OPTIONAL FEATURES - OPTICAL FLATS & MONOCHROMATIC LIGHT
Optical flats are optical-grade, lapped and polished glass pieces
which are extremely flat (about 25 nanometres). They are used in
combination with a monochromatic light to determine the flatness
of the lapped valve seat or disc. When the valve seat or disc and
optical flat are placed under the monochromatic light, the surface is
illuminated and the light waves reflect off both the bottom surface
of the flat and the surface of the seat. The reflected waves interfere,
creating a pattern of interference fringes (Newton’s rings), visible
as light and dark bands. The spacing between the fringes is smaller
where the gap is changing more rapidly, indicating a departure from
flatness in one of the two surfaces, in a similar way to the contour
lines on a map. A flat surface is indicated by a pattern of straight,
parallel fringes with equal spacing, while other patterns indicate
uneven surfaces. Two adjacent fringes indicate a difference in
elevation of one-half wavelength of the light used, so by counting
the fringes differences in elevation of the surface can be measured to
millionths of an inch. The delivery includes a laminated poster with
operating instructions and indicative guidelines of levels of flatness.
