Testing Flatness

download Testing Flatness

of 21

Transcript of Testing Flatness

  • 7/31/2019 Testing Flatness

    1/21

    October 1, 2009 Copyright R. E. Parks 2009 1

    Testing flats or plane surfacesand metallic, reflective coatings

  • 7/31/2019 Testing Flatness

    2/21

    October 1, 2009 Copyright R. E. Parks 2009 2

    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 sleeksSurface texture and mid-spatial frequency roughness and

    orange peel

    Learn about cleaning

    When is it necessary and when it should be avoided

    Introduction to coatingMetal reflective coatings and overcoats

    Fixture marks and coating defects

  • 7/31/2019 Testing Flatness

    3/21

    October 1, 2009 Copyright R. E. Parks 2009 3

    Where is absolute flatnessimportant?

    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 doesnt matter;

    System can be compensated by re-focusingIf 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 testAnother method is the three flat test

  • 7/31/2019 Testing Flatness

    4/21

    October 1, 2009 Copyright R. E. Parks 2009 4

    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

  • 7/31/2019 Testing Flatness

    5/21

    October 1, 2009 Copyright R. E. Parks 2009 5

    Three flat test - example

    If all 3 have same powerthen 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

  • 7/31/2019 Testing Flatness

    6/21

    Idea of absolute testing

    Idea used to be unique to optics but now in allprecision work

    Machinists rule; test device 10x better thantolerance

    In optics, test optics no better than optic undertest

    How are test optics errors removed from opticunder test

    Basically, two tests are done, one with opticmoved Then test maps compared to see what errors moved

    with optic

    For complete separation, three tests needed One movement along each of two axes

  • 7/31/2019 Testing Flatness

    7/21

    Schematic of test optics errorremoval

    September 17, 2009 Copyright R. E. Parks 2009 7

  • 7/31/2019 Testing Flatness

    8/21

    Testing system fold flat for figure

    Need collimated beam large enough to cover clearaperture Or test flat the size of clear aperture

    Flat held in interferometer test fixture with threefingers 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

  • 7/31/2019 Testing Flatness

    9/21

    1st step setting up interferometer

    September 17, 2009 Copyright R. E. Parks 2009 9

    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

  • 7/31/2019 Testing Flatness

    10/21

    2nd step, aligning optic under test

    September 17, 2009 Copyright R. E. Parks 2009 10

    Pair of dots are reflection from both sides of plane window with wedge

    Align by placing one or the other dot behind the crosshairsDimmer dot tends to be from rear surface

    Misaligned, left, and rear surface dot aligned, right

  • 7/31/2019 Testing Flatness

    11/21

    3rd step, check focus

    September 17, 2009 Copyright R. E. Parks 2009 11

    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

  • 7/31/2019 Testing Flatness

    12/21

    4th step, measure

    September 17, 2009 Copyright R. E. Parks 2009 12

    View on monitor screenData will depend on settings

    entered on control screen

    Image has 75764 good

    data points

    Vertical wedge fringes more

    easily seen hereBlank areas around edge

    due to hot glue blocking

    view

  • 7/31/2019 Testing Flatness

    13/21

    Control screen with profile

    September 17, 2009 Copyright R. E. Parks 2009 13

  • 7/31/2019 Testing Flatness

    14/21

    Control screen with Zernikepolynomials

    September 17, 2009 Copyright R. E. Parks 2009 14

  • 7/31/2019 Testing Flatness

    15/21

    Problems with testing flats

    September 17, 2009 Copyright R. E. Parks 2009 15

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

  • 7/31/2019 Testing Flatness

    16/21

    Interference between surfaces

    September 17, 2009 Copyright R. E. Parks 2009 16

    Very parallel (1-2 fringes) but non-flat window

    Interference between surfaces affects phase unwrappingProblem obvious in fringe pictures, maybe a coating problem also

  • 7/31/2019 Testing Flatness

    17/21

    Interference between surfaces only

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

    Wedge fringes only gottenby removing transmission

    flat and using

    interferometer as an

    illumination and imaging

    system only

    Cannot phase shift sinceno 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

  • 7/31/2019 Testing Flatness

    18/21

    Details of figure testing

    Test on vibration isolated table; two fringe printthrough Couple part under test to interferometer

    Keep fold flat close to reference flat; air turbulence

    Use reference flat with same reflectivity as testpart; contrast

    For phase shifting interferometer, break out onefringe

    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 useZernikes Just refer to p-v and rms results

  • 7/31/2019 Testing Flatness

    19/21

    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 reallyvisible then

    Use bright light like microscope illuminator, LEDflashlight 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 greylook due to scattered light

    Scratches are long defects where glass surface has

    fractured

  • 7/31/2019 Testing Flatness

    20/21

    Inspection for texture and orangepeel

    September 17, 2009 Copyright R. E. Parks 2009 20

    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

  • 7/31/2019 Testing Flatness

    21/21

    Cleaning

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

    In general, dont

    For inspection of surface quality it will be necessaryIf 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 toavoid contamination from previous wipe

    Keep tissue handy because sometimes an apparent defect is dirt

    Dont want to reject surface by mistaking dirt for a defect