Abstract: In an optical measuring apparatus so structured as to hold a CCD camera and a laser displacement gauge so that they can be three-dimensionally driven at the same time on a stage, a jig including two straight, non-parallel line segments which can be measured by the CCD camera and the laser displacement gauge in the projection plane along the Z-axis direction is mounted on the stage; the straight line segments are measured in the projection plane by the CCD camera and the laser displacement gauge to obtain formulas of these segments; the obtained formulas are subjected to the arithmetic operation to calculate an offset value of the X and Y plane coordinate centers between the CCD camera and the laser displacement gauge; and the offset value is used as position calibration data of the CCD camera and the laser displacement gauge to perform position calibration of the measurement data.

Abstract: The present invention provides a method of synthesizing polypropylene terephthalate with no more than 5 ppm acrolein and no more than 3 ppm allyl alcohol. The method comprises direct esterification of terephthalic acid with 1,3-propanediol in the presence of 30 to 200 ppm titanium in the form of an inorganic esterification catalyst containing at least 50 mol % TiO.sub.2 precipitate, blocking the esterification catalyst after esterification by adding 10 to 100 ppm phosphorus in the form of a phosphorus-oxygen compound, and then performing precondensation and polycondensation in the presence of 100 to 300 ppm antimony in the form of a conventional antimony polycondensation catalyst and optional conventional coloring agent.