How to Choose Optical Spherical Lenses For Imaging?

Plano-Convex Lenses are the best choice for focusing parallel rays of light to a single point, or a single line in the case of cylindrical lenses. This lens can be used to focus, collect and collimate light. It is the most economical choice for demanding applications. The asymmetry of these lenses minimizes spherical aberration in situations where the object and image are located at unequal distance from the lens. The optimum case is where the object is placed at infinity (parallel rays entering lens) and the final image is a focused point. Although infinite conjugate ratio (object distance/image distance) is optimum, plano-convex lenses will still minimize spherical aberration up to approximately 5:1 conjugate ratio. For the best performance, the curved surface should face the largest object distance or the infinite conjugate to reduce spherical aberration.

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Bi-Convex Lenses are the best choice where the object and image are at equal or near equal distance from the lens. When the object and image distance are equal (1:1 magnification), not only is spherical aberration minimized, but also coma, distortion, and chromatic aberration are identically canceled due to the symmetry. Bi-convex lenses function similarly to plano-convex lenses in that they have a positive focal length, and focus parallel rays of light to a point. Both surface are spherical and have the same radius of curvature, thereby minimizing spherical aberration. As a guideline, bi-convex lenses perform within minimum aberration at conjugate ratios between 5:1 and 1:5. Outside this magnification range, plano-convex lenses are usually more suitable.

Plano-Concave Lenses are the best choice where object and image are at absolute conjugate ratios greater than 5:1 and less than 1:5 to reduce spherical aberration, coma, and distortion. Plano-Concave lenses bend parallel input rays so they diverge from one another on the output side of the lens and hence have a negative focal length. The spherical aberration of the Plano-Concave lenses is negative and can be used to balance aberrations created by other lenses. Similar to the Plano-Convex lenses, the curvature surface should face the largest object distance or the infinite conjugate (except when used with high-energy lasers where this should be reversed to eliminate the possibility of a virtual focus) to minimize spherical aberration.

Bi-Concave Lenses are the best choice where object and image are at absolute conjugate ratios closer to 1:1 with converging input beam. The output rays appear to be diverging from a virtual image located on the object side of the lens; the distance from this virtual point to the lens is known as the focal length. Similar to the Plano-Concave lenses, the Bi-concave lenses have negative focal lengths, thereby causing collimated incident light to diverge. Bi-Concave lenses have equal radius of curvature on both side of the lens. They are generally used to expand light or increase focal length in existing systems, such as beam expanders and projection systems.

Positive Meniscus Lenses are designed to minimize spherical aberration and are generally used in small f/number applications (f/number less than 2.5). The Positive Meniscus Lenses have a larger radius of curvature on the convex side, and a smaller radius of curvature on the concave side. They are thicker at the center compared to the edges. Positive meniscus can maintain the same angular resolution of the optical system while decreasing the focal length of the other lens, resulting a tighter focal spot size. A positive meniscus lens can be used to shorten the focal length and increase the numerical aperture of an optical system when paired with another lens. For the best performance, the curved surface should face the largest object distance or the infinite conjugate to reduce spherical aberration.

Spherical Lens Material Options

Lens Type N-BK7 UV Fused Silica CaF2 MgF2 ZnSe Crown/Flint Plano-Convex Bi-Convex Plano-Concave Bi-Concave Achromatic Doublet Cylindrical Lenses Plano-Convex Plano-Concave

Coatings

Optical coatings are generally applied as a combination of thin film layers on optical components to achieve desired reflection/transmission ratio. Important factors that affect this ratio include the material property used to fabricate the optics, the wavelength of the incident light, the angle of incidence light, and the polarization dependence. Coating can also be used to enhance performance and extend the lifetime of optical components, and can be deposited in a single layer or multiple layers, depending on the application. Newport’s multilayer coatings are incredibly hard and durable, with high resistance to scratch and stains.

Anti-Reflection Coating (AR coating)

Newport offers an extensive range of antireflection coatings covering the ultraviolet, visible, near infrared, and infrared regions. For most uncoated optics, approximately 4% of incident light is reflected at each surface, resulting significant losses in transmitted light level. Utilizing a thin film anti-reflection coating can improve the overall transmission, as well as minimizing stray light and back reflections throughout the system. The AR coating can also prevent the corresponding losses in image contrast and lens resolution caused by reflected ghost images superimposed on the desired image.

