Understanding Surface Quality Specifications for Sapphire Windows

Custom fabricated sapphire windows & lenses which are chemically inert and operate over a wide spectrum for use in blood gas monitors, centrifuges, and other instruments are available from AOG Sapphire Optics Co.,Ltd.

Surface Quality of Sapphire Windows

The surface quality of an optical surface describes its cosmetic appearance and includes such defects as scratches and pits, or digs. In most cases, these surface defects are purely cosmetic and do not significantly affect system performance, though, they can cause a small loss in system throughput and a small increase in scattered light. However, certain surfaces, however, are more sensitive to these effects such as: (1) surfaces at image planes because these defects are in focus and (2) surfaces that see high power levels because these defects can cause increased absorption of energy and damage the optic. The most common specification used for surface quality is the scratch-dig specification described by MIL-PRF-13830B. The scratch designation is determined by comparing the scratches on a surface to a set of standard scratches under controlled lighting conditions. Therefore the scratch designation does not describe the actual scratch itself, but rather compares it to a standardized scratch according to the MIL-Spec. The dig designation, however, does directly relate to the dig, or small pit in the surface. The dig designation is calculated at the diameter of the dig in microns divided by 10. Scratch-dig specifications of 80-50 are typically considered standard quality, 60-40 precision quality, and 20-10 high precision quality.

Surface Roughness (Ra, Rz, Rq, Rt)

Measured in micrometers (μm) or nanometers (nm), surface roughness quantifies the texture of a surface.

Common standards include Ra (arithmetical mean deviation) and Rz (average peak-to-valley height).

Waviness

Represents the broader, more widely spaced irregularities on a surface.

Measured in terms of Wt (total waviness height) or Wa (waviness average).

Defects and Imperfections

Includes scratches, pits, dents, and inclusions that can affect performance.

Optical components often use a "scratch-dig" specification, such as 40-20 or 20-10, defined by MIL-PRF-13830B.

ISO 4287 & ISO 25178

Define roughness parameters and 3D surface texture measurement techniques.

ASME B46.1

Provides a framework for surface roughness evaluation in the United States.

MIL-PRF-13830B

Governs surface quality specifications for optical components, focusing on scratch and dig classifications.

Optics and Photonics

High-precision optics require extremely low roughness and minimal defects.

Aerospace and Automotive

Surface quality affects aerodynamics, friction, and durability.

Medical Devices

Ensuring biocompatibility and preventing bacterial adhesion.

Semiconductor Manufacturing

Ultra-smooth surfaces are crucial for high-performance chips and wafers.