Strange pattern in Angulon 90 6.8 front element - balsam separation?

I just received a Schneider Angulon 90 6.8, with a rather strong pattern in it's front element (like a number of c-shaped rainbow arches - see images.) My guess is it's balsam separation? Any thoughts would be much appreciated, especially if you think I'm wrong and it's something different!

Yes separation... a tough lens to glue given the diferential in lens elemsnts radius

Cheers!!!

Rui Lourosa

Lens separation. It's not difficult to repair with access to proper tools.

True, but these are cheap enough it's probably not worth the hassle, just get another one.

Angulons seem particularly prone to separation, if Ebay is any indication. Bid/buy carefully if you can't inspect the lens in person.

That means there's a thermal expansion coefficient mismatch between the two elements of the doublet used in the design. What happens is that two CTE-mismatched elements expand and contract over temperature. This induces stress in the bond joint. If the CTE mismatch is slight (my design rule of thumb is 2 ppm/C or less), then the bond can accommodate the stresses without failure. If the mismatch is higher with subsequently higher stresses, then the bond will fail as seen here after years of experiencing normal temperature variations.

It can also happen because of poor surface prep, but if the issue is systemic with this particular model then that points to a design issue.

It rarely happens systemically now because of the wide selection of glasstypes available today, but in the past choices of glass with all the desirable properties were more limited.

My 90mm Angulon did this as well, after I bought it. My guess is it came to the hot, humid south and that caused it.

It flares really badly when oblique light falls on the front element. Otherwise there's no issue.

These are partial Newton's Rings, which shows there is a tiny air gap in the effected area. The physical optical cause is interference between reflected light wavelengths. Nice indication of the different wavelengths of visual light. You can actually work out the width of the air gap, using the wavelength of light.

This is almost always a problem with hardening synthetic glues - balsam retains a lot of flexibility, allowing the surfaces to slide back and forth.

A slight segue (with apologies)....but a small number of the lenses I've owned over the years (typically "2nd to 3rd Tier" lenses) have exhibited newton's rings only under fluorescent light - nothing under tungsten or natural light. Any idea why this might be?

Actually, synthetic adhesives are superior to Canada balsam in almost every way... particularly because tailorability of the chemical formula has provided for a very wide selection of adhesives. If something you need isn't available, companies like Dymax will custom make an optical adhesive.

If Canada balsam was superior, it would still be widely used in the optical industry today. It isn't.

Use it when repairing doublets that were originally bonded with Canada balsam, if you're DIY'ing and have no UV curing capability.