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 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