The high surface quality of an optical component is imperative, particularly in laser applications and optics using UV wavelengths. Sometimes imperfections, such as scratches, can occur during manufacturing or handling. Poor surface quality directly impacts cost. Over-specifying components or continuing to manufacture optics with poor surface quality can directly impact your bottom line. There are two commonly used standards to specify surface quality: the U.S. Military Performance Specification MIL-PRF-13830B and ISO 10110.
What Are the Specs of U.S. Military Performance Specification MIL-PRF-13830B?
The U.S. Military Performance Specification MIL-PRF-13830B standard is a visual inspection, so its subjective nature can differ depending on who is performing the inspection. Using this standard, the inspector will assign the component known as a “scratch” and a “dig” number. The scratch number will always be one of these assigned numbers: 10, 20, 40, 60, or 80. These numbers refer to the brightness of scratches on the component.
However, this isn’t an exact measurement, and it is determined individually by the inspector.
The dig number, however, is measurable. A dig is a pit in the surface of the optic. To calculate the dig number, it calculates the diameter of the largest surface dig on the component, represented by 1/100 mm. For example, a dig diameter of .06 would have a dig number of 60, and a dig diameter of .04 would have a dig number of 40. This calculation is always performed the same.
After the scratch and dig numbers have been determined, it’s time to calculate how many imperfections are allowed within the component
- Scratches. Generally, the length of the largest scratches can not exceed ¼ of the diameter of the optic. The sum of all scratches is also multiplied by the ratio of each scratch. The diameter of the surface of the scratch cannot be larger than half of the maximum scratch specification. Suppose there is no scratch with a maximum allowable scratch size (as described above). In that case, the sum of the number of scratches multiplied by the ratio of the length of each scratch to the scratch’s diameter cannot be larger than the maximum scratch specification.
- Digs. The sum of digs cannot be two times larger than the maximum dig specification.
Typical industrial-grade quality scores are often 80-50, but precise applications will require scores of 10-5 for the utmost in precision.
What Is the ISO 10110-7 Standard?
The U.S. Military Performance Specification MIL-PRF-13830B standard is commonly used because it’s quick and economical, but it is not as precise of an inspection because of the subjective nature of the visual inspection. ISO 10110 is much more precise; however, it requires using differential interference contrast (DIC) microscopy, which requires an investment. It is also a more time-consuming method.
ISO 10110 does not differentiate between scratches and digs and classifies them both as surface imperfections. ISO 10110 informs you of the number of imperfections allowed and a grade number, which equals the square root of the area of the maximum allowed imperfection. This is referred to as the dimensional method. However, sometimes ISO drawings can indicate quality, and this is a visual inspection similar to MIL-PRF-13830B. MIL-PRF-13830B is a better choice for some industries because of its efficiency and cost-effectiveness but is useful for more specific components.
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