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Surface roughness is one component of describing how the shape of a surface deviates from its ideal form, where higher values correspond to rougher surfaces while lower values indicate the surface is smooth. He Rui optics Roughness describes high spatial frequency errors, meaning very small deviations on the order of Angstroms (10-10 m). Understanding the surface roughness of optical surfaces is critical to control light scatter, as scatter can be proportional to the surface roughness of the optics. Light scattering and absorption from surface roughness have a significant impact on applications such as high-power laser systems, which can negatively impact the efficiency and laser damage threshold. In addition to impacting damage threshold, scattered high-power laser radiation can be a safety hazard for anyone near the system, as the light is being redirected in unintended directions. The current standard utilized for surface roughness is ISO 10110-8, which defines how surface roughness should be analyzed and specified.

Surface Finish, shown in Figure 1 as "P3":

This variable indicates the surface finish. For simple polishing specifications, it could either be a G, for ground surface, or P to indicate it is optically polished. The grade of the polishing will be assigned from 1-4 according to the degree of smoothness in terms of microdefects per 10mm scan as shown below in Table 1:

Table 1: Indication of the degree of smoothness in terms of microdefects

Statistical method used, shown in Figure 1 as "Rq 4":

This will indicate the statistical method used to measure the surface roughness, followed by the value.

Spatial bandwidth, shown in Figure 1 as "1/0.003":

This specifies the spatial bandwidth ranging from the upper bound to the lower bound.

Spatial Frequencies and Frequency Groups

When quantifying the surface texture of an optical component, it is important to define what level of spatial resolution is being measured. Surface texture, or the complete shape of a surface, can be broken down into three main spatial frequency groups: roughness, waviness, and figure.

Figure 2 shows how surface figure, waviness, and roughness together characterize all of the ways a surface deviates from its ideal shape. Figure describes the overall shape of the surface and is the largest scale, or largest spatial frequencies, which will be analyzed. The errors described by figure are on the order of tenths of mm to cm. Waviness measures mid-spatial frequency errors describing features on the order of µms to mm. Roughness is the smallest form of error and describes closely-spaced abnormalities in the surface texture on the order of tenths of Angstroms to tens of µm.