Key Questions to Ask When Ordering Optical Flat Mirror

To simplify the process when purchasing flat optics, we will discuss the top six factors from a manufacturer&#;s perspective, all of which affect the three major areas of the purchasing decision: quality, delivery, and cost.

Michael Naselaris, Sydor Optics Inc.

Please visit our website for more information on this topic.

Flat optics generally perform three main functions: They transmit light (windows), reflect light (mirrors), and fold light (prisms). While most optical manufacturers make spherical and flat optics, a smaller percentage focus specifically on making only flat optics.

Knowing the intended use of the optical component, shared by about half of all customers, helps the manufacturer to understand which specifications are the most critical to the optic&#;s performance, and to discuss specifications that might have been overlooked or overspecified, as they all could be cost and/or delivery drivers.

Common flat optical components used in the UV, visible, and IR spectrums include:

                                              &#; Debris shields                                 &#; Reference surfaces                        &#; Encoder disks                                 &#; Reticles                        &#; Filters                                 &#; Substrates                        &#; Gratings                                 &#; Wafers                        &#; Lightpipes                                 &#; Wave plates                        &#; Mirrors                                 &#; Wedges                        &#; Optical flats                                 &#; Windows

Optical materials

 The first and foremost item to consider is the optical material. Important factors include homogeneity, stress birefringence, and bubbles; all of these affect product quality, performance, and pricing. Homogeneity plays a major role with transmissive optics. As homogeneity decreases, so does the optician&#;s ability to achieve the desired transmitted wavefront specification. Stress birefringence, on the other hand, affects the mechanical stability of optics requiring surface flatness. Bubbles could affect cosmetics if they break the surface during grinding and polishing stages.

Other relevant factors that can impact processing, yield, and pricing include chemical, mechanical, and thermal properties, along with the form of supply. Optical materials can vary in hardness, making manufacturability difficult and processing cycles possibly lengthy. The ideal optical materials for flat optics manufacturing are BK7, Borofloat, and fused silica. Other optical materials require careful handling and special processing techniques, as they can easily stain or may be sensitive to environmental changes, such as temperature and humidity.

Keep in mind that often, equivalent material types can be used interchangeably. Some engineers will document a specific material (e.g., Schott&#;s N-BK7), whereas others may state a preferred material and add the clause, or equivalent, to their specifications. Having this option may shorten lead times and even decrease pricing to some extent.

The following optical materials and their equivalents are popular for flat optics:

 

                                              &#; B270          &#; Silicon                        &#; BK7          &#; Sapphire                        &#; Borofloat®          &#; ULE®                        &#; Crystalline material, such as
      CaF2, MgF2, BaF2          &#; Zerodur®                        &#; Filter glass          &#; Zinc selenide                        &#; Fused quartz and fused silica          &#; ZSinc sulfide                        &#; Germanium         

Reflected wavefront vs. transmitted wavefront, or both

One issue requiring clarification in approximately one in four inquiries involves reflected wavefront and transmitted wavefront. Prints received are often vague enough to make one question the intent of the reflected wavefront, which is the accuracy of the surface with respect to a reference plane. Transmitted wavefront is the permissible wavefront deformation involving the surface flatness of both surfaces, the parallelism of the optic, and the homogeneity of the optical material. For the most part, flat optics require reflected wavefront or transmitted wavefront, with the primary exceptions being plate beamsplitters and prisms. Prints oftentimes state the word flatness, yet the function may be that of a window, thereby requiring transmitted wavefront as the ideal specification for optical performance. Mirrors, on the other hand, require reflected wavefront as the key indicator of performance in this respect. Head-up displays, for example, are plate beamsplitters and require both good optical transmission and optimal reflected wavefront for performance.

Filter glass on a double-sided machine. 

For more information, please visit ruiqi.

The terms used for specifying reflected wavefront and transmitted wavefront are optical in nature &#; waves and fringes (half-wave) and lately more requests are in terms of nanometers &#; but on rare occasions, surface flatness may be specified as a mechanical callout in microns (0.001 mm). It is important to distinguish the difference between two commonly used specifications: peak to valley and rms. Peak to valley is the maximum measurement and the worst-case scenario, taking into account the difference between the surface&#;s lowest and highest points. It is by far the most widespread flatness specification used today. A more accurate measurement of surface flatness is rms, as it takes into account the entire optic and calculates deviation from the ideal form. Traditionally, optical flats have measured surface flatness in fringes; today, however, laser interferometers at 632.8 nm measure most optical components.


The clear aperture, also known as the usable aperture, is important. Normally optics are specified with an 85% clear aperture. For optics requiring larger clear apertures, attention must be taken during the production process to extend the performance area closer to the part&#;s edge, making it more difficult and costly to fabricate.

