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76 Part II: Metrology
As industrial processes are automated, gauging must keep pace. Automated gauging
is performed in two general ways. One is in-process or on-the-machine control
by continuous gauging of the work. The second way is post-process or after-themachine
gauging control. Here, parts coming off a machine are passed through
an automatic gage. A control unit responds to the gage to sort pieces by size, and
adjusts or stops the machine if parts are found out of limits.
Interferometry
Part II.B.2
Light waves of any kind are of invariable length and are the standard for ultimate
measures of distance. Basically, all interferometers divide a light beam and send
it along two or more paths. Then the beams are recombined to show interference
in some proportion to the differences between the lengths of the paths. One of the
simplest illustrations of this phenomenon is the optical flat and a monochromatic
light source of known wavelength.
The optical flat is a plane lens, usually a clear, fused quartz disk, around 51–
254 mm (2–10 in.) in diameter and 13–25 mm (.5–1 in.) thick. The faces of a flat
are accurately polished to nearly true planes; some have surfaces within 25 mm
(.000001 in.) of true flatness.
Helium is commonly used in industry as a source of monochromatic or singlewavelength
light because of its convenience. Although helium radiates a number
of wavelengths of light, that portion that is emitted with a wavelength of 587 nm
(.00002313 in.) is so much stronger than the rest that the other wavelengths are
practically unnoticeable.
Principle of Operation. The principle of light-wave interference and the operation
of the optical flat are illustrated in Figure 8.2a, wherein an optical flat is
shown resting at a slight angle on a workpiece surface. Energy in the form of light
waves is transmitted from a monochromatic light source to the optical flat. When
a ray of light reaches the bottom surface of the flat, it is divided into two rays. One
ray is reflected from the bottom of the flat toward the eye of the observer while
the other continues on downward and is reflected and loses one-half wavelength
upon striking the top of the workpiece. If the rays are in phase when they reform,
their energies reinforce each other, and they appear bright. If they are out of
phase, their energies cancel and they appear dark. This phenomenon produces
a series of light and dark fringes or bands along the workpiece surface and the
bottom of the flat, as illustrated in Figure 8.2b.
The distance between the workpiece and the bottom surface of the optical flat
at any point determines which effect takes place. If the distance is equivalent to
some whole number of half wavelengths of the same monochromatic light, the
reflected rays will be out of phase, thus producing dark bands. This condition
exists at positions X and Z of Figure 8.2a. If the distance is equivalent to some odd
number of quarter wavelengths of the light, the reflected rays will be in phase with
each other and produce light bands. The light bands would be centered between
the dark bands. Thus a light band would appear at position Y in Figure 8.2a.
Since each dark band indicates a change of one-half wavelength in the distance
separating the work surface and flat, measurements are made very simply
by counting the number of these bands and multiplying that number by one-half
the wavelength of the light source.