Inspection of Optical Components

Oct. 2, 2009

Why inspect?

• Often don’t get what you’ve asked for

• You want to see what quality from vendor

• Easier to catch errors in beginning– If system together you don’t know where to

look for the trouble

• For precision systems will do re-comp based on as-built dimensions

• Assemble with new spacings to optimize

Optics quality only goes downhill

• Optics will be nearly pristine when received• You want to keep them that way• Nothing you do to them will make them better• Use gloves• Set down on soft, clean surfaces• Use shims when making mechanical measurements• Do not re-use tissue when cleaning• Clean edges and bevels first• Note: Coatings are generally harder than glass

Inspection for surface defects

• Use a flat black background• May want to dim room lights• Use a bright point source of light• Let light reflect back toward eye just outside of

pupil and view against black background• Align scratches with tape on source and view

against black background

Quantitative stress measurement

See ISO 11455 Determination of birefringence

Bubbles and inclusions

Bright light makes bubbles and inclusions sparkle against dark background

Carousel projector makes a good light source

Haze inspection (contamination)

Use narrow beam illumination to avoid flooding eye

Good method to see if surface completely polished out

Inhomogeneity

• Must be done interferometrically

• Often done with oiled on plates and index matching oil

• Requires skill to do correctly

• Largely unnecessary due to glass quality

Striae samples

Striae viewing set up

Importance of striae orientation

• Glass used to be made in pots – striae formed in layers• If lens oriented correctly, parallel to face inspected, striae

did not affect OPD• With prisms it was different – light traveled different paths• It was necessary to polish and inspect in two or three

directions – obviously more expensive• Glass now made in continuous slabs and is much freer of

striae, inclusions and inhomogeneity

Figure via test plate

• Test plates are delicate, can be ruined by dirt• Clean each half with solvent – isopropyl• Place dry tissue between halves and gently pull

out while watching for fringes• If there are lots of fringes there is still dirt in gap• Lift apart gently and clean again and repeat with

tissue• Estimate power and irregularity for both pair

Centering

• Use the fixture used in the centering lab

• If using for real set balls up outside CA

• Also indicate outside clear aperture

• Calculate tilt in lens based on dimensions

Surface imperfections

• View using bright light looking against dark background• Scratches most visible aligned with dark/bright line• Find biggest dig or pit, and scratch• Classify scratch/dig by visibility against paddle standard• Try to estimate total area obscured by defects per ISO• Visibility versus metric standard?

Mechanical dimensions

• Diameter – use care not to chip edges– Bigger problem with small or no bevel

• Estimate bevel face widths• Center thickness – always use shim to avoid damage

– Measuring in most cosmetically obvious part of lens

• Measure sag to flat – CT minus height to CX side– Again, use shim under indicator tip

Data for re-comp

• Radii• Center thickness• Sag to flat• Melt data for index – you have to ask for it when

glass ordered• These are extra measurements for supplier

– Not normally done– You pay for them in the glass price

• Recomp to find optimum lens spacings