Surface Quality and Defect Density
Surface quality is often called out using the MIL-C-675C and MIL-F-48497 terminology as a “scratch and dig” set of numbers. These describe the maximum allowable defect size with a number of qualifications such as defect density. It should be noted that MIL-C-675C and MIL-PRF-13830B are “active” specifications while MIL-F-48497 and MIL-F- 48616 are “inactive” meaning that they are intended to be used to support or reprocure existing items.
Characterization of defect density is an active area of development because of rapid advances in detector and imager technology. Optics near the focal plane of a detector are often poorly described by conventional scratch/dig metrics, and this application is occurring more frequently. Applying the actual lighting and observation standards described by MIL specs often confuses users, who perceive a poor quality of inspection technique and may be surprised by the “allowed” defects. Also, most producers and consumers of optics utilize more stringent inspection procedures internally in order to drive process improvement and to characterize optics more thoroughly than the MIL spec standard requires.
However, the MIL spec defect ratings are perfectly adequate for optics used far from a focal plane and in the MWIR/LWIR. In the case of more demanding technical requirements, a detailed discussion is beneficial between system designers and optics producers.
Unmanned aerial systems and their applications are an attractive new growth area for advanced optics, which are at the heart of UAS capabilities. Multispectral imagers and optical systems that image in multiple wavelength ranges are increasingly being used. Sapphire optical windows are an excellent fit for the harsh environment and multiple wavelength requirements encountered in UAS service. As shown, the new BBAR coatings provide increased transmittance over the wide wavelength range that sapphire allows, permitting technological advances in optical system design.