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Thread: The 'Pits'

  1. #1
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    Jun 2008
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    The 'Pits'


    Could someone help us out please? Everybody here talks about 'coating pits' as though they are fully familiar with what they are, but when interrogated, I don't think anyone is really sure at all. Googling the topic is not helpful.
    The objects which we refer to as 'pits' has, typically, a small, dark, round centre, and is surrounded by a more diffuse area. The view through a microscope looks like the view of a lunar volcano seen from a great height.

    An old document on coating blemishes, from Colts, states that a pit is caused by a coating build up over and around a small particle.

    However, other sources explain 'pits' as coating overflows around cavities in the lens surfaces. The cavities are, presumably, due to lens surfacing operations. In this case, the term 'pit' would be an accurate description.

    There is a third, less helpful definition, where any hardcoat speck whatsoever, is categorised as a 'pit'.

    Any explanations, or opinions, which would help us,would be gladly received.

    Best Regards,


  2. #2
    Ghost in the OptiMachine OptiBoard Silver Supporter Quince's Avatar
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    Jan 2016
    Sebago ME
    Optical Laboratory Technician
    In my experience, a pit is a flaw in the lens that is inverted. Usually caused by power issues during generation.

    What I find more common, and maybe people are lumping these together, are coating defects that would be caused by particles being fused to the lens during coating. This could be dust or any other debris that might make its way onto the lens before the coating has cured and is now sandwiched in.
    Have I told you today how much I hate poly?

  3. #3
    Master OptiBoarder OptiBoard Silver Supporter lensmanmd's Avatar
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    Oct 2016
    Optical Wholesale Lab (other positions)
    Pits can also be from generating flaws, normally caused by dull diamonds. Most of the time, these can be removed via fining and will occur on CR39 and hi-index. If not caught after fining, they can mimic coating flaws.

  4. #4
    Master OptiBoarder
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    Mar 2010
    north of 49
    Dispensing Optician
    You mentioned a "volcanic" like appearance, this might also be a gas bubble created during coating?
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