Cosmetic surface quality describes the level of defects that can be visually noted on the surface of an optical component. Specifically, it defines state of polish, freedom from scratches and digs, and edge treatment of components. These factors are important, not only because they affect the appearance of the component, but also because they scatter light, which adversely affects performance. Scattering can be particularly important in laser applications because of the intensity of the incident illumination. Unwanted diffraction patterns caused by scratches can lead to degraded system performance, and scattering of high-energy laser radiation can cause component damage. Over specifying cosmetic surface quality, on the other hand, can be costly. The most common and widely accepted convention for specifying surface quality is the U.S. Military Surface Quality Specification, MIL-0-13830A, Amendment 3.

As stated above, all optics in this catalog are referenced to MIL-0-13830A standards. These standards include scratches, digs, grayness, edge chips, and cemented interfaces. It is important to note that inspection of polished optical surfaces for scratches is accomplished by visual comparison to scratch standards. Thus, it is not the actual width of the scratch that is ascertained, but the appearance of the scratch as compared to these standards. A part is rejected if any scratches exceed the maximum size allowed. Digs, on the other hand, specified by actual defect size, can be measured quantitatively. Because of the subjective nature of this examination, it is critical to use trained inspectors who operate under standardized conditions in order to achieve consistent results.

The scratch-and-dig designation for a component or assembly is specified by two numbers. The first defines allowable maximum scratch visibility, and the second refers to allowable maximum dig diameter, separated by a hyphen; for example, 80–50 represents a commonly acceptable cosmetic standard. 60–40 represents an acceptable standard for most scientific research and commercial applications. 10–5 represents a precise standard for very demanding laser applications.

Scratches

A scratch is defined as any marking or tearing of a polished optical surface. In principle, scratch numbers refer to the width of the reference scratch in ten thousandths of a millimeter. For example, an 80 scratch is equivalent to an 8-μm standard scratch. However, this equivalence is determined strictly by visual comparison, and the appearance of a scratch can depend upon the component material and the presence of any coatings. Therefore, a scratch on the test optic that appears equivalent to the 80 standard scratch is not necessarily 8 μm wide. If maximum visibility scratches are present (e.g., several 60 scratches on a 60–40 lens), their combined lengths cannot exceed half of the part diameter. Even with some maximum visibility scratches present, MIL-0-13830A still allows many combinations of smaller scratch sizes and lengths on the polished surface.

Digs

A dig is a pit or small crater on the polished optical surface. Digs are defined by their diameters, which are the actual sizes of the digs in hundredths of a millimeter. The diameter of an irregularly shaped dig is 1/2#(length plus width): 50 dig = 0.5mm in diameter 40 dig = 0.4mm in diameter 30 dig = 0.3mm in diameter 20 dig = 0.2mm in diameter 10 dig = 0.1mm in diameter. The permissible number of maximum-size digs shall be one per each 20mm of diameter (or fraction thereof) on any single surface. The sum of the diameters of all digs, as estimated by the inspector, shall not exceed twice the diameter of the maximum size specified per any 20-mm diameter. Digs less than 25 micrometers are ignored.

Edge Chips

Lens edge chips are allowed only outside the clear aperture of the lens. The clear aperture is 90% of the lens diameter unless otherwise specified. Chips smaller than 0.5mm are ignored, and those larger than 0.5mm are ground so that there is no shine to the chip. The sum of the widths of chips larger than 0.5mm cannot exceed 30% of the lens perimeter. Prism edge chips outside the clear aperture are allowed. If the prism leg dimension is 25.4mm or less, chips may extend inward 1.0mm from the edge. If the leg dimension is larger than 25.4mm , chips may extend inward 2.0mm from the edge. Chips smaller than 0.5mm are ignored, and those larger than 0.5mm must be stoned or ground, leaving no shine to the chip. The sum of the widths of chips larger than 0.5mm cannot exceed 30% of the length of the edge on which they occur.

Cemented Interfaes

Because a cemented interface is considered a lens surface, specified surface quality standards apply. Edge separation at a cemented interface cannot extend into the element more than half the distance to the element clear aperture up to a maximum of 1.0mm . The sum of edge separations deeper than 0.5mm cannot exceed 10% of the element perimeter.

Bevels

Although bevels are not specified in MIL-0-13830A, our standard shop practice specifies that element edges are beveled to a face width of 0.25 to 0.5mm at an angle of 45°±15°. Edges meeting at angles of 135° or larger are not beveled.

Coating Defects

Defects caused by an optical element coating, such as scratches, voids, pinholes, dust, or stains, are considered with the scratch and-dig specification for that element. Coating defects are allowed if their size is within the stated scratch-and-dig tolerance. Coating defects are counted separately form substrate defects.