Surface quality specifications refer to the type and amount of allowable imperfections on each of the coated or uncoated surfaces of an optical component. Although some surface imperfections are purely cosmetic, many can introduce unwanted scattering or make the optical filter more susceptible to laser induced damage, resulting in decreased system performance.
Surface quality specifications are typically called out according to a set of standards defined by the United States Military, the American National Standards Institute (ANSI), or the International Organization for Standardization (ISO). Definitions, evaluation methods, and evaluation time can all vary depending on which standard is used.
Most military and ANSI standards call out scratch-dig surface quality specifications as a set of numbers that correspond to the respective size of the scratch or dig. In the case of MIL-PRF-13830B, evaluation is performed by visual comparison between the imperfections on an optical filter and a reference part using specific lighting conditions outlined in the document. While the dig value roughly corresponds to the diameter of the imperfection, the scratch value only refers to the apparent width of the scratch when compared to the standard. For example, a common scratch-dig specification of 60-40 means that no scratches can appear wider than the ‘60’ comparison standard and no digs can have a diameter larger than 0.4 mm.
Once all the scratches and digs on an optical filter are classified, MIL-PRF-13830B lists further requirements for the scratches and digs that fall into the maximum allowable category as determined by the specification. These are:
- The sum of the length of maximum size scratches must be less than ¼ of the diameter of the part
- When a maximum size scratch is present, the ratio of scratch length to part diameter is multiplied by the corresponding category number for each scratch and the sum must be less than half of the maximum category number (or less than the maximum category number if no maximum size scratches are present)
- The number of maximum size digs must be less than the part diameter divided by 20
- The sum of diameters of all digs must be less than twice the product of the number of maximum size digs multiplied by the maximum dig category number
Recommended specifications vary by application and system requirements, but in general, an 80-50 scratch-dig specification is usually sufficient for low to moderate-performance filters, while higher-performance filters should be specified as 60-40 or better. Filters used in laser applications commonly require a specification of 40-20 or better.
ANSI/OEOSC OP1.002-2009 was created as a more universal alternative to the military standards and gives the option of calling out specifications using either the visibility method or the dimensional method. The visibility method is nearly identical to the methods outlined in MIL-PRF-13830B but with added clarity surrounding test procedures. The dimensional method assigns a specification letter to each scratch width and dig size category. The letter categories correspond to those outlined in MIL-C-48497A, but ANSI/OEOSC OP1.002-2009 also includes the added option to call out the specifications according to maximums other than those listed in the pre-defined categories. The visibility method is much more commonly used than the dimensional method.
Unlike the MIL and ANSI standards, ISO 10110-7 surface quality specifications are purely quantitative. This standard does not distinguish between scratches and digs, instead broadly classifying both as surface imperfections. These are specified in terms of the total number allowed and a maximum grade number, which equals the square root of the measured area of the surface imperfection, given in mm. Optionally, the prefixes C, L, and E can also be used to respectively call out imperfections in the coating, long scratches (>2 mm), and edge chips.
It should be noted that surface quality specifications can dramatically affect cost. While it is important to select surface quality specifications according to system requirements, over-specifying can lead to decreased yield and increased cost. Likewise, choosing a labor-intensive surface quality standard can also increase overall cost.
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