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.