INTRODUCTIONWhat do a mobile phone, a subnotebook, a nearly invisible hearing aid, a microgear wheel and a medical device have in common? They all are, or have, microcomponents in them.

Complex components such as electronic components, gears, micro pumps andsurgical implants are now commonly manufactured in the µm range using micromachining, EDM injection moulding and other techniques. The challenge is howto measure these 'intelligent miniatures' in order to confirm the quality.

But that is not enough — it is also necessary to have effective and traceable 3Dsurface measurement and characteristics that document the quality of the surfacesimultaneously with its form.

The smaller geometric forms of micro structured components become, thehigher is the challenge in meeting and measuring the required tolerances ofbores, gates, grooves and other forms. Also, tribological studies on frictionsurfaces with high precision geometries have become increasingly important asmanufacturers strive to make products with long lives.

Good surface quality with dimensionally accurate geometry are also the basis forlow wear of micro bearings or micro gears, for example.

It is mainly the surface of different materials, steep slopes and complexgeometries that have been a challenge for optical measurement devices: inpractice the most common topographies are of high-tensile components such ascarbide, carbon, ceramics and hardened steel. These materials often showvarying reflective properties combined with complex forms and steep flanks. Byusing competent optical measuring systems manufacturers are able to have asystem that measures independently of those reflections with software thatautomatically adjusts to the different surface conditions and delivers repeatableand traceable measurement results.

SOME TASKS CAN NO LONGER BE SOLVED BY TACTILE MEASUREMENTIt is a paradox: On one hand physical limits are being reached by tactile measurement systems.On the other hand, optical systems, which have not yet reached their limits, havenot been widely taken up in production measurement and quality assurance.

Why is this? It is a fact that modern fine detailed functional geometries have become too

spotlight METROLOGY

OPTICAL 3D

MICRO

METROLOGY

Brian Kyte, Alicona

“The repeatable and high-resolution

measurement

of micro manufactured components has become

increasingly important in quality assurance.”

Mark Raleigh CEO, EDM Department, Barlett Il, USA

“The traceability of measurement results to a common

and well known reference is an absolutely essential

factor. Otherwise no significant comparison with other

measured values, certified standards or well known

thresholds, or target values can be achieved.

Without the traceability no binding statement can

be made about the matching of two components.”

Prof. Richard Leach, Principal Research Scientist, NPL, UK

Figure 1. An optical 3D micro coordinate measurement device to measure form and roughness

complex to be measured with a tactile system. The results are simply not robustenough and vary depending on the measurement position.

Also, tactile systems cannot offer any information on the uncertainty of themeasurement obtained, whereas some optical based systems can offerinformation of the repeatability of every single 3D point. Therefore, the user getsan estimated measurement uncertainty with every measurement point.

Applications in the engineering sector, such as the measurement of shafts andseals, typify the problem. A tactile system, for example, would damage a sealand would neither provide characterisation data on a granular surface norfunctional data on a honed surface. These measurement tasks can only beachieved by optical methods. More and more manufacturers recognise thepossibilities provided by optical measurement techniques and have started to usethese techniques for quality assurance. However, this is still more validlaboratory, rather than in production.

But why is it that optical measurement methods have been rarely used in aproduction near environment where there is both a need and the technologyavailable to do so?

This is not a result of missing or fully developed products or solutions; it hasbeen the result of inadequate standards to satisfy the needs of the users.

Tactile systems are well established and have a wide range of standards andreference tools, making the methods comfortable to use, with traceable results.But with optical measurement, a young and relatively unknown alternative, effortshave not been completed to help the user decide which settings or parameterscan be used to achieve optimal or traceable measurements.

For tactile instruments a number of standards and reference tools are available toverify the accuracy of measurement results. These standards are still needed inthe world of optical instruments, and they are being developed. Most companieshave only focused on the development of their products, leaving out furtheraspects of providing usability, such as guidelines and standardisation. Only a fewvisionary forces from industry and research are looking beyond their technicalhorizons to counteract this drawback.

ROUGHNESS MEASUREMENT MATTERS FOR DENTAL IMPLANTSAreal based measurements are also a feature generally not available on a tactilemeasurement system. And on complex geometries such as screwed dentalimplant this is very important.

The surface characteristics of a dental implant are crucial as it is the decisivefactor for the successful and permanent adhesion with the jaw bone. In order togrow together quickly and sustainably, the surface of the implant has to have thenecessary characteristics to allow protein binding. Optical 3D measurementprovides the numerical verification of this and consequently the verification as towhether the implant has the required surface quality for a successful treatment.

Dr. Frank Rupp works with Prof. J. Geis Gerstorfer, the expert in the field ofsurface modification of implants. They are located at the Medical Materials &Technology section at the Prosthodontics department at University HospitalTübingen, Germany.

Due to the large measurement area, this measurement can only be meaningful ifareal, and not line based measurement are used. As a supplier of areal based 3Dmeasurement across large areas, Alicona is an innovative pioneer in this field.

