface in reflection. The measuring head focuses in the center of curvature

b) Measurement of the bottom surface taking the radius of the curvature refractive index and lens thickness into account


10

In addition, the orientation of the best-fit axisitself represents a robust indicator for thealignment of the sample with respect to thereference axis.

For this application the dedicated softwaremoduls MultiLens® has been developed. MultiLens® provides the exact position coordi-nates of all centers of curvature in space. Themeasured data support further analysis of thelens system and additionally provide theanalysis features:

• Orientation of lenses/groups of lenses with respect to each other

• Orientation of lenses/groups of lenses with respect to a best-fit optical axis of a superordinated reference (e.g., the overall opti-cal axis)

• Orientation of lenses/groups with respect to an outer mechanical reference

Principle of the Measurement of Air Spac-ings and Center Thicknesses

The TRIOPTICS OptiCentric® 3D feature is anupgrade to the standard OptiCentric® instru-ment that enables the measurement of air

gaps between two lenses and lens thickness-es. Thus, it yields a different set of parametersfor the evaluation of mechanical errors in op-tical assemblies. Lens air spacings and centerthicknesses are defined as the geometricaldistance between neighboring optical sur-faces along the reference axis of an opticalsystem (e.g. optical axis, symmetry axis of thelens barrel etc.). A highly accurate method todetermine the positions of the optical sur-faces is low coherent interferometry.

In general, low coherent interferometry isbased on the determination of the position ofzero group delay difference within an interfer-ometer system. In an interferometer, e.g., aclassical Michelson-type (Fig. 2a), the lightcoming from a low coherent light source,e.g., a spectrally broadened light source issplit by a beam splitter into an object and areference wave. The object wave illuminatesthe lens system along its optical axis. In everysurface, a fraction of the incoming lightbeam is reflected. This light is guided to aphoto detector where it is superimposed withthe reference light that is varied in the opticalpath length by an optical delay line. Whenanalyzing the resulting intensity as a functionof the delay (Fig. 8), interference patterns areobserved whenever the optical path lengths

A

B

C

low coherent light source

photo detector

photo de

tector signa

l

moveable mirror

position encoder

encoder position

Fig. 8: Principle of low coherence interferometry


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in the two interferometer arms coincide.There, the group delay difference equals ze-ro. Thus, the location of every object surfacecan be determined from the positions of themaximal modulation in the interference pat-tern.

An important prerequisite for the detection ofevery single optical surface is a sufficient in-tensity of the back-reflected light. Besides theoptimized focusing of the object beam toobtain reflections with balanced intensitiesfrom all surfaces, it is essential to align an ac-curate coincidence of the measurement axisof the low coherent interferometer and theoptical axis of the sample. The particularpatented (Patent EP000002458321A1) advan-tage of TRIOPTICS OptiCentric® 3D is the com-bination of the low coherent method withtechniques for the centration measurement.The centration values are used for the deter-mination of the optical axis of the lens system,e.g., the best-fit axis of all the centers of cur-vatures or just of selected lens elements. If theorientation of the axis of the low coherent in-terferometer with respect to the reference ax-is is known, the sample axis can be aligned tothis.

For testing single lenses and completed as-semblies that are only transparent in the in-frared range, TRIOPTICS provides measure-ment heads specifically designed for the in-frared wavelength ranges that are equippedwith focal-plane array infrared image sensorsfor ease of use. Typical applications are e.g.testing of lenses and assemblies for thermalimaging, military applications or industrialprocess control made from lens materials likee.g. Ge, Si, ZnSe, ZnS or CaF2.

Infrared Spectral Ranges

The infrared wavelength range is divided inthree distinct ranges for imaging applications.These bands exhibit high transmission of lightthrough air and are separated by strongly ab-sorbing bands in the spectrum. The threeimaging infrared ranges are: short wave in-frared (SWIR) from 0.9-1.7 µm, medium waveinfrared (MWIR) from 3-5 µm and finally longwave infrared (LWIR) from 8-12 µm wave-length. Due to limitations in detector technol-ogy, so far no image sensor can cover allthree wavelength ranges, however a LWIR sys-tem can also cover most MWIR lenses andvice versa.

