Optical manufacturing and testing is the process of manufacturing and testing

optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultravio ...

components. It spans a wide range of manufacturing procedures and optical test configurations. The manufacture of a conventional spherical

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

typically begins with the generation of the optic's rough shape by grinding a glass blank. This can be done, for example, with ring tools. Next, the lens surface is polished to its final form. Typically this is done by

lapping Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them, by hand movement or using a machine. Lapping often follows other subtractive processes with more aggressive material removal as a first ste ...

—rotating and rubbing the rough lens surface against a tool with the desired surface shape, with a mixture of abrasives and fluid in between. Typically a carved pitch tool is used to polish the surface of a lens. The mixture of abrasive is called slurry and it is typically made from cerium or zirconium oxide in water with lubricants added to facilitate pitch tool movement without sticking to the lens. The particle size in the slurry is adjusted to get the desired shape and finish. Types of lapping include planetary lapping, double-sided lapping, and cylindrical lapping. During polishing, the lens may be tested to confirm that the desired shape is being produced, and to ensure that the final shape has the correct form to within the allowed precision. The deviation of an optical surface from the correct shape is typically expressed in fractions of a

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

, for some convenient wavelength of light (perhaps the wavelength at which the lens is to be used, or a visible wavelength for which a source is available). Inexpensive lenses may have deviations of form as large as several wavelengths (λ, 2λ, etc.). More typical industrial lenses would have deviations no larger than a quarter wavelength (λ/4). Precision lenses for use in applications such as

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

interferometer Interferometry is a technique which uses the '' interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...

s, and

holography Holography is a technique that allows a wavefront to be recorded and later reconstructed. It is best known as a method of generating three-dimensional images, and has a wide range of other uses, including data storage, microscopy, and interfe ...

have surfaces with a tenth of a wavelength (λ/10) tolerance or better. In addition to surface profile, a lens must meet requirements for surface quality (scratches, pits, specks, etc.) and accuracy of dimensions.

Fabrication techniques

*Glass blank manufacturing ** Batch mixing **

Casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or ...

techniques ** Annealing schedules and equipment ** Physical characterization techniques **

Index of refraction In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

measurements and calculation of melt pedigree * Diamond shaping techniques ** Diamond wheel curve generation processes and equipment ** Diamond edging processes and equipment * Loose grit fabrication techniques: ** Rough grinding ** Fine grinding **

Polishing Polishing is the process of creating a smooth and shiny surface by rubbing it or by applying a chemical treatment, leaving a clean surface with a significant specular reflection (still limited by the index of refraction of the material accordi ...

and figuring * Glass moulding techniques ** Precision glass moulding Unconventional techniques include single-point diamond turning (SPDT) and magnetorheological finishing (MRF).

Free-abrasive grinding

Free-abrasive grinding is a technique to grind down the surface of a material before polishing. It involves the use of small particles of grit to grind away small chips of material from the surface of an optical workpiece. The grit particles are known as free abrasives. The particles are added to a liquid slurry, which goes between a grinding plate and the material. Sliding motions between the grinding plate and the material are used. After grinding, there is a small amount of surface roughness, which is based on the size of the grit. There is also a small amount of fracturing below the surface of the material, known as subsurface damage (SSD). To reduce the amount of surface roughness and subsurface damage, additional grinding at a smaller grit size can be done. Typically, two or three stages of grinding are used, with the second and third stages having a size that is decreasing. For example, a typical set of grit stages is 30 micrometer, then 15 micrometer, then 9 micrometer. An alternate set of typical grit stages is 20 micrometer, then 12 micrometer, then 5 micrometer. Types of abrasives include

aluminium oxide Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several Aluminium oxide (compounds), aluminium oxides, and specifically identified as alum ...

, industrial diamond, and

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A wide bandgap semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder a ...

. Diamond is typically only used for grinding down very hard materials, or for certain crystals.

