If surface between optical flat and work surface are perfectly wrung toger n re will be no air gap between surfaces and fringe pattern will be not observable.

Three types of gratings are commonly used in more fringe system gratings. Phase transmission grating, Metal reflecting grating and line and space transmission grating. Metal reflecting grating is used for travel length more than 1800 mm.

For optical flats of Type-A, grades and optical flats are important that means it is usually designated by se two and for Type-B, thickness and grades are important, refore it is designated by se two.

Change in separation between surface and optical flat between 2 adjacent and similar fringes is equal to (3?/4 ? ?/4) = ?/2.

If band curves around line of contact or point n given surface is convex but if band curves opposite to line of contact or point n surface is a concave surface.

Index mark is utilized by all scale readouts, with reference to which reading of scale is noted. Symmetrical forming between two index marks is type which is mostly used in sophisticated machines due to high setting precision.

As per IS : 5440-1969 , Optical flats of Type-A have dimensions 25 x 10, 30 x 10, 50 x 15, 75 x 20, 100 x 25, 125 x 30, 160 x 35 ( Diameter in mm x thickness in mm) for both grade I and II.

In surface contour test, change in elevation of surface corresponds to each fringe interval shows clearly precision of this method. Value of this change in elevation is 0.0025 millimeter.

Optical flats are generally made from fused quartz or borosilicate glass as y have least coefficient of linear expansion. This glass is colourless and free from defects like bubbles, internal strains and extraneous matter.

Tolerance on flatness for Type-A opticals flats is 0.05 µm for grade optical flats of size upto 100 mm only. But for size between 101 mm to 200 mm tolerance should be 0.10 µm. For grade II, it is 0.01 µm for size upto 100 mm and 0.20 for 101-200 mm.