bentec services limited (canada)

        key to advanced device fabrication;                                           

define and control non-spherical wavefronts;                                   

tuned Fresnel phase lenses; groove width and depth variation; multi-focal systems;

aspherical optical design; mould tool inserts; polymer and silicone optics.

Niche markets and highly confidential exclusive partnerships are our interests,

together with the development of enabling technologies.

Uniqueness: hyper-aspherical surfaces are independent of conic constant and aspherical coefficients. Standard optical software requires pre-defined optical surfaces. In contrast, hyper-aspherical surfaces are a dynamic consequence of pre-defined wavefronts - key to advanced device fabrication. Bentec Services Limited (Canada) offers free software related to specific projects such that we illustrate the optical system incorporating hyper-aspherical surfaces - thus providing immediate results and removing some complexities of aspherical surface design. Specified wavefronts may be imposed as input, output or between existing surfaces and may be simultaneously combined - multiplexed. Aspherical, light weight, (low inertia) optics are our interests.

Aspherical optics production via replication is cost effective but the initial cost of masters and tooling is significant.

 bbennet2@telus.net - contact me directly using this email address

Services: customized and innovative designs, using

Bentec Hyper-Aspherical and Zemax Software.

 

Generalized Design Guidelines:

Typical Specifications for Injection Moulded Optics

 

Low cost

Commercial

State of the art

Focal Length +/- 3 to 5% +/-2 to 3% +/-0.5 to 1%
Radius of curvature +/- 3 to 5% +/-2 to 3% +/-0.5 to 1%
Irregularity 6 to 10 f/in 2 to 6 f/in 0.5 to 2 f/in
Scratch/Dig 80/50 60/40 40/20
Centration +/-3 min +/-2 min +/-1 min
Thickness +/-0.005" +/-0.002" +/-0.0005"
Diameter +/-0.005" +/-0.002" +/-0.0005"
Repeatability 1 to 2 % 0.5 to 1% 0.3 to 0.5%

Diameter to Centre Thickness Ratio

2:1 Difficult to  mould, loose tolerancing recommended;

3:1 Moderately easy to mould, commercial tolerancing;

5:1 High Precision tolerancing achievable.

Smaller, Lighter, Faster, Cheaper

Power (Collection Efficiency): Design freedoms provided by polymer optics permit an approximate 1.78 energy collection advantage over glass: Example: A high index SFL6, all spherical glass element will have an f# of around 0.8 maximum relative aperture (effective useful relative aperture is 1.0) for an effective focal length of 15 mm. A bi-aspherical moulded polycarbonate element can be designed with f#  0.6 maximum relative aperture for a 15 mm efl. This lens will collect 1.78 times the energy of the glass element.

Colour Correction: Aspherics and Diffractives provide correction with fewer elements.

Challenges of Mould Insert Fabrication: Improvements in tool fabrication techniques permit construction of complex tooling architectures with improved micro-finishes.

Compared to Glass, some benefits of polymer mouldings:

  • Mechanical mounting flanges, self-registration, alignment features, snap features, and tabs provide an ability to automate assembly;
  • Mechanical repeatability to high accuracy, in comparison to ground and polished glass lenses;
  • Aspheric, conic or toroidal surfaces are as easy to produce as spherical;
  • Moulded micro-structured surfaces - such as diffractives.

Performance:

Issues of dN/dT (index vs. temperature) and the restricted refractive index range of polymers can be corrected by either compensation in  mechanical design or employing micro-structured surfaces for temperature and chromatic dispersion.

Dispersive diffractives may be used to correct colour. The high index and negative Abbe values of diffractive surfaces help reduce monochromatic aberrations and may be used to athermalize the design.

Optical Performance: Glass has variable surface accuracies after finishing whereas injection moulded optics have repeatable surface accuracies - lens to lens repeatability of 0.5% is typical. Complex shapes used in laser scanner systems that cannot be manufactured in glass can be moulded and coated as one unit. Optical accuracy may be held to 5 waves or less. Micro finishes of less than 70 Angstroms are possible - hence low scatter.

Good reasons for using polymers:

Polymers provide a repeatable level of performance at lower cost.

  • Raw material is less expensive;
  • Variety of mounting and alignment features can be moulded as part of the optic element - permitting automated assembly;
  • Multicavity moulds reduce production costs if there is enough volume to justify higher tooling investment;
  • Lens to lens repeatability - less than 0.5%;
  • Surface accuracy variations less than 1 fringe, depending on size;
  • Dimensional tolerances in the tenths of thousandths of an inch.

