5 Things to Know Before Buying 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.

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

Selecting the right lens for your imaging application is important to achieving optimal performance. Lenses come in various shapes and forms, each with its own unique characteristics and advantages. Understanding the differences between spherical and aspheric lenses can help you make an informed decision that meets your specific needs. In this blog, you will learn more about the intricacies of both lenses, including their design, how they work, their applications, and the main considerations in choosing the right lens for an optical system.

A sphere-shaped lens features an even curvature across its entire surface and is relatively inexpensive and easy to manufacture, aspherics being more so. However, Spherical lenses may suffer from an effect called Spherical Aberration which causes light rays passing through their edges not focusing correctly in comparison with those passing through its center; images produced can appear blurry due to this phenomenon using wider apertures or high magnification magnification levels.

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Refraction occurs when light rays pass through spherical lenses which bend them as they pass. Their basic principle lies within their circular design: light entering such lenses interact with its curvilinear surface, leading them either towards convergence (convex lenses) or divergence (concave lenses).

Here’s a more detailed look at how spherical lenses work, based on their type:

Focusing the light through a spherical lens depends upon its curvature, refractive indices of materials used in its construction and wavelengths of light that pass through it. Spherical lenses suffer from distortion due to their uniform curve; light hitting their edges being refracted more than those striking its center, thus leading to different focus locations along an optical axis.

Aspherical lenses work by controlling the direction that light rays pass through through a process known as refraction, similar to how spherical ones do, yet feature significant variations in surface curvature; their profiles tend to be more complex than spherical ones which typically feature uniform curvatures; as such they’re better at correcting aberrations (especially spherical) more effectively due to non-uniform surface curvatures; as such they focus light more precisely onto one focal point; correct aberrations while correct aberrations more effectively due to non-uniform surface curvatures as opposed to uniform curvatures featured by their counterparts spherical counterparts which feature uniform curvatures; they also focus light more efficiently onto one point when focused onto one point than traditional counterparts would allow.

Take a close look at an aspherical lens to witness its functionality:

Finding an aspherical or spherical lens suitable to your needs requires considering several key aspects, particularly within photonics. Photonics is an expansive field that encompasses everything from telecom systems and laser beam systems through medical photonics as well as sensors requiring lenses – this comprehensive guide can assist in selecting an appropriate type of lens in photonics applications.

In the field of photography, aspheric lenses are prized for their ability to minimize distortion and provide high image clarity, making them essential in professional-grade cameras and high-end smartphones. They help achieve sharp images with accurate focus, important for detailed photography and videography. Spherical lenses, while not as advanced in reducing aberrations, are commonly used in entry-level cameras where cost-efficiency is a priority.

VR and AR systems demand lenses that can deliver a wide field of view with minimal distortion. Aspheric lenses are well-suited for these applications due to their ability to provide clear and immersive visuals, enhancing the user experience. The precision in aspheric lenses ensures that users perceive virtual objects with minimal optical flaws, which is critical for maintaining realism and immersion in VR and AR environments.

In display technologies such as projectors and augmented reality displays, the choice between spherical and aspheric lenses can impact image quality and device compactness. Aspheric lenses help in producing uniform and high-quality images across the entire display surface, while spherical lenses might be used in more cost-effective solutions where high precision is not as important.

Aspheric lenses, due to their complex manufacturing process and materials, can be more delicate and susceptible to damage if not handled properly. They require careful handling and storage to maintain their precision and performance. On the other hand, spherical lenses, being simpler in design and construction, tend to be more robust and less prone to damage, making them a durable option for rugged applications and environments.

The maintenance requirements for aspheric lenses are typically higher due to their complex surface profiles, which can make cleaning and alignment more challenging. Special tools and techniques might be needed to ensure they remain in optimal condition. Spherical lenses, with their simpler curvature, are easier to clean and maintain, reducing the time and cost associated with their upkeep.

Selecting the right lens type for your imaging application involves a thorough understanding of the specific requirements and constraints of your project. Spherical lenses offer simplicity and cost-effectiveness for less demanding applications, while aspheric lenses provide superior optical performance for high-precision tasks. By considering factors such as clarity, field of view, compactness, cost, and supplier capabilities, you can make an informed decision that meets your needs. Innovations in lens technology continue to expand the possibilities, making it an exciting time for developments in optical systems.

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