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Environmntal Standards

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					             Testing flats or plane surfaces
            and metallic, reflective coatings




October 1, 2009       Copyright R. E. Parks 2009   1
                          Objectives
  Learn methods of testing flats for figure and absolute flatness
          Where is absolute flatness required and where not
          Concept of “absolute” testing
  Learn about inspecting for surface finish quality
          Grey, pits and full polish
          Scratches, digs and sleeks
          Surface texture and mid-spatial frequency roughness and
          “orange peel”
  Learn about cleaning
          When is it necessary and when it should be avoided
  Introduction to coating
          Metal reflective coatings and overcoats
          Fixture marks and coating defects


October 1, 2009          Copyright R. E. Parks 2009                 2
     Where is absolute flatness important?
  A flat is a spherical surface of infinite radius
  A test for flatness is measuring the difference between two “flats”
             Does a zero difference means the two are flat?
             And does it matter?
  If a plane mirror is used at normal incidence it usually doesn’t matter;
             System can be compensated by re-focusing
  If a plane mirror is used at non-normal incidence, it will matter:
             This introduces astigmatism since footprint of beam is elliptical
             Longer one way than other, more sag one way

  Also suggests how truly flat mirrors should be tested;
          Test at non-normal incidence, Ritchey-Common test
          Another method is the three flat test




October 1, 2009               Copyright R. E. Parks 2009                         3
          Ritchey-Common test for flatness




     Plane mirror under test is used to rotate center of curvature of concave
     spherical mirror
     Footprint of beam on plane mirror is roughly elliptical so sag in plane of
     page is greater than perpendicular to page if flat has power
     Orientation of astigmatism will indicate cc or cx power

October 1, 2009               Copyright R. E. Parks 2009                          4
                  Three flat test - example
                                                     If all 3 have same power
                                                     then the difference
                                                     (match) is the same.
                                                     Therefore if all 3 are
                                                     truly flat the matches
                                                     will be zero in all cases
                                                     If there is a difference in
                                                     power it becomes
                                                     obvious the matches will
                                                     be different




October 1, 2009         Copyright R. E. Parks 2009                                 5
                Idea of absolute testing
• Idea used to be unique to optics but now in all precision work
• Machinist’s rule; test device 10x better than tolerance
• In optics, test optics no better than optic under test
   – How are test optics errors removed from optic under test
• Basically, two tests are done, one with optic moved
   – Then test maps compared to see what errors moved with optic
• For complete separation, three tests needed
   – One movement along each of two axes
• All precision optics measurements based on this principle
   – For λ/4 optics the interferometer reference flat is good enough
   – Could also test against an optical flat
     Schematic of test optics error removal




September 17, 2009   Copyright R. E. Parks 2009   7
        Testing system fold flat for figure
• Need collimated beam large enough to cover clear aperture
   – Or test flat the size of clear aperture
• Flat held in interferometer test fixture with three fingers
   – Use lightest force possible to just hold flat stable
• Align reference flat to interferometer
   – Two dots superimposed
• Align fold flat to reference flat
   – Two dots superimposed behind reference flat dots
   – Switch to measure mode
   – Check that fold flat is in focus, look at edges
• Mask test aperture to clear aperture on fold flat
• Measure
          1st step setting up interferometer
Install transmission flat (TF), reference surface, in bayonet mount
In “Align” mode, align TF return dot behind cross hair
Monitor screen with mis-aligned, then aligned return spot




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          2nd step, aligning optic under test
     Pair of dots are reflection from both sides of plane window with wedge
     Align by placing one or the other dot behind the crosshairs
     Dimmer dot tends to be from rear surface
     Misaligned, left, and rear surface dot aligned, right




September 17, 2009           Copyright R. E. Parks 2009                       10
                     3rd step, check focus
    Once aligned, switch to view mode to see fringes
    Generally, view will need focusing to avoid diffraction at edge
    Barely visible fine, vertical fringes are wedge between the two surfaces




