# Environmntal Standards by hilen

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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”
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

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