Aspherical surfaces: it costs no more to mould an asphere than it does a spherical element.

Diffractives: are an example of a product difficult and expensive to fabricate in glass - whether it be a rotationally symmetric kinoform surface, multiphase binary stepped surface or grayscale diffractive structure.

Benefits of polymer optics: Lower material weight; Integrated mounting features; Optical glasses typically are 2 to 2.5 times heavier than corresponding polymer lenses; Aspheric and diffractive surfaces permit reduced numbers of elements. 

Temperature Effects: Although the refractive index changes with temperature (and humidity) to a greater extent than glass, the more significant effect in polymers is thermal expansion - causing the radii and thickness of the optic to change - thus shifting the point of focus. Typical glasses have coefficients of expansion of 0.7 X 10-7 /K, whereas plastics such as Acrylic have 7 x 10-7/K, a factor of 10 greater. Thermal compensation designs (athermalization) are possible using diffractives or compensating mechanical structures in the lens housing/spacing. Example: The use of a higher temperature coefficient non-optical polymer such that lens separation and image distances physically change but the image plane remains constant. 

Alternatively the combination of glass/polymer elements (hybrids) employing a thin polymer layer, reduces overall expansion.

Mechanical Design Guidelines

Thickness: Shrinkage compensation is less complex for a thin lens and moulding cycle times are shortened - reducing cost. Thicker cross sections will increase cycle times and increase cost as well as make it more difficult to hold tight surface figure. Extreme variations in thickness may create uneven flow characteristics that may induce uneven cooling and change optical figure. Rule of thumb: Keep the diameter to thickness ratio greater than 6:1 and flange width to 2.5 times the edge thickness.

Structural Elements: Flanges; Spacers; Tabs; Slots; Brackets and Snaps may be integrated to the optical element - resulting in a single-piece design that eliminates mounting hardware and simplifies assembly and alignment. 

The shape and size of the structural elements outside the optic surfaces are important - allowing shrinkage effects to occur in the flange area and not in the critical optic aperture - thus preserving optical performance to the edge of the specified critical aperture.

Clear Aperture: If no flange is permitted, the clear aperture should be no closer to the edge than 1.5 times the edge thickness.

Symmetry: within the moulding aids flow characteristics.

Flat Surfaces: have a tendency to sink when the polymer cools. If possible, add a surface of power - even as little as a meter (radius of curvature). If a flat surface is required to compensate for sag longer cycle times and other process changes are required to create acceptable flatness.

Gate: Consider factors that determine which type of gate design is best suited to the optic element configuration and surface figure required. These range from fan gates to small buried sub-gates which automatically shear off as the mould opens.

Gate Vestige: When detaching the part from the runner some vestige of material is left on the part. This can be compensated by creating a small flat section at the gate area - which could also be used as a clocking feature if rotational alignment is necessary. 

 Draft Angle: (minimum of 0.5 degrees)  The degree to which side walls are tapered - in order to extract the part from the cavity.

Number of Cavities: Increasing the number of cavities per shot will lower the part cost but increases the mould cost. This will increase the degree of difficulty of balancing moulding characteristics of these cavities. The size, shape of part, volume of parts and tightness of optical tolerances desired, will limit the practical number of cavities used for a particular part configuration.

Shrinkage: Thermoplastic shrink rates for commonly used materials vary from 0.001" to 0.006"/F. It is important to compensate for shrinkage precisely. However it is often not possible to model shrinkage. 

Polymer is compressed in the cavity at injection and more material is added to compensate for shrinkage during cooling. Where there is uncertainty, moulds are usually built with 'steel safe' to allow for minor corrections after the first round of parts has been made and measured. This enables material to be gradually removed from the tool in order to approach the required final part dimensions.

Housings/Structures: Maintain a uniform wall cross-section thickness, even at the corners; Avoid overly thin / thick cross sections or combinations of extremes in cross sections. Avoid sharp corners which localizes stress concentrations.

Holes:  The flow of material around holes can cause weld lines. Shear in the polymer material during the fill of the cavity induces polymer orientation and birefringence.

High power negative lenses are problematic. Flow of material around the thicker edge section fills first and last in the center - causing weld lines where the flow fronts meet - resulting in distortion of surfaces and poor optical figure control.

Assembly Considerations: Press fits; Snap fits; Cementing; Heat staking; Ultrasonic welding.

Limitations of Glass: Advantages of glass optical elements are temperature stability, Abbe values and a greater index range for colour compensation over a wide wavelength range. Cost and repeatability are limitations however.

