What are the surface quality requirements of optical glass squares?

Scratch requirement

Scratch depth: For high-precision optical glass squares, such as optical elements used in lithography equipment and high-end microscopes, the surface scratch depth is usually required to be controlled below 0.1 μ m.. This is because even tiny scratches will cause scattering and reflection when light passes through, thus affecting the clarity and contrast of imaging.

Length and number of scratches: In a certain detection area, the allowable length and number of scratches are also strictly limited. For example, in a square inspection area with a side length of 10mm, the number of scratches with a length greater than 1mm may not exceed 1, and these scratches cannot be located in the key imaging area of optical elements, such as the central part of the lens.

Pitting requirement

Diameter of pitting: pitting is a small pit on the glass surface, and its diameter directly affects the optical properties. Generally speaking, the maximum diameter of pits is usually required to be less than 0.05 mm. For some optical glasses that require extremely high imaging quality, such as astronomical telescope lenses, the diameter of pits may be controlled below 0.01 mm.

Density of pits: In the unit area, the distribution density of pits is also required. For example, on the glass surface of each square centimeter, the number of pits (the diameter is greater than the specified value) may not be allowed to exceed 5, so as to ensure the uniform spread of light and avoid dark spots in imaging caused by too many pits.

Roughness requirement

Roughness after grinding: After grinding process, the surface roughness (Ra value) of optical glass square should generally reach 0.01-0.05μm m.. This roughness range can make the glass surface have better flatness, provide a good foundation for the subsequent polishing process, and also reduce the diffuse reflection of light on the surface.

Roughness after polishing: The Ra value of the polished glass surface is required to be higher, usually reaching 0.001-0.005 μ m.. This super-smooth surface can effectively reduce the reflection loss of light and improve the optical transmittance, which is very important for optical systems that need high-definition imaging, such as photographic lenses and lithography objectives.

Smoothness requirement

Flatness: the flatness error of optical glass square is generally required to be within 0.002 mm. This is because the poor flatness will lead to the deflection of light when it passes through the glass, which will distort the imaging. For large optical components, such as the main mirror of astronomical telescope, the flatness requirement is more stringent, which may reach within 0.0005 mm.

Parallelism: If the optical glass cube is used in a multilayer optical system, such as the objective lens group of a microscope, the parallelism error of its two opposite surfaces is usually controlled within 0.003 mm.. Excessive parallelism error will cause astigmatism and other aberrations, which will affect the imaging quality.

Smoothness requirement

The glass surface should have a good finish, without stains, oil stains, water stains and other pollutants. These pollutants will change the optical properties of glass, such as refractive index and reflectivity. Before optical coating, the requirement of smoothness is more stringent, because pollutants will affect the adhesion and uniformity of coating, and then affect the overall performance of optical components.

Non-stress concentration area requirements

Stress concentration areas should be avoided on the surface of optical glass squares. Stress concentration may be caused by uneven cooling and excessive grinding pressure during machining. These areas will lead to local changes in the refractive index of glass, resulting in birefringence, thus affecting the imaging accuracy and stability of the optical system. The stress concentration on the glass surface can be detected by polarized light detection and other methods, which requires that the stress concentration should be controlled at a very low level to meet the requirements of optical applications.