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Applications | Atometrics White Light Interferometer for Optical Lenses

2023-11-16

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With the development of science and technology, optical lenses are widely used in people's daily life. Improving the product quality and performance of optical lenses plays a vital role in enhancing the competitiveness of modern optical industry, and to improve the product quality and performance of optical lenses, not only rely on advanced processing technology, but also cannot be separated from precision testing technology. Surface roughness, face shape, radius of curvature and other parameters are important factors affecting the quality control and performance of optical lenses, and choosing an inspection method with high precision, fast speed and easy operation can help to reduce the scrap rate of optical lenses, and further enhance the competitiveness and core technology of the optical lens industry.

一、Classification of optical lenses


According to the shape of the lens, it can be divided into two categories: spherical lenses and aspherical lenses.

1、Spherical lens It is a lens that has a spherical surface on one side and a flat surface on the other, or both the inner and outer surfaces are spherical. Spherical lenses are generally categorized into concave and convex lenses. Spherical lenses are subject to the phenomenon of aberration, therefore, aspherical lenses that can reduce, complement, and correct aberrations and distortions have come into being.

2、Aspherical lenses It is a lens in which the radii of the points on the face shape are not the same as determined by the multiple image height equations, and the curvature of the surface of the lens is not perfectly circular. This type of lens can be used in optical systems to improve image quality by correcting a variety of aberrations, expanding the field of view and improving the performance of the optical system, thus improving the discriminatory ability of the optical system. One or several aspherical lenses can replace many spherical lenses, thus reducing the production cost, simplifying the production process and structure, and reducing the weight of optical products to a certain extent. Aspherical lenses have gradually become one of the most widely used optical components in optical products.

  1. Aspherical technology is generally used in high-end cameras, astronomical telescopes, precision microscopes, optical measuring instruments and other optical instruments to make their imaging clearer and more accurate.

  2. In the field of semiconductor lasers and fiber optic communications, aspherical technology can improve the focusing and coupling efficiency of the beam, and improve the performance and stability of the equipment.

  3. In the field of medical devices, aspheric lenses can realize the precise positioning and focusing of laser beams to improve the accuracy and safety of laser surgery; they can also be applied in the manufacture of medical devices such as artificial crystals and eyeglass lenses to improve the effect and comfort of vision correction.

  4. In the field of aerospace and aviation, aspheric lenses can improve the imaging accuracy and stability of space optical devices such as satellites and telescopes, and adapt to extreme space environments. In aircraft, missiles and other aeronautical devices, aspheric technology can also improve the performance of optical guidance systems and enhance the accuracy of navigation and guidance.



二、Precision Measurement Requirements for Optical Lenses


Surface finish Sa is the "arithmetic mean of the absolute values of the deviations of the contours of all the peaks and valleys" in the sampled area. The smaller the surface roughness, the smoother the surface of the object, and Sa is the "arithmetic mean of the absolute values of the profile deviations of all peaks and valleys" within the sampled area.

Shape of face is the shape and curvature of the surface of an optical lens. Different surface shapes will have different effects on the propagation and focusing of light. In the field of optics, the indicator of the quality of the surface shape of an optical surface is generally the PV value, also known as the peak-to-valley value, which generally indicates the deviation between the actual surface of the lens and the ideal sphere, and is a more comprehensive indicator of surface error. Generally speaking, the smaller the PV value is, the flatter the surface of the object is and the higher the processing quality.
Curvature radius It refers to the distance between the apex of an optical lens and the center of curvature and is an important parameter in the development and production of optical components. Precise measurement of the radius of curvature determines the length of the optical path of light as it passes through the lens, thus helping to monitor the parameter requirements at all stages of R&D, production and quality control.
In order to achieve the expected precision standard of optical lenses, several precision measurements and trimming will be carried out during the processing of optical lenses to detect whether the product quality standard is met, and the product precision, quality and performance of optical lenses will be improved through several iterations of grinding, lapping, polishing and other maneuvers. Therefore, precision measurement is an indispensable step in the R&D, production and manufacturing of optical lenses.


（1）Ultra-smooth machined components:
In the field of precision optics, elements with Ra values <0.3 nm are generally referred to as ultra-smooth (supersmooth) elements. In optical systems, ultra-smooth processed elements effectively reduce light scattering by virtue of extremely low surface roughness and non-destructive surfaces.
The following is an example of measuring the surface roughness of an ultra-smooth lens:


（2）Microlens:

Microlenses are a common miniature optical element used in optical systems to focus and disperse optical radiation. Microlens array is an array structure composed of multiple microlenses, which not only has the basic functions of focusing and imaging of traditional lenses, but also has the advantages of high integration and small cell size. This structure enables efficient control and processing of light, and is widely used in optical communications, optical imaging, laser processing, and other fields.


(3) Fresnel lenses:

Fresnel lenses, also known as threaded lenses, have a lens surface that is smooth on one side and etched with a series of concentric circles ranging from small to large on the other side. This concentric circle texture is based on the requirements of light interference, perturbation, relative sensitivity, and angle of reception.

The following is an example of Fresnel lens radius of curvature and face shape PV value measurement:


（4）Columnar mirror：

Cylindrical mirror is a special aspheric lens, with special optical properties such as changing the size of the imaging dimension, which can effectively reduce chromatic aberration and spherical aberration, and is widely used in various optical products. With the development of science and technology, the requirements for cylindrical mirror parts are getting higher and higher, at this time the precision measurement technology plays a key role.

The following is an example of the measurement of the radius of curvature and the PV value of the face shape of a cylindrical mirror：The following is an example of the measurement of the radius of curvature and the PV value of the face shape of a cylindrical mirror:


（5）Concave mirror, convex mirror：

Concave mirrors are spherical mirrors that use the inside of a sphere as the reflective surface, and convex mirrors are spherical mirrors that use the outside of a sphere as the reflective surface. Concave mirrors play a role in convergence of light, often used in satellite dishes, radar, lamps, telescopes and other products; convex mirrors play a role in dispersion of light, often used in the need to expand the field of view of the product, such as turning mirrors, wide-angle mirrors and so on.

The following are examples of concave mirror surface roughness and face shape PV value measurements:


The Atometrics White Light Interferometer AM-7000 series has sub-nanometer accuracy.

Maximum RMS repeatability up to 0.002nm, with a large range of piezoelectric ceramic devices, maximum scanning speed of 400μm/sec.

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White light interferometer is based on white light interference principle, through the optical interference phase measurement, any magnification can be obtained under the detection accuracy of less than 1nm.

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