![]() ![]() The length and width of scratches, and the dig (hole) diameters shall be determined by use of interferometry, microscopic measuring devices, calibrated precision comparators, or similar applicable precision measuring devices. In this case, unlike MIL-PRF-13830B, the actual width is being used to characterize a scratch. Scratch letters were defined as follows (sec. But for opaque coatings, it adopted designations consisting of “two letters separated by a hyphen.” Dig letters were defined as follows (sec. Central area of high-quality eyeglasses.Īnother military standard covered the surface quality of the coating of optical parts.⁴ For transparent coatings, the standard applied the method and two-numeral designation of the MIL-0-13830 standard. Some examples of ratings appropriate for various uses: 80-50Ĭosmetic. Working standards from commercial suppliers differ slightly from maker to maker, so it is a good idea to include in a surface quality specification the name of the manufacturer of the working standards to which the part is to be compared. Everyone else must rely on commercially available copies of the prototypes, a set of which costs several thousand dollars. The military and its vendors can refer to these prototypes, or to working standards directly calibrated against them. For each scratch number (80, 60, 40, 20, 10) there is a pair of prototypes, one defining the maximum allowable visibility for that scratch number and another defining the lowest allowable visibility. The actual prototypes are kept at the Picatinny Arsenal in New Jersey. Army ARDEC drawing “Surface Quality Standards for Optical Elements (scratch and dig) Drawing C7641866.” (Revision R, as of 2010). The scratch standards are described in U.S. ![]() But the scratch number is not a direct measurement of any dimension of the scratches, but of scratch visibility. The MIL standard includes other requirements, such as that the combined length of the biggest scratches can't be more than ¼ the diameter of the lens. Because of the nature of the visual matching procedure, it is possible for different evaluators to assign different scratch numbers to the same part. The evaluator picks the best match, essentially matching the brightnesses of the two surfaces by eye. It is arrived at through a method similar to the one machinists use to rate the smoothness of a machined surface, by visual comparison to a set of calibrated samples.³ MIL-PRF-13830B describes dark-field lighting conditions under which the part being evaluated is to be viewed together with working scratch standards. The scratch number is a less objective measurement. The standard only allows one dig of the stated maximum size per 20mm of lens diameter, and there are additional constraints, such as that if the dig number is 10 or less, every dig edge must be at least 1 millimeter away from the edge of its nearest neighbor. So, if a lens surface is rated 80-50, somewhere on that surface is a pit 0.5mm in diameter, about half a pinhead. ![]() In the case of irregular shaped digs the diameter shall be taken as the average of the maximum length and maximum width.² The dig number is the diameter, measured in hundredths of a millimeter, of a circle which would enclose the biggest dig in the surface.ĭig numbers are the actual diameters of defects allowed, specified in units of 1/100mm. The first is the scratch number and the second the dig number.¹ The rating is formatted as two numbers separated by a hyphen, or sometimes a slash, for example 80-50. In the United States, since 1954 optical surface quality has usually been described following military standard MIL-PRF-13830B and its predecessors. By scattering light these imperfections can, for example, lower the contrast in a photographic image or even, if the light source is a high power laser, damage the equipment. A manufacturer would find it hard to sell a camera lens with a visible scratch on the front objective, even though the scratch had no noticeable effect on photographs. “Digs” are pits.Ī scratch-and-dig evaluation of an optical part addresses two concerns. The processes by which lenses are made inevitably leave imperfections. #Iso 10110 scratch dig isoBy ISO standard 10110-7, e.g., 5/1 x 0.05 L5 x 0.004įigures of merit for the surface quality of an optical component, such as a lens or prism. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |