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scratch dig analysis of optical elements

Introduction. Optical systems must perform as designed. Specification or drawing gives requirements and deviation allowance.Numerous defects can be acceptable.. The Challenge:How many defects can there be?How serious can the defects be?Remember

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scratch dig analysis of optical elements

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    1. SCRATCH & DIG ANALYSIS OF OPTICAL ELEMENTS Michael J. Broyles OPTI 521 College of Optical Sciences University of Arizona Fall 2006 Revised October 2010 This presentation covers general inspection of scratch and dig analysis. Other defects, specific lens configurations and equipment may cause some variation.This presentation covers general inspection of scratch and dig analysis. Other defects, specific lens configurations and equipment may cause some variation.

    2. Optical components vary in purpose from simple to very complex. Often a tiny lens might be all the difference for a large elaborate optical system to work or not. Engineering drawing will define the degree of quality that must be maintained. An element may look terrible and still be acceptable to use. Optical components vary in purpose from simple to very complex. Often a tiny lens might be all the difference for a large elaborate optical system to work or not. Engineering drawing will define the degree of quality that must be maintained. An element may look terrible and still be acceptable to use.

    3. Objectives Step-by-step procedure for inspecting and evaluation surface quality of an optic.

    4. Performance Specification Optical Components for Fire Control Instruments: General Specification Governing the Manufacture, Assembly, and Inspection of

    5. Other References MIL-F-48616, Filter (Coatings), Infrared Interference: General Specification for AII EKV Optical Inspection: Optical Component Scratch, Dig and Debris Inspection Criteria

    6. Definitions SCRATCH – any marking or tearing of the polished or coated surface.

    7. Definitions, cont… DIG – a small rough spot or pit on the polished or coated surface.

    8. Definitions, cont… BUBBLE, OPACITY, INCLUSION – defects within the substrate. These are classified as digs.

    9. Definitions, cont… CLEAR APERTURE– Area of concern of the optical element. Often 90% of element diameter.

    10. Definitions, cont… EDGE CHIPS – Actual chips on the edge of the optical surface. Edge chips which extend into the clear aperture shall be considered as digs.

    11. Definitions, cont… RETICLE – a removable microscope eyepiece that has precision markings. These markings are used to determine the sizes of defects when they are viewed through the microscope.

    12. General Inspection Requirements Handle optic with care-avoid damage. Wear gloves. Ensure work surface is cleared of non-essential items, clean and free of debris, wiped down if needed. No streaks, smears, stains, blotches, discoloration allowed on optic which will be in a focal plane.

    13. Equipment Reticle Width lines are scratch standards. Circles are dig diameter standards.

    14. Reticle Markings Letter codes on reticle

    15. Scratch Standards Instead of a reticle, a set of scratch standards can be used. The scratch on an element is visually compared to the standard. These are similar to surface finish standards.Instead of a reticle, a set of scratch standards can be used. The scratch on an element is visually compared to the standard. These are similar to surface finish standards.

    16. Procedure Inspect IAW MIL-PRF-13830. Paragraphs 4.2.2.1 or 4.2.2.2. Calibrate the reticle. Map the Optic. Compare the scratches. Compare the digs. Evaluate scratches and digs. Accept or reject the optic.

    17. Inspect IAW MIL-STD 13830

    18. Reticle Calibration

    19. Map the Optic With unaided eye and then microscope. Map the general location of flaws on the optical map. Exact scale is not important. Simple sketch.

    20. Scratch Comparison Locate the scratch to be sized through the microscope with the reticle. Move scratch under the width standard that just covers the scratch. Compare scratch length using length scales on reticle. Record width and length information on the map. Document as width / length.

    21. Dig Comparison

    22. Map the Artifacts Note the diameter is 1.5” for the example.Note the diameter is 1.5” for the example.

    23. Optic Evaluation Typical drawing call-out is “80-50”. “80” is maximum allowed scratch width or max scratch number. “50” is maximum allowed dig diameter or max dig number. Drawing also defines clear aperture or diameter.

    24. Scratch Evaluation Diameter, D Max scratch number, NM Number of scratches, i Length of each scratch, Li Scratch number of each scratch, Ni Combined length of all max width size scratches cannot exceed Ľ the optic dia The sum of the products of the scratch numbers time the ratio of their length to the diameter shall not exceed ONE HALF the max scratch number. Re-write the equation with the product of the scratch number and scratch length on the left side of the equation for easier evaluation using tables.Combined length of all max width size scratches cannot exceed Ľ the optic dia The sum of the products of the scratch numbers time the ratio of their length to the diameter shall not exceed ONE HALF the max scratch number. Re-write the equation with the product of the scratch number and scratch length on the left side of the equation for easier evaluation using tables.

    25. Scratch Evaluation Diameter, D Max scratch number, NM Number of scratches, i Length of each scratch, Li Scratch number of each scratch, Ni AGAIN Combined length of all max width size scratches cannot exceed Ľ the optic dia The sum of the products of the scratch numbers time the ratio of their length to the diameter shall not exceed the max scratch number. Re-write the equation with the product of the scratch number and scratch length on the left side of the equation for easier evaluation using tables. AGAIN Combined length of all max width size scratches cannot exceed Ľ the optic dia The sum of the products of the scratch numbers time the ratio of their length to the diameter shall not exceed the max scratch number. Re-write the equation with the product of the scratch number and scratch length on the left side of the equation for easier evaluation using tables.

    26. Scratch Points Table

    27. Scratch Evaluation

    28. Dig Evaluation The possible number of max size digs shall be one per any given 0.78 inch diameter on the optical surface. The sum of the diameters of all digs shall not exceed twice the diameter of the max dig size.The possible number of max size digs shall be one per any given 0.78 inch diameter on the optical surface. The sum of the diameters of all digs shall not exceed twice the diameter of the max dig size.

    29. Dig Accumulation Table Accumulation number is used to simplify the addition of the actual dig diameters.Accumulation number is used to simplify the addition of the actual dig diameters.

    30. Dig Evaluation

    31. Accept or Reject Ľ diameter Product of ˝ the scratch number times the diameter Product of the scratch number times the diameter 2 times the max dig diameterĽ diameter Product of ˝ the scratch number times the diameter Product of the scratch number times the diameter 2 times the max dig diameter

    32. Conclusion Tables are optional in their use and format.

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