What are the requirements for the grinding accuracy of optical glass lenses?

The requirements of grinding accuracy of optical glass lenses are reflected in many aspects, which are very important to ensure the performance of lenses in various optical equipment. The following are introduced from the dimensions of dimensional accuracy, shape accuracy, surface roughness and optical performance accuracy:

dimensional accuracy

Diameter and thickness: The diameter and thickness of the lens should be strictly controlled within the specified tolerance range. For example, the diameter tolerance of common camera lens may be required to be controlled within ±0.05mm to ensure that the lens can be accurately installed in the lens assembly, and to avoid the optical performance degradation or mechanical assembly problems caused by dimensional deviation. For the thickness, different lenses have different requirements. For example, the thickness tolerance of the lens of a mobile phone camera may need to be controlled at ±0.03mm to ensure that the refraction and propagation of light passing through the lens meet the design requirements, so as to achieve a clear imaging effect.

Aperture and groove depth: If there is an aperture or groove on the lens, its dimensional accuracy is equally critical. Take some complicated optical instrument lenses as an example. The aperture on the lens is used to install other optical elements or mechanical parts. The aperture tolerance may be controlled within ±0.01mm to ensure the matching accuracy with other parts, and avoid the unstable installation or inaccurate position of optical elements caused by aperture deviation, thus affecting the performance of the entire optical system. For the groove on the lens, such as the clamping groove used for positioning or fixing, the groove depth tolerance may need to be controlled at ±0.02mm to ensure the tightness and stability of the clamping groove and other components, and to ensure that the relative positions of the components of the optical system remain unchanged during the working process, thus ensuring the stability of the optical system.

Shape accuracy

Sphericity: For spherical lenses, sphericity is an important index to measure their shape accuracy. Sphericity requires that the lens surface be as close to the ideal sphere as possible. For example, in high-precision astronomical telescope lenses, the sphericity error may be required to be controlled within 0.0005 mm. This is because even the slight deviation of sphericity will lead to the deviation of refraction and reflection of light on the lens surface, which will cause aberrations in imaging, such as spherical aberration and coma aberration, which will seriously affect the observation accuracy and imaging quality of astronomical telescopes.

Flatness: The flatness of a planar lens requires that the lens surface be an ideal plane. In some optical measuring instruments, such as plane lens used in plane interferometer, the flatness error may need to be controlled within 0.0002 mm. This is because the deviation of flatness will lead to the change of the propagation path of light on the lens surface, which will affect the shape and position of interference fringes and lead to errors in the measurement results. Therefore, for this kind of high-precision optical measuring instrument, the flatness of the plane lens is extremely high to ensure the accuracy and reliability of the measurement results.

Aspheric accuracy: Aspheric lens can effectively correct aberration and improve imaging quality because of its unique shape. For aspheric lenses, the shape accuracy is more stringent. For example, the deviation of aspheric lens used in high-end digital camera lens from ideal aspheric surface may be required to be controlled within 0.0001 mm. This is because the surface shape of aspheric lens is complex, and the slight shape deviation will have a significant impact on the propagation and imaging of light. If the shape accuracy of aspheric lens is not up to standard, it will lead to insufficient aberration correction, which will reduce the imaging quality and cause problems such as blur and distortion. Therefore, for high-end optical imaging equipment, the shape accuracy of aspheric lens is extremely high to ensure high-quality imaging effect.

surface roughness

Ra value requirement: surface roughness is usually expressed by Ra value, which reflects the error of micro-geometry of lens surface. Different uses of optical glass lenses have different requirements for Ra value. For example, the Ra value of lenses used in ordinary optical microscopes may be required to be controlled between 0.05 μ m and 0.1 μ m.. This is because in the imaging process of ordinary optical microscope, when the light passes through the lens, if the surface roughness of the lens is large, it will cause the light to scatter on the lens surface, thus reducing the contrast and clarity of imaging. Therefore, in order to ensure the imaging quality of ordinary optical microscope, there are certain requirements for the Ra value of lens. For lenses in high-end laser optical systems, the Ra value may need to be controlled between 0.01 μ m and 0.03 μ m.. This is because in the laser optical system, the laser beam has the characteristics of high energy, high directivity and high monochromaticity, which requires extremely high surface quality of the lens. If the surface roughness of the lens is large, the laser beam will be scattered and absorbed on the lens surface, which will lead to the energy loss and beam quality degradation of the laser beam, and may even cause the lens surface to be damaged by absorbing too much laser energy. Therefore, in order to ensure the performance and stability of the high-end laser optical system, the requirements for the Ra value of the lens are extremely strict.

Consistency of micro-morphology: In addition to the requirements of Ra value, the consistency of micro-morphology of lens surface is also very important. This means that the microscopic geometry of the lens surface should be as uniform as possible over the entire lens surface. For example, in the large-size primary mirror of astronomical telescope, because of its large size, the propagation path of light on the lens surface is long. If the micro-morphology of the lens surface is inconsistent, it will lead to uneven propagation of light on the lens surface, which will cause aberration in imaging and seriously affect the observation accuracy and imaging quality of astronomical telescope. Therefore, for large-size optical lenses, the consistency of their surface micro-morphology is extremely demanding. In order to ensure the consistency of the micro-morphology of the lens surface, it is necessary to use high-precision grinding equipment and advanced processing technology to grind the lens surface evenly and carefully, and at the same time, it is necessary to carry out strict quality control and testing in the processing process to ensure the consistency of the micro-morphology of the lens surface meets the requirements.

Optical performance accuracy

Refractive index accuracy: refractive index is one of the important optical parameters of optical glass, which determines the propagation speed and refraction angle of light in glass. For optical glass lenses, the refractive index needs to meet strict accuracy requirements. Different types of optical glass and lenses with different uses have different requirements for refractive index accuracy. For example, the refractive index accuracy of optical glass lenses used in some high-precision optical metrology instruments may be required to be controlled within 0.0005. This is because the refraction angle of light passing through the lens is an important basis for measurement in optical metrology instruments. If the refractive index accuracy of the lens is not up to standard, it will lead to the deviation of the refraction angle of the light in the lens, which will lead to the error of the measurement result and seriously affect the measurement accuracy and reliability of the optical metrology instrument. Therefore, for high-precision optical metrology instruments, the refractive index accuracy of lenses is extremely high. In order to ensure the refractive index accuracy of lenses, it is necessary to strictly control the purity, proportion and melting process of raw materials in the production process of optical glass to ensure that the produced optical glass has a stable and accurate refractive index. At the same time, in the process of lens grinding, it is also necessary to strictly control the processing technology and environment to avoid the refractive index change of the lens due to factors in the processing process, so as to ensure that the refractive index accuracy of the lens meets the requirements.