Flat Optics: Considerations When Buying
Flat Optics: Considerations When Buying
Michael Naselaris, Sydor Optics Inc.
With competitive price and timely delivery, CLZ sincerely hope to be your supplier and partner.
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.
&#; 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.
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.
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 .
What to look for in an optical components company: Part 2
We&#;ve written previously about the qualities that buyers should look for when seeking a company to supply optical components or services. Although some of these qualities are as obvious as value for money and a commitment to precision delivery, others are more specific to the sector and the particular demands of optical components, such as a track record in the field and the quality of materials used.
In this second article, we&#;ll outline a few more of the factors that buyers of optical components and services should look for in the optics company they turn to:
UK Manufacturing Base
Dealing with an optics company that has its own UK manufacturing base offers a range of advantages. The first of these is that the company won&#;t be relying on long and sometimes fragile supply chains to deliver the components which their clients need. This, in turn, will reduce the transportation costs, a saving that can be passed on to consumers in the form of lower prices or recycled into production spending to drive innovation and quality.
Having a UK manufacturing base will also enable faster delivery times and greater control over communication. In simple terms, the manufacturers of the components you need will be available to answer any questions you have during your working hours because they&#;re within a few hundred miles away rather than several thousand.
Small Quantities Available
Some optics companies can deliver the components buyers need because they rely on economies of scale and bulk orders to finance their operations. For many buyers, however, this is less than ideal because they only need a handful of specialist items manufacturing. In some cases, the requirement might even amount to no more than a single bespoke component for use in a nice area of expertise such as forensic science.
The willingness of an optics company to handle orders for small quantities will show a willingness to meet customer requirements in full, no matter how bespoke those requirements are, an ability to specialise for the duration of an individual project, and flexibility which will be reflected in everything else they do.
Fast Turnaround
Although the provision of optical components and services is a highly specialised sector, the best companies never lose sight of the fact that their customers work in an incredibly wide range of sectors, from retail and entertainment to specialist medicine, defence and energy.
One thing that all of these industries have in common is that they often need to have optical components delivered in time to meet extremely tight deadlines. Make sure that the optical components company you deal with is able to promise a fast turnaround on the products you need without any drop in quality. They should be able to point to examples of them having done this in the past and be willing to guarantee the turnaround your business needs.
Shipping All Over the World
We work in a truly global marketplace and even many companies which are based in the UK deal with customers and third-party partners located in all corners of the globe. If the optical components company you work with has a track record of shipping items to different parts of the world then this means that they will already have the systems and processes in place to meet genuinely global delivery requirements. This could involve existing partnerships with delivery companies, the maintenance of supply chains, and knowledge of the market conditions in different territories.
If you are looking for more details, kindly visit Optical Mirrors supplier.
Engineering Services
The provision of specialist engineering services is a vital aspect of the optical components company you choose to work with. In the first instance, it means that they will be able to deliver all aspects of even the most complex components in-house. It&#;s difficult to overstate the importance of in-house delivery of this kind, as it guarantees the quality of the component from beginning to end, without the risk of having to call on third-party providers for specific aspects. It also enables faster turnaround of projects and completely open communication from first enquiry through to delivery.
The ability to deliver engineering services also means that bespoke optical components can be provided, from scratch if needed, providing specialist solutions in highly niche sectors.
Flexible Communication
The optical components company you choose to work with should be able and willing to communicate and liaise in the way that suits you. For some customers that will mean an ongoing, two-way communication with a sales representative able to offer a consultative approach and advice and support as the project is being delivered. This is more likely to be the case for those customers who need to have a specialised, bespoke item delivered.
In other cases &#; such as when ordering a standard component from existing stock &#; it may be quicker and more convenient to simply place the order online, without needing to speak to anyone. The optical components company you choose should be happy to offer both types of delivery and all points in-between.
Source from a network of certified suppliers
The highly specialised nature of much optical component provision means that it is sometimes going to be necessary to bring in parts from third party suppliers. This is particularly likely to be the case if the project has to be turned around to a particularly tight deadline. The company you work with should already have a trusted network in place to ensure that this can happen at speed and that any parts which are sourced meet a quality certification such as ISO :.
With this network of suppliers ready and waiting, delivery of a component or project will never be delayed because a particular aspect isn&#;t already in stock.
Testing Capability
Meeting wider quality certifications is one thing, but the very best optical components companies should also have a rigorous in-house testing department of their own. This will mean that they can meet standards of delivery and quality that far exceed those set by external quality controls and that the equipment, methods, and personnel they employ to deliver your components are never allowed to slip below an extremely high threshold.
Internal testing capability will also mean that the company you work with can tweak the tests carried out to ensure that the component in question is perfectly suited for work in your specific sector.
R & D
If an optical components company has its own dedicated Research and Development (R and D) department in place, then it shows they are committed to more than simply churning out the basic components the majority of customers need. Engaging in R and D of optical components indicates a commitment to innovation and excellence, and a willingness to work beyond the parameters of specific projects to constantly build on and push the quality of what&#;s being delivered.
If your optical components company has its own dedicated R and D facility then you can be certain that the techniques, methods, and materials they use are always going to be at the cutting edge of the sector. As a client, you will always be getting the latest and the best in optical components, and innovation that cuts costs, improves quality, and tightens delivery times.
About UQG Optics
UQG Optics has been providing optical components and solutions for over 65 years, and now supplies to clients around the world in over 50 countries.
We hold an extensive range of optical components in stock for fast delivery, as well as offering custom optics from our manufacturing facility in Cambridge. For larger volumes, we source from trusted and certified suppliers around the world, with all testing done in our own in-house facility.
Our product range includes:
In addition to this, the optical materials offered by UQG Optics range from a range of general sheet glasses to high precision coated sheet optical filters.
Our glass raw materials, machined blanks, polished sheet glasses and coated sheets are sourced from leading manufacturers such as Schott, Heraeus and Corning, and we hold full batch identification of every glass stocked so we can supply full traceability on your finished component.
Next Steps: To find out more about Optical Components from UQG Optics, or any of our other optics call us on or our sales team at .
The company is the world’s best Optical Mirrors custom supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.