Enhanced evanescent prism coupling systems and methods for characterizing stress in chemically strengthened curved parts

A evanescent prism coupling system configured to determine at least one of a refractive index profile and at least one stress-related characteristic in a chemically strengthened (CS) substrate formed by an ion-exchange (IOX) process and having a curved surface and a near-surface index region (NSIR), comprising:

• a light source system that forms a focused light beam that travels along a first axis;
• a coupling prism interfaced with the curved surface of the CS substrate to form a coupling interface that receives the focused light beam and forms a reflected light beam that travels along a second axis and that embodies a mode spectrum of the NSIR;
• a detector system arranged along the second axis and comprising in order from the coupling prism:
• i) a detector optical system;
• ii) a polarizer having transverse magnetic (TM) and transverse electric (TE) sections;
• iii) a digital detector;
• and
• wherein the detector optical system is located an axial distance l from the coupling interface and an axial distance L from the digital detector to define a length ratio 0.01 ≤ l/L ≤ 0.2 to form a mode spectrum image at the digital detector.

A method of determining at least one of a refractive index profile and at least one stress-related characteristic of a curved chemically strengthened (CS) substrate using an evanescent prism coupling system having an adjustable focus, comprising:

• a) calibrating the evanescent prism coupling system by adjusting the adjustable focus to a first focus that forms an in-focus calibration mode spectrum image of a reference CS substrate to establish a calibrated system for measuring the reference CS substrate;
• b) using the calibrated system, forming an in-focus mode spectrum image for the curved CS substrate by adjusting the adjustable focus to a second focus due to a focus shift caused by using the curved CS substrate in the calibrated system for measuring the flat CS substrate;
• c) determining a raw value of at least one stress-related characteristic of the curved CS substrate based on the in-focus mode spectrum image for the curved CS substrate using a corresponding at least one calculation for the at least one stress-related characteristic for the flat CS substrate;
• and
• d) adjusting the raw value of the at least one stress-related characteristic to obtain a corrected value by multiplying the raw value by a correction factor based on the change in the adjustable focus from the first focus for the flat CS substrate to the second focus for the curved CS substrate.