When not to use Polymer: If optical figures tighter than 1 fringe are required and if extreme temperature ranges are required - 40 C or greater operating range.

A durable 'hard coat' may be required to provide better scratch resistance. The 'hard coat' application involves a spin-on chemical solution followed by curing. Acrylic hardcoat treatment additionally enables surface adherence for the AR coating that otherwise was not obtainable. A chemical 'hard coat' of a few µm thickness provides most of the scratch resistance.

Polymers diffuse moisture continuously.

Polymeric materials are ideal for creating moulded mirror substrates of complex shapes. Advanced illumination systems in automobiles, aircraft, and compact display units make maximum use of the fit-to-form and lightweight advantages of plastic reflectors. Such surfaces, produced in high volume, are metallized (aluminum) and the metal is protected from abrasion and corrosion with a dielectric layer system (silicon monoxide or alumina). Generally, the rms surface roughness requirements are not as rigid as for refractive optics

Manufacturing Considerations:

Shrink rates occur in the range of 0.001 to 0.006 in./in. Compensation of shrink rates is important and may be achieved empirically -  by initially starting with a mould of smaller dimensions than the designed dimensions of the actual optic. The undersized tool (with steel safe) is then run and fabricated components produced - from which shrinkage measurements may be accurately established. The tool insert may then be machined accurately accounting for polymer shrinkage. In this way and through Process Control techniques the dimensions of the polymer moulding may be consistently maintained. 

Spherical and plano inserts are generally fabricated from chromium-alloy stainless steel. 50 to 54 Rockwell C-hardness rating range prior to polishing. 

Aspheric inserts: A best-fit curve is generated on a stainless steel substrate which is then subjected to a nickel plating process - electrolytic or electroless.  This process deposits a layer of nickel - up to 0.5 mm thick. Diamond turning produces the final aspherical surface in the nickel. Diamond turned inserts typically exhibit RMS surface roughness values of less than 50 Angstroms.

Diamond grinding provides another method of producing mould tool inserts but now may be used on ferrous metals such as stainless steel. Less surface accuracy is achieved compared to the diamond-turned product. 

Component Tolerances: Repeatability is one advantage of the injection moulding process. Tolerances are dependent upon part geometry, size, mould material, mould tool insert design and construction.

Coatings: Vapour deposition is used to apply antireflective, conductive, mirror and beam splitter coatings. 

Antireflection coating - A Magnesium Fluoride l/4 single layer coating on a polymer surface reduces reflectance from about 4% to 1.5%. Broadband coatings comprising three or four layers may reduce reflectances to less than 0.5% across the visible spectrum. Narrowband, multi-layer antireflection coatings may yield surface reflectance to less than 0.2%.

Reflective coatings - Typical coating metals include aluminum, silver and gold. Aluminum coatings provide surface reflectances greater than 88% across the visible spectrum and gold coatings greater than 95% from 700 to 1000 nm.

Aspherical surfaces: Prudent placement of an aspheric surface can reduce element count in some designs or relax certain fabrication tolerances in others. The location of an aspherical element may be critical. Ideal shapes for plastic optical components are those that maintain a near uniform wall thickness - due to consideration of shrinkage and birefringence. Strong meniscus, bi-convex and bi-concave shapes should be avoided to achieve high-quality, high-yielding polymer optics.

Birefringence - For systems in which polarization control is paramount the optical designer must properly choose the location and shape of components. Various process parameters may be adjusted for use of polymers with fairly low birefringence - yielding components that exhibit qualitatively high extinction ratio components as viewed through crossed polarizers.

Thermal effects - The thermal differential index of refraction coefficient of optical polymers is approximately an order of magnitude greater than that of glass. For this reason high-performance lens systems that require large temperature-band operating conditions are more suited for hybrid glass-polymer designs. Although the temperature band for all-plastic lenses may be quite limited, this characteristic depends strongly upon resolution criteria.  A comparable design can often be achieved with the introduction of a minimal amount of glass elements.

Mechanical design - Most polymer optical components have a flange around the circumference of the optic that performs several functions: - prevents cosmetic defects, provides additional mechanical rigidity and provides a mechanical mounting surface. Occasionally the flange incorporates an integrated spacer.

Typical components suited to polymer lens manufacture: - Low cost and repeatability are the most significant benefits of polymer optics. There are other potential benefits that are achievable: Complex apertures and component geometries; Off-axis aspheres; Surface aperture recesses; Multiple-surfaces (>3) in a single component.

