How to determine the number of interference fringes
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Different levels of flatness in the sealed end face result in different types of patterns. Figure 16 shows several images that meet the flatness requirements, while the various images that do not meet the requirements are not shown. Figure 16a shows straight stripes, indicating that the flatness of the sealed end face is close to that of an optical plano-plate; clearly, this level of flatness is the highest ； Figure 16b shows curved fringes, with the number of interference fringes being one or less, that is, less than 0.3 μm ； Figure 16c also shows curved fringes; there are two interference fringes, and the flatness is within 0.6 μm ； Figure 16d shows concentric interference fringes; if this is used to check the flatness after grinding a flat surface, it indicates that the inner edge of the end face is higher while the outer edge is lower. The greater the number of concentric circles, the larger the difference in height between the inner and outer edges. For seals under low pressure, the number of concentric circles should not exceed 3 (with a flatness tolerance of 0.0009 mm); otherwise, the leakage at the seal surface will be excessive. In balanced seals under high pressure, the sealing surfaces may be pushed apart; therefore, if concentric rings appear, there should not be more than 2 of them. Flatness inspection must be carried out after polishing; otherwise, the surface finish will not be sufficient and the striations will not be visible, especially in the case of graphite rings. In addition, the end faces of the flat plate and the sealing ring must be cleaned thoroughly, free of moisture and dust. The flat plate should be handled with care to avoid damaging its surface. To ensure the accuracy of testing, the flat plate should also be calibrated regularly.
Interference fringes are a phenomenon that occurs when two light waves meet in an interferometer; one of these light waves usually comes from a reference surface (typically a known optical plane), while the other wave comes from the surface being tested. When these two light waves meet, they form alternating bright and dark stripes in space due to coherence; these are interference fringes. By observing and analyzing these interference fringes, the flatness of the surface under test can be inferred. Based on your description, the following steps can be used to determine the number of interference fringes and the flatness: 1. Prepare an optical flat (plane plate) and the sealed end face to be tested (sample), making sure that both are clean, free of moisture and dust. 2. Place the sealed end face to be tested on an optical plane, and use an interferometer to project a coherent light source onto these two surfaces. 3. Observe the interference fringes produced. If the stripes are straight (as shown in Figure 16a), this indicates that the flatness of the specimen is close to that of an optical plane, meaning it has a high degree of flatness. 4. If the stripes are curved (as shown in Figures 16b and 16c), the flatness is evaluated by observing the degree of curvature and the number of stripes. Generally, each complete interference fringe represents a height difference of half a wavelength. So, based on the wavelength of the light source you use, you can calculate the flatness. For example, for light with a wavelength of 0.3 micrometers, the number of fringes is less than 1 (i.e., the height difference is less than 0.3 micrometers); in Figure 16c, there are two interference fringes, indicating a flatness within 0.6 micrometers. 5. If concentric interference fringes are observed (as in Figure 16d), this indicates a significant height difference between the central and peripheral parts of the specimen. The greater the number of concentric circles, the larger this difference in height. Based on specific requirements (such as the standards for flatness in terms of sealing), decide whether to accept this level of flatness. 6. Finally, determine the standard for the number of stripes based on your sealing requirements (such as pressure level, etc.). For example, in the case of seals with low pressure, no more than 3 concentric circles are allowed (with a flatness requirement of 0.0009 mm) ； For balanced seals under high pressure, do not exceed 2 concentric circles. To ensure the accuracy of measurements, the flat plate should be regularly calibrated to verify its flatness and condition, in order to maintain accurate results. .