In ultrasonic calibration, which quantity is defined by establishing the index point?

• A. Index point
• B. Verification of wedge angle
• C. Sensitivity calibration
• D. Resolution

Answer: (A)

Setting up the index point establishes the zero reference on the time (and thus distance) axis of the ultrasonic display. This reference anchors how travel time translates into material depth, using the known sound velocity. By aligning the front-surface or a known reflector to a fixed position, you define the point from which all subsequent echoes are measured. With this origin in place, the instrument can accurately convert two-way travel times into distances, which is the essence of the calibration step that the index point provides.

Other calibration steps serve different purposes: verifying wedge angle checks the geometry of the probe path, sensitivity calibration adjusts the signal level to fit the display range, and resolution concerns the ability to distinguish closely spaced features—none of these establish the time-zero origin used for depth measurement.