For an overview about compatible materialsand the wavelength regions covered by theavailable measurement heads please referto Fig. 9. Silicon is a special case as the trans-parent region depends on the doping leveland dopant type, so a LWIR head might be asuitable depending on application.

Infrared optics Typical

Special Considerations ofthe Infrared Spectral Ranges


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Differences to VIS from the Operator Side

In contrast to VIS systems, the light emittedfrom the focused autocollimator head can-not be seen by the naked eye which is how-ever no problem in practice for aligning thesample. Apart from that, the operation of theinfrared systems is not different to the VIS sys-tems, so a user can be quickly trained for anew wavelength range.

Differences to VIS from a Technical Perspec-tive

From a technical side, apart from using suit-able optics and illumination sources in themeasurement heads, the most important dif-ference between visual and infrared range isthat in the infrared range every object, includ-ing the sample, emits light in this wavelengthregion, so the instrument needs to compen-sate for the thermal background before tak-ing a measurement. This is done automatical-

ly by the software and requires no operator in-tervention. Also, the contrast between back-ground and illuminated areas is lower than inthe VIS, so specialized image processing al-gorithms are used to reach the required highresolution.

OptiCentric® in the standard reflection moderelies on back-reflection from the lens surface,so the light intensity of the reflected reticuleimage strongly depends on the type of coat-ing used. Typically, all infrared imaging lensesare AR-coated, however there is a wide varia-tion in efficiency which the instruments com-pensates by adjusting illumination power andshutter times where available.

In general, the typical accuracy of the cen-tering error measurement is approximately 1 µm, slightly higher than for the instruments inthe VIS, which is due to the longer wavelengthand larger pixel size of the cameras used inthe autocollimators.

.1 .2 .3 .4 .5 .7 1 2 3 4 5 7 10 20 30 40 50 70 100.6

Wavelength (µm)

VIS SWIR MWIR LWIR LWIR laser

Gallium Arsenide GaAs

Zinc Selenide ZnSe

Zinc Sulfide „Cleartran“ ZnS

Zinc Sulfide ZnS

Germanium Ge

Silicon SiSilicon Si

Calcium Fluoride CaF2

Fig. 9: Comparison of transparent regions of typical infrared lens materials and the regions covered by available OptiCentric® measurement heads. *Depending on doping level and dopant type

OptiCentric® Measuring Heads

*

Setup

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OptiCentric® Principle Setup ofthe Instruments

SWIR MWIR LWIRLASER

VIS

OptiCentric®with Single Head

Measuring Heads

OptiCentric®with Dual Head

Lens rotation deviceswith vacuum chuck

Lens Rotation Devices

Air bearing

OptiSurf® OptiSpheric®

for infraredAspheroCheck Distance Sensor Lever Gauge

Options and Modules

Selection of OptiCentric® Modular Product Group for Infrared Lens Measurement

Setup

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Centration Measurement of all Variants ofSingle Lenses and Objective Assemblies

OptiCentric® instruments are designed tomeasure centering errors and other parame-ter of lenses and optical systems. All instru-ments can be used to carry out measure-ments using the reflection mode. Measure-ments in transmission mode are possible if asecond collimator is fitted into the instrument.

OptiCentric® is of modular design, so that theinstruments are upgradeable and compati-ble with each other. To make the selection ofthe suitable equipment easier, the main in-strument components are presented in thefollowing.

OptiCentric® Measuring Head

The optical measur-ing head of the in-strument is designedto view and quantifythe size of the cen-tering error in singleand multiple opticalcomponents.

The measuring headconsist of an auto-collimator equippedwith additional headlenses. During themeasurement, themeasuring head isfocused in the im-age plane of thesample (transmissionmethod) or in the

center of curvature of the lens surface undertest (reflection method). The majority of in-frared applications can be measured with avisual measurement head in reflection.

The autocollimators used in the OptiCentric®infrared instruments are designed for dedicat-ed infrared use: The light source is typically a

broadband thermal source or an infraredlaser for higher illumination power, the retic-ules and lenses are made from suitable in-frared lens materials and finally, a thermalimaging camera is used instead of a con-ventional CCD camera to acquire the im-ages. For the LWIR and SWIR range, uncooledcameras are used, whereas in the MWIRrange cooled cameras are needed. TheMWIR cameras are fitted with self-containedStirling-type cryocoolers so that no manual fill-ing of dewars with liquid nitrogen is required.

