Occasionally, a patient will have a cataract that is too dense to achieve a reliable axial length measurement via the Zeiss IOL Master™, or a patient may present with a difference in the eye axial length between the two eyes. When either of these situations arises, an independent test called an Immersion Ultrasound A-scan is performed. The Immersion Ultrasound A-scan involves the patient being seated in a semi-reclined position, and an ultrasound probe with a water bath is placed on the corneal surface. The ultrasound unit measures the time it takes for the ultrasound wave to travel from the probe tip to the retina and back. A one-dimensional scan is produced measuring eye axial length.