Identification and reading of interference fringes (test results) of flat crystal detector for flatness tests of mechanical seals?

The flat crystal detector is often used in the flatness tests of mechanical seals. The phenomenon of interference effect of light waves is usually used to get flatness values that cannot be tested by ordinary measuring tools.
① Straight Bands：The image of dry streaks shown below are straight bands detected with an optical flat（=flat crystal） under sodium light. Straight band images show that the measured flatness is very high, close to the ideal value, and the flatness deviation is less than 0.029um. Pls see figure 1 below：

② Concentric interference fringes：The interference fringes are concentric, indicating that the surface  being measured has a slight curvature. The picture below shows the concentric interference fringes detected with an optical flat （=flat crystal） under a sodium lamp. Pls see figure 2 below：

③ Qualified Interference Bands：As shown in the picture below, 1, 2, 3, 4, 5 are qualified interference bands while  6, 7, 8, 9 are unqualified interference bands. Pls see figure 3 below：

To judge whether the surface to be tested is concave or convex, you can press lightly on the edge of the flat crystal with your fingers, and the light interference fringes will move to the outer periphery and indicate the condition is convex, otherwise it is concave. The former is called high aperture and the latter is called low aperture. In actual measurement, as long as the interference fringes on the sealing ring  of the mechanical seal are very regular and there is no sudden change, the seals all can be used.

④ The end face is uneven. The picture below is the image when the flatness error is 2.7um when the sealing surface of the mechanical seal is tested by a flat crystal under a sodium light lamp. This local flatness error of the sealing surface is caused by poor grinding and polishing. In this case, the leakage of the seal is serious. Pls see figure 4 below：

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