10 Things to Consider When 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

How to Select a Spherical Lens

Lenses are an integral component in most optical systems, where they are used to focus, collimate, expand, collect and image light. Many optical tasks require several lenses in order to achieve an acceptable level of performance. This selection guide will review the singlet spherical lens shapes offered by CVI Laser Optics and offer some practical guidance on determining the best material type and lens quality for your application. For a complete discussion of lens theory, use, and aberrations please refer to the Fundamental Optics and Gaussian Beam Optics sections of the Technical Guide found online.

Lens Shape

CVI Laser Optics offers four lens types for converging or focusing light. A biconvex lens is the classic symmetric lens, possessing two convex surfaces of equal radii. Biconvex lenses have positive focal lengths and form both real and virtual images. It is the best singlet lens for imaging at unit magnification; spherical aberration is minimized, and coma, distortion, and transverse chromatic aberration exactly cancel each other out for a perfectly made lens (longitudinal chromatic aberration is not corrected). This is true regardless of the material used or wavelength, although use of a remote stop can reduce the degree of cancellation. Aberrations increase as conjugate ratios (object distance/image distance) depart from unity. Bi-convex lenses can also be used for focusing applications, in particular when a lower f-number (ƒ /CA) is required, even if they do not have the best shape for this conjugation. They are recommended for virtual imaging of real objects and for positive conjugate image ratios from approximately 0.2 to 5 (note that these values are wavelength sensitive).

Away from unity, the singlet lens shape that best minimizes spherical aberration at a given conjugate ratio is called a bestform lens, in which the two convex sides are of different radii. The marginal rays are equally refracted at each of the lens/air interfaces for this shape, and surface-reflection loss is minimized. Another benefit is that absolute coma is nearly minimized for bestform shape, at both infinite and unit conjugate ratios. It does not, however, perform well in wide-field applications, except in very specific configurations (with a meniscus for instance). At infinite conjugate ratio, the best form lens is not the optimum singlet shape, since it results in high field curvature. Positive bestform lenses are of exceptional performance and provide the smallest spot size available in a singlet lens.

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When working at infinite or near-infinite conjugate ratio, plano-convex lenses with the convex side toward the infinite conjugate perform nearly as well as the best-form lens. Convex on one side and flat on the other, these positive focal length lenses cost much less to manufacture than a bestform lens. This lens shape also exhibits near minimum transverse spherical aberration and near-zero coma when used off-axis. Longitudinal aberration is low, but is only minimized when using a bestform lens.

If bulk light collection is required at minimal cost, an aspheric glass condenser lens may be the solution. Aspheric lenses provide better performance by reducing aberrations when used in low f-number, high-throughput applications. One surface is aspheric; the second surface is flat, or spherical-convex. A flat second surface minimizes aberration, while a spherical-convex second surface provides the lowest f-number and highest transmission. Only single-layer MgF2 antireflection coatings are recommended for these lenses due to the steep curvature of the surface. The molding and felt-polishing processes used to manufacture these lenses economically, together with the use of optical crown glass, makes these lenses best suited to less demanding light collection applications. They are not recommended for imaging, precise focusing, or high power applications, but are ideal for gathering low-power light with minimal aberration.

CVI Laser Optics offers two lens types for diverging or expanding light. A bi-concave lens is a symmetric lens, possessing two concave surfaces of equal radii. Biconcave lenses have negative focal lengths and form only virtual images which can be seen through a lens. They are used in laser beam expanders, optical character readers, viewers, and projection systems to diverge collimated incident light.

Like their convex counterparts, plano-concave lenses can reduce aberrations as compared to bi-concave lenses, depending on the configuration. They have a negative focal length and are often used to expand light or to increase focal lengths in optical systems, as they diverge collimated incident light. When working at infinite or nearinfinite conjugate ratio, plano-concave lenses with concave side toward the infinite conjugate reduce spherical aberration, and coma. Additionally, the negative spherical and chromatic aberration that plano-concave lenses exhibit can be used to balance the aberrations resulting from other lenses within a system.

