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Optical Flat Interpretation/Selection

How to Select an Optical Flat


What is an Optical Flat?

An optical flat is a piece of glass that has been precision polished to a specific flatness on one or both sides and is used as a reference (test plate) against which the flatness of an unknown surface can be compared. In practice, an optical flat can be used to test and certify metal or glass surfaces that have been lapped or polished such as metal rings or seals, optical mirrors and optical windows, prisms, filters and more. A single sided optical flat (single surface optical flat) is polished on only one side and the opposing side is generally left clear, but not precision polished. A double sided optical flat (dual surface optical flat) is precision polished on both sides and may be used as an optical window.

How Does an Optical Flat Work?

Optical flats are polished to a known flatness and used in conjunction with a monochromatic light source. The optical flat is placed on top of the work to be tested under a monochromatic light of a specific wavelength, typically He-Ne which has a wavelength of 632.8nm and is equivalent to 0.000024 inches per wave. An interference (fringe) pattern of light and dark bands are formed due to the air gap between the work surface and the optical flat. The change in thickness of the air gap or wedge shows the direction and shape of the interference bands. The curvature and distance between bands determines how flat the work surface is compared to the known surface of the optical flat.

To determine the surface flatness of a test piece (work surface) using the interference pattern formed by an optical flat, gently place the optical flat on top of the work piece. Both the working surface of the test piece and the optical flat should be clean and free of dust. Care should be taken to not slide the optical flat across the surface of the part being tested as it may cause damage (scratches) to the optical flat. Scratches or damage to the surface of the optical flat can cause it to become less flat and therefore affect the results of the test. Next use a soft object, such as the new eraser of a pencil, to apply pressure to various points across the surface of the optical flat. Applying gentle pressure will reduce the air gap between the optical flat and the working surface of the part being tested. As the air gap is reduced, a fringe pattern (light and dark bands) will appear. Continue to apply light pressure to spread the fringes so that only a few are visible. Then imagine two straight and parallel lines, one between the ends of any one fringe and the other at the top of that same fringe. The number and curvature of fringes located between the two lines can be used to determine flatness.

optical flat techinical information
1/2 wave optical flat top viewoptical flat top view
Flatness Error of One Fringe
(1/2 wave)
(top view of optical flat)
Test Piece
Perfectly Flat
(top view of optical flat)

Selection of an Optical Flat

Optical flats can be made of several low expansion materials such as fused silica, fused quartz. Zerodur© or Clearceram© Fused silica and fused quartz provide low thermal expansion and are ideal for high wear applications. Zerodur© and Clearceram© are glass-ceramic materials with extremely low thermal expansion and are an ideal choice for applications where temperature fluctuation is a concern. BOROFLOAT©33 may also be considered as a less expensive alternative for test plates.

An optical flat must be more accurate (flatter) than the surface of the work or piece being tested. For example, if the surface being tested is flat to 1/4 wave or 1/2 of a light band, the optical flat used to determine if the work or test piece is flat should be 1/10 wave or 1/5 of a light band. The optical flat should be larger than the part being tested so that an accurate reading of the entire work surface is obtained.

Optical flats come in many different accuracies, but most commonly 1/4 wave, 1/10 wave and 1/20 wave. In addition to describing the accuracy of an optical flat in terms of waves (as above) accuracy may be specified in terms of light bands, microinches, fringes or inches. The table below summarizes how the accuracy of an optical flat may be specified.

Waves Lambda Light Bands Microinches Fringes Inches
1/4 wave λ/4 1/2 of a light band 6.228 microinches 0.5 .000006"
1/10 wave λ/10 1/5 of a light band 2.3913 microinches 0.2 .000002"
1/20 wave λ/20 1/10 of a light band 1.2457 microinches 0.1 .000001"

Optical flats are commonly round in shape. However, depending on their application, custom shapes and features may be required. Our optic capabilities also include crafting custom shapes including; square, rectangular, beveled and truncated optical flats. We also have the ability to drill custom holes and can provide coatings to increase the interference (fringe) pattern. In addition, we offer custom wooden storage cases for the protection of the optical flat when not in use.

Keep in mind, understanding your application can help you to select the right optical flat without over specifying your requirements thereby saving you time and money. Choosing the appropriate material, defining the correct accuracy and determining whether the flat should be polished on one or both sides should be considered. If you are still uncertain as to how to specify the correct optical flat, Advanced Optics has over 50 years of experience and can help you define your requirements and wisely choose the correct optical flat for your application.

Optical Flat Certification

Advanced Optics optical flats are measured using either a master test flat traceable to the National Institute of Standards and Technology (NIST) or peak to valley using a Zygo Interferometer.

Optical Flat Resurfacing

Optical flats become worn and abraded from use and accuracy is lost. This degradation in performance is due to a decrease in the distinctness of the interference pattern, and to a change in figure caused by scratches on either side of the optical flat. Resurfacing or re-polishing the surface of an optical flat back to its original accuracy can be less costly than replacement. Advanced Optics maintains an optical flat refinishing service with a fast turnaround time.

Have a question? Give us call and tell us about your project. (262) 548-1155