We describe a modified three-flat method. In a Cartesian coordinate system, a flat can be expressed as the sum of even–odd, odd–even, even–even, and odd–odd functions. The even–odd and the odd–even functions of each flat are obtained first, and then the even–even function is calculated. All three functions are exact. The odd–odd function is difficult to obtain. In theory, this function can be solved by rotating the flat 90°, 45°, 22.5°, etc. The components of the Fourier series of this odd–odd function are derived and extracted from each rotation of the flat. A flat is approximated by the sum of the first three functions and the known components of the odd–odd function. In the experiments, the flats are oriented in six configurations by rotating the flats 180°, 90°, and 45° with respect to one another, and six measurements are performed. The exact profiles along every 45° diameter are obtained, and the profile in the area between two adjacent diameters of these diameters is also obtained with some approximation. The theoretical derivation, experiment results, and error analysis are presented.
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