TRIOPTICS offers three different types of illumi-nation sources: cold light/halogen for VIS andSWIR, broadband infrared for MWIR and LWIRand laser illumination at 10.2 µm for LWIR.With the broadband sources, a cross shapedreticle is usually used, while with the coherentlaser source a pinhole is used. The lasersource for the LWIR is recommended for Multi-Lens® measurements on complex LWIR as-semblies as it delivers much higher illumina-tion power than the broadband source.

For advice on selecting illumination sourcesand heads please refer to your TRIOPTICS rep-resentative.

For LWIR, TRIOPTICS recommends the broad-band infrared head for measurements involv-ing single lenses or measurements in transmis-sion, whereas for MultiLens® measurements oncomplex assembles the laser illumination isneeded to reach the required illuminationpowers. In the case of laser illumination, theinstrument is equipped with a protective hous-ing including the necessary interlocks to guar-antee the eye safety of the operator.

OptiCentric® measuring heads are availablewith either 57 or 38 mm outer tube diameterand can be easily interchanged on the samesystem within minutes. No extra alignment isnecessary.

OptiCentric® measuringhead

Setup

15

Lens Rotation Devices

As the measurement of centering errors re-quires a stable reference axis, suitable lensholding and rotary devices are crucial.

The accuracy of the centering measurementrelies on the quality of the two basic compo-nents of the instrument:

• Optical measuring head

• Lens rotation device

Many optics manufacturers underestimatethe importance of the lens rotation devices inensuring the required accuracy. Since the useof accurate CCD-cameras and complex im-age processing software provides a high ac-curacy of optical measuring heads, thelargest error source in lens centering andmounting is in many cases the lens rotationdevice. Many of the lens rotation devicesused in the optical manufacturing do not pro-vide a reference rotation axis which is suffi-ciently stable.

High Precision Lens Rotation with Air Bearings

For applications requiring highest accuracy,OptiCentric® can be equipped with an ultra-precision rotary air bearing table. The rotaryair bearing provides lens rotation facilities andan ultra-stable and ultra-accurate reference(rotation) axis for the measurement of center-ing errors. Typically, the run-out of the rotationaxis is <50nm. For alignment needs the airbearing is equipped with a stable tilt andtranslation stage.

The air bearing features a central hole formeasuring in transmission mode or for check-ing top and bottom surfaces in reflectionmode using a dual measuring head, or forthickness measurement with OptiSurf®. Theprecision rotation motor provides smooth ro-tation - without any vibrations - and, if neces-sary, accurate angular positioning. For a longoperational life, the air bearings are suppliedwith a complete air conditioning control unitincluding a membrane dryer, filter andmanometer.

For the pre-alignment of the lenses to a givenreference axis a tilt and translation tables canbe used with the air bearing.

Technical Specification of the Measuring Heads

Wavelength of the Measured Material

Range VIS SWIR MWIR LWIRLaser

Wavelength range in µm 0.4-0.7 0.9-1.7 3.5-4.8 10.2

Accuracy in µm (typical) 0.2 1.0 2.0 2.0

Illumination Halogen Halogen Broadband CO2 Laser

OptiCentric® 100 with air bearing and tilt and translationtable

Setup

16

Motorized Lens Rotation Device with VacuumChuck

The vacuum unit is a rotation device ideallysuited for cost-effective single lens measure-ment in reflection.

It measures the centering error of single lens-es with the outer edge as the reference. Thelens is rotated using a motor-driven frictionwheel against a V-block. The lens is held inposition by vacuum.The control of all functions such as pressure,rotational speed and vacuum pump is inte-grated in one control unit. A compressed airline system is not required. The vacuum rota-tion fixture exhibits an excellent level of accu-racy and reproducibility. Possible errors relat-ing to the roughness of the outer cylinder are

averaged out so that the results have a veryhigh repeatability.

Stands with Travel Mechanism

The optical measuring head is mounted ontoa motorized vertical stage for the positioningof the focal plane of the collimator to a cer-tain center of curvature position (for ReflectionMode) or to a focal length (for TransmissionMode).

