Understanding Surface Quality Specifications based on U.S. Standard MIL-PRF-13830B
The surface quality of an optical component is an evaluation of the surface imperfections that may be caused in the manufacturing or handling process. In most cases these defects have little to no adverse effect on the overall system performance in imaging or light gathering applications; only small reductions in throughput and small increases in scattered light are typical. There are certain surfaces, however, that are more sensitive to these defects such as surfaces at image planes because these defects are in focus and surfaces that see high power levels because these defects can cause increased absorption of energy and damage the optic. Governing bodies for rating surface quality are:
- ANSI/OEOSC OP1.002-2009; For Optics and Electro-Optical Instruments –Optical Elements and Assemblies –Appearance Imperfections*
- ISO 10110-7:2008; Optics and photonics -- Preparation of drawings for optical elements and systems -- Part 7: Surface Imperfection Tolerances*
- ISO 14997:2003; Optics and optical instruments -- Test Methods for surface imperfections of optical elements
- MIL-PRF-13830B; General specification governing the manufacturing, assembly, and inspection of Optical Components for Fire Control Instruments
*Edmund Optics’s custom optical components can be manufactured to these standards
Edmund Optics adheres to the most common method of rating surface quality: the U.S. Military Performance Specification MIL-PRF-13830B, which uses “scratch and dig” numbers based on calibrated standards prescribed therein. It determines the caliber of scratches and digs and sets guidelines limiting the allowable amount of each imperfection based on the component’s size and its scratch and dig number.
The scratch and dig numbers are unrelated two-digit numbers typically separated by a hyphen: scratch number first, followed by dig number. These numbers are determined by visually comparing the brightness of scratches and diameter of digs with those on the calibrated standard while the component and standard are each under the lighting conditions prescribed in the performance specification. The specified scratch and dig numbers indicate that the component will not have scratches brighter or digs larger than the corresponding numbers on the calibrated standard.
The scratch number is one of the following arbitrary numbers: 10, 20, 40, 60, or 80, where brightness of scratches increases from 10 to 80. It is not an exact measurement, only an indication of the best match of component scratch brightness with calibrated standard scratch brightness. The dig number, however, is a measureable quantity: the diameter of the largest component dig, given in 1/100 of millimeters. Therefore, a component with a 0.4mm diameter dig is represented with a dig number of 40, one with a 0.2mm diameter with a dig number of 20, and so on (Figure 1).
Figure 1: An illustration of scratch-dig evaluation
Once the scratch and dig numbers have been determined, the following will set the limitations of allowable defects:
The sum of all scratch length with the specified scratch number (LSN) will not exceed one fourth of the diameter of the optic. For non-circular optics, the diameter of a circle with equal area to that of the optic should be used.
The total number of allowable maximum size digs (N) will not exceed the diameter divided by twenty.
The sum of all dig diameters (d) will be less than or equal to twice the total number of allowable maximum sized digs (N) multiplied by the specified Dig Number (D)
An optic with a 100mm diameter with specified Surface Quality of 60-40, based on the above limitations, can have several 60 scratches whose total length is not more than 25mm. It may not have more than 5 maximum sized digs of 0.4mm (40 Scratch Number), and the sum of the diameters of all digs must not exceed 4mm.
Scratch-dig specifications of 80-50 are typically considered standard quality, 60-40 precision quality, and 20-10 high precision quality.