Interferometry Study Notes for Instrumentation Engineering

By BYJU'S Exam Prep

Updated on: September 25th, 2023

Interferometry is a branch of optics that involves the use of interference patterns to measure and analyze properties of light and other wave-like phenomena. In the context of instrumentation engineering, interferometry has a wide range of applications, including the design and calibration of precision measuring devices such as spectrometers, telescopes, and microscopes.

In this article, you will find the Study Notes on Interferometry which will cover the topics such as Introduction, Interference of Light, Light sources for interferometry, Interferometry Applied to Flatness Testing, Interferometers, and Laser Interferometers.

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Interferometry Study Notes: Introduction

Interferometry study notes for instrumentation engineering cover the fundamental principles and practical applications of interferometry, including interferometric techniques, interferometers, and interferometric data analysis. With clear explanations and examples, these notes provide a comprehensive overview of interferometry and its role in instrumentation engineering, making them a valuable resource for students in the field.

  • Light is considered an electromagnetic wave of sinusoidal form. When two monochromatic light beams combine they undergo the phenomenon of interference. Therefore, the resultant light rays carry the characteristics of both monochromatic light sources. The amplitude and hence the brightness of the resultant beam also becomes different from the original ones.
  • By analyzing differences in phase, which also corresponds to the difference in wavelength (length), the distance between the two light sources can be calculated. The phenomenon of light used in this way has given rise to the branch of dimension metrology called Interferometry.
  • Interferometry involves testing of flatness, surface contour, and determination of the thickness of slip gauges, etc. Interferometers are commercially available instruments for calculating small differences, constructed on the basis of principles of interferometry.

Light Sources for Interferometry

A variety of light sources are available for interferometry work but the selection of proper sources for any application depends on the requirements or results to be obtained by the interferometer, cost, and convenience. Characteristics of various light sources are summarized below:

  • Mercury: It is a less expensive source having high intensity, and the green line can be easily isolated with filters. Since natural mercury contains several isotopes, each isotope emits light whose wavelength is slightly different from the other. As a result, natural mercury light source radiates a mixture of slightly different but close to each other wavelengths that can be treated as monochromatic light.
  • Mercury 198: It is a pure isotope produced by neutron bombardment of gold. It is considered to be one of the best sources of very sharply defined wavelengths, and fringes are visible with path differences up to 500 mm. Light is emitted when mercury 198 is excited by a microwave-produced electric field. It is the international secondary standard of wavelength.
  • Cadmium: It is the only natural material producing a spectral line (red) almost completely monochromatic. It can be conveniently used up to a path difference of about 200 mm. Cadmium 114 is the official secondary international standard of length.
  • Krypton: It is used in some instruments for its advantage of being easily excited. It is not as monochromatic as krypton-86 because Krypton is a mixture of isotopes. It can be used up to the path difference of 375 mm.
  • Krypton 86: Krypton-86 lamp produces spectral lines of different wavelengths and, therefore, an elaborate monochromator is required to separate them. Further, its excitation takes place at very low temperatures. So this lamp is used only in standardizing laboratories. It enables the fringes to be observed with maximum path differences nearing up to 800 mm.
  • Thallium: As 95% of its light is emitted at one green wavelength, it can be used over a reasonable path difference without the use of the filter.
  • Sodium: It is used only in applications where interference path difference does not exceed a few hundred wavelengths. Usually, the yellow sodium light is used which contains two separate but closely spaced lines of equal intensities.
  • Helium: Orange line of helium is used where the path difference is not large.
  • Gas Lasers: Lasers are highly monochromatic the light that enables the interference fringes to be observed with enormous path differences, up to 100 million wavelengths.

Interferometry Applied to Flatness Testing

Flatness is defined as the geometrical concept of a perfect plane. It is an important function in the construction of many technical components where accuracy is a required criterion. For example, controlled flatness is required to provide full contact with a mating part. Flatness is a precondition for the parallelism of the nominally flat surface. It is a reliable boundary plan for linear dimensions. It also provides locating planes for dependable mounting or assembly of manufactured parts.

