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Form and Wavefront Metrology

Working Group 4.21

Polarimetric calibration of quartz control plates

Quartz control plates serves to calibrate polarimeters and saccharimeters. They are used as transfer standards by manufacturers of polarimeters and in the sugar and pharmaceutical industry, to check polarimetric instruments within the scope of quality management.

Polarimetry determines the rotation of the plane of polarization (optical rotation) of optically active materials (here: quartz control plates) at specific light wavelengths. Quartz control plates have defined rotary powers as a function of their thickness and temperature, as well as of the wavelength of the light used. These rotary powers are determined by a precision polarimeter and documented with a calibration certificate.

The calibration is performed in accordance with the recommendations of ICUMSA (International Commission for Uniform Methods of Sugar Analysis) or OIML (Organisation Internationale de Métrologie Légale).

A calibration first consists of a preliminary test in which the fundamental calibration capability is checked. The quartz control plates must meet the required minimum requirements with respect to their dimensions, optical pureness, flatness, parallelism of the faces and optical axis errors. After that, the actual measurement value - the optical rotation – is measured with the precision polarimeter.

The measurement uncertainty of the polarimeter amounts to 0.001° (k=2).

The principle of the set-up of the polarimeter consists of the light source, a polarizer with angle measuring instrument (P), the quartz control plate (QP), a Faraday modulator (FM), an analyser (A), a photodetector (PD) and a contoller and evaluation unit.

Fig. 1:Set-up of the precision polarimeter
Fig. 2: The precision polarimeter for the calibration of quartz control plates