Manufacturing requirements for optical systems are comprised of mechanical and optical parameters.
Mechanical parameters are fairly straightforward and consist of items such as flange diameter, center thickness (CT), radius of curvature and clear aperture (CA). An additional mechanical consideration for plastic optics is center to edge ratio due to the implications this has on final form due to shrink.
Optical parameters are much less known to non-optical staff and consist of focal length, surface form (Power/Irregularity), material imperfections, cosmetics (scratch/dig), centering deviation (concentricity, beam deviation, wedge or total indicated runout) and surface roughness.
Tolerances normally achievable for glass components may not be appropriate for plastic optics. During the optical design process the base values and tolerances for both mechanical and optical parameters are established. They may vary greatly depending upon the system performance requirements, the assembly method selected, the materials of manufacture and the design experience of the optical engineer performing the design. For example, mechanical and optical specifications and tolerances may be loosened if the optical design engineer can design the system to allow active alignment of components during assembly.
While many of these design parameters may be controlled very tightly when designing with glass and metal this comes at great cost as each component is machined individually. Alternately with the use of plastic molding technology the mechanical and optical properties are built into the tooling and with injection molding may be replicated at higher production rates and lower cost for each component.
Appropriate optical design is performed using design and simulation software such as Zemax Optics Studio, Code V, Oslo or several other optical modeling platforms. Design parameters and tolerances should be applicable to the specified material and manufacturing process planned for the system design.
Specifying design parameters and tolerances that are too loose may result in inadequate system performance. On the other hand, specifying design parameters and tolerances that are too tight may result in significant increase in the project cost or even project failure due to inability to manufacture components to specification.
Optical tolerances should be specified for each optical surface individually. The most common method used for describing an optical part is prescribed within ISO 10110, an international standard titled, Preparation of drawings for optical elements and systems. This standard prescribes not only how optical drawings should appear, but also how constructional data and tolerances should be specified to communicate optical requirements in a standardized method thereby minimizing potential confusion.
Typical optical tolerances and design parameters applicable to plastic optics are listed in the table below:
| Term | Definition | Parameters | Standard | Precision |
|---|---|---|---|---|
| Focal length | The distance from the optical surface to the plane on which the image comes to focus. | Focal length (%) | ±5 | ±2 |
| Radius of curvature | The radius of the optical surface. | Radius of curvature (%) | ±3 - 5 | ±2 - 3 |
| Surface form | Tolerance defines maximum allowable deviation of the surface from its nominal shape. In spherical optics, the most common terms used to specify this error would be: | |||
| Power (overall departure from nominal) | Power (fringes) | 10 | 5 | |
| Irregularity (localized departure from nominal) in fringes | Irregularity (fringes/10mm) | 4 | 2 | |
| Cosmetics (surface imperfection) tolerances | Surface imperfection tolerances describe such defects as scratches and pits (digs). In most cases, these surface defects are purely cosmetic. However, in certain cases they may affect system performance. The most common method used to define surface imperfection tolerances is the scratch-dig specification described by MIL-PRF-13830B, a widely used surface quality standard for optical components. | Scratch / dig | 80 / 50 | 60 / 40 |
| Center thickness | The thickness of the lens at the center. This parameter is a major factor in controlling focal length. | Center thickness (mm) | ±0.1 | ±0.05 |
| Flange diameter | The outside diameter of the lens. This controls alignment in a classical optical design with optics placed within a barrel. | Flange diameter (mm) | ±0.1 | ±0.05 |
| Centering | Centering requirements typically refer to the position of one surface of a lens with respect to the second surface of the lens. Centering is, typically, specified in terms of beam deviation, wedge angle, total indicator runout (TIR), or concentricity. | Concentricity (mm) | 0.1 | 0.05 |
| Surface roughness | Surface roughness or surface texture measures small scale irregularities on a surface. It's usually expressed in units of Ra (average roughness) or RMS (root mean square). | Surface roughness (ÅRMS) | >75 | >60 |
| Clear aperture | The light gathering area of the optical surface. Clear aperture should be offset from the physical edges of the part by at least 0.5 - 1 mm due to the edge roll-off effect and coating fixture marks. | |||
| Material imperfections | Material imperfections that may affect optical performance include stress birefringence, bubbles, inclusions, inhomogeneity, and striae. Due to the nature of manufacturing processes used for plastic optics, control of material imperfections is a challenge. | |||
| Surface treament and coating | Optical coating specifications should include performance requirements expressed as a percentage of reflectance and/or transmission over a specific spectral range and range of angles of incidence. |
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