Colleagues:

Joy to all, and thank you for your awesome support of our Optomechanical Engineering 2017 Conference in August.  Just wait ’till you see the program.

Now, back to business.  Fractures in glass optics seem ubiquitous. 

So… the optomechanical engineer has a problem.  He’s nominally responsible for protecting the glass elements in the optical systems he designs.  But there’s very little (if any) information available on the structural properties of the glasses that optical designers specify.  And the guidance for the engineer on using the available data is all-over-the-map, from the incomprehensible to the impossible.  No wonder most engineers use simple “rules-of-thumb.”

That’s where I started, “Keep the tensile stresses under 2,000 psi.”  But then the glass broke anyway!  So I started testing the glass objects to 4,000 psi.  I broke a few in testing but those that went into service are still in service, as far as I know.  I didn’t get to make many, they were too big and heavy.  A colleague who specialized in space-based ISR systems confided to me that he kept the stresses under 500 psi!  That’s when another colleague gave me a copy of “Reconnaissance and Surveillance Window Design Handbook” (AFAL-TR-75-200). 

Voila!  Section 7.3.1 is the perfect introduction for the engineer to “Allowable Stresses in Glass.”  It covers fabrication process controls, slow crack growth through stress corrosion (from moisture) and estimating the service life by integrating the stress corrosion equations for eight glasses.  The Wizard’s green curtain is drawn back disclosing all of his secrets and Dorothy dances down the yellow brick road and back to safety in Kansas.

Every optomechanical engineer needs a copy of that Handbook to help him protect the optical glass that has been entrusted to him by the optical designer.  It guides the design, analysis and fabrication of glass optical elements.  Perhaps the engineer should enter the required structural properties, including the fracture toughness and stress corrosion constants, on the lens drawings (think the yellow brick road).  The Handbook’s drawback is that only eight glasses are treated and some of those have since been re-formulated to remove toxic elements.

The glass suppliers also need copies of the Handbook so they’ll know what the engineer is requesting and, maybe someday, put the information in their glass catalogs and data sheets.

Joy!  Spring is just around the corner.  Ahh… Kansas in Springtime!

Al H.
3-10-17