Tolerances
Tolerance is a critical factor impacting the performance and cost of an optical system. Optical components usually require much tighter tolerances than that commonly associated with mechanical components

Cosmetic Surface Quality
Cosmetic surface quality describes the level of defects that can be visually noted on the surface of an optical component. The most common and widely accepted convention for specifying surface quality is the U.S. Military Surface Quality Specification, MIL-0-13830A. These standards include scratches, digs, grayness, edge chips, and cemented interfaces.

scratch : 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.

Dig: 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 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 20mm 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 0.5 mm are ignored, and those larger than 0.5 mm are ground so that there is no shine to the chip. The sum of the widths of chips larger than 0.5 mm cannot exceed 30% of the lens perimeter. Prism edge chips outside the clear aperture are allowed.

Cemented Interfaces: 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 1.0 mm. The sum of edge separations deeper than 0.5 mm cannot exceed 10% of the element perimeter.

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

Surface Accuracy
When attempting to specify how closely an optical surface conforms to its intended shape, a measure of surface accuracy is needed. Surface accuracy can be determined by interferometric techniques. Traditional techniques involve comparing the actual surface to a test plate gage. In this approach, surface accuracy is measured by counting the number of rings or fringes and examining the regularity of the fringe. Modern techniques for measuring surface accuracy utilize phase measuring interferometry with advanced computer data analysis software.
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Surface Flatness
Surface Fatness is simply surface accuracy with respect to a plane reference surface and is specified in terms of the wavelength f light, e.g. Lambda/2 at 633nm, which represents the degree of power, with sometimes a form-error addition, eg Lambda/5 at 633nm, which defines the allowed irregularity of the fringe pattern. It is used extensively in mirror and optical flat specifications.

Wavefront Error
Sometimes the best specification for an optical component is its effect on the emergent wavefront. This is particularly true for optical flats, collimation lenses, mirrors, and retroreflectors where the presumed effect of the element is to transmit or reflect the wavefront without changing its shape. Wavefront distortion is often characterized by the peak-to-valley deformation of the emergent wavefront from its intended shape. Specifications are normally quoted in fractions of a wavelength.

Centration
The mechanical axis and optical axis exactly coincide in a perfectly centered lens. There are typically two types of mounting configurations, edge mounting and surface mounting. With edge mounting, the mechanical.

Parallelism
Parallelism of a window can be specified in terms of angle, usually as minutes of arc. Sometimes it is in terms of thickness variation or "run-out". With conventional polishing techniques where no particular control of this parameter is involved, parallelism of less than 10 to 20 arc minutes can be expected. Where double-sided techniques are involved, parallelisms obtained are usually not measurable by normal methods. In some cases, to avoid multiple reflections between the faces, a small wedge angle may actually be specified.

Other special requirements are upon customer design!