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Optical Flats


Knight Optical can supply precision optical flats to meet individual requirements.

Optical flats have either one or two highly polished surfaces which are used as reference surfaces to compare against the flatness of an unknown test or work surface. They are used in conjunction with a monochromatic light source which Knight Optical has available as stock products which you can find here. The monochromatic light has a helium filled tube source which eliminates all colours except a “yellowish” orange

When the reference surface of the optical flat is carefully placed on to the test surface (ensuring no dirt is on either surface), interference fringes are produced. These light and dark bands that are present are known as interference fringes and their shape represents how flat the part being tested measures. Dependent on how many rings are formed or how straight the presented lines are is an indication of how flat the tested part is. The rings present are called “Newton’s rings” they are an interference pattern created by the reflection of light between two optical surfaces.

Important features of optical flats:

  • one or two precision polished surfaces, with high flatness
  • High quality surface finish, free from inclusions and bubbles
  • Highly parallel
  • Low coefficient of thermal expansion (CTE)

Due to their high quality surface finish and flatness, optical flats are also often used in interferometers as a precision flat reference mirrors once an optical coating has been applied, Knight Optical can supply reference mirrors for the use in interferometry.

If these do not meet your individual requirements, Knight Optical can also supply custom made optical flats, including larger diameters, coatings and different substrates such as borosilicate.

ZERODUR® is a non-porous lithium aluminium silicon oxide glass-ceramic, produced by Schott AG. It has a number of key properties:

  • Extremely low coefficient of thermal expansion (CTE) for a wide temperature range
  • Excellent CTE homogeneity throughout the total volume
  • Very low content of imperfections
  • Extremely smooth surface with residual roughness below 1 nm
  • Excellent chemical stability
  • Low helium permeability
  • Lightweight due to the manufacturing process
  • Transmits well between 400-2800nm

It is the material of choice for an array of industries and applications including:

  • Aerospace – in ring laser gyroscopes and as the chosen substrate for optical components such as prisms and mirrors.
  • Astronomy – as a mirror blank substrate for large astronomical telescopes
  • Metrology – optical flats for testing gauge blocks and reference standards in measurement instruments such as interferometers.

ZERODUR® is also the substrate of choice for a vast range of coated optical components such as convex mirrors, plane mirrors and 30°/45° fold mirrors

Knight Optical has a vast stock of ZERODUR® and fused quartz single-sided optical flats ranging from 26mm to 302mm in diameter, with thicknesses between 6mm and 40mm, typically with precision flatness of lambda/10 and lambda/20. We can also offer single-sided or double-sided ZERODUR® and fused quartz optical flats in three additional grades: lambda/4, lambda/8 and lambda/16 to meet all applications.

All our stock and custom optical flats are tested on our Zygo Verifire XPZ interferometer system fitted with the latest Mx® software. They are then cleaned and checked for surface imperfections by our highly trained QA department. Each optical flat comes with a protective wooden box and interferograms with its precise flatness measurement.

Contact our multilingual technical sales team and discover how Knight Optical’s high quality Optical Flats and service can improve your instrumentation and supply chain experience.

Our 2018/2019 Catalogue

Inside Knight Optical’s catalogue you’ll find details of the industries we serve as well as the many other services we can offer. It also showcases hundreds of our most popular products, supported by useful technical information, to help you choose the right part for your application.
Click below to view our digital catalogue now.

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