Optical coatings requirements are stated in the drawing of the optical element or in the coating specification, which is an integral part of the purchasing requirements for an optical element and are based on the functionality of the element and the environmental conditions in which the optical system operates.
Optical, mechanical, electrical, and durability properties are tested on witness samples (test disc) that undergo the same, entire procedure of cleaning and coating as the actual elements; they are also part of the same coating and material growth batch. The witness samples should represent precisely the characters and quality of the actual coated elements. The witness samples are mostly round, a 1-inch (25.4 mm) diameter, and a thickness that is ~1 mm or the same as the actual elements (with the same surface quality).
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