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Fast non-contact 3D form measurement of aspheric optics Your turn-key solution for ultra precision metrology
Fast non-contact 3D form measurement of aspheric optics
Your turn-key solution for ultra precision metrology
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Content
LuphoScan platforms are interferometric, scanningmetrology systems based on MWLI® technology(multi-wavelength interferometry). They are designed to perform ultra precision non-contact 3D form measurements mainly of rotationally symmetric surfaces such as aspheric lenses.
Contents
BenefitsMain features of LuphoScan metrology platforms ........03
Measurement principleOptical metrology based on MWLI technology .............04
Measurement accuracyHigh accuracy due to unique reference concept ...........05
SoftwareIntuitive software for easy control and analysis ...............06
ApplicationsMaximum flexibility with LuphoScan technology ...........07
LuphoSwapComplete form error characterisation ....................... 08–09
Software modulesAugmented flexibility through software add-ons .. 10–11
LuphoScan modelsVariety of platform configurations ................................. 12–13
SpecificationsTechnical Data ...........................................................................14–15
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Benefits
LuphoScan systems offer crucial benefits with regard to 3D form measurements of high quality optical surfaces
•Investigationofanyrotationallysymmetricsurface Aspheres, spheres, flats and slight freeforms
•Ultrahigh,reproducibleaccuracy ≤±50nm
•Almosteverymaterial Transparent, specular, opaque, polished, ground
•Largesphericaldepartures Unrestricted,e.g.canmeasurepancakeorgullwingsurfaces,andprofileswithpointsofinflection
•Steepslopes Upto90°(i.e.measurementofhemispheres)
•Highlyflexible* Measures segmented surfaces, annular optics, rectangular surfaces, surfaces with diffractive structures, axicons
•Completelenscharacterisation* Lensthickness,wedgeerror,decentreerror,lens–mountpositioning
•Diameters Upto260mmor420mm
•Fastmeasurementspeeds E.g.1:58min(Ø=30mm,Roc=60mm,100points/mm2), or5:29min(Ø=130mm,Roc=150mm,50points/mm2)
*Add-ons,seepages8-11
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Measurement accuracy
Optical metrology based on MWLI technology
FeaturesLuphoScan platforms enable straightforward form measurements of aspheres, spheres, flats and slight freeforms. Key benefits of the systems include fast measurement speeds, high flexibility with regard to uncommonsurfaceshapes(e.g.flatapexesorprofileswithpointsofinflection),andmaximumobjectdiametersupto420 mm. Due to the employed MWLI® sensor technology various different surface types such as transparent materials, metal parts, and ground surfaces can be scanned.
Measurement principleThe scanning process is accomplished by meansofanMWLIpointsensor(MWLI–multi-wavelengthinterferometer)andfourprecision stages. The MWLI point sensor continuously measures the distance to the objectsurfaceundertest.Objectsarerotatedbymeansofa360degreerotarystage(C),while the position of the sensor is controlled by2linearstages(enablinghorizontal(R)andvertical(Z)movements)and1rotarystage(T).Instandardoperationmodethesensoris presented normal and equidistant to the surface. It is controlled to follow the profile of an ideal counterpart of the specimen. During ameasurementtheCstagerotatestheobjectand the other stages move the probe so as to perform a spiral scan over the whole surface (seefigure).Theresultantpointcloudrevealsshapedeviationsanddefectsoftheobjectsurface.
The LuphoScan measuring platform
Deviation of an asphere from its design shape
Movement of the objectsensor in LuphoScan platforms
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Measurement accuracy
High accuracy due to unique reference concept
Measurement accuracyDue to the use of a sophisticated arrangement of reference sensors and a unique reference frame concept, the systems enable ultra high measurement accuracies betterthan±50nm(2σ).
Reference frame conceptEach LuphoScan platform comprises an Invar reference frame that can be considered as an open loop metrology frame. Three reference sensors together with one cylindrical and two plane mirrors facilitate continuous determinationofthepositionoftheobjectsensorwithinthis frame. In accordance with the Abbe principle, this concept enables compensation of all first order errors duetothemechanicalR,ZandTaxes.Thereferenceframeconceptinconjunctionwiththeextremelyhighaccuracy of the MWLI® sensor technology and an ultra precise C stage guarantee a form measurement accuracy betterthan±50nm(2σ)andareproducibilitybetterthan±20nm(2σ).
Positioning of opticsBy default the platforms come with a hydraulic expansion chuck.Alensstillstucktotheexpansionarborfromits production process, can be measured straight away. Onejustputsinthelens,tightenstheexpansionchuck,invokesapreviouslydefinedmeasurementprocedure,and starts the measurement. The whole process can easily beaccomplishedinlessthanaminute.Theadjustment
accuracy of this solution is sufficient for all standard measurement situations. Tilt and shift of the specimen are automaticallyidentifiedandcorrected.Unmountedlensescanbeheldina3-jawchuck(optional).
