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Looking For a Custom Optical Solution? Enquire

Custom Optical Flats

custom optical flats, quartz optical flat, zerodur optical flats, precision optical flats, low expansion optical flatsOptical flats have one, or two, highly polished reference surfaces which are used to determine the flatness of an unknown test or work surface. They are often used in conjunction with a monochromatic light which works predominantly in one wavelength, often with a sodium (589.1nm) or helium (632.8nm) lamp.
Knight Optical holds a vast stock of precision optical flats manufactured from quartz and Zerodur® with lambda/10 or lambda/20 surface flatness. These come in both single- or double-sided alternatives.
We can also supply custom-made optical flats to better suit your application. Whether it is different dimensions, flatness, substrate, or the addition of a coating, Knight Optical can assist you with your enquiry. Please note, the substrate material will depend on the application and the desired flatness.
Below are our typical manufacturing capabilities, however these are always expanding so please do not hesitate to contact our technical sales team if your individual requirements do not fall within these:




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(arc mins)


Optical flats

10 to 600mm+

up to 1/20 @ 633nm (single or double sided)

< 5

< 60/40

All optical flats are tested on our Zygo Verifire XPZ interferometer to ensure they meet the required flatness specification. Knight Optical can provide an interferogram upon request for reference. Once the precision optical flats have been tested in our dedicated metrology laboratory, they are visually inspected and packaged within wooden boxes that provides protection whilst the optic is not in use.
Please enquire with our dedicated sales team for further information.  

Additional Information

  • Optical flats often have the following features:
    • One, or two, surfaces polished to a precision flatness.
    • High quality surface finish, free from inclusions and bubbles.
    • Low coefficient of thermal expansion (CTE).
  • Various materials can be used for optical flats including fused quartz, fused silica, Zerodur, Clearceram, BK7 and borosilicate.
  • The material used will be dependent on the application, however due to the nature of their use the materials need to be durable but can be polished to a high degree.
  • Zerodur is a glass-ceramic developed by Schott, and its coefficient of thermal expansion (CTE) is extremely low, making it an ideal material for precision applications, including astronomy and aerospace, where the glass properties need to stay homogenic when there are fluctuations in temperature.
  • The datasheet for Zerodur can be found here.


Knight Optical can also apply a wide range of coatings to your optical flats to enhance their performance to suit your application:

  • Antireflective (AR) coatings, including broadband coatings, to optimise the transmission at certain wavelengths or across a range.
  • Mirror coatings which enable the optical flats to be used as reference mirrors, Knight Optical can coat to a range of different wavelengths.

Quality Assurance

  • The PV and RMS of our single and double-sided optical flats are tested on our Zygo Verifire to ensure they meet the tight specifications required of optical flats.
  • The parallelism is measured on our Trioptics PrismMaster which is accurate to 3 arc seconds.
  • Reflection scans to confirm the performance of an applied coating are conducted on our Agilent Cary 7000 Spectrophotometer which can perform spectral scans at multiple angles of incidence and different polarisations.
  • All optics are 100% visually inspected before packaging, our staff are trained to ISO 10110 and MIL-0-13830A, and can check down to 10/5 scratch/dig which is especially critical in laser applications.
  • Our optical flats are packaged in wooden boxes to ensure extra protection when they are not in use.


  • Primarily, optical flats are used to determine the flatness of a desired surface. When the optical flat’s reference surface is in contact with the work surface, it can form an air wedge (or air gaps) where the two surfaces are not fully in contact. Light will interact differently (due to interference) based on the thickness of the air gap.
  • When measuring the flatness this way, a monochromatic light is often used to make the interference pattern produced clearer and the flatness can be given at a certain wavelength.
  • Optical flats with a mirror coating applied are often used in Twyman-Green interferometers as reference mirrors, where a high surface flatness is critical.

Our 2022/2023 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.
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