A large double-sided machine.
Parallel or wedged

Components such as filters, plate beamsplitters, wafers, and windows are required to be of very high parallelism, whereas prisms and wedges are intentionally wedged. The method of grinding and polishing plays an important role in the manufacturer&#;s ability to achieve the parallelism specs. For parts requiring exceptional parallelism (<1 arc second) and transmitted wavefront (<1 wave), double-sided grinding and polishing is the best method to use. Parallelism can be easily measured using an interferometer.

Tropel wafer interferometer.
Wedges and prisms require angled surfaces at demanding tolerances and are usually processed via a much slower process using pitch polishers. Pricing increases as angle tolerances become tighter. Wedge is specified in degrees, minutes, and seconds, and occasionally it will be stated as a thickness measurement at the parts&#; thin edge, thick edge, or center. Wedge-angle tolerance of several arc seconds falls into the higher level of precision, whereas tolerances of minutes or degrees fall into the medium and looser levels of precision. Typically, an autocollimator, goniometer, or a coordinate measurement machine is used for wedge measurements.

Side view of a pitch polisher.

Dimensions and tolerances

Size, in conjunction with other specifications, will dictate the best processing method, along with the size of equipment to use. Although flat optics can be any shape, round optics seem to achieve the desired specifications more quickly and uniformly. Overly tightened size tolerances can be the result of a precision fit or simply an oversight; both have an adverse effect on pricing. Bevel specifications are at times overly tightened, also resulting in increased pricing.

Surface quality

Surface quality is influenced by cosmetics, also known as scratch-dig or surface imperfections, as well as surface roughness, both with documented and universally accepted standards. In the U.S., MIL-PRF-B is popular with an increasing use of ISO-7 or its American-based counterpart ANSI/OEOSC OP1.002. 

Top view of a pitch polisher.

Scratch-dig is represented with two numbers (e.g., 20-10) that generally fall into predetermined sets, such as 20-10, 40-20, 60-40, etc. The first number is arbitrary and denotes the scratch appearance, best matched to a calibrated standard. The second number refers to the dig size, which is designated in 0.01-mm increments. Scratch-dig values of 80-50 and above refer to commercial quality, 60-40 refers to general optics quality, and surface qualities of 20-10 and 10-5 are utilized more for laser optics and high-end optics applications. Lower numbers mean a higher level of precision and increased pricing. Keep in mind that, as the area of a part increases the difficulty, achieving a higher level of precision for scratch-dig increases difficulty at an even greater rate.

metrology that can objectively determine cosmetic thresholds for each of the existing cosmetic standards has become more prevalent and is taking the pre-existing subjectivity out of cosmetic evaluation.

Surface roughness refers to the overall texture of an optical surface and can influence the production process or the need for different or additional polishing steps to achieve lower surface roughness requirements, both having cost implications. Surface roughness generally falls into five categories: superpolishing (&#;1 angstrom rms), high-precision laser grade (1 to 5 angstroms rms), standard optics (5 to 15 angstroms rms), commercial optics (15+ angstroms rms) and those with no specification. The detail to remember is that lower roughness equals higher price. Generally, surface roughness is measured with noncontact optical profilometers. One universal and often overlooked problem with roughness specifications is the omission of a measurement&#;s length.

Quantity

For the most part, the smaller the quantity, the higher the processing costs per piece and vice versa. Quantities too low may involve lot charges, as a group of components may need to be processed to properly fill and balance the machine to achieve the desired specifications. The goal is to maximize each production run to amortize processing costs over the largest quantity possible. Although the same optical component can be made using different processing methods, the dominant one is usually indicated by the quantities and specifications.

A small double-sided machine.

The most commonly used processes for flat optics involve double-sided polishing and single-sided polishing on pitch polishers. Double-sided grinding and polishing, a batch-type procedure, can process both faces of the optic simultaneously for parallel optics. Economical batch sizes are determined by the size of the optic and the machine size. Pitch polishing, however, is a more time-consuming process generally utilized for requirements specifying fractional wave surface flatness and/or improved surface roughness. Double-sided polishing is deterministic, involving hours, while pitch polishing may involve days for the same quantity of parts. If transmitted wavefront and/or total thickness variation are your primary specifications, double-sided polishing is best, whereas polishing on pitch polishers is ideal if reflected wavefront is of primary importance.

Trends

Over the past several years, we have observed a few trends regarding precision flat optics. More and more customers are making the assumption of quality, in turn making delivery more important than pricing for the most part. The assumption of quality needs to be substantiated with more questions to ensure that the proper metrology and levels of verification are used. Increasingly, the demand is for thinner and thinner optics, along with tighter surface flatness and higher levels of cleanliness.

