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Aspherical lenses with a complex surface profile can be designed to reduce or minimize spherical aberration and also other aberrations such as astigmatism, field curvature, and distortion. In optical systems, multiple spherical elements can be replaced by a single asphere without a loss in performance. In many cases, using aspherical lenses can significantly improve the system performance and also reduce the system weight, size, and manufacture cost.

Differential interference contrastmicroscopy

Because of the complex surface profiles, the fabrication of aspherical lenses are more difficult as compared with conventional sphere lenses. Several methods can be used to produce aspheric lenses. Each of these methods has its own unique advantages and limitations.

For decades, the method of grinding and polishing one aspheric lens at a time has been the standard manufacturing process. Most recently, computer numerically controlled (CNC) precision polishing can automatically adjust tool dwell parameters to form the aspheric shape accurately. When higher quality polishing is required, magneto-rheological finishing (MRF) can be adopted because of the high removal rate to correct the surface profile. While other manufacturing techniques require high-cost molds, precision polishing method utilizes standard tooling, which make it be the first choice for prototyping and low volume production.

Phase contrastmicroscopy

Similar to grinding and polishing, single point diamond turning (SPDT) can be used to manufacture single lenses one at a time. Diamond turning offers a high level of precision in the quality of the optical surfaces but is somewhat limited in choice of materials. However, the tool size used in SPDT is significantly smaller than in precision polishing, producing surfaces with improved surface finishes and form accuracies. Standard optical glasses cannot be shaped through diamond turning, whereas plastics, metal, and crystals can.

Fluorescence microscope

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Many factors such as wavelength requirements of the application and manufacturing cost will determined the material selection.  Below is a summary of materials that can be use with each manufacturing process.

Transmission electronmicroscopy

Comparing with molded plastics lenses, molded glass lenses have greater consistency in performance from lens to lens due to the thermal stability, high optical quality and greater durability of glass.

Type of microscope

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With a compound microscope, dark field is obtained by placing an occulting disk in the light path between source and condenser. A cheap set of occulting disks can be prepared by cutting circular pieces of black electrical tape ranging from dime-size up to a diameter that equals the width of the slide, and sticking them to the slide in a row. The circles should be spaced well apart. A specimen is placed on the microscope stage as usual, and the illumination should be made as uniform as possible. If there is an aperture diaphragm in the condenser (contrast lever), it should be opened up wide. After focusing at low power, the slide with occulting disks is placed in the light path between source and condenser, bringing it as close to the bottom of the condenser as it will go.

I set this up on the crummiest little piece of garbage microscope I could find, and it looked very good. A relatively new student-model microscope should give a much better effect.

Dark fieldscattering spectroscopy

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A spherical surface is defined by a single radius of curvature. Aspherical lenses have at least one surface that is not spherical. Optical Engineers typically consider the aspheres to be non-spherical, rotational symmetric surfaces. The radius of curvature of an asphere gradually changes from the center of the lens to the edge.

I would start with the largest disk, sliding it around until it is directly in the center of the light path. Increasing the illumination should then produce a good dark field effect. To optimize, first try stopping down the field diaphragm to get the best contrast between background and specimen. Try to match the size of the occulting disk to the field diameter, so that the edge of the disk is just outside the field of veiw - smaller disks are appropriate for higher power objectives. Vertically, the disk should be a close to the condenser as possible, to make the contrast the greatest. On microscopes with built-in dark field equipment, the view is so impressive because the occulting disk is built into the condenser - very close and focused. After testing the set-up this way, a stand might be rigged to fit under the microscope, so the slide can be placed in position without holding it. Something that 'grabs' the condenser and supports the occulting disks would be ideal. The less the students have to mess with, the better.

Dark field optics are a low cost alternative to phase contrast optics. The contrast and resolution obtained with inexpensive dark field equipment may be superior to what you have with student grade phase contrast equipment. It is surprising that few manufacturers and vendors promote the use of dark field optics.

Dark field illumination is most readily set up at low magnifications (up to 100x), although it can be used with any dry objective lens. Any time you wish to view everything in a liquid sample, debris and all, dark field is best. Even tiny dust particles are obvious. Dark field is especially useful for finding cells in suspension. Dark field makes it easy to obtain the correct focal plane at low magnification for small, low contrast specimens. Use dark field for

A dilute suspension of yeast cells makes a good practice specimen for dark field optics, particularly when cultured with living Paramecium.