Sample Holder

Depending on the applications OptiCentric®provides a large variety of sample holders:self-centering holders, vacuum retention de-vices, hydrostatic chucks, etc.

OptiCentric® 100 with rotation device, head lenses, tilt and translation table (TRT 200), Contoller

Application

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Measurement Examples for the Most Com-mon Infrared Lenses

In order to find the OptiCentric® measure-ment configuration that fits exactly with yourneeds here are some typical measurementtasks and their solutions presented.

Centration Testing of Single Lenses

Measurement Task: Centration testing of non-VIS-transparent or -transparent single lenseswith OptiCentric®. The required instrument isequipped with a lens rotation device and avisual measurement head.

Measurement: The lens under test is placedonto the ring chuck support of the lens rota-tion device. Thus, the center of symmetry of

the bottom surface is fixed in space. Whenthe lens rotates with its edge against the V-block, the lens rotation axis is given by thegeometrical center of the circumference andthe center of curvature of the bottom sur-face. In this way the rotation axis correspondsdirectly to the reference axis for the determi-nation of the wedge error. So the remainingparameter to be measured is the center ofcurvature position of the top surface whichcan easily be tested by OptiCentric® in reflec-tion mode.

Conclusion: Single lens centration testing withOptiCentric® in reflection mode with the lensrotation device allows the evaluation of in-frared lenses with a cost effective instrumentwithout the need for expensive infrared de-vices.

Instrument Configuration: OptiCentric® withlens rotation device with vacuum chuck.

Application

Infrared optics measured with OptiCentric®

Application

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Centration Testing of Highly Sensitive SingleLenses

Measurement Task: The easiest way to protecthighly sensitive lenses like calcium florid lensesagainst scratches is a minimum of handlingduring the production and testing process.

The OptiCentric® Dual instrument in combina-tion with an air bearing and a non-contactdistance sensor fulfill this requirement, insteadof using the lens rotation device with V-block.

Independently from the material properties ofthe lens under test an OptiCentric® Instrumentwith cost-effective visual measuring heads isused.

Measurement: The lens is carefully placed inthe lens holder. During the measurement thedual autocollimators measure the center ofcurvature positions of the top and bottom sur-faces; the distance sensor measures the radi-

al run-out of the lens edge. In the analysis thesoftware determines the reference axis fromthe center of the circumference and the bot-tom surface. The measured offset of the topsurface to this axis gives the lens’ centering er-ror.

For lenses that are transparent in the VISrange the centering error can alternatively bemeasured in transmission. Therefore, the topautocollimator measures the run-out of thefocus spot when the lens under test is illumi-nated with collimated light by the bottom(auto-) collimator.

Conclusion: A minimum of handling protectsthe sensitive surfaces of the lens. Both sur-faces are measured in one step with cost ef-fective visual measurement heads.

Instrument Configuration: OptiCentric® 100Dual or OptiCentric® 300 Dual with distancesensor.

Distance sensor measures edge of a highly sensitive lense

Application

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Centration Measurement of Infrared SingleLenses with Plane Reference Flange

Measurement Task: Lenses with plane refer-ence flange are often used in infrared opticalsystems. The preferred instrument to measurethese lenses is the OptiCentric® Dual, with twovisual measurement heads and lens rotationdevice with vacuum chuck.

Measurement: The lens is placed with thesupport flanges onto the ring chuck and ro-tates against the V-block.

The upper autocollimator focuses in the cen-ter of curvature of the top surface. The lowerautocollimator focuses in the center of curva-ture of the lower surface. Thus, the measure-ment is accomplished in reflection.

In this case the reference axis given by thecenter of symmetry of the circumference andthe normal of the support flange. The result ofthis measurement gives information about thecentering errors of both surfaces with respectto the reference.

For VIS-transparent lenses the centration canbe measured in transmission as well.

Conclusion: Both surfaces are measured inone step with cost-effective visual measure-ment heads and lens rotation device withvacuum chuck

Instrument Configuration: OptiCentric® Dualwith vacuum chuck

OptiCentric® 100 Dual

Typical lens with plane reference flange

Application

20

Centration Testing of Aspherical Lens Surfaces

Measurement Task: The use of aspheric sur-faces in optical systems’ design allows toachieve better spot size performance, or al-ternatively achieve similar performance whileusing fewer elements in the system. Therefore,especially for infrared optics aspherical sur-faces are commonly applied.