METROLOGY spotlight

“A measurement value without a

measurement of uncertainty is useless.”

Dr. Ulrich Neuschaefer-Rube,

Coordinate Metrology Departmen PTB, Germany

“Thanks to them necessary standardisation is

slowly but surely being reviewed in international

committees. With EN ISO 25178 the first steps have

been made. Hopefully these driving forces keep going

and ensure that in near future optical measurement

techniques will find its true place in production.”

Prof. Jörg Seewig, Head of Institute for Measurement and

Sensor-Technology, TU Kaiserslautern, Germany

“Everything needs to be measured that is in

contact with the bone. And it is especially

important that I can measure the

roughness on the root of the thread.”

Dr. Frank Rupp, Medical Materials & Technology

section at the Prosthodontics department,

University Hospital Tübingen, Germany

Figure 2. Automatic CAD comparison measurement Figure 3. Detailed micro form measurement displayed in true colour — measureflanks exceeding 80º — courtesy of EDM Department Inc.

spotlight METROLOGY

Dr. Rupp has focused his investigations primarily on the condition of the surfaceof the implant in order to guarantee the best possible bone integration. Thisresearch would be severely hampered if optical 3D measurement was notavailable to him for areal and functional parameters.

This is because the surface roughness dictates how permanently an implantmates together with the jaw bone. With areal based optical 3D measurement theroughness and surface characteristics can be measured wherever it is required,including the root of the thread of the implants.

SUMMARYWe blindly trust in the timekeeping of a Swiss clock, it has a written and provenhistory. It is important to prove this precision in modern measurementtechniques. Proving the measurement accuracy is achieved by traceableinternational reference standards. For a measurement, to be trusted it must berepeatable and traceable. Only comparison with standards of the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany, or the NationalPhysical Laboratory (NPL) in Teddington, UK, can certify the accuracy of themeasurement results.

Precision is not all. It mustbe traceable. The tactilemeasurement techniqueconforms to a number ofestablished standards thatconfirm the accuracy of themeasurements made.

But tactile measurementtechniques are notcomprehensive enough forthe micro structured surfaces that are produced nowadays, and, for opticalmeasurement devices, standards for proving the accuracy have been missing.

Until recently, there was no option available that allows tracing back the accuracyof an optical roughness measurement to a certified standard; instead, it wascommon practice to use standards from tactile devices. Alicona’s CEO Dr. StefanScherer commented on this problematic practice: “Most standards are designedonly for tactile sampling. To trace back an optical measurement by using astandard made for the tactile instrument class is a rather questionable venture.Two processes and techniques are mixed up that should be observed differently.”

That is the reason why Alicona, the 3D specialist company, has taken care that

this lack of appropriate standards is now a thing of the past. Alicona’sspecifically developed standard is traceable to the PTB. This means that theaccuracy of several ISO-certified optical measurement techniques can be proved.

Prof. Jörg Seewig from the Institute of Measurement and Sensor-Technology atthe Technical University Kaiserslautern, Germany, noted that one of the mostfundamental requirements for science and industry is fulfilled: “This newstandard 100% fitted to the need of the optical measurement technique.”

THE OPTICAL MEASUREMENT TECHNIQUE REQUIRES INTERNATIONALLY VALIDSTANDARDS AND GUIDELINESIndustry and science will see their expectations fulfilled by this ‘certified’standard for the traceability of optical measurements. Therefore, one of theirrequirements for acceptance the optical measurement technique has beenachieved, but there is still a lot to be done: “Tactile measurements are trusted asnot only standards are available but also engineer practices and guide lines onhow to proceed. In the optical measurement world this certainty is missing asthere are few engineer practices and guidelines available. This is one of the thereasons why the user cannot judge if the system he has chosen is the correct

option for his application,”concluded Dr. UlrichNeuschaefer-Rube from thecoordinate measurementtechnique department of thePTB.

There are various opticalmethods available to companiessuch as FocusVariation, WhiteLight Interferometry and laserbased systems offering different

methods and results. Micro components with steep walls and complexgeometries are more suited to Focus Variation, whereas smooth surfaces arebetter suited to the other techniques.

This article is written by Brian Kyte director of Alicona UK Ltd.Brian has wide experience in the area of 3D optical metrology both in terms of principles andapplications in many application areas, particularly in the field of micro machining, where he has beenworking for two years.Brian started Alicona UK in 2005 having worked with Alicona almost since its inception in 2002 and hasbeen influential in the development path of the company. Prior to this he was Sales Director at ISSGroup Services, specialising in Optical and Electron Microscopy. His previous experience includessurface analysis instrumentation with the VG group of companies.www.howtomeasuremicrocomponents.com

Figure 4. Deviation measurement of an injection valve using the geometric primitive of a cone Figure 5. Steep angle measurement of a high-precision injection mouldedcomponent — measure smallest details of large areas

Figure 6. Form measurement of a micro precision milling cutter — courtesy of FBK — Technical University Kaiserslautern