Polishing

Optics are polished in a slurry of abrasive particles, a fluid carrier, and optional additives. Types of abrasive particles that can be used include

cerium(IV) oxide Cerium(IV) oxide, also known as ceric oxide, ceric dioxide, ceria, cerium oxide or cerium dioxide, is an oxide of the rare-earth metal cerium. It is a pale yellow-white powder with the chemical formula CeO2. It is an important commercial produc ...

, diamond, aluminum oxide, and

colloidal silica Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase. It may be produced by Stöber process from Tetraethyl orthosilicate (TEOS). Properties Usually they are suspended in an ...

. Optional additives include suspension agents, lubricants, and detergents.

Materials

There are various materials that can be used for optical components, including various types of glass,

fused silica Fused quartz, fused silica or quartz glass is a glass consisting of almost pure silica (silicon dioxide, SiO2) in amorphous (non-crystalline) form. This differs from all other commercial glasses, such as soda-lime glass, lead glass, or borosil ...

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

, and crystal quartz. Calcium fluoride (CaF2) can be used as an optical material, although it is easily fractured and scratched. Materials for infrared optical components include zinc selenide (ZnSe),

zinc sulfide Zinc sulfide (or zinc sulphide) is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it mainly occurs as the mineral sphalerite. Although this mineral is usually black because of various i ...

(ZnS), and

gallium arsenide Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a Zincblende (crystal structure), zinc blende crystal structure. Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits, monoli ...

(GaAs).

Specifications

The specifications for optical components vary based on their type: Specifications for prisms include pyramidal error, beam path, beam displacement and deviation, base angle, roof edge chips, wavefront, and polarization. Specifications for

aspheric lens An aspheric lens or asphere (often labeled ''ASPH'' on eye pieces) is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens. ...

es include base radius with tolerance, conic and polynomial coefficients, best-fit sphere reference, sag table reference, sag error tolerance, slope errors versus bandwidth, wavefront per specified test, tilt, and decenter. Optical coating specifications include apertures, reflection, transmission, absorption, phase shift, adhesion, abrasion resistance, and damage threshold. In order to avoid the irrecoverable loss of going under minimum thickness, opticians strive to meet all other specifications for an optical component at the maximum allowable thickness within tolerance.

Surface quality

Surface quality is the condition of the surface of an optical component. It indicates the presence of imperfections, such as scratches and pits. It is typically rated according to scratch-dig (S-D) specifications. Standards for specifying surface quality include the U.S. Military Performance Specification MIL-PRF-13830B and ISO 10110. MIL-PRF-13830B was formerly MIL-O-13830a. Other standards include MIL-C-48497a and MIL-F-48616, which are formally inactive and apply only to coatings. All three of these military standards lack specifications for statistical surface parameters, such as root-mean-square roughness, slope error, and ripple. An extension and improvement to MIL-PRF is the ANSI/OEOSC OP1.002 standard.

Testing techniques

* Direct surface profile measurement * Direct surface surveying (no intervening optics, for example Foucault knife-edge test,

Ronchi test In Optics, optical testing a Ronchi test is a method of determining the surface shape (Figuring, figure) of a mirror used in Optical telescope, telescopes and other Optics, optical devices. Description In 1923 Italian people, Italian physicist Vasc ...

, Caustic test) * Auxiliary optics ( null correctors,

computer-generated hologram Computer-generated holography (CGH) is a technique that uses computer algorithms to generate holograms. It involves generating holographic interference patterns. A computer-generated hologram can be displayed on a dynamic holographic display, or i ...

s, etc.) * Interferometric testing The Fizeau interferometer is the standard type of interferometer that is used in optical fabrication. Stitching interferometry can be used for testing aspheres. It involves performing subaperture tests that are stitched together into a single-high resolution image.

Notes and references

*Malacara, D., ''Optical Shop Testing - 2nd Edition'', John Wiley and Sons, 1992, {{ISBN, 0-471-52232-5

External links

Virtual Lens Plant, Canon Camera Museum
Instructional videos of the processes, within a flash web interface. Manufacturing and testing Optical components, fabrication Glass engineering and science