PRINCIPLES OF INTERFEROMETRY |
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| Interferometers are measurement devices that rely on a seldom observed characteristic of light; its wavelength, to resolve linear differences to less than one millionth of an inch (0.000001 in.). The wave nature of light, which makes measurements of this magnitude possible, is always present but seldom observed under ordinary conditions. Only by making two or more light waves interact with each other in the same region of space can the wave nature of light be observed. This interaction is known as interference, and nearly all instruments that make use of it are called Interferometers. The most common use for Interferometers is to determine the degree of flatness of polished surfaces such as Gage Blocks, Optical Flats, Mirrors, etc.
The reason interference is not normally observed is that it can only occur if the interacting light waves come together in a condition known as coherence. That is, the waves maintain a constant phase relationship. This normally occurs only when the interfering waves originate at the same point simultaneously. Accordingly, many Interferometers make use of some type of beam divider that splits an incoming wave into two parts that travel different paths before being recombined. This type of Interferometer is known as an amplitude-splitting Interferometer. All Interferometers manufactured by Davidson Optronics are amplitude-splitting devices and, |
with the exception of the D-304 Twyman and Green Interferometer, are based on principles established by Louis Fizeau, the noted 19th century French Physicist. An amplitude splitting Fizeau Interferometer is diagrammed in Figure 1.
Illumination for the system is provided by a monochromatic light source and pinhole. Placement of the pinhole in the focal plane of the lens ensures that light will emerge from the lens 'collimated' or parallel to the optical axis of the system. A beamsplitter follows the pinhole and directs the light toward the lens. |
Following the lens, light is next incident on the divider plate. So named because this is where 'amplitude division' takes place.
The bottom surface of the divider plate is the reference surface of known flatness and is the surface to which each test piece will be compared. In order to understand how interference is created, the path of the incident light as it interacts with both reference surface of the divider plate and test piece will be explained. Figure 2 shows the divider plate configuration in greater detail. Two areas of the divider plate will be examined in order to illustrate the two extreme cases of interference: Constructive and Destructive. Light incident on the divider plate is shown as a series of rays (arrows). Note the rays incident at point A on the reference surface. Some of the incident intensity is reflected upward into the optical system. A greater portion is transmitted and strikes the component under test at point B. Here again, some of the light incident on the test piece is reflected upward, reentering the optical system. As the component of the incident light reflected from point B combines with the light reflected from a second ray incident at point C, interference occurs. These two components then retrace a path through the optical system and are finally imaged for viewing by the eyepiece. The patterns created by interference are known as Interferograms. A typical Interferogram is shown in Figure 3. |
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| Exploded view of the divider plate - test piece interface. Interference is created when the ray reflected from point B combines with that reflected from point C. The length of path ABC determines the type of Interference produced. | For example, if ABC is equal to an integral number of whole wavelengths, the reflected light waves recombine in phase and a bright band or Constructive Interference is seen. (Left side of divider plate) | If, however, ABC is equal to an odd number of half-wavelengths (1/2, 3/2, . . 7/2,. . . 99/2, etc.), the reflected light waves recombine 180 degrees out of phase and a dark band or Destructive Interference is seen, (Right side of divider plate) |
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