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Aspherical Lenses

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Aspherical Lenses
Aspherical Lenses
  • Aspherical Lenses

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    Aspheric lenses have its advantages in a range of applications where eliminated spherical aberration or (and) single element design is required. CASTECH offers CNC precision-polished aspherical and acylindrical lenses, with or without anti-reflection (AR) coatings. Our CNC polished aspheric lenses are iteratively ground and polished by a software supported computer-controlled processing center to provide better controlled surface quality, irregularity and focal length deviation.

     

    •  Material from DUV to IR 

    •  Custom dimension and shapes

    • ​​​​​​ ​Diameter from 5 to 200 mm

    • ​​​​​​ ​Clear aperture: Central 90% of diameter

    •  Irregularity : λ/10@632.8nm

    • ​​​​​​ ​Surface quality: 10-5 S/D

    • ​​​​​​ ​Centration: 3′

    • ​​​​​​ ​Standard and custom coating options available 

    • ​​​​​​ ​High laser damage threshold

     

    Metrology
    CASTECH employs rigorous verification procedure to guaranty the high performance of our each aspheric lens.Taylor Hobson Form Talysurf PGI profilometer allows contact measurement in grinding process while Taylor Hobson LuphoScan 260 non-contact 3D optical surface profilometer performs ultra precision optical surfaces form measurement in polishing process.

     

    Taylor Hobson Form Talysurf PGI

    Measure range: from 0.1 to 200 mm

    Resolution ratio: 0.8nm

    Taylor Hobson LuphoScan 260 for irregularity measurement

    Measure range: from 5 to 260 mm

    High surface accuracy: ±50nm

     
     

    What a significant work aspherical lenses can do.

     

    Thin lenses match perfectly with the fancy frames to provide style and function for the short sighted.  Excellent images with exquisite details are captured instantly with a wide-angle zooming camera or even a cellphone. Vivid and realistic scenes amaze users in virtual environments.  All these seemingly magical achievements are made possible by aspherical lenses. Aspherical lenses already have a huge impact at all aspects of our daily life in the above examples and in many other applications including LiDAR, Head Up Displays, Medical Instruments, 5G Communication, IR Instruments and many more.

     

    What are aspherical lenses?

     

    In 1638, Johann Kepler experimented with aspherical surfaces on lenses, which made it possible to obtain spherical aberration free images in both near and far distances. His efforts gradually laid the foundation of aspherical optics.

     

    An aspherical lens is an optical component whose surface is determined by multinomial higher-order equations and has a different radius at each point on the surface. When the incident light enters the aspherical lens the light will focus on one small point. This characteristic eliminates aberrations to obtain high-quality optical image. The multinomial higher-order equation is given as follows:

    This equation describes the surface as the deviation from a cone, where:

    Z is the verticality of the surface,

    Y is the height of the light,

    C is the curvature of the surface on the optical axis.

    A4, A6, A8... are the 4th, 6th, 8th...aspherical surface coefficient,

    k is conic constant.

    The relationship between the shape of an aspherical surface and K is as follows:

     

    Conic Constant k

    Aspherical shape

    k=0

    spherical surface

    k=-1

    parabolic surface

    k<-1

    Hyperbolic surface

    -1<k<0

    Ellipsoidal surface, the focus is on the optical axis

    k>0

    Ellipsoidal surface, the focus is on the perpendicular of the optical axis

     

    The difference between aspherical lenses and spherical lenses

     

     Spherical aberration is inherent which cannot be eliminated by a single spherical lens. However, a single aspherical lens can correct spherical aberration, coma, astigmatism, field curvature and other aberrations while reducing the loss of light energy so as to obtain a high-quality image and other outstanding optical properties. Lets take two comparable lenses with 50mm focal lenth (532nm wavelength) in below figures for example to see the difference in focusing performance between a single spherical lens and aspherical lens.

    From a system design perspective, the aspherical lens has a simpler structure, larger field of view and a higher image quality than the spherical system. Let's look at a Cooke triplet lens design as an example to see the merits of an aspherical lens. As we can see from the image below, the photograph of CASTECH Park, which was also simulated optically in a 30 degree field-of-view designed environment, can be obtained by replacing only the fifth surface of the triplet lens with an aspherical surface, resulting in smaller edge distortion, higher image quality and a more realistic picture.

     

     

    Aspherical lenses offer great advantages but have been historically difficult to produce.  CASTECH has developed precision manufacturing techniques, including CNC Machining and advanced metrology, to produce precision aspherical lenses and acylindrical lenses for many different applications.

     

    CNC machining and measurement technique for aspherical lenses

     

    CASTECH has established an advanced high-precision aspherical CNC (Computer Numerical Control) machining and testing center, with the ability to design, process and test high-precision aspherical lenses and acylindrical lenses applied to laser and other precision optical applications. High precision aspherical lenses and acylindrical lenses are obtained through the repetitive numerical control polishing and non-contact measuring conducted by the program of a CNC machining and testing center. CASTECH offers both standard and customized aspherical products whose diameters range from φ 5 to φ 200mm, surface quality exceeds 20/10 and surface irregularity better than λ /4.

     

    The high-precision measuring equipment is the "eye" of high-precision aspherical lens processing. A Taylor Hobson PGI is used in the fine grinding process, while LUPHOScan 260 non-contact measurement is used in the polishing process. The measurement aperture is up to φ260mm, and the accuracy is better than± 50nm.       

     

     

    Ion Beam Figuring (IBF) for aspherical lenses

     

    CASTECH owns a cutting-edge IBF (Ion Beam Figuring) machining center. With the help of a high-precision non-contact measurement equipment, the aspherical surface irregularity test data is imported into the computer, and the simulation software accurately controls the residence time of the ion beam in a specific area, which can achieve high-performance polishing in a short time. This process can produce an aspherical lens with surface irregularity better than λ / 10.

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