CalibrationAllrequiredreferenceobjectsareincluded.Thecalibrationprocedure for adapting the platform to temperature changes can easily be conducted by the user. The complete cycletakesaround15min.
Reference frame and layout of probe stages (R, Z, T) and object stage (C)
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Software
Screenshot of theLuphoScan software
Intuitive software for easy control and data analysis
Data input and measurement procedureLuphoScanplatformscomewithasoftwarepackagethatenables complete control of the system, predefinition of measurement procedures, analysis of measurement results, and print out of test reports. Measurements usually start by putting in a surface description, e.g. by entering the radius of curvature, conic constant, and even and uneven aspheric coefficients of the part under test. Subsequentlythesoftwaredisplaysthetrajectoryoftheobjectsensor,enablingastraightforwardcheckwhetherthe correct description has been entered. In addition, the software provides an easy way for comparison with aSAGtable.Thedatadensitycanbeuseradjustedandthe program suggests an optimal setting for minimal measurement times. During measurements a countdown shows the remaining time.
Data analysisAfter a measurement has been finished a topview, a 3D view and cutaways of the error map are shown. Several tools are available for analysing the data. Tilt and shift
compensation,aswellasbestfitsubtraction(sphericoraspheric)canbeseparatelyswitchedoffandon.Furtherfeaturesincludesophisticatedadjustmentofaperturesand various filtering tools, such as low pass or high pass filters,filtersforeliminatingpeakscausedbydustparticlesetc. The measurement data can be interpreted as being obtainedfromapolishedorgroundsurface(afterthemeasurement!).Errorscanbedisplayedperpendicularwith respect to the ideal surface or in the direction of the optical axis of the tested surface. Of course, all standard parameterssuchasPower,PV,RMS,andZernickevaluesare displayed.
Data exportMeasurement data can be exported as 3D sets or 2D linescans.BesidesthenativesoftwareformatsX,Y,Z,dNordZ(3D)andX,Z,dNordZ(2D)variousfurtherformatsareavailable.Forinstance,datacanbeexportedinZygosMetroProXYZformat(3D)andTaylorHobsonsMODandPRFformats(2D).Theseformatsenablestraightforward utilisation in the production line, e.g. for corrective polishing.
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Applications
LuphoScan technology offers maximum flexibility
Main field of applicationLuphoScan platforms are designed to measure 3D topologies of rotationally symmetric surfaces such as concave and convex aspheric lenses. Due to the stage layout most of the lenses that fit in the machine can be measured. There are no restrictions with regard to sphericaldeparture,uncommonapexshapes(e.g.flatapexes),steepslopes,orprofilesthatexhibitinflectionpoints. In standard operation mode the theoretical description of the surface under test has to be provided, based on the radius of curvature, the conic constant and theasphericcoefficients(evenanduneven).Bydefaulttheplatform’ssoftwarepackageenablesmeasurementofpolished or ground parts.
Ground lensesThe unique MWLI approach also enables measurement of rough surfaces such as ground lenses. The platforms are therefore able to measure a lens in all its states from the green body through to the ultra precision polished lens with the same measuring machine.
Special shapesAlthough the platforms are designed to measure rotationally symmetric parts, they are also able to measure slight freeform parts, if the departure from an aspheric, spheric or flat shape is small. Examples include ellipsoidal X-ray mirrors or beam shaping elements.
Further applicationsBesides the standard measurement applications, it is also possible to utilise LuphoScan platforms for complete characterisation of optical elements by means of the LuphoSwapextension(seepage8-9).Thistoolfacilitatesthicknessmeasurementoflenses,aswellasdeterminationof wedge and decentre errors. In addition, several add-on software modules are available that enable straightforward measurement of discontinuous optics such as segmented surfaces including rectangular parts, annular optics, or surfaceswithdiffractivesteps,andaxicons(seepage10-11foracompletelist).
Measurement of a segmented aspheric lens
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LuphoSwap – Extension
Complete form error characterisation of optical parts by LuphoSwap extension
AchievementsLuphoSwap is an extension available for LuphoScan 260 and LuphoScan 420 platforms that enables complete characterisation of both surfaces of a lens. The two surfaces are measured successively. A unique measurement concept enables absolute correlation of the results measured on both sides. That is, at the same time as the form errors are measured this tool determinestheexactlensthickness,thewedgeanddecentre errors of the two surfaces and their rotational orientation. In addition, the lens–mount positioning can be assessed. This powerful tool is based on the absolute measurement capability of the LuphoSmart sensor technology, a unique holder concept, and on an additional(runout)referencesensor.