Zygo IR interferometer.
My recommendation would be to call your optics vendor. Share more information up front in the quoting process and be open to suggestions, as your vendor may point out key cost, delivery, and quality drivers. This will yield the most effective and accurate quotation, as compared to just sending an .

,Step Feature Specification Characteristics / Benefits Limitations 1 Specify the Quantity Quantity Required &#; The larger the quantity of pieces that can be used in an application, the less expensive each part becomes as material, labor and coating charges can be divided over the total number of parts.

&#; Advanced Optics has the ability to modify catalog/overrun optical mirrors (when possible) to reduce costs and lead times. Small number of prototypes may be more expensive due to lot charges for glass and coating. 2 Select the Material Soda-lime Glass
&#; Commonly known as float glass. &#; Least expensive of all glass types. &#; Can be polished 1-3 waves/inch. &#; May be tempered making it 3 times stronger than non-tempered glass. &#; Softer than borosilicate glass making it easily scribed and broken.
&#; Cannot be precision polished and is available in commercial grade only (1-3 waves/inch).
&#; Has the lowest thermal shock and chemical resistance of all glass materials used to fabricate optics.
&#; Not as scratch resistant as borofloat, quartz or fused silica. BOROFLOAT®33 &#; Borofloat®33 is a borosilicate glass with a low thermal expansion.
&#; Good all around general purpose mirror substrate that is moderately priced.
&#; Easier to polish than harder materials such as fused quartz, fused silica or Zerodur® and is much less costly. &#; May be polished down to λ/10, but is not suitable for polishing down to λ/20.
&#; 2-3 times more costly than float glass (soda-lime glass).
&#; Not as thermally shock resistant as fused quartz or fused silica.
&#; Cannot be fully tempered like soda-lime glass.
&#; Not suitable for extreme high temperature conditions and will not hold its shape over 450° C for long periods of time. N-BK7® &#; Common borosilicate crown glass know for its low bubble and inclusion content.
&#; Economically priced, may be used as an optical mirror substrate, but more commonly used in the manufacture of optical windows. N-BK7 is not recommended for applications where thermal shock is a factor. Viosil &#; Viosil is a synthetic quartz glass substrate manufactured by ShinEtsu.
&#; The absence of bubbles and inclusions make it an excellent window substrate.
&#; It offers excellent chemical resistance, mechanical strength and high heat resistance. Carry glass only up to .250&#; thick. Fused Silica &#; Made from a synthetically derived silicon dioxide that is extremely pure.
&#; It is a colorless, non-crystalline silica glass.
&#; The main difference between fused silica and fused quartz is that the former is composed of a non-crystalline silica glass while the latter is composed of a crystalline silica glass.
&#; Advantages of fused silica over fused quartz include; greater ultraviolet and infrared transmission, a wider thermal operating range, increased hardness and resistance to scratching and a lower CTE which provides resistance to thermal shock over a broad range of temperatures.
&#; As opposed to other less costly glasses, the surface figure (flatness) of optical mirrors made of fused silica are not at risk in applications that expose the material to coatings applied at high temperatures or applications that require the material to remain flat at high and/or varying temperatures.
&#; Fused silica is also chemically resistant and provides superior transmittance in the UV spectrum when compared to fused quartz.
&#; Fused silica comes in many grades with the most common being 2G. Please visit Corning&#;s Quality Grade Selection Chart for further information. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses.
&#; The homogeneity of fused silica exceeds that of crystalline fused quartz, however standard 2G (UV grade) material has a higher OH content which cause dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. Quartz &#; Made from naturally occurring crystalline quartz or silica grains whereas fused silica is entirely synthetic.
&#; Fused quartz and fused silica are both extremely pure materials and have very low thermal expansion rates. However, fused quartz is more cost effective.
&#; Known for its incredible thermal shock resistance, chemical resistance and for being an excellent electrical insulator.
&#; Fused quartz has more metallic impurities and a lower OH content than standard UV grade fused silica which has dips in transmission at 1.4µm, 2.2µm and 2.7µm. These dips can be eliminated by using a more expensive grade of IR fused silica. &#; Very hard glass making it more difficult to fabricate than float or crown glasses.
&#; Raw material is more costly than float or crown glasses, but less expensive than fused silica.
&#; Fused quartz shares many of the same advantages of fused silica with the exception of metallic impurities found in the mined, natural quartz or silica sand. These impurities inhibit the materials ability to transmit well in the UV spectrum. ULE® Low Expansion Glass ULE® is a titania-silicate glass with near zero expansion characteristics that have made it the material of choice in unique applications such as machine tool reference blocks, gratings, interferometer reference mirrors, and telescope mirrors. Low expansion glasses offer unique characteristics that make them the material of choice for certain applications, although the material tends to be more costly than its float or crown glass counterparts. ClearCeram®-Z ClearCeram®-Z is a glass-ceramic material that offers an ultra low thermal expansion and is Ohara's equivalent to Zerodur® which is manufactured by Schott. Low expansion glasses offer unique characteristics that make them the material of choice for certain applications, although the material tends to be more costly than its float or crown glass counterparts. ZERODUR® &#; Glass-ceramic material which has a yellowish tint.
&#; Extremely low thermal expansion coefficient which approaches zero allowing it to be used to produce mirrors that retain their surface figures in extremely cold environments such as space.
&#; The CTE of Zerodur® is lower than ULE, fused quartz and fused silica.
&#; Known for its low level of bubbles and striae, internal stress and its excellent chemical resistance. &#; Yellow tint.
&#; Low expansion glasses offer unique characteristics that make them the material of choice for certain applications, although the material tends to be more costly than its float or crown glass counterparts. 3 Determine the Size/Shape Round
Rectangular
Square
Custom Round provides the best opportunity for obtaining flatness/accuracy. Square, rectangular and custom shapes provide more challenges to maintaining surface flatness. 4 Refine your Mechanical Tolerances Defines the acceptable limits of both size and thickness required for an application. Specified in inches or mm and typically given a +/- value.
&#; Round: Provide tolerance for diameter.
&#; Rectangular/Square: Provide tolerance for LxW.
&#; Thickness: Provide tolerance for thickness. &#; Tighter tolerances for diameter and LxW are typically easier to hold than for thickness.
&#; Extremely tight tolerances available, but may require specialized techniques and can reduce yield leading to increased costs.
&#; Loosening your tolerances can reduce costs. 5 Establish the Correct Accuracy Commercial grade
1-3 waves/inch

