Prism coupling systems and methods employing light-blocking members

A method of determining at least one stress characteristic of a chemically strengthened substrate having a surface and a near-surface waveguide, comprising:

• optically coupling a measurement light into and out of the near-surface waveguide using a coupling prism, wherein the measurement light incident on the coupling prism includes a light beam generated by a light source system;
• digitally capturing a mode spectrum image of the chemically strengthened substrate using the measurement light coupled out of the near-surface waveguide through the coupling prism that travels over an optical path having a low-angle region and a high-angle region and is incident on a digital detector;
• and
• blocking at least a portion of light traveling in the low-angle region without substantially blocking the light traveling in the high-angle region,
• wherein the digitally captured mode spectrum has a first contrast in the absence of said blocking and has a second contrast due to said blocking, and wherein the second contrast is at least 10% greater than the first contrast, and
• wherein the blocking is performed using a light-blocking member, and an elevation light-blocking angle corresponding to a height of the light-blocking member is from greater than 17.4° to less than 25.3°, and the elevation light-blocking angle is measured from a tangent line tangent to the surface of the chemically strengthened substrate at a location where the optical path impinges the surface of the chemically strengthened substrate to an outer periphery of the light-blocking member further from the tangent line, and the light-blocking member extends from a location on the tangent line for the height to the outer periphery.