Lenses are the core components of all optical imaging systems, and their imaging quality is not only determined by the substrate material and surface processing precision, but also deeply affected by optical coating technology. Optical coating is a thin film layer deposited on the lens surface by physical or chemical methods. It is by no means a decorative process, but a key technology that directly determines the light transmission efficiency, contrast level, color reproduction accuracy and anti-interference ability of the optical system. Without optimized coating design, even the most precisely machined lens will suffer from severe light energy loss and imaging degradation due to Fresnel reflection.
By depositing thin film materials with specific refractive index and thickness on the lens surface, the reflected light beams generated at each interface of the film layer interfere and cancel each other out, thereby reducing reflected light and increasing transmitted light. By adjusting the number of film layers, material composition and thickness design, different optical functions such as anti-reflection, high reflection, beam splitting and spectral filtering can be realized.
Anti-reflective (AR) Coating: The most widely used coating for imaging lenses. Single-layer AR coating can reduce the reflectivity of a single wavelength to below 1%; broadband multi-layer AR coating can control the average reflectivity of the entire visible light band (400–700nm) below 0.5%. It directly improves image brightness, enhances picture contrast, suppresses ghosting and flare caused by reflected light, and improves the signal-to-noise ratio of imaging. For complex multi-lens systems, AR coating is a necessary condition to ensure effective imaging.
High-reflection (HR) Coating: Used for reflective optical systems such as mirrors and prisms. High-reflection coating can increase the reflectivity of specific wavelength bands to over 99%, reducing light energy loss during reflection. In imaging systems such as telescopes and periscopes, high-quality HR coating directly determines the final imaging brightness and light energy utilization.
Spectral Filter Coating: Including band-pass, long-pass and short-pass filter films. It selectively transmits light of specific wavelengths and blocks unwanted light, which is widely used in biomedical imaging, machine vision and spectral detection systems. Such coatings can effectively filter out stray light of irrelevant wavebands, improve imaging specificity and reduce background noise.
Beam Splitter Coating: It splits the incident light into two paths of transmission and reflection according to a specific intensity ratio, and is commonly used in binoculars, beam splitters and 3D imaging systems. The uniformity of the splitting ratio directly affects the brightness balance of the two imaging paths.
Protective Functional Coating: Such as hard carbon film, anti-fog film, hydrophobic and oleophobic film. Although they do not directly change the optical imaging parameters, they protect the lens surface from scratches, corrosion and fogging, maintain long-term stable imaging quality, and extend the service life of the lens in harsh environments.
The actual effect of the same coating design is highly dependent on the process control level, and small deviations in process parameters will cause significant changes in imaging performance:
Film Thickness Accuracy: The optical thickness of each film layer must be precisely controlled to the order of nanometers. A thickness deviation of more than 5% will cause the central wavelength of the coating to shift, resulting in a decrease in transmittance at the target wavelength and color cast of the image. For multi-layer broadband coatings, thickness errors of each layer will accumulate, eventually leading to a significant decline in overall optical performance.
Film Uniformity: The coating thickness on the whole lens surface must be highly consistent. Poor uniformity will lead to different transmittance in the central and edge areas of the lens, resulting in uneven brightness of the imaging field, vignetting and color deviation at the edge of the picture. For large-diameter lenses, ensuring radial uniformity of the coating is one of the key process difficulties.
Film Stress and Adhesion: Excessive film stress will cause micro-deformation of the lens substrate, introduce additional optical aberration, and reduce imaging resolution and shape accuracy. Poor adhesion will lead to film peeling and cracking, which directly damages the imaging effect and even causes the lens to be scrapped.
Surface Smoothness: The surface roughness of the coating must be kept at an atomic level. Excessive roughness will cause light scattering, reduce image contrast, and form stray light spots under strong light irradiation.
Reasonable coating matching should be carried out according to the use scenario and imaging requirements to achieve the best cost performance:
Medical endoscope lenses: High-performance broadband AR coating is required to ensure high transmittance in the full visible light band, and anti-fog and biocompatible coatings are required to adapt to the humid environment in the human body and disinfection procedures, so as to ensure clear and true color imaging during surgery.
Industrial optical lenses: Hard protective AR coating is preferred. While ensuring imaging clarity, it improves the wear resistance and corrosion resistance of the lens surface, and adapts to harsh industrial environments such as dust, oil fume and high temperature.
Photographic and cinematographic lenses: Ultra-wideband AR coating with extremely low reflectivity is adopted to maximize contrast and color reproduction, suppress ghosting and flare under complex light conditions, and meet the high requirements of commercial imaging for picture quality.
Laser optical lenses: Special single-wavelength AR coating is used to achieve ultra-high transmittance (over 99.5%) at the laser working wavelength, reduce energy loss and thermal effect, and ensure the beam quality and focusing accuracy of the laser system.
Optical coating technology is an indispensable part of modern lens design and manufacturing. It breaks through the inherent optical limitations of lens substrate materials, and comprehensively improves the imaging quality of lenses from multiple dimensions such as light energy utilization, contrast, color reproduction and anti-interference ability. With the continuous development of coating processes such as ion beam sputtering and atomic layer deposition, and the application of new film materials, optical coatings will move towards higher precision, wider spectral range and stronger environmental adaptability, providing stronger support for the upgrading of various optical imaging systems.
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E-mail: info@aosoptics.com
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Add: Yiju Road High-Tech Zone, Changchun City, Jilin Province, China
