Measurement solutions for optics manufacturing

Measurement Solutions for Optics Manufacturing

Testing of Lens Systems

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Finished lens assemblies undergo final inspections to evaluate image quality using MTF or to assess opto-geometric properties such as focal lengths. If target values are not met, identifying whether systematic or random faults are at play, particularly for high-quality lens systems, becomes essential. Systematic faults can be pinpointed via a centration test, allowing for corrective actions. Knowing which lens impacts the overall quality can help adjust tolerances, whereas random faults may only necessitate replacing specific faulty elements.

Optical Fabrication

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In optical workshops, various processes like cutting, grinding, lapping, and polishing are employed to fabricate lenses, prisms, and mirrors with high-quality surfaces.

This discussion mainly covers optical glasses, the most common materials in optics. However, adapted methods are available for other materials such as plastics and crystalline substrates. For specialized optical components like microlenses, gradient-index lenses, diffractive optics, and waveguides, consult dedicated articles.

Fabrication of Optical Elements Made of Glasses

Glass Fabrication

Articles about optical glasses explain common glass materials used in optics. Here, we assume inorganic glasses, distinct from organic glasses used for plastic optics.

Fabricating optical glasses involves mixing purified compounds in powder form, heating them to produce a glass melt, and cooling them slowly until solidification. Highly purified materials and thorough mixing processes avoid inhomogeneities and contamination from containers.

A controlled slow cooling process is crucial for achieving complete solidification and annealing, which removes internal mechanical stress. The final density and resulting refractive index depend on the annealing rate, which may be as low as 2 K/h. Consistently applying this rate ensures the desired optical quality for specific glass types.

Manufacturers often supply optical glasses as blocks, strips, or rods for components like lenses or prisms. Some suppliers also offer processed forms such as plates, cut prisms, or pressed blanks.

Production of Blanks

The initial step post-glass production is fabricating blanks with approximate dimensions and a slight oversize. For volume production, blanks might be produced through embossing or molding. For smaller volumes, cutting or trepanning from larger glass blocks is common.

Special glass cutting machines with circular diamond saws are typically used. The diamond particles in the saw blade are extremely abrasive and can cut through optical glasses with high reliability, minimal material loss, and reduced breakage. During the sawing process, clean water cools the blade and removes glass dust, preventing inhalation of particles by operators.

Generation

Post-cutting, curve generators prepare plane or curved surfaces closer to the final shape, usually within 0.1 mm. Due to the brittle nature of glasses, adapted mechanical machining methods are necessary.

The specifics depend on the optical component. For lenses, cup-shaped grinding wheels with diamond grit held in a metal or resin matrix are often used. This method naturally produces spherical surfaces, while other processes are required for aspheric lenses, such as diamond turning.

Lapping and Polishing

The generation process causes surface damage; hence, further refinement is needed. Lapping methods, which involve grinding with abrasives, remove the damaged surface layer and achieve the required surface shape.

The process begins with loose abrasive lapping to remove the damaged top layer, achieving surface shapes within a few micrometers. Successive pellet lapping steps involve finer abrasives for gradual surface refinement.

Optical polishing follows, using cerium oxide powder in a synthetic pad, achieving surface deviations below λ/10. Superpolishing methods may be applied for exceptionally high surface quality.

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Edging

Edging processes peripheral regions for precise mounting. For lenses, this ensures the lens can be easily mounted with the correct optical axis orientation.

Adaptation to Different Materials

To achieve high quality results, all processes must be tailored to specific materials. Optical glasses vary widely in mechanical properties; some are soft, others hard, impacting processing methods.

Selection of optical glasses considers factors like refractive index, chromatic dispersion, availability, and cost. Mechanical properties alone cannot dictate material choice for fabrication.

Crystalline Materials

Processing crystalline materials such as sapphire, semiconductors, laser crystals, or nonlinear crystal materials involves similar methods as optical glasses, with parameter adaptations.

Additional considerations include the axis orientation of single crystals.

Plastic Optics

Plastic materials, or organic glasses, are also prevalent in optics. Their advantage lies in processing costs rather than material savings. Simplified processes like embossing or molding are often sufficient without extensive further processing, unlike inorganic glasses. While optical quality may be lower, it suffices for many applications, including high-quality imaging in smartphone cameras. Aspheric and free-form optics are also easier to produce.

Volume vs. Custom Optics

Different methods apply based on quality specifications and production volumes. Custom optics may require cutting from blocks with more waste if suitable blanks are unavailable. Computer-controlled machines, offering flexibility, are employed extensively. Manual processes may be necessary for small quantities due to setup constraints of automated machinery.

Optical Characterization

Optics manufacturing demands precise optical characterization. Instruments like optical profilometers and various interferometers measure surface topographies and assess fabricated components.

Characterization aids in quality control and can be integrated into the fabrication process. In manual processes, measurements guide further steps, while computer-controlled equipment uses characterization data in streamlined production, balancing quality and cost, especially in volume production.

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