Measurement principleAs in standard operation mode, the theoretical description (includingRoc,conicconstantandasphericcoefficients)is entered – but now for both sides. The lens under test is then mounted in the special LuphoSwap holder and the measurement of the first surface is started as usual. Subsequently the lens with the holder is turned over as shown in the figure and the second surface is measured as usual. In addition to the standard scanning procedure, theobjectsensoriscontrolledinawaytomeasuretheexact orientation of each surface with respect to the calibrated LuphoSwap holder. In this way the software can automatically determine all geometric parameters (thickness)anderrorsofthetestlens(form,wedgeanddecentre).Hence,usingthistoolisaseasyasperformingany other measurement of the same lens in standard LuphoScan platforms.
LuphoScan 260 with installed LuphoSwap lens holder
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LuphoSwap – Extension
Object sensor, Pos 1:Measuring lens
Object sensor, Pos 2:Referencing position
Additional sensor:Measuring runout
The turning of the LuphoSwap holder including a lens
Objectdimensions
Diameter 5 … 70 mm
Thickness 2 … 25 mm
Maximal slope ±60°
Objectmaterials Transparent(e.g.glass),Opaque(e.g.ceramic,metal)
Lensthicknessdetermination ±0.5μm
Wedgeerrordetermination(Roc=50mm) ±5’’
Orientation of both optical surfaces Wedge, Decentre
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Measurement accuracy
Augmented flexibility through software add-ons
Module 1: Segmented lensesThis module allows for testing lens segments such as rectangular optics. The platforms still control the stages to perform continuous spiral scans. Signals that do not stem fromanobjectsurfaceareautomaticallyidentifiedandomitted.
•Underlyingsurfaceshapes:Aspheres,Spheres,Flats
•Formmeasurementaccuracy:±50nm(2σ)
•Automatedsurfacemasking
•Canbecombinedwiththe“Annularlenses”module andthe“Axicons”module
Axicons with angles from 0 up to 90 degrees Asphero-diffractive lens Annular asphere
Deviation of a segmented asphere from the design curvature
Measuring an aspheric lens segment
Several add-on software modules are available for LuphoScan platforms that enable straightforward measurement of various, discontinuous optics. The modules are based on the absolute measurement capability of the MWLI sensor technology. They facilitate 3D form measurements of segmented lenses, annular lenses, axicons, cones, and asphero-diffractive lenses. Every module comes with a sophisticated data analysis tool. In addition, all the standard data options are available, and of course, themodulesworkforeverymaterialthatcanbetested in LuphoScan platforms.
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140 mm140 mm
4 µm
45 mm45 mm
3 µm
50 mm50 mm
3,5 µm
Software modules
Module 2: Annular lensesThis add-on equips LuphoScan platforms with adapted measurement procedures in order to assess annular lenses with same form measurement accuracy as for standard aspheric lenses.
•Objectshapes:Aspheres,Spheres,Flats
•Formmeasurementaccuracy:±50nm(2σ)
•Largesphericaldepartures
•Adjustableinnerandouterdiameters: 0…260 mm or 420 mm
•Canbecombinedwith“SegmentedOptics”module
Module 3: AxiconsAdds measurement capability that enables highly accurate assessment of form and angular errors of axicons.
•Objectshapes:Axicons,truncatedcones
•Formmeasurementaccuracy:±25nm(2σ)
•Anglemeasurementaccuracy:upto0.001°
•Determinationofslope(Bestfitslope)
Module 4: Asphero-diffractive lensesThis tool enables non-contact inspection of the 3D deviations of asphero-diffractive lenses from the underlying aspheric design shape. Diffractive steps can be automatically removed.
•Underlyingsurfaceshapes:Aspheres,Spheres,Flats
•Diffractivestructures:Verticalsteps
•Stepheightsupto±600μm
•Arbitraryno.ofzonesperlens
•Formmeasurementaccuracy:±50nm(2σ)
•Determinationofstepheights
•Removalofstepsduringanalysis
Deviation of an aspheric annular lens from the design curvature
Deviation of an axicon from the corresponding ideal cone
Raw 3D measurement data of a diffractive lens with vertical surface steps
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LuphoScan models
Variety of platform configurations
Platform sizesThe LuphoScan platform technology is available in three differentsizesandindifferentmeasuringconfigurations.Theplatformsizedeterminesthemaximalobjectdiameter that can be measured by a system. Maximal measurable diameters are 260 mm and 420 mm. In addition, depending on the customer’s main application a system can be configured for measuring a larger variety of concave or convex parts.
