What is Optical Flat – Working, Applications, Advantages & Disadvantages

What are Optical Flat ?

An optical Flat is an instrument which is used for testing the flatness of workpieces, gauge blocks, micrometer anvils, etc. Such measurements are based in interferometry ; the science of measuring with light waves

Optical flat
optical flat

Read : Autocollimator Working principle

Optical Flat construction

The optical Flat is made from natural quartz and consists of a flat piece, circular in section and manufactured in a variety of diameters and thickness. A typical size is 59 mm in dit and 15.875 mm thick. The faces of an optical Flats are true on one side only.

Optical Flat working Principle

The working principle of optical flats is based on the principle of interference. Before allowing a monochromatic light to pass through the area of contact, an optical flat is placed on the surface to measure its flatness.
There will be no gap between the two surfaces if the test surface is perfectly flat. In cases where the test surface is not flat, interference fringes form and are visible as dark and light bands.

The surface elevation is calculated by counting the fringe differences.

How Does an Optical Flat Works

In use the flat is placed upon the object to be tested and a light having a single color (monochromatic light) is thrown upon it. Helium is most commonly used in industry as a source of monochromatic or single wave-length light because of its convenience.

The principle of light wave interference and the operation of the optical flat are illustrated in Fig. 16.38 wherein an optical flat is shown resting at a slight angle on a workpiece surface. The ray of light penetrates the flat, and as it reaches the bottom surface it is divided into two rays.

One ray is reflected back to the eye from the bottom surface of the flat, while the other is reflected back from the top surface of the object being tested. 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 are dark. The rays have the same wave-length, but the second ray will lag behind the first ray by an amount equal to twice the space between the flat and the work.

If this space is equal to half a wave-length, the second ray will be in step with the first one when it join it again in air. If it is a full wave-length, the second will also join the first one. But, if the distance is equal to one-fourth or three fourths of a wave-length, the second ray will be a half wave length out of step with the first one and will neutralize it when they join and create a dark band.

Since each dark band indicates a change of one-half wave length in distance separating the work surface and flat, measurements are made very simply by counting the number of these bands or interference lines and multiplying that number by one half wave-length of the light source.

Method of Measuring a Ball Using an Optical Flat and Gauge Block

This procedure may be illustrated by the use of Fig. 16.39, which shows an optical flat, being used in connection with a gauge block of known size to check the height of a steel ball. The gauge block is placed on to a toolmakers’ flat and the optical flat is placed on top with the ball in position.

If the parts are not of the same size, the optical Flat will be tilted slightly, and parallel interference lines will appear on the top surface of the gauge block. The difference in height can be determined from the number of bands that appear and may be calculated by the following formula :

H = 1/2 – wave-length * N * (L/W)


N = number of bands appearing on width of the block.

L = distance between contact points.

W = width of the precision block.


The main applications of optical flats are listed below:

1. Calibration of flatness of various optical surfaces
2. Inspection of gauge blocks
3. Testing of filters, mirrors, and prisms
4. Spectrophotometry

Types of Optical Flat

  1. Type A
  2. Type B

Type A

It has only one surface flat. The working surface of this type of flat is indicated by a an arrow head on the cylindrical surface pointing towards the working surface.

This type of flat are used for testing the flatness of precision measuring surfaces of flats, slips etc.

Type B

It has both surface flat and parallel to each other. This type of flat are used for testing measuring surfaces of micrometers, measuring anvils.

Flatness test of Optical Flat

> The flatness of the working surface of an optical flat is tested by comparing its flatness with a master flat of known flatness.
> The testing is usually carried out by observing the interference fringes using monochromatic light source.
> If the optical flat under test has a perfectly flat surface then the fringes observed will be straight line.

Parallelism Test

> The parallelism of working surfaces of type B optical flats can be tested by Fizeau Interferometer method.
> In this method light from mercury vapor lamp is focused on to an opening in the eyepiece and is partly reflected by beam splitter.
> The reflected light strikes the collimating lens which collimates it and throws it further on the flat under test. The flat under the test is placed on the table provided.
> The table is arranged such that beam strikes the surfaces of the flats perpendicularly.

> After reflection from the two surfaces of the flat under test, the beam retrace its path.
> Due to interference of the light reflected at the two parallel surfaces of the optical flat, interference fringes are formed which can be observed from the eyepiece of the auto-collimator.
> These interference fringes helps in determining the degree of flatness of two surfaces of the flat.

Care in the use of Optical Flat

Before using the optical Flat, it should be insured that both flat and work piece are clean and free from dust, oil, dirt and finger prints.

The flat should be only rested carefully on the workpiece.

What is an optical flat used for?

Optical flats are primarily used as highly flat reference surfaces in interferometers to check the flatness of optical elements such as optical windows, laser mirrors, prisms, optical filters, or laser crystals.

What is an optical flat made of?

A disc made of crystal or glass with one surface finished to extremely precise flatness. Contact with the precisely mirror-finished workpiece surface causes interference fringes for light waves to be observed, and the degree of flatness of the mirror-finished surface can be determined.

How accurate is an optical flat?

The flatness of any specific region of a surface is determined by drawing two parallel imaginary lines, one between the ends of any one fringe and the other at the top of that same fringe. The number of fringes between the lines can be used to calculate flatness.

What are the limitations of optical flat?

The optical flat is in close proximity to the test sample, resulting in scratches on both.

Check Out Other Important Topics

IC EngineImportant PDFsBoilersSynergy Maritime ExamNaval ArchMEO Class 4
Interview QuestionsDifference BetweenTypes of PumpsAuxiliary MachinesTypes of ValvesHome

Leave a Reply

Your email address will not be published. Required fields are marked *