The fringes act very much like the lines on a topography map, where the fringes are always perpendicular to the wedge between the surfaces. When wringing first begins, there is a large angle in the air wedge and the fringes will resemble grid topography-lines. If the fringes are straight; then the surface is flat. If the surfaces are allowed to fully wring and become parallel, the straight fringes will widen until only a dark fringe remains, and they will disappear completely. If the surface is not flat, the grid lines will have some bends in them, indicating the topography of the surface. Straight fringes with bends in them may indicate a raised elevation or a depression. Straight fringes with a "V" shape in the middle indicate a ridge or valley running across the center, while straight fringes with curves near the ends indicate edges that are either rounded-off or have a raised lip.
If the surfaces are not completely flat, as wringing progresses the fringes will widen and continue to bend more. When fully wrung, they will resemble contour topography-lines, indicating the deviations on the surface. Rounded fringes indicate gentle sloping or slightly cylindrical surfaces, while tight corners in the fringes indicate sharp angles in the surface. Small, round circles may indicate bumps or depressions, while concentric circles indicate a conical shape. Unevenly spaced concentric circles indicate a convex or concave surface. Before the surfaces fully wring, these fringes will be distorted due to the added angle of the air wedge, changing into the contours as the air is slowly pushed out.
A single dark-fringe has the same gap thickness, following a line that runs the entire length of the fringe. The adjacent bright-fringe will indicate a thickness which is either 1/2 of the wavelength narrower or 1/2 of the wavelength wider. The thinner and closer the fringes are; the steeper the slope is, while wider fringes, spaced further apart, show a shallower slope. Unfortunately, it is impossible to tell whether the fringes are indicating an uphill or downhill slope from just a single view of the fringes alone, because the adjacent fringes can be going either way. A ring of concentric circles can indicate that the surface is either concave or convex, which is an effect similar to the hollow-mask illusion. There are really three ways to test the surface for shape, but the most common is the "finger-pressure test." In this test, slight pressure is applied to the flat, to see which way the fringes move. The fringes move away from the narrow end of the wedge. If the testing surface is concave, when pressure is applied to the center of the rings, the flat will flex a little and the fringes will appear to move inward. However, if the surface is convex, the flat will be in point-contact with the surface in that spot, so it will have no room to flex, and the fringes will remain stationary, merely growing a little thicker. If pressure is applied to the edge of the flat, the opposite happens. If the surface is convex the flat will rock a little, causing the fringes to move toward the finger. However, if the surface is concave the flat will flex a little, and the fringes will move away from the finger toward the center. Although this is called a "finger" pressure test, a wooden stick or some other instrument is often used to avoid heating the glass (with the mere weight of a toothpick often being enough pressure).
Another method involves exposing the flat to white light, allowing rainbow fringes to form, and then pressing in the center. If the surface is concave, there will be point-contact along the edge, and the fringe there will turn dark. If the surface is convex, there will be point-contact in the center, and the central fringe will turn dark. Much like tempering colors of steel, the fringes will be slightly brownish at the narrower side of the fringe and blue on the wider side, so if the surface is concave the blue will be on the inside of the rings, but if convex the blue will be on the outside.
The third method involves moving the eye in relation to the flat. When moving the eye from a zero-degree angle of incidence to an oblique angle, the fringes will appear to move. If the testing surface is concave, the fringes will appear to move toward the center. If the surface is convex, the fringes will move away from the center. To get a truely accurate reading of the surface, the test should usually be performed in at least two different directions. The fringes only represent part of a grid, so a valley running across the surface may only show as a slight bend in the fringe if the fringe is running parallel to the valley. However, if the flat is rotated 90 degrees and retested, the fringes will run perpendicular to the valley and it will show up as a row of "V" or "U" shaped contours in the fringes. By testing in more that one orientation, a better map of the surface can be made.
The fringes act very much like the lines on a topography map, where the fringes are always perpendicular to the wedge between the surfaces. When wringing first begins, there is a large angle in the air wedge and the fringes will resemble grid topography-lines. If the fringes are straight; then the surface is flat. If the surfaces are allowed to fully wring and become parallel, the straight fringes will widen until only a dark fringe remains, and they will disappear completely. If the surface is not flat, the grid lines will have some bends in them, indicating the topography of the surface. Straight fringes with bends in them may indicate a raised elevation or a depression. Straight fringes with a "V" shape in the middle indicate a ridge or valley running across the center, while straight fringes with curves near the ends indicate edges that are either rounded-off or have a raised lip.
If the surfaces are not completely flat, as wringing progresses the fringes will widen and continue to bend more. When fully wrung, they will resemble contour topography-lines, indicating the deviations on the surface. Rounded fringes indicate gentle sloping or slightly cylindrical surfaces, while tight corners in the fringes indicate sharp angles in the surface. Small, round circles may indicate bumps or depressions, while concentric circles indicate a conical shape. Unevenly spaced concentric circles indicate a convex or concave surface. Before the surfaces fully wring, these fringes will be distorted due to the added angle of the air wedge, changing into the contours as the air is slowly pushed out.
A single dark-fringe has the same gap thickness, following a line that runs the entire length of the fringe. The adjacent bright-fringe will indicate a thickness which is either 1/2 of the wavelength narrower or 1/2 of the wavelength wider. The thinner and closer the fringes are; the steeper the slope is, while wider fringes, spaced further apart, show a shallower slope. Unfortunately, it is impossible to tell whether the fringes are indicating an uphill or downhill slope from just a single view of the fringes alone, because the adjacent fringes can be going either way. A ring of concentric circles can indicate that the surface is either concave or convex, which is an effect similar to the hollow-mask illusion. There are really three ways to test the surface for shape, but the most common is the "finger-pressure test." In this test, slight pressure is applied to the flat, to see which way the fringes move. The fringes move away from the narrow end of the wedge. If the testing surface is concave, when pressure is applied to the center of the rings, the flat will flex a little and the fringes will appear to move inward. However, if the surface is convex, the flat will be in point-contact with the surface in that spot, so it will have no room to flex, and the fringes will remain stationary, merely growing a little thicker. If pressure is applied to the edge of the flat, the opposite happens. If the surface is convex the flat will rock a little, causing the fringes to move toward the finger. However, if the surface is concave the flat will flex a little, and the fringes will move away from the finger toward the center. Although this is called a "finger" pressure test, a wooden stick or some other instrument is often used to avoid heating the glass (with the mere weight of a toothpick often being enough pressure).
Another method involves exposing the flat to white light, allowing rainbow fringes to form, and then pressing in the center. If the surface is concave, there will be point-contact along the edge, and the fringe there will turn dark. If the surface is convex, there will be point-contact in the center, and the central fringe will turn dark. Much like tempering colors of steel, the fringes will be slightly brownish at the narrower side of the fringe and blue on the wider side, so if the surface is concave the blue will be on the inside of the rings, but if convex the blue will be on the outside.
The third method involves moving the eye in relation to the flat. When moving the eye from a zero-degree angle of incidence to an oblique angle, the fringes will appear to move. If the testing surface is concave, the fringes will appear to move toward the center. If the surface is convex, the fringes will move away from the center. To get a truely accurate reading of the surface, the test should usually be performed in at least two different directions. The fringes only represent part of a grid, so a valley running across the surface may only show as a slight bend in the fringe if the fringe is running parallel to the valley. However, if the flat is rotated 90 degrees and retested, the fringes will run perpendicular to the valley and it will show up as a row of "V" or "U" shaped contours in the fringes. By testing in more that one orientation, a better map of the surface can be made.
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