The LuphoScan 260 and 420
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Measurement accuracy
Platform designA LuphoScan measuring system consists of the actual measuring platform, a measuring table, controller of actuators, a protective cover, and a control and evaluation computer. The measuring platform itself is placed on a breadboard that is uncoupled from mechanical vibrations by means of a pneumatic vibration damping system with automatic re-leveling. The granite base of the platform allows for very little influence on the measuring system due to fluctuations in temperature. The yellow reference frame to which both linear reference mirrors are attached is made from Invar and thus has a low thermal expansion coefficient. Additionally, the linear reference mirrors are equipped with protective elements against air turbulences, in order to reduce the effects of air fluctuations on the distance measurement.
UsabilityThe platforms are designed for use in production environments. Due to uncoupling from mechanical influences and due to employing materials with low thermalexpansioncoefficientsinconjunctionwithshort measurement times, the systems guarantee highly accurate, reproducible measurement results also when placed in the production hall. A temperature stability of ±1°Cistypicallysufficient.Aftercommissioningincludinga basic calibration, a user can easily execute a 15-minute calibration procedure enabling the platform to adapt to temperature changes.
LuphoScan configurations
Models LuphoScan260(A,B) LuphoScan420(A,B)
Maximumobjectdiameter 260 mm 420 mm
Maximal SAG heightsConvex 55 mm, 50 mm 80 mm, 75 mm
Concave -20 mm, -30 mm -30 mm, -50 mm
Maximaldiameterwith90°slope 75 mm, 55 mm 105 mm, 65 mm
Hydraulicexpansionchuck HD25 HD25, HD40
Maximumobjectweight 25kg 50kg
Machinedimensions(wxdxh) 85 cm x 100 cm x 186 cm 100 cm x 115 cm x 186 cm
Measurementtime(3Dtopology)
Plane, ∅=25mm 0:55min(16points/mm2),1:36min(100points/mm2)
Sphere,Roc=60mm,∅=40mm
2:19min(16points/mm2),2:36min(100points/mm2)
Sphere,Roc=(±)80mm,∅=80mm
4:10min(16points/mm2),5:18min(100points/mm2)
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Specifications
Measurement system
Models LuphoScan 260 LuphoScan 420
Machine type 4-axis(3mechanicalbearings,1airbearing)
Measurement principle Scanning point interferometry
Sensor technology Fibreopticsbasedmulti-wavelengthinterferometer(MWLI®)
Scanningmode(3D) Spiral, equidistant, normal
Measurementvolume(diameterxheight) 260 mm x 75 mm 420 mm x 100 mm
Maximum tilt 90°
Referencesystem
3 MWLI® sensors
Invar frame
1storderR,Z,Taxiserrorcompensation
Object parameters
Surface shapes Aspheric, spheric, flat, slight freeform
Surface finish Polished, rough, transparent, specular, opaque
Reflectivityrange 0.1 % ... 100 %
Spherical departure Unrestricted(objectsensorfollowsidealprofile)
Maximaldiameterwith90°slope 75 mm 105 mm
Maximumobjectdiameter 260 mm 420 mm
Maximumobjectweight 25kg 50kg
Machine characteristics
Objectmount Hydraulicexpansionchuck(HD25orHD40),optional:3-jawchuck(D=22…200mm)
Internal data rate 2500Hz
Wavelength range 1530 nm ... 1610 nm
Laser classificationClass 1
Continuouswaveoutput(CW),<1mW
Machinedimensions(wxdxh) 85 cm x 100 cm x 186 cm 100 cm x 115 cm x 186 cm
Machine weight 450kg 600kg
Compressed air requirement 6..10bar,20litre/min
Electrical power requirement 230VAC,50/60Hz,<700W
Technical data
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Specifications
85 cm*
186
cm*
100 cm*
Data handling
Parameter input Asphericcoefficients(even,uneven)
Measurement data 3D, 2D linescan
Data export formats
3DZygoMetroProXYZ,
X,Y,Z,dN(ASCII,binary),X,Y,Z,dZ(ASCII,binary)
2DTaylorHobsonMOD,TaylorHobsonPRF,
X,Z,dN(ASCII,binary),X,Z,dZ(ASCII,binary)
Data analysis3Dsurfacevisualisation,adjustablecross-section,2Dgraphics,filtering(LPF,HPF,Gaussian),best-fitradius,asphericfit,PV,RMS,tangential&radialerrors,measurementreport(PDF)
Measurement characteristics
Accuracy(2σ)(angleofincidence≤±1°)
polished Ra<1μm 1μm≤Ra≤3μm
±50nm ±250nm ±1μm
Longitudinal resolution 0.1 nm
Spotsize 4μm
Lateral resolution (points per mm2) (adjustable:)0.1...2×105
*DimensionsofLuphoScan260
Copyright© 2015 • Taylor Hobson • LuphoScan_21 September
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