The three-flat method by interferometry is commonly used to measure the flatness of standard flat surfaces for high-precision interferometers.
The ambient temperature fluctuation during the measurements by the three-flat method was within 0.1°C.
An absolute line profile could be measured only along a rotation axis on the reference or the detected flat by the three-flat method.
Absolute flatness of three silicon plane mirrors have been measured by a three-intersection method based on the three-flat method using a near-infrared interferometer.
The absolute line profile of the silicon mirror cannot be measured by the three-flat method when a visible light is used.
They also measured the absolute line profiles of three silicon plane mirrors for standard flats through the use of the three-flat method by near-infrared interferometry [10].
When there was a peak in relative line profiles for one combination of two flats, the corresponding peak could appear in all absolute line profiles of A, B, and C flats in the three-flat method.
In the three-intersection method, the reference flat (the B flat) was rotated around the z-axis, or the detected flat (the C flat) was shifted from the combination B and C flats in the three-flat method.
To measure the absolute line profile of the silicon mirror by the three-flat method, an interferometer with a light source where the silicon mirror is transparent must be constructed, and only three silicon mirrors are used to measure the absolute line profiles.
Three silicon plane mirrors (A, B, and C flats) were combined in pairs with different positional combinations (transmission reference A and detected B, A and C, and B and C) in the interferometer and used for calculation of the absolute line profile of each silicon plane mirror by the three-flat method [2].
A predicting model was developed, which took advantage of flat mapping method to avoid unnecessary calculations.
