The Dimensional Laboratories are primarily concerned with maintenance of the primary standard for the unit of length and disseminating traceability for dimensional measurements. The metre, one of the fundamental SI units, was redefined in 1983 as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second”. This definition has been realized in the TÜBİTAK UME Wavelength Laboratory with reference stabilized lasers. Having achieved traceability at the primary level by using these lasers, the Dimensional Laboratories transfer the unit “metre” to accredited laboratories and industry. The study areas of the dimensional laboratory can mainly be divided into following groups.

**GAUGE BLOCKS AND INTERFERENTIAL LENGTH MEASUREMENTS**

The length of materials can be directly compared with internationally recommended wavelength standards using interferometry. This guarantees a link-up of the quantity of length in compliance with the definition of the SI unit, the metre, and with the recommendations made for its realization. Using interferometry, the wavelengths of a laser along a path alongside a body are counted and interpolated, which makes an unusually large measuring range possible, from several dozen metres to fractions of nanometres.

The highest accuracy grade gauge blocks are calibrated at UME using interferometry. The gauge blocks up to 300 mm are calibrated by an automatic gauge block interferometer which compares the length of the gauge block against the very accurately known wavelength of two frequency-stabilised He-Ne lasers. Gauge blocks from 100mm up to 1000mm are calibrated by High accuracy Long Gauge Block Comparator or home made Köster Interferometer located in the length laboratory. The mechanical comparators are also available for calibration of gauge blocks of lower accuracy grades.

Flatness measurement of optical surfaces (such as optical flats, mirrors, or platens) and calibration of corner cubes are performed using Zygo Verifire Flatness Interferometer. Diameters of parts up to 150mm can be accommodated.

**ANGLE MEASUREMENTS**

The SI unit of the plane angle is the radian (rad.) and it is defined as the angle subtended at the centre of a circle by an arc, length of which is equal to the radius. In industry, the unit “degree (°)” is used for angle measurement. It is obtained by the 360th division of the full circle, which is in fact 2 p rad. angle. A degree is divided into sixty minutes ('), and then a minute is further subdivided into sixty seconds ("). There is no primary artefact standard for the angle. It is determined by dividing a full circle as equally as possible. This is done by the use of so-called self-calibrating methods.

The national standard for angle measurements in UME is realised utilising a precise air bearing rotary table equipped with Heidenhein ERP 880 encoder (0.001” resolution). Precise and small increments can be achieved using nano position control mechanism of the table. High Resolution (0.005”) autocollimator (Elcomat HR) is used in conjunction with the precise table for calibration of high accuracy angle standards (such as polygons and angular tables) applying the self-calibrating methods. In addition, Moore indexing tables, autocollimators, polygons, angle gauge blocks and levels are available for various angle measurements.

An angle can also be obtained by trigonometric calculation of length measurements. Using a sine-bar or similar type of instrument, small angles can be generated to calibrate spirit levels and electronic levels. UME has made various sine bars for calibration of such instruments. Sine bars can be used for realisation of SI Unit angle with an uncertainty of 0.01” (50 nanoradian), calibration of autocollimators and levels.

**SURFACE TEXTURE AND NANOMETROLOGY**

Surface texture affects the mechanical and physical properties of parts significantly. Required surface texture on the parts can be obtained by choosing and monitoring the suitable manufacturing process. In this way, required physical properties on the parts such as friction, wear, fit, seal, fatigue, adhesion of coatings, electrical and thermal contact, and even optical properties such as glass transparency, etc., can be adjusted by manufacturing design.

Measurements of surface roughness are made using stylus equipment (Mahr Perthometer Concept) at UME and all of the surface-roughness parameters can be determined (Ra, Rq, Ry, Rz, etc.). Besides, geometry, roughness and depth standards are calibrated with high accuracy in accordance with ISO 4287. The contact stylus instrument is traceable to the unit metre through our reference standards.

Nanosensors are calibrated using plane mirror and differential interferometers. The new work in this area is underway for mask and line scale measurements. We are constructing a system for precise mask and line scale measurements. The system contains nanoscale 2D positioning system (300mm x 400mm precise air bearing stage made out of zerodur the position of which is controlled with nanopositioners and 2-axes differential laser interferometer) and digital microscope with magnification up to 150X.

Used as a transfer standard for the calibration and for providing traceability chain for the devices used in the area of dimensional nanometrology, optical gratings are needed to be calibrated. Laser diffractometer for calibratıon of grating pitch standards which uses optical diffraction principle and Littrow configuration along with UME reference angle encoder system, is realized.

**GEOMETRICAL STANDARDS AND FORM MEASUREMENTS**

The form features of parts are significantly important for the fittings. They are also important for high accuracy dimensional artefacts as the dimensional accuracy also depends on the form of parts. Simply, the form error can be described as the deviation of the shape of the manufactured part from the relevant ideal geometric shape. Geometrical forms are described mathematically and there are no primary artefact standards. Therefore, measurement systems, which can reproduce the geometric forms without reference to any physical artefact, can be used for high accuracy measurement of the form parameters. This is performed with the help of so-called error-separation methods (i.e. reversal techniques), or by means of physical references, such as the direction of gravity (plumb line) or the directionality of a light beam.

Straightness and squareness measurements are performed with a CMM machine using the reversal technique. Measurement values taken by CMM are transferred to the special software developed by UME. The error of the CMM is separated and straightness or squareness (or both) error of the artefact is determined.

Work is in progress for new straightness measurement facilities. We are constructing a straightness measurement device out of a 1m precise air bearing slide.

The flatness of surface plates up to a number of square metres in size is measured by electronic levels and software specially developed for this purpose.

Roundness and cylindiricity measurements are performed using Mahr MMQ40 and MFU 800 form measuring instruments. Error-separation method is applied for high accuracy measurements.

Gauges are calibrated on the Mahr 828 CiM length-measuring machine modified at UME. The machine fitted with a remote control mechanism and temperature control case is used for high-accuracy grade calibrations of setting ring and plug gauges. Use of different purpose measuring heads also allows measurement of thread and gap gauges. Diameters from 2mm to 300mm (external diameters, down to 0.1mm) can be measured using a gauge-block substitution technique. The achievable uncertainty can, depending on the gauge, be less than 0.2 µm.

**3-DIMENSION MEASUREMENTS (CO-ORDINATE METROLOGY)**

Co-ordinate measuring machines are becoming more important in industrial production due to their various applications in particular for measurement of form and distance at the same time (geometric dimensioning and tolerancing). The activities of UME in the field of coordinate metrology include the calibration of gauges and artefacts (particularly cylindrical gauges, taper gauges, thread gauges, taper thread gauges, gear standards and special-purpose gauges) the verification of workpieces and the investigation of calibration procedures.

**TOPOGRAPHIC AND INDUSTRIAL MEASUREMENTS**

A homemade 5m bench is used for calibration of tapes and rules. Large range of optical tooling equipment such as precise optical levels, teodolites, telescopes and total stations are used depending on the industrial demands and applications. Optical measurements and levelling can be performed on site.