  • Interferometry is one of the precise methods for the calculation of flatness. In this method, monochromatic light is allowed to fall on an optical flat which in turn is placed on the surface at a small angle whose flatness is to be calculated. Optical flats are transparent, flat, circular sections of small thickness usually made of clear fused quartz or Pyrex.
  • The optical flat is placed on the surface to be inspected in such a way as to create interference bands observable under monochromatic light. The resulting band pattern permits the object’s flatness conditions to be evaluated.



Interferometers are optical instruments used for measuring flatness and determining minute differences in length by direct reference to the wavelength of light. Basically, an interferometer is constructed using the same principle as an optical flat. The disadvantages of optical flats are overcome here by some refined arrangements.

  • The fringes formed can be oriented to the best advantage in interferometers. Also, there is an arrangement to view the fringes directly from the top, thus avoiding any distortion due to incorrect viewing. This makes their uses easier and faster than optical flats.


  • There are a number of interferometers available. But most of them are designed employing slightly different methods to accomplish the same result.
  • The light sources from a single color light source are collimated into parallel by a lens. When these rays reach the partially silvered surface of mirror A, about half of the light is reflected toward mirror B, and the other half passes through the silvered surface towards the workpiece and table surface.
  • Thus, the light rays are divided and directed along two different paths. These divided light rays fall on the surface and mirror B and are then reflected back to mirror A.
  • Since they are of the same wavelength and emerge from the same source, they will show the phenomenon of interference.
  • Some light from mirror B passes through the partially silvered surface towards the eye, and some light from the workpiece and table surfaces also are reflected towards the eye.
  • If the path difference of these two rays is even multiple of the half of the wavelength of the light source, they undergo constructive interference and the light rays reinforce each other, and the workpiece and the table surface appear to be ordinarily illuminated. However, if the path of the light reflected from the workpiece surface differs in length from that of the light reflected from mirror B by odd multiples of half wavelength, the workpiece surface appears to be dark because of destructive interference.
  • The surface of the table also appears to be dark if the same situation is applied to it.


  • In order that the operator can make a measurement with an interferometer, the table is tilted slightly by a very small angle. This causes a series of interference fringes to appear on the surfaces of the workpiece and table.
  • In case, the surface of the workpiece is at a certain angle with the base plate, the fringe pattern will be


  • Here the error is indicated by the amount by which the fringes are out of parallelism with those on the base plate.

Laser Interferometers

The measuring capacity in interferometers of the lamp of a single wavelength as a source of light is limited because of their low resolution and short measuring range. If the light source is replaced by a laser source, measurement can be done over a long distance because it facilitates to maintain the quality of interference fringes over a long distance. Since the laser is a highly monochromatic coherent light source that follows all the principles of light, the fringes formed due to interference of the laser are very sharp, accurate, and precise.


Uses of a Laser Interferometer

Since laser interferometer produces very thin, straight beam, they are used for measurements and alignment in the production of large machines.

  • They are also used to calibrate precision machines and measuring devices.
  • They can also be used to check machine setups. A laser beam is projected against the work and measurements are made by the beam and displayed on a digital readout panel.
  • Because of their very thin, straight-beam characteristics, lasers are used extensively in construction and surveying. They are used to indicate the exact location for positioning girders on a tall building or establishing directional lines for a tunnel being constructed under a river.
  • Laser interferometers can also be used in glass feature measurements.
  • The main advantages of a laser interferometer are as given below:
    • Laser interferometers have high repeatability and resolution of displacement measurement.
    • They give high accuracy (0.1μ) of measurement.
    • It facilitates maintaining a long-range optical path (60 m).
    • Laser interferometers are easy to install.
    • There is no chance of deterioration in performance due to aging or wear and tear.

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