MIL-PRF-13830B
located on each surface of an optical element shall not exceed one quarter the
diameter of that element.
3.5.2.1.1 Maximum combined lengths of scratches. When a maximum size
scratch is present, the sum of the products of the scratch numbers times the
ratio of their length to the diameter of the element or appropriate zone shall not
exceed one half the maximum scratch number. When a maximum size scratch
is not present, the sum of the products of the scratch numbers times the ratio
of their length to the diameter of the element or appropriate zone shall not
exceed the maximum scratch number.
3.5.2.2 Noncircular shaped element. The computing diameter of element
shapes other than circular shall be that of a circle of equal area. Scratches
beyond the free aperature of any element as given on the optical system
drawings or detail drawings shall not be considered when applying the
appropriate formula specified in 3.5.2.1.1.
3.5.2.2.1 True roof surfaces on prisms. True roof surfaces on prisms shall
be considered equivalent to a single surface equal to the sum of the individual
roof areas for purposes of scratch and dig computation, except that the roof
edge shall not be considered in the summation of the length of the allowable
scratches. Scratch and dig tolerances for roof prisms are set on the basis that
the equivalent surface above is viewed from the air side. (3.7.10.1).
3.5.2.2.2 Surface quality, central zone. Areas of surfaces whose specified
scratch qualities are 20 or better shall have no more than 4 separate scratches
in any 1/4-inch diameter circular area. This requirement does not apply for
scratches smaller than number 10.
3.5.2.3 Surface quality, outer zone. Surface quality outside the free
aperture of any element shall be considered 80-50, unless otherwise required.
3.5.2.4 Coating scratches. Coating scratches, scratches which do not
penetrate the glass surface, shall be within the same limits specified in 3.5.2.
Coating scratches shall be considered separate from the substrate scratch
requirements.
3.5.3 Digs.
3.5.3.1 Dig designation. Dig numbers are the actual diameters of defects
allowed, specified in units of 1/100mm. In the case of irregular shaped digs the
diameter shall be taken as the average of the maximum length and maximum
width.
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