September 17, 2009           Copyright R. E. Parks 2009                        11
                     4th step, measure
                                                    View on monitor screen
                                                    Data will depend on settings
                                                    entered on control screen
                                                    Image has 75764 “good”
                                                    data points
                                                    Vertical wedge fringes more
                                                    easily seen here
                                                    Blank areas around edge
                                                    due to hot glue blocking
                                                    view




September 17, 2009     Copyright R. E. Parks 2009                             12
                 Control screen with profile




September 17, 2009      Copyright R. E. Parks 2009   13
   Control screen with Zernike polynomials




September 17, 2009   Copyright R. E. Parks 2009   14
                     Problems with testing flats




     Noise due to coherent source partially removed with Ring of Fire, lower

September 17, 2009          Copyright R. E. Parks 2009                         15
             Interference between surfaces




Very parallel (1-2 fringes) but non-flat window
Interference between surfaces affects phase unwrapping
Problem obvious in fringe pictures, maybe a coating problem also




September 17, 2009         Copyright R. E. Parks 2009              16
        Interference between surfaces only
                                                  Wedge fringes only gotten
                                                  by removing transmission
                                                  flat and using
                                                  interferometer as an
                                                  illumination and imaging
                                                  system only
                                                  Cannot phase shift since
                                                  no reference surface
                                                  Can shift by tuning laser
                                                  source
                                                  Time delay source is way
                                                  of phase shifting to
                                                  measure wedge,
                                                  effectively also measuring
                                                  transmitted wavefront
                                                  Fringes do not move when
                                                  window tilted
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                Details of figure testing
• Test on vibration isolated table; two fringe print through
   – Couple part under test to interferometer
• Keep fold flat close to reference flat; air turbulence
• Use reference flat with same reflectivity as test part; contrast
• For phase shifting interferometer, break out one fringe
   – Reduce tilt to near zero; tilt will introduce false coma
   – For test plate need 5 or so fringes to judge power and irregularity
• In analysis over elliptical aperture, do not use Zernikes
   – Just refer to p-v and rms results
   – Even Zernike power or focus term will be incorrect; non-orthogonal
   – For specifications, rms number best reduces effect of outliers
             Inspection of surface finish
• In general, test against dark background
   – With fold mirror, more difficult because of ground surface background
   – Mirror will be coated for high reflectivity; defects really visible then
• Use bright light like microscope illuminator, LED flashlight
   – Reflect light back until just outside eye pupil when looking through lupe
   – This type examination best for checking complete polish, orange peel
   – If in complete polish, specular surface will have a “grey” look due to
     scattered light
• Scratches are long defects where glass surface has fractured
   – Digs are round defects caused by bubbles, or residual grinding damage
   – Sleeks are long defects where there is no fracture but damage
     Inspection for texture and orange peel
    A straight line separation of bright and dark is best type of source
    Scratches readily apparent if aligned parallel to light/dark line with
    scratch over dark area. Scratch seen by scattered light against dark
    Similarly, orange peel seen at grazing incidence against dark/light line
              Boundary will show a waviness like shadows of orange peel
    A bare fluorescent line bulb with black paper behind is excellent source




September 17, 2009           Copyright R. E. Parks 2009                        20
                                Cleaning
    In general, don’t
    For inspection of surface quality it will be necessary
    If obviously dirty, blow off, raise with soap and water, blow dry
    If still dirty, use edge of folded tissue wet with IPA, or acetone
    Wear gloves or finger cots, otherwise grease from fingers will contaminate
    Wipe from above center to edge, discard tissue and use another for
                 next wipe
    Idea is to avoid dragging contamination from edge onto surface and to
                 avoid contamination from previous wipe
    Keep tissue handy because sometimes an apparent defect is dirt
    Don’t want to reject surface by mistaking dirt for a defect




September 17, 2009           Copyright R. E. Parks 2009                      21

				
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