Unlike all the tube inspection techniques  which operate on magnetic or electromagnetic principles the IRIS, technique is based on ultrasonics.  As shown in the attached illustration a beam from an ultrasonic transducer is reflected from a mirror set at 45 degrees so that the reflected ultrasonic beam impinges on the tube I.D. at right angles. Part of this beam is then reflected from the tube I.D., while the remainder is transmitted through the wall thickness and is reflected from the tube O.D. This technique can detect defects with a diameter down to 1.5 to 2.0mm and defect depth can be measured to an accuracy of less than 0.1mm. The time difference between the two reflected signals is then used to measure the tube wall thickness. The inspection of the tube can be viewed in B, C and D-scan.