Measure the feature that the question and specification concern. State cap or body, dome or straight wall, axial and circumferential position, and whether the reading represents one wall or combined thickness. A cap-dome result does not describe the entire shell.
Keep the location map, conditioning and support with the result. A dome and sidewall reading concern different features, even when taken on the same shell.
Only comparable features and adequately characterised methods support that expectation. Contact geometry and loading affect mechanical readings; optical section measurements can depend on boundary selection, refractive index and calibration. Assess these influences for the actual shell and method before expecting agreement.
When results disagree, compare the feature, support, contact conditions, optical boundary definition and calibration. Evaluate uncertainty for the complete method, rather than using catalog instrument accuracy alone. Optical suitability also depends on the shell’s actual optical properties.