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Optical Flats

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  • An optical flat is a flat surface which is precisely polished. It can be used as a reference, against which the flatness of unknown surface may be compared. Measurement of the surface flatness of polished surfaces can be determined visually by comparing the variations between a work surface and the surface of an optical flat. These optical flats can be used in many applications, such as, inspecting of gauge blocks for wear and accuracy, as well as testing various components, including windows, prisms, filters, mirrors, etc. They can also be used as an extremely standard flat in interferometers.

BTMT offers both single and dual surface optical flats in either BK7A or Fused silica. We offer two different levels of accuracy for our

single surface flats:λ/10 and λ/20. For our dual surface flats,only λ/10 accuracy is available.

Material: N-BK7 ZKN7 Fused Silica
Index of Refraction(587.6nm): 1.5168 1.5080 1.4586
Abbe Number(νd): 64.20 61.05 68.00
Density(g/cm3): 2.52 2.43 2.21
Coefficient of Expansion: 66x per ℃ 42x per ℃ 2.1x per ℃
Surface Quality: 60/40 60/40 60/40
Surface Accuracy: λ/10 & λ/20 λ/10 & λ/20 λ/10 & λ/20
Back surface accuracy: λ λ λ
Parallelism: 5-10 arcmin 5-10 arcmin 5-10 arcmin
Clear Aperture: >90% of central diameter >90% of central diameter >90% of central diameter
Bevel: 1.0x45° 1.0x45° 1.0x45°


Perfect Flat of the inspected piece

The surface accureacy is indicated by the amount of curve and spacing between the interference fringes.


Straight,parallel, and vevnly spaced interference fringes indicate that work surface flatness is equal to or higher than the accuracy of the reference surface.