Lightfield microscopy

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Suspensions of cells and samples of pond water look spectacular in dark field. While specimens may look washed out and lack detail in bright field, protists, metazoans, cell suspensions, algae, and other microscopic organisms are clearly distinguished and their details show up well. At 100x you can readily see bacteria, even distinguish some structure (rods, curved rods, spirals, or cocci) and movement. Non-motile bacteria look like vibrating bright dots against a dark background. Motile bacteria can be seen moving in a definite direction, sometimes remarkably fast. In pond water samples you may find Spirillum volutans, a very large (up to 0.5 mm) motile spiral bacterium.

The most notable advantage of using an aspherical lens is the reduction in spherical aberration commonly seen in spherical lenses. Spherical aberration occurs when a lens is not capable of focusing all the incident light rays on the exact same point. Any spherical optical surface, even a perfectly designed and manufactured one, will still exhibit spherical aberration. This inherent defect of a spherical surface causes incident light rays to focus at different points and create a blur.  By adjusting the surface constants and aspheric coefficients, the surface of an asphere can be designed to eliminate spherical aberration. Aspheres provide true diffraction-limited spot sizes and the lowest wavefront error. Figure 2 shows a spherical lens in a focusing system with significant spherical aberration when compared to an aspheric lens.

Due to the advantages in improving the optical performance and/or reducing the number of lens elements of optical systems, aspheres are increasingly being used in optical systems for all kinds of applications including laser focusing/collimating, LED illumination, optical metrology, optical imaging, digital cameras, CD players, high-end microscopy instruments, etc.

Microinjection molding or polymer molding method has been used to produce hybrid precision glass-polymer lenses by adding a fairly thin layer of aspherized optical polymer to a conventional glass lens such as achromatic lens. The molding happens at room temperature instead of at a high temperature, there is less stress induced in the mold. The tooling costs are low because of the easy fabrication of the molds. Glass-polymer hybrids are widely used in optical systems because of the light weight and low cost for the production part. The polymer is also not as durable as glass, making this is an unideal solution for surfaces that will be exposed to harsh environments.

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For optical engineers, it is important to understand the manufacturing techniques and choose the best manufacturing method with considering the lens application, performance requirements, development cost, sample cost, production part cost, and project timeline.

Figure 1 shows the surface profile comparison of an aspherical lens a spherical lens. Unlike the spherical lens with a constant radius of curvature, the biggest feature of aspheric lenses is that the radius of curvature continuously changes with the radial distance from its optical axis. An aspheric lens can be designed to minimize aberrations or improve performance of optical systems by adjusting the conic constant and aspheric coefficients of the curved surface of the lens.

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Precision glass molding is a manufacturing technique of heating the glass material in a high temperature until it becomes malleable enough to be pressed uniformly into an aspheric mold. After the glass and molds gradually cools down to the room temperature, the resulting lenses maintain the shape of the mold. The initial cost of creating precision glass molds is high. However, once the mold is finished the incremental cost for each lens is lower than that of standard manufacturing techniques. This manufacturing technique is especially suitable for high volume production of high quality aspherical lenses.

Dark field microscopy

To view a specimen in dark field, an opaque disc is placed underneath the condenser lens, so that only light that is scattered by objects on the slide can reach the eye (figure 2). Instead of coming up through the specimen, the light is reflected by particles on the slide. Everything is visible regardless of color, usually bright white against a dark background. Pigmented objects are often seen in "false colors," that is, the reflected light is of a color different than the color of the object. Better resolution can be obtained using dark field as opposed to bright field viewing.

You don't need sophisticated equipment to get a dark field effect, although the effect is most dramatic when the occulting disk is built into the condenser itself. You do need a higher intensity light, since you are seeing only reflected light. At low magnification (up to 100x) any decent optical instrument can be set up so that light is reflected toward the viewer rather than passing through the object directly toward the viewer.

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In addition to molded glass lenses, molded plastic lenses are also available. Plastic injection molding is injecting the molten plastic into the aspheric mode for producing plastic aspheric lenses. Compared with glass, plastic does not have good thermal stability and scratch resistance. While this method cannot produce high precision aspheric lenses, it does allow for mass production at extremely low cost following relatively high up-front tooling charges. Plastic lenses are describes as low cost, low weight and ease of adding mounting features. Plastic aspheres are widely used in optical systems that require moderate quality and low thermal stability.

To set up a dissecting microscope for "dark field" viewing, the specimen should be placed over an opening so that light reflects only from surfaces between cover slip and slide, not from a surface beneath the slide. You may need to make a stand to hold the slide. The surface beneath the opening should be a flat black. Turn off any built-in illuminator. Aim a high-intensity light source toward the specimen at an angle, from the top or side through a glass dish or jar.

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