JML Optical

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Single lens elements, known as “singlets,” are the basic elements in every optical system. They are used to manipulate narrow laser beams or to build prototype imaging systems. They are also used as magnifiers, relay lenses and field lenses. Singlets are used in combination to achieve high-quality system performance. 

Optical systems engineers and lens designers usually match a singlet to an application by specifying five primary characteristics:

1. Surface contour

2. Material from which it is made

3. Cross-sectional bending, or shape, of the element (ratio of the curvatures of the two refracting surfaces)

4. Thin lens attributes (focal length, diameter, and f-number)

5. Thickness

 Influencing these choices is the designer’s concern about aberrations, performance and ease of manufacture.

Surface Contour
Surface contour refers to the geometric shape of a polished surface of refraction. Two examples are “spherical” and “aspherical.” The surfaces of a spherical element are as round as the surface of a perfect sphere. On the other hand, the term “aspherical” is commonly used to describe the surface if the contour deviates from that of a sphere.

Figure 1

Different contours produce different optical performance. Designers usually specify lens elements with spherically contoured surfaces because their performance-to-cost ratio is higher than lens elements with other contours such as aspheres.

Material

For the lens designer, material allows a significant degree of freedom. Different materials exhibit different optical, mechanical and chemical properties. Choice of material is made partly on the basis of the wavelength of light that is used in the application. Many different materials transmit near-ultraviolet, visible and near-infrared radiation (350nm - 2500nm). The selection shrinks dramatically for applications in the deep ultraviolet and far infrared regions of the spectrum.

Another factor that influences a designer’s choice of material is image quality. Two optical properties of a material that can be used to control the aberrations of a lens are index of refraction and chromatic dispersion (Abbe number). The index of refraction affects a singlet’s monochromatic aberrations, while the chromatic dispersion affects its chromatic aberrations.  The optical engineer will also match material to the operating environment of the application. For example, some materials, such as Pyrex®, can withstand rapid changes in temperature while other materials, such as fused silica, can withstand temperatures of 1000°C for extended periods of time.

Bending (Shape)

The term “bending” refers to the ratio of the curvatures of the two refracting surfaces of a lens. Examples include meniscus or plano-convex bendings. “Shape” is a term often used interchangeably with “bending.”

Designers can employ this technique to maximize the performance of a singlet when it is used individually.  Bending can also change a singlet’s back focal length by altering the location of its principal planes. When several elements are combined in an optical system or compound lens, bending can be used to help balance their aberrations and produce higher quality performance.

Thin Lens Attributes

Three basic numbers describe the ideal image-forming characteristics of a lens: focal length, diameter and f-number. These “thin lens attributes” serve as a measure of performance. Focal length is a measure of the refractive power of a lens. Diameter is a measure of light gathering power. The f-number, defined as the ratio of focal length to diameter, is a measure of limiting resolution and image brightness.

Choice of thin lens attributes depends on requirements of the application such as light-gathering power, spot size in the focal plane, MTF, total working distance from object to image and back focal length.

Thickness

Lens thickness is one of the determinants of the optical path of a ray traveling through a lens and therefore is one of the characteristics that can influence lens performance. It also is important in determining the edge thickness (mounting surface) of a lens.

Spherical Lens Elements

Spherical lens elements derive their name from their spherically contoured refracting surfaces. They are the most widely used type of singlets. Imaging performance of an individual spherical singlet is limited by significant monochromatic and polychromatic aberrations.  When used in combination, however, one singlet can cancel aberrations caused by another. The result is a system of higher-quality performance. It is important to note that extremely high-quality performance often requires specially designed components.

Material

JML stocks mainly spherical singlets in four types of glass: BK7, fused silica, Pyrex, and CaF2. Each glass offers distinct advantages in different applications:

BK7 Glass n_d 1.517 _d 64.2

BK7 glass, the most commonly used material for spherical singlets, transmits light throughout the near-ultraviolet, visible and near-infrared spectra (350nm-2500nm).

Fused Silica n_d 1.458 v_d 67.6: Fused silica transmits deep ultraviolet light down to 200nm. This glass will also transmit throughout the visible and near-infrared. Its extremely high durability makes it a material of choice for a wide range of temperatures, chemical environments and laser power.

Pyrex n_d 1.473 u_d 65.7

Pyrex glass is well known for its stability in a wide range of temperatures and its resistance to chemical attack. Pyrex singlets are used in unusually harsh environments. Transmission through Pyrex is similar to that of BK7 (350nm-2500nm), although its optical homogeneity is somewhat less than that of BK7. Corning Glass made the name “Pyrex” a household name when it created its famous line of cookware and laboratory vessels. Since Corning’s introduction of this durable form of borosilicate glass, other manufacturers have produced the same type of glass with different trademarks and trade names. Schott America uses the trade name “Tempax®” for its special borosilicate glass; Schott is world renowned for its precision optical glass. JML uses Tempax and Pyrex 7740 for its lenses.