Lens Materials

Aside from lens shape, the material a lens is made from has the greatest impact on its performance. Not only does it determine the transmission properties, refractive index, laser damage threshold, thermal coefficient, durability and weight, but it also impacts cost. It even imposes practical limits on manufacturing tolerances, especially surface cosmetic quality. CVI Laser Optics utilizes five different materials to manufacture our catalog singlet lenses, but other materials such as magnesium fluoride, sapphire, Infrasil, Suprasil, or different grades of the materials listed below are available on a custom basis. We can also manufacture many of our lenses with custom dimensions and focal lengths.

N-BK7 is a lead- and arsenic-free borosilicate crown glass that is used widely in the optics industry. It has excellent transmission from 350 nm – 2.0 μm, good thermal expansion coefficient, moderate laser damage threshold, and is relatively low in cost. It is a hard glass that stands up well to handling, with good chemical resistance.

Our fused silica is Standard Grade Corning , a synthetic form of fused silica manufactured by flame hydrolysis to extremely high standards. Its ultra-low impurity content is evident in the wide transmission range of 180 nm – 2 μm and its high laser damage threshold. It does not fluoresce in response to wavelengths longer than 290 nm, and in general exhibits good resistance to radiation darkening from ultraviolet, x-rays, gamma rays, and neutrons. It also boasts excellent thermal properties, including a wide operating temperature range, low thermal coefficient, and resistance to thermal shock. Fused silica lenses from CVI Laser Optics have increased hardness and resistance to scratching, resulting in better surface quality than their N-BK7 equivalents.

KrF and ArF grade fused silica is fused silica that has been manufactured with near-zero defects for use with either KrF lasers at 248 nm or ArF lasers at 193nm. This material has very low levels of inclusions, bubbles, striation and striae, and minimal variations in index of refraction. Its exhibits the lowest level of laser induced fluorescence of compared to most glasses, strong resistance to radiation-induced defect generation, and higher UV laser damage threshold than CaF2. Internal transmittance is also guaranteed at high levels at their respective wavelength of use. It is one of the more expensive UV material options, particularly at larger diameters and available as a custom option.

N-SF11 is a type of lead- and arsenic-free Schott glass with much higher refractive index than N-BK7, resulting in greater focusing power and reduced spherical aberration. Its transmission is best in the visible and near-infrared, from 500 nm – 2.5 μm. Its thermal and hardness characteristics are comparable to N-BK7.

CaF2 is a cubic single-crystal material with transmission spanning the deep UV through infrared, 150 nm – 8 μm. It can be mined or manufactured synthetically, but is higher in cost than other lens materials, particularly in the high purity forms used for deep-UV applications. That being said, its excellent transmission and high laser damage threshold in the deep UV makes it the material of choice for many excimer laser applications. Lenses manufactured with CaF2 tend to have slightly lower surface quality than their N-BK7 or fused silica equivalents per its soft nature.

Optical crown glass is a low-index, commercial-grade glass. Its index of refraction, transmittance, and homogeneity are not controlled as carefully as in optical grade glasses like N-BK7. It transmits from 200 nm – 2 μm, with best performance from 400 nm – 1.5 μm. Though its thermal expansion coefficient is reasonable and its hardness makes it robust to handling, its laser damage threshold is low.

Lens Quality

Once the lens shape and material has been selected, the next step is to determine the lens quality required. This will depend on the application and performance needed, but factors to consider include laser damage threshold, degree of scatter, surface figure, and focal length tolerance.

Standard lenses possess a surface irregularity of λ/4 to λ/2 before coating, and are manufactured to a minimum surface quality of 60-40 to 40-20 scratch and dig, depending on material. They are an economical solution for many applications, but lower surface accuracy impacts resolution, and laser damage threshold is not as high. CVI Laser Optics offers standard singlet lenses in both N-BK7 and fused silica. These lenses are available uncoated, or with a selection of antireflection coating options from the UV to near-infrared.

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