The surface quality of an optical part is a measure of evaluating defects such as "scratch", "dig", etc. that may occur on the surface during manufacturing or during use. uneven surface quality can cause faster damage due to laser beam. Select the proper surface quality for the application is also a cost-critical criterion and over specification can result in unnecessary cost increases. Common criteria to defining the surface quality of lenses include U.S. Military Performance Specification MIL-PRF-13830B and ISO 10110-7.
U.S. Military Performance Specification MIL-PRF-13830B uses "Scratch" and "Dig" numbers to define surface quality based on predefined standards. It is specified as one of the numbers 10, 20, 40, 60, and 80 and increases in brightness as the number goes up. This number is indicated the scratches and brightness of the calibrated specification, not the exact measurements. Basically, the Scratch inspection is performed in a darkroom with individual lighting, but it is conducted as a subjective visual inspection, so the results may slightly differently it depends on the inspector. Dig Number is a measurable number that indicates the largest Dig diameter on the surface of the material and is expressed in 1/100 of a scale. If Dig diameter is 0.2mm, Dig Number is 20. The scratch and Dig values are defined, determine the number of faults that are acceptable. The total number of Digs(N) allowed does not exceed the diameter divided by 20.¨ç Please make sure that sum of the Scratch lengths (Lsn) with the specified scratch number and ensure that the sum does not excess quarter of the lens diameter. ¨è The total number of Digs allowed (N) does not excess the diameter divided by 20. ¨é The sum of all dig diameters is (d) to doubled the total number of Digs allowed or less than multiplied dig number by the specified. The Scratch-Dig specifications are widely reviewed standard quality for many optical applications. Typical optical products use 40-20 (Scratch-Dig) and optical products in the laser field define high quality criteria at 20-10 or 10-5 levels. Allowable value for surface defects is depend on the wavelength and UV lasers require 10-5 levels of high quality.
In the aforementioned MIL-PRF-13830B is an efficient inspection, but the accuracy of the result may be insufficient depend on the subjective check of the inspector. A more objective inspection method, ISO 10110-7 defines quality based on the size and frequency of physical imperfections. However, it takes more time and cost than MIL-PRF-13830B. Using a microscope with a small FOV lens the entire of the test lens is imaged by measurement several times. Inspect measured imaging for surface imperfections not expressed in Scratch and Dig. When the dimensional specification method is used, the drawing indication for number and size of surface imperfections that are permissible within the test region of an optical surface is 5/Ng x Ag and for localized surface imperfections in optical assemblies, it is 15/Ng x Ag Ng is the number of allowed imperfections. Ag is the grade number which is equal to the square root of the area of the maximum allowed imperfection, expressed in millimetres as defined in ISO 14997.
The total area of covered by imperfections are calculated as follows. AreaTotal = Ng X (Ag)2 ISO 10110-7 basically defined surface quality in a dimensional through Ng and Ag, but it can also define surface quality in a Visibility inspection such as MIL-PRF-13830. The benefits of using both Dimension and Visibility methods can be used in a variety of applications.