The heart of interference comparators of the "Kösters-Type" is a Kösters-prism which acts
as beam splitter. This way the beams are folded. For this reason such comparators are more compact compared
to other interference comparators (Twyman-Green type). The PTB Kösters-Comparator is used for primary
length calibrations of gauge blocks up to 1 m.
Fig. 1 shows a scheme of the "old" PTB Kösters-Comparator with visual fringe evaluation.
Here the Cd-lamp has already been replaced by three stabilized lasers. In this traditional setup interference
fringes at the position of a vacuum cell where adjusted by a slightly variation of the air pressure.
This way it was possible to trace the wavelength of the light back to the vacuum wavelength. For this reason
the (old) Kösters-Comparator was also called vacuum wavelength comparator.
Fig. 1
In the new design, the visual observation was replaced by a camera system featuring 12-bit dynamics.
Five frames of intensity data, where equidistant phase steps are applied from frame to frame
(phase stepping interferometry), are used to calculate the interference phase map using an appropriate algorithm.
The phase step is made by a defined slightly tilting of wedged plates by a piezo-actuator. The evaluated phase
map includes a newly designed refractometer cell and the interference of the gauge block with platen.
Fig. 2
The massive aluminum body of the Kösters-Comparator is temperature stabilized by water thermostats
(new: including the top cover). In this way a very homogenous temperature distribution is achieved.
Defined temperatures can be set, typically ranging from 15 °C to 25 °C.
The temperature is measured at the gauge block itself. In addition the temperature of the air is measured
accurately. Especially when steel gauge blocks are to be measured a highly accurate temperature measurement
is essential, since a temperature variation of only 1 mK leads to a length change of approximately 11 nm per m.
Therefore, the temperature is measured extensively by using an AC-bridge with pt25 standard thermometers
(fix point calibrated according to ITS 90) and a special designed thermo couple measurement system.
This way the temperature is measured with an uncertainty of less than 1 mK.
Fig. 3 shows an example for measurements at a 1 m steel gauge block. The offset length at 20 °C
is subtracted in this plot. The red data points in the bottom part indicate several measurements at certain
temperatures. The solid line represents a polynomial fit curve of the 2nd degree. Fig. 3, top, displays the
deviation of the data from the fit which is in the range of only view nm. From such measurements as a
function of temperature, the temperature dependent coefficient of thermal expansion (CTE) of long gauge blocks
can be measured with high accuracy.
Bild 3
- Decker, J. E.; Schödel, R.; Bönsch, G.:
"Considerations for the evaluation of measurement uncertainty in interferometric gauge block
calibration applying methods of phase step interferometry",
Metrologia, 41, L11-L17 (2004)
- Decker, J. E.; Schödel, R.; Bönsch, G.:
"Next generation Kösters Interferometer",
Proc. of SPIE 5190, 14-23 (2003)
- Bönsch, G.; Schuster, H. J.; Schödel, R.:
"Hochgenaue Temperaturmessung mit Thermoelementen",
Technisches Messen 68, 550-557 (2001)
- Darnedde, H.:
"High-precision Calibration of Long Gauge Blocks Using the Vacuum Wavelength Comparator"
Metrologia 29, 349-359 (1992)
© Physikalisch-Technische Bundesanstalt
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