Precision polished λ/4 or λ/10


Precision polished λ/10 or λ/20 Commercial grade mirrors are generally made from less expensive materials such as soda-lime glass and borofloat.

Working grade mirrors are polished either λ/4 or λ/10 and most often made of Borofloat®33 or N-BK7.

&#; Precision grade mirrors are polished either λ/10 or λ/20 and are typically made from harder glass materials such as quartz, fused silica or Zerodur®.
&#; To achieve the best accuracy, optical mirrors are polished in a 6:1 aspect ratio (diameter to thickness). The higher the ratio, the greater probability the glass will distort during the manufacturing process. When the glass is deblocked after polishing, mirrors with non-standard aspect ratios may spring as they do not have the stability to hold surface flatness.
&#; Advanced Optics manufactures precision grade mirrors with non-standard aspect ratios. Achievable surface accuracy is dependent on choice of substrate and thickness of material. 6 Specify the Surface Quality Provide the required
Scratch and Dig 80-50: Commercial grade mirrors, suitable for non-critical applications, easily manufactured, lowest cost.

60-40 or 40-20: Working grade mirrors, precision quality, suitable for most scientific and research applications as well as low to medium power lasers, intermediate price point.

20-10 or 10-5: Precision grade, suitable for high power lasers, highest cost. Extremely tight tolerances available, but may require specialized techniques and can reduce yield leading to increased costs. 7 Provide Parallelism (if required) Amount of wedge or variation in thickness allowed over the surface of a part.

It is defined in arc minutes (an angular measurement that is 1/16th of a degree) or arc seconds where 60 arc seconds is equal to 1 arc minute.
Advanced Optics can hold parallelism of < 2 arc seconds.

Want more information on Optical Flat Mirror? Feel free to contact us.

Extremely tight requirements for parallelism require specialized manufacturing techniques which may reduce yield and increase manufacturing costs. 8 Define the Clear Aperture/ Edge Bevel Requirements The clear aperture is the percentage of useable area of an optical mirror.

An edge bevel or safety chamfer is applied around the edge of an optical mirror.
Normally 90% or advise requirement.


An edge bevel or safety chamfer is applied around the edge of an optical mirror to eliminate sharp edges and reduce edge chips caused by cutting of the glass. Typically between .010"-.040" face width at 45 degrees depending on size of part, please advise preference and tolerance. Very small edge bevels with tight tolerances will add additional costs. 9 Choose the Proper Coating Metallic and Dielectric coatings available for the UV-VIS-NIR regions. &#; Provide the wavelength(s) of interest and % reflectivity required.
&#; Provide the intended AOI (angle of incidence) for the optical mirror. Custom coatings for a small quantity of parts may add additional expense. 10 Customization The following attributes can be added to customize your mirror. &#; Shapes: Provide drawing of custom shape.
&#; Holes and Notches: Provide location, size with tolerances.
&#; Custom Bevels: Provide location, depth and angle.
&#; Custom Coatings: Provide expected % of reflectivity over wavelength(s) of interest and AOI (angle of incidence). Additional features may add lead time and cost.