TOFD (Time of Flight Diffraction) has been a popular ultrasonic testing method for many years. It has associated with it, limitations associated with the pulse duration. The pulse-duration (ring-time) of the transmitted pulse is relatively easy to understand as the source of the lateral wave dead zone. The lateral wave is considered the earliest arriving signal in the TOFD image and extends to a depth equivalent to the trigonometric pulse-duration in the Pythagorean equation. The rationale for calculating the backwall dead zone is less easy to comprehend as it has us subtract the pulse duration from the back wall depth. CIVA and photoelastic images are used in an effort to understand why the pulse is considered to occur prior to the backwall to cause the dead zone. CIVA is also used to help understand how the equations found in the standards are only empirical approximations.