CaF2 refractive index 1.401 at 4.8 microns

Calcium fluoride is often used in the 2-6 micron region, although it is transparent from the ultraviolet region to 9 microns and has a relatively low absorption coefficient. Due to its low refractive index it can often be used without an antireflection coating. It is, however, a relatively soft material and is not resistant to thermal or mechanical shock.

Bending

Within each category of glass type JML has classified spherical singlets by bending. “Bending,” also called “shape,” refers to the ratio of the curvatures of the first and second refracting surfaces of a singlet. Therefore a singlet with surfaces of equal curvature is of shape 1:1.  If the first surface is curved more strongly than the second by a factor of 2, then its shape is 2:1.

Use of the word “bending” derives from a poetic interpretation of the mental process of changing the curvatures of a singlet while maintaining its focal length.  While searching for the ideal shape of a singlet in a given application, a lens designer will mentally “bend” a singlet as if it were made of rubber. Figure 2 illustrates the basic shapes found in positive and negative lenses.

Figure 2

Each bending produces a slightly different image quality and back focal length. Choice of bending depends on required performance for a specific magnification and the system layout.

Positive Focal Lengths

Plano-Convex

The most commonly used singlet is plano-convex.  Remember to orient a plano-convex lens with its convex surface facing the more distant conjugate.

Bi-Convex (Equi-Convex)

If your application requires imaging at 1:1 (1× magnification), then a bi-convex singlet with the same curve on both sides (also known as equi-convex because the radii of the first and second surfaces are equal) will produce better-quality imagery than spherical singlets of other shapes.

Meniscus

Between the cases of 1× magnification and collimated beams lie most remaining applications in optical engineering. For these intermediate magnifications, positive meniscus lenses produce the best-quality images. For example, a positive meniscus lens can be used to slightly decrease the focal length of a sophisticated compound lens without seriously degrading its performance.

Historically, positive meniscus singlets were designed for the ophthalmic industry. Series of these lenses were produced with small increments between their focal lengths in order to measure precisely and to correct farsighted vision.

Positive meniscus lenses are sometimes used in combination with plano-convex singlets to create higher quality condensers and magnifiers.

Best-Form

Consider the need to focus a collimated beam such as a laser beam. The smallest spot size obtainable at best focus with an individual spherical singlet requires a singlet with a special shape determined by focal length, thickness and f-number. JML’s Best-form laser lens conforms to this ideal specification (see Figure 3).

Figure 3

Negative Focal Lengths

Plano-Concave

The most commonly used singlet of negative power is plano-concave. A classic application can be found in some Galilean telescopes where it serves as the eyepiece and creates a collimated beam for the observer’s eye.

Bi-Concave (Equi-Concave)

For applications requiring virtual imaging at 1:1 (1× magnification), the bi-concave singlet produces results with fewer aberrations than negative singlets of other shapes. Virtual images are associated with diverging beams of light. Unlike real images, virtual images cannot be seen on imaging media such as film or CCDs.

Meniscus

Between the cases of 1× magnification and collimated beams are most remaining applications in optical engineering. For these intermediate magnifications, negative meniscus lenses produce the best-quality virtual images when compared to other singlets. For example, a negative meniscus lens can be used to slightly increase the focal length of a sophisticated compound lens without seriously degrading its performance.

Historically, negative meniscus singlets were designed for the ophthalmic industry. Series of these lenses were produced with small increments between their focal lengths in order to measure precisely and to correct nearsighted vision.

Thin Lens Attributes (focal length, diameter, f-number)

Within each classification of bending, JML offers singlets of many different thin lens attributes. Lenses are listed according to their focal lengths.

Figure 4

Combination of Spherical Singlets

Performance can be enhanced when two singlets are used to replace one singlet of best shape. Remember that truly high-performance systems are rarely designed by matching elements from a catalog. Instead, their components must be specially designed.

A classic example of enhanced performance is found in the case of 1:1 imaging. A pair of plano-convex lenses with convex curves facing each other produces an image of better quality than that from a bi-convex singlet (see Figure 4).

The rule of thumb in this situation: choose two plano-convex singlets of the same focal length. Their focal lengths must be twice the focal length of the lens to be replaced because the focal length of a pair of identical lenses is half the focal length of each one alone.