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Newport offers three types of AR coating designs to choose from, the Single Layer Magnesium Fluoride AR coating, the Broadband Multilayer AR coating, and Laser Line AR V-coating. A single layer Magnesium Fluoride AR coating is the most common choice that offers extremely broad wavelength range at a reasonable price. It is standard on achromats and optional on our N-BK7 plano-convex spherical lenses and cylindrical lenses. Comparing to the uncoated surface, the MgF2 provides a significant improvement by reducing the reflectance to less than 1.5%. It works extremely well over a wide range of wavelengths (400 nm to 700 nm) at angles of incidence less than 15 degrees.

Broadband Multilayer AR coating improves the transmission efficiency of any lens, prism, beam-splitter, or windows. By reducing surface reflections over a wide range of wavelengths, both transmission and contrast can be improved. Different ranges of Broadband Multilayer AR coating can be selected, offering average reflectance less than 0.5% per surface. Coatings perform efficiently for multiple wavelengths and tunable laser, thereby eliminating the need for several sets of optics.

V-coatings offer the lowest reflectance for maximum transmission. With its high durability and high damage resistance, Laser line AR V-coating can be used at almost any UV-NIR wavelength with average reflectance less than 0.25% at each surface for a single wavelength. Valuable laser energy is efficiently transmitted through complex optical systems rather than loss to surface reflection and scattering. The trade off to its superior performance is the reduction in wavelength range. AR.33 for nm is available from stock on most Newport lenses. All other V-coating can be coated on a semi-custom basis.

Coating Wavelength Range
(nm) Reflectance Cost Features AR.10
Broadband
245–440 Ravg <0.5% Moderate Only available on UV fused silica lenses MgF2
Broadband
Broadband
400–700 Ravg <1.5% Low Available on achromats, KPX series, and Cylindrical lenses AR.14
430–700 Ravg <0.5% Moderate Best choice for broadband visible applications AR.15
Broadband
250–700 Ravg <1.5% Moderate Great choice for broadband UV to visible applications AR.16
Broadband
650– Ravg <0.5% Moderate Excellent for NIR laser diode applications AR.18
Broadband
– Ravg <0.5% Moderate Ideal for telecom laser diode applications V-Coat Multilayer, AR.27 Laser Line
532 Rmax <0.25% High Highest transmission at a single wavelength V-Coat Multilayer, AR.28 Laser Line
632.8 Rmax <0.25% High Highest transmission at a single wavelength AR.33
Laser Line
Rmax <0.25% Moderate Highest transmission at a single wavelength

The optical lens is an optical component used by movie cameras, projectors, cameras, and cameras to generate images, and consists of multiple lenses. It is an indispensable component in the machine vision system, which directly affects the quality of the imaging, and affects the realization and effect of the algorithm. Optical lenses can be divided into short-focus lenses, medium-focus lenses, and telephoto lenses in terms of focal length; wide-angle, standard, and telephoto lenses in terms of field of view; fixed-aperture fixed-focus lenses and manual aperture fixed-focus lenses in terms of structure. Auto-iris fixed-focus lenses, manual zoom lenses, auto-zoom lenses, auto-iris electric zoom lenses, electric three-variable (aperture, focal length, and focus) lenses, etc. So what is a spherical lens?

1. What is a spherical lens?

The convex or concave surface of the lens is shaped like a section cut from a sphere, which is a spherical lens. The convex or concave mirror for myopia or hyperopia has only one radius of curvature, the convex surface of the inner TC lens for astigmatism has one radius of curvature, and the concave surface has two perpendicularly intersecting radii of curvature. A mirror whose reflective surface is part of a sphere is called a spherical mirror (a part of a spherical shell—a spherical cap). A lens whose lenses are all composed of spherical lenses is a spherical lens. Spherical lenses are more common in the low-end market. Its advantage is low cost.

2. Types of spherical lenses

Spherical lenses are divided into convex mirrors and concave mirrors. 1. Concave mirror: A spherical mirror that uses the inner side of the sphere as the reflecting surface is called a concave mirror. 2. Convex mirror: A spherical mirror that uses the outside of the spherical surface as the reflecting surface is called a convex mirror.

3. Imaging comparison between spherical lens and aspheric lens

Spherical lens means that both the inner and outer sides of the lens are spherical, or one side is a spherical lens and the other half is a flat lens. The surface shape of the aspheric lens is determined by the multi-image high-order equation, and the radii of each point on the surface shape are different. When the general spherical lens is used for imaging, it cannot ideally focus the object object to a point, and various aberrations will appear. Therefore, its image quality is generally relatively poor. In order to improve the imaging quality, different types of concave and convex lenses are generally used to cancel out the aberrations; while using aspheric lenses, the aspheric surface can effectively improve the imaging quality of the lens.

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