Aspherical infrared lenses are preferablymeasured with an OptiCentric® Dual instru-ment equipped with AspheroCheck® Module.This configuration achieves the best resultsand allows for a simple handling.

Measurement: The lens is put with the aspheri-cal surface to the top in the lens holder. Dur-ing the measurement the lens rotates and thethree parameters required for centration test-ing are measured:

• Center of curvature position of the spheri-cal bottom surface

• Paraxial Center of curvature position of the top aspherical surface

• Eccentricity of the outer edge of the as-pherical surface under rotation

These three measurements are accom-plished at the same time during the rotationof the lens: The lower measuring head focuses in thecenter of curvature of the spherical bottomsurface and measures its centration. The cen-tration of the paraxial area of the upper sur-face is measured with the help of the uppermeasuring head in reflection mode as well.Third, the AspheroCheck Module measuresthe eccentricity of the aspherical surface.

OptiCentric® with AspheroCheck® Module

focussing autocollimator

sample

height Adjustment (z)

lateral position (y) high precision rotary device (air bearing)

AspheroChecksensor

Application

21

Conclusion:The results of these measurements of aspheri-cal infrared lenses give information about:

• Orientation of the asphere with respect to the primary reference axis of the measure-ment system (corresponding to the axis of rotation)

• Orientation of the top asphere with respect to the axis of bottom sphere and center of circumference

• In case of a double-sided asphere: relativeorientation of the two aspherical surfaces (Fig.: 10b)

Instrument Configuration: OptiCentric® 100Dual or OptiCentric® 300 Dual with Asphe-roCheck Module

OptiCentric® measuring an infrared lens

Fig. 10a: Lens with an aspherical and spherical surface Fig. 10b: Lens with two aspherical surfaces

Axis of asphere

Axis of asphere #2

Optical axisAxis of asphere #1

Center of theparaxial areaof the asphere

Asphericalsurface Aspherical

surface #1

Asphericalsurface #2

Center of curvature ofthe sphere

Spherical surface

Measuredangle

Measured angle

Measured shift

Application

22

Assembly of Optical Systems

Measurement Task: The precise assembly oflenses in a barrel is a decisive step during theproduction of objective lenses.

An OptiCentric® instrument with an air bearingand distance sensor strongly improves the ac-curacy of the production process and theperformance of the final product.

Measurement & Assembly: At first the barrelaxis is aligned with the help of a distance sen-sor (e.g. a lever gauge) to the axis of the ro-tary air bearing. The first lens is placed into thebarrel. The autocollimator measures the cen-ter of curvature position of the top surface

Process, How to Assemble an Optical System

with respect to the reference axis. Then thelens can be realigned in magnitude and di-rection according to the measured values.

Depending on the mechanical design of thesample the lens is fixed in position e.g. by us-ing retaining rings or glue.This procedure is iteratively repeated for allfurther lens elements until the optical system iscompleted.

Conclusion: An optimized mechanical align-ment of optical infrared systems is reachedusing an OptiCentric® instrument.

Because of the step-by-step measurementand alignment procedure of freely accessiblelens elements, here a cost-effective visualmeasurement head can be applied withoutconsidering the lens material properties.

Instrument Configuration: OptiCentric® 100 orOptiCentric® 300 with distance sensor.

Measuring the lens barrel with the lever gauge

Assembly of Optical Systems

Application

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Characterization of Complex Lens System

Measurement Task: In the quality inspection oflens systems, testing solely the final opticalperformance is often not sufficient as it doesnot reveal the causes of potential substan-dard performance. Instead, a full opto-me-chanical characterization of the samples isrequired to identify potential issues in the as-sembly process.

An OptiCentric® 3D Instrument equipped withan infrared measurement head, rotary airbearing, tilt and translation table, MultiLensand OptiSurf Module fulfills these require-ments.