The case of a simple condenser is another example in which performance of a singlet can be improved with the addition of a second singlet. A meniscus lens is used with a plano-convex lens as shown in Figure 5.

There is no simple rule of thumb, as there is in the earlier example, to guide the experimenter. Nevertheless, acceptable results may be realized if the two lenses are located very close to each other and the total power of the two is equally divided between them. The concave surface of the positive meniscus lens must face the source of light and the flat surface of the plano-convex lens must face the meniscus element.

Figure 5

 Aspheric

Aspheric singlets derive their name from the non-spherical contour of their refracting surfaces. An aspherical surface follows that of a sphere in the central portion of the lens but a slight deviation from a spherical figure generally causes the cross-section to flatten near its edges (Figure 6).

By deviating from a strictly spherical figure, an asphere can be designed to perform without the spherical aberration caused by spherical contours. The result is a lens element with superior performance.

Figure 6

One of the more common applications for aspheres is found in condensing systems. Molded aspheres efficiently relay the energy from the light source into the imaging components of the product. Designers of slide projectors and movie projectors often employ aspheres in their condensing systems to maximize the brightness and quality of their projected images.  JML molds aspheres out of high-quality crown glass and fire-polishes the surfaces. These lens elements have excellent homogeneity and surface quality.

Mounted Aspheric Lenses

This line of compact, high quality, glass (Corning C0550, nd=1.605004, ud=50.4) aspheres is an alternative to conventional microscope objectives. These mounted aspheres are perfect for use anywhere you might otherwise use a microscope objective. They are especially good for low F number applications such as coupling light into or out of optical fibers or collimating or focusing laser diodes.

Best-Form Laser Lens Elements

When an application demands the best concentration of energy for the least expense, Best-form laser lens elements produce the finest results. When operated at fast and moderately fast f-numbers, these specially designed lens elements focus a collimated beam to a spot whose diameter is smaller than the spot size achieved with plano-convex, bi-convex (equi-convex) or meniscus singlets.  Best-form lenses bridge the gap in lens performance between basic singlet shapes and achromats.

Designed with the aid of sophisticated computer programs, bi-convex lenses are nearly plano-convex. Their small deviation from exact plano-convex shape produces their unique focusing power. In practice, performance is optimized when the more strongly curved surface faces the incoming beam.

Applications for Best-form laser lenses share one common requirement: smallest focal spot for a single lens element. These applications usually involve lasers.

Lenses of this shape have been used to increase the pumping efficiency of dye lasers. In this application one laser is used to activate or “pump” another. The first laser must be focused into the organic dye to cause the laser action. The smaller spot size of a Best-form laser lens creates a higher energy density and more efficient use of the pumping laser.

Laser welding, cutting and engraving require intense concentration of power created by small spot sizes, and more precise positioning of the beam can be achieved with smaller spot sizes.

Laser beam quality can be improved with a technique called “spatial filtering.” A collimated laser beam is often marred by dust and dirt. These defects in the beam can be filtered out by focusing the beam into a small spot and sending it through a pinhole aperture. The metal into which the aperture is drilled blocks the energy contained in high spatial frequencies created by the dust. On the other side of the pinhole, a lens recollimates the emergent beam, which exhibits a smooth, unblemished cross-section. Best-form laser lenses produce better spatial filtering than other singlets.

Photographic images can be enhanced by spatial filtering in much the same way. In fact, some experimental machine vision systems use spatial filtering to enhance specific details and reduce post-processing time of their images.

In addition to direct focusing applications, Best-form laser lenses can be found in Keplerian beam expanders and reducers. Their superior focusing properties contribute fewer aberrations to a beam, therefore, a beam whose size has been adjusted with a pair of Best-form lenses will be of higher quality than a beam whose size has been adjusted with a pair of singlets of different shape.

For applications requiring the smallest possible spot size with a singlet, Best-form laser lenses are the ideal choice. JML offers this series of lenses in BK7, Grade A glass.

Fresnel

Augustin Jean Fresnel developed the first Fresnel lens in the early nineteenth century. Today, his invention can be found in optical systems where design criteria call for light weight or small size.

Unlike traditional lenses, Fresnel lenses do not employ smooth-surface contours to focus rays of light. Instead, the surface of a Fresnel lens is molded into many circular, concentric ridges. The symmetry of these concentric ridges is similar to that of a dart board (Figure 7).

Figure 7

These circular ridges give the Fresnel lens a zigzag or saw tooth cross-section. Each saw tooth creates a tiny prism. By choosing appropriate powers for these prisms, designers can define the focal length and control image quality.