Examples of unique polymer applications include: off-axis scanning parabolas; gold-coated barcode scanning octagons; medical arthroscope prisms - which comprise refracting, beam-deviating and reflecting surfaces all within one component. Mounting surfaces may be incorporated into components.

For design examples, see D. Buralli and G.M. Morris (1991), "Design of diffractive singlets for monochromatic imaging," Appl. Opt. 30 (16) 2151-2158

The single-point diamond turned 'insert' fabrication process for diffractive elements has improved considerably since the early 1990s. This improvement in quality combined with advances in precision moulding has yielded components with diffraction efficiencies greater than 95 percent at the desired wavelength and incident angle.

Although injection moulding produces high-fidelity diffractive elements performing close to theoretical predictions, diffraction efficiency vs. incident wavelength and angle still detrimentally affect contrast in visible broadband systems. These effects dictate proper optical system design and analysis for successful product implementation.

Polymer optics offer several advantages in many optical systems: lower-cost, aspherical surfaces, integrated components and complex aperture or multi-surface elements. The successful application of polymer optics to an engineering problem results from an integration of the opto-mechanical design process, tooling construction, component fabrication and surface coating deposition.

   Applications:

Micro-optics

Integrated optics

Hartmann sensors

Microlens arrays

Robotic guidance

Focal plane lenslet arrays

Micro-electro-optics

Optical switches

Laser diode arrays

Beam shaping

Optical testing 

Opthalmic lenses

Collimate & expand beam

Aberration correction

Bifocal contacts

Beam homogenizer

Null Correctors

Laser scalpel

Medical imaging

Shaped cutting and welding

Edge Detection 

Optical data storage

Laser machining

Packaging alignment

Optical processing

Laser hole drilling

Textile alignment & cutting

Heads up displays

Night vision goggles

Surface Profiling

Head mounted displays

Reconnaissance systems

Contour mapping 

Aft-Imager

Optics 3-D goggles

Machine vision

Circuit board inspection

Fanout gratings

Laser printers

Depth measurements

Beam Splitters

Color copiers 

Manufacturing inspection

Wavelength multiplexing

Laser entertainment

Polarization components

Quarter-wave plates

Laser material processing 

Polarizing beam splitter

Half-wave plates

Product marking

Linear polarizers

Retarders

Scanning systems

Static filters

Laser mode tuners

Optical computing/matched filters

Tunable filters

Laser end mirrors

Chirped grating couplers

Narrowband filters

Modans Laser

doppler velocimetry 

Circles, lines, etc.

Color separation gratings

Hybrid achromats 

Laser cross hairs

Projection systems

Apochromatic doublets

Gun sights

Fiber optic communications

Replace aspheric elements

Lenses

Fourier Transform lens

IR systems 

Wide-field imaging

Anamorphic (cylinder) lenses

Thermal imagers

Aberration correctors

F-Theta scan lenses

Friend/foe identification 

Athermalized optical systems

Optical interconnects

Anti-Reflection Structures

hyper-aspherical optical design

hyper-aspherical surfaces are independent of conic constant and aspheric coefficients and steer definable wave-fronts.

Ref: Stavroudis, O.N. 1987, Tracing Wavefronts: Can it be done? SPIE Vol.766 'Recent trends in optical design'  pp 18-26 

 

Links:

UK Patent Office World Intellectual Property Organization US Patent and Trademark Office Australia Patent, Trademarks and Designs
Canadian Patent Database European Patent Office Japanese Patent Office Patent Storm
www.osa.org Optics Express www.optics.org www.opticnotes.com
Science Magazine Measurement Science and Technology Journal of Optics A: Pure and Applied Optics Applied Physics Letters
Wikipedia Roget's Thesaurus www.gratinglab.com  

Fresnel lens examples using hyper-aspherical optical surfaces

imaging, 0.4 mm boPET lens, where each facet is hyper-aspherical. In this case the facets face the 'shorter' conjugate focus.

back focal length 200 mm, centre thickness 3.5 mm, half aperture 100 mm. Facets face the 'longer' conjugate focus. lens of hyper-aspherical facet surfaces.

wide-angled lens. Each facet independently spans a portion of the field. Facets face the 'shorter' conjugate focus.

mirror: F# = 2 parallel light focused. consequently these hyper-aspherical facets are paraboloid in nature.

0.4 mm, internally reflecting hyper-aspherical mirror. parallel rays pass internally and out of the plano surface, toward a focus.

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