Measurement: After the objective lens to betested is placed onto the sample table, thecenter of curvature positions of all single lenssurfaces are measured following the MultiLensconcept. Therefore, the autocollimator itera-

tively focuses into the center of curvature po-sitions (respectively their image positions ac-cording to the MultiLens calculation) of allsample surfaces and measures their relativeposition with respect to the reference axis. Inthe following analysis, the relative orientationof the single elements’ or groups’ axis is evalu-ated with respect to each other or a me-chanical barrel axis.

As the precise coincidence of sample andmeasurement axis is a particular requirementfor the measurement of the center thicknessand air gaps, data from centration testing isused for the precise alignment of the sampleaxis with the tilt and translation table. Then, the axial lens surface positions aremeasured.

Conclusion: OptiCentric® 3D 100 Infrared isthe only instrument which accomplishes thecomplete opto-mechanical characterization

of infrared lens systemsand delivers detailed in-formation about the as-sembly of the instrument.

Instrument Configuration:OptiCentric® 3D 100 In-frared, OptiCentric® 3D300 Infrared or OptiCen-tric® 3D Dual. If the objec-tive lens only consists oflenses that are transpar-ent in the visible range,standard VIS measure-ment heads can be used.

OptiCentric® 3D 100 Infrared measuring an infrared lens

Application

24

The functionality of OptiCentric® Instruments isextended with the help of extension modules.For the measurement of infrared lenses, espe-cially MultiLens, AspheroCheck, OptiSurf® andOptiSpheric® Modules are recommended.

For further reading about OptiCentric® Exten-sion Modules TRIOPTICS recommends the Op-tiCentric® Brochure.

Optical Head with Low Coherence Interfer-ometer

For a complete opto-mechanical characteri-zation of an optical system OptiCentric® in-struments are optionally equipped with theLow Coherence Interferometer OptiSurf(Patent EP000002458321A1) The combinationof OptiCentric® and OptiSurf® is called Opti-Centric® 3D and allows for highest accuracy:

• Lens centering errors down to 0.1µm

• Air spacing and center thickness of less ±1µm

Fig. 11: OptiCentric® 3D, the different steps of the measurement process

OptiCentric® 3Dmeasurement head

Measurement axis

optical axis of the optical system (tilted and shifted)

OptiSurf® low coherence interferometer measurementhead

rotation device

tilt andtranslationtable

1. Optical axis of the sample is shifted and tilted

2. Reference axis of OptiCentric® 3D and optical axis of the sample coincide

3. Center thickness and air gaps aremeasured with OptiSurf®

OptiCentric® Extensions for Infrared Lens Measurement

OptiCentric® Extensions

25

The following accessories are offered to ex-tend the use of OptiCentric® instruments:

• Motorized Centering and Cementing• Revolving Turret (manual/ motorized)

• Tactile and non contact distance sensors

• Extension of Measurement Range, large setof head lenses available

• Calibration Check

• Alignment Set

The advanced software is designed to workwith Microsoft Windows® systems. It fulfills theneed of the optical shop for easy, intuitive op-eration and features a number of options toaccommodate a large variety of specific re-quirements.

The software modules „Centration in Transmis-sion“ and „Centration in Reflection“ provide anoutstanding accuracy even in difficult mea-suring situations such as poor contrast, anti-reflection coated surfaces, very small lenses,etc. With the OptiCentric® Dual instrument thesoftware optionally provides simultaneousmeasurement data with two live images forboth lens surfaces. The software features se-lectable options to adapt the system to differ-ent hardware configurations e.g. infraredmeasurement head or different reticule pat-terns (bright cross, dark cross, pinhole, etc.).

Several different measurement units like mm, arcsec, etc. can be selected. To in-crease the production efficiency, the opti-mized process parameters can be saved forfuture use.

The standard OptiSurf with 1.3µm measuringwavelength covers the majority of the infraredmaterials. A model working at longer wave-length for the measurements of all infraredmaterials will be available soon.

MultiLens

This comprehensive software module is usedfor measuring the centering errors of com-pletely mounted optical assemblies. The in-spection of mounted objective lenses pro-vides precise data about the assembly quali-ty. In this way, the MultiLens® software becomesan indispensable tool for optimizing the man-ufacturing process. The MultiLens® moduledelivers in a nondestructive way the completeinformation about the individual centering er-rors of all elements inside of an assembly.