Fresnel lenses can be molded from precision optical grade acrylic. Typical applications are condensing systems with small amounts of residual heat and for which a large aperture or thin profile is critical.  Applications for Fresnel lenses include TV projection, LCD projection units, 3 dimensional photography, solar energy systems, infrared alarms and detection systems point-of-sale scanners.

Overhead projectors incorporate a Fresnel lens as part of their illumination systems.  Usually the Fresnel lens is visible just below the glass platen onto which the transparencies are placed. To evenly illuminate the image of the transparency on the screen, the condensing system must direct light through all parts of the transparency into the projection lens that is mounted on the arm above the platen. A more conventional lens could be used for the task, but its weight and expense would be prohibitive if it were made with the requisite 81⁄2 by 11-inch aperture.

Another application for a Fresnel lens can be found in the design of large-format view cameras. A view camera contains a large ground-glass focusing screen. To improve uniformity of brightness across the entire screen, a Fresnel lens can be placed against the ground glass on the side facing the camera’s lens. The Fresnel element functions as a field lens by intercepting light already focused at the screen and refracts it toward the viewer’s eye. Rays that would be lost because of their large angles of incidence near the edge of the focusing screen are thus captured and contribute to image brightness.

JML offers injection molded type Fresnel lenses.

Cylindrical

Cylindrical lenses derive their name from their cylindrically contoured refracting surface, which is circular in shape when viewed in cross-section. Unlike spherically symmetrical lens elements, such as spherical singlets, cylindrical lenses allow light rays to converge (positive cylinders) or diverge (negative cylinders) in only one cross-section or meridian (see Figure 8). Accordingly, the theory of performance for spherical singlets also applies to cylindrical singlets, but only along the axis with curvature or power. In fact, a cylindrical lens element may be abstractly considered as one-half of a spherical element (see Figure 9); two crossed, mutually perpendicular cylinders of equal power behaving in tandem as one spherical singlet.

Figure 8

Figure 9

Cylindrical lenses are either plano-convex (converging; positive focal length) or plano-concave (diverging; negative focal length) in form and rectangular in shape.  Plano-convex cylindrical lenses, like their spherical counterparts, find applications as simple focusing optics and collimators in a single meridian. Similarly, plano-concave cylindrical lenses are used for beam expansion along a single axis.  With cylindrical lenses, point sources can be imaged as lines and vertical magnification can be different from horizontal magnification. Applications are especially numerous in imaging systems referred to as “anamorphic,” a term which applies to the optics found in laser scanners, laser diode systems, spectrophotometers and projectors.

Examples of cylindrical lens usage in industry can be found from the world of computer science to Hollywood. Many laser printers employ laser diodes as their source of light. Laser diodes emit asymmetrical beams, for example 5° x 22° divergence; they are elongated and elliptical in cross-section. Cylindrical lenses

can be used to transform the elliptical beam into a circular beam by expanding or compressing one axis of the light bundle at a different power from the other.  Widescreen movie camera and projection systems employ cylindrical lenses to expand the horizontal dimension of the picture on the screen. As a matter of fact, some wide-screen projectors are designed to reproduce the horizontal dimension with slight residual compression.  The resulting illusion is one in which the actors appear slimmer on screen than they are off screen!

Our negative cylindrical lenses are made of BK7 glass. Our positive cylindrical lenses are available made of BK7 glass or fused silica. Plano-convex shapes produce lenses with positive focal lengths and plano-concave cylindrical lenses have negative focal lengths.

Micro-Optics and Micro Ball Lenses

Micro-optics is a relatively new class of optics that deserves special mentioning.  With today’s emerging technology and market push toward miniaturization of products, there is an increasing need for very small, precision optics. Applications such as medical diagnostic and surgical products, optical fiber coupling and laser diode collimating and focusing requirements have sparked the necessity for lenses, cylinders, mirrors and prisms as small as 1 and 2mm in diameter or length.

JML is filling the needs of this new optical market by supplying “off the shelf” and custom micro-optics to the precision optical, laser and electronic communities.

In addition to our standard micro-optics, many other types and shapes (e.g., rod, tunnel lenses and mirrors, cubes, cones and prisms) are available as special order items.

Sapphire Ball Lenses

Sapphire ball/half-ball lenses are used for applications that require hard, scratch resistant optics. Typical applications include fiber optic couplers, bar code readers, photo diodes and detector type equipment.

Sapphire has high mechanical strength, temperature stability, wear resistance and chemical inertness.

Sapphire transmits in the UV, visible as well as infrared ranges so it can be used in a wide variety of optical applications and spectral ranges. JML’s ball lenses are made from synthetic sapphire which has the same chemical composition as natural sapphire, but far fewer impurities.

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