AspheroCheck®

AspheroCheck® (Patent 10 2006 052 047.5-51) is a hardware and software module de-signed to measure the orientation of an as-pherical axis with respect to a defined refer-ence axis. The module is available as an up-grade for all OptiCentric® Systems equippedwith a rotary air bearing sample stage.

OptiSpheric®

With the extension module OptiSpheric®, themeasurement functionality of an OptiCentric®instrument can be extended by the followingoptical parameters:

• EFL - Effective Focal Length

• BFL - Back Focal Length

• FFL - Flange Focal Length

• Radius of Curvature

• MTF on-axis

Available for all spectral ranges.

Accessories

Software

Software

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Key Features

• Live view monitoring of the camera image from all measuring heads

• Real time measurement

• Vector display of the size and direction of the centering error

• HTML certificate output

• Display of tolerance ranges for quick go/no go decisions

• Automatic adjustment to the sample reflectivity and correction of the thermal background

• Dedicated algorithms for high accuracy measurement of infrared optics

• Automatic calibration procedure by meansof a calibrated sample, the calibration canbe verified at customer site

• Artificial crosshair for the initial alignment assistance

Furthermore, almost every upgrade includesa software part which enables the software toanalyze the measurement results.

OptiCentric® Software: Dual live image

Overview Configurations

27

OptiCentric®

OptiCentric® 100

OptiCentric® 100 Dual

Spectral Range

VIS

SWIR

MWIR

LWIR Laser

Bearings

Air bearing AB 100 (sample diameter 0.5-225 mm)

Air bearing AB 300 (sample diameter 0.5-600 mm)

Vacuum unit (sample diameter 1-5 mm)




Vacuum unit extension (sample diameter 125 - 200 mm)

Measuring Head

Single Head

Dual Head

Range of radii or effective focal length of the sample

0 to ± 400 mm and plane

0 to ± 2000 mm and plane

Measurement value analysis

PC evaluation

Measurement head linear stage

Automated positioning, PC controlled

OptiCentric® extension

OptiSurf®, instruments called OptiCentric® 3D

MultiLens® (centering error measurement of aspherical lenses)

AspheroCheck® (centering error measurement of aspherical lenses)

OptiSpheric®/OptiSpheric® Infrared (EFL, BFL, FFL, MTF)

Standard features

Optional

Technical Data

Order Information

28

Order Information

OptiCentric® Order Number

OptiCentric® 4-100-02

OptiCentric® 100 4-400-02

OptiCentric® 100 Dual 4-405-02

OptiCentric® 300 4-401-08

OptiCentric® 300 Dual 4-405-60

OptiCentric® with Infrared Measuring Head

OptiCentric® 100 SWIR 4-400-02 SWIR

OptiCentric® 100 MWIR 4-400-02 MWIR

OptiCentric® 100 LWIR 4-400-02 LWIR LASER

Upgrades for OptiCentric® VIS Instruments

Upgrade OptiCentric® VIS for the SWIR spectral range (0,9-1,7µm, broadband) SWIR UP

Upgrade OptiCentric® VIS for the MWIR spectral range (3-5µm, broadband) MWIR UP

Upgrade OptiCentric® VIS for the LWIR spectral range (8-12µm, laser) LWIR UP LASER

Extension Moduls Order Number

MultiLens® 4-400-90

MultiLens® IR 4-400-90-IR

AspheroCheck® 4-400-99

OptiSurf® with 1.3µm light source Depending on measurement length

OptiSpheric® Infrared 4-600-380 / on request

Accessories Order Number

Motorized lens rotation device with vacuum Chuck 4-400-90

Extension of measument range ± 2000 4-400-91

Alignment tool set 4-400-99

Manual revolving turret 4-300-074

Precision, motorized and software controlled revolving turret 4-400-86

Complete device for calibration check 4-600-380

Notes

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Notes

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TRIOPTICS GmbH . Optische Instrumente Hafenstrasse 35-39 . 22880 Wedel

Tel.: +49-4103 - 18006 - 0Fax: +49-4103 - 18006 - 20

E-Mail: [email protected] . www.trioptics.com© 2014 TRIOPTICS GmbH . All rights reserved

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