*3.2.2. First prototype system*

**Figure 6.**

(a)

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(b)

**Figure 5.** (a) – Active system block diagram with arrays (b) – Active system schematic block diagram

For the feasibility study of the active direct imaging concept, a 2-D version of the 3-D DUVD sonoptics was chosen. However, it is important to note that as for the 3-D sonoptics, the same acoustooptical relationships could be maintained by using cylindrical lenses to give an image field similar to a B-scan. Because of the flexibility introduced by the sampling and retransmit‐ ting acoustic arrays, the operation of the lenses could be emulated by a number of different ways; e.g. by electronic focusing to represent both lenses, using solid cylindrical lenses, or by a combination of both. It should be emphasized that once the lens characteristics required are implemented, there is no further special requirement for dynamic focusing as the entire image field will be in focus at one instant of time requiring only one excitation pulse to produce the image of the whole object field.

After investigating different possibilities, the following configuration shown in Figure 7 below was used for the first prototype.

**Figure 7.** Active system configuration with conversion of DUVD sonoptics to maintain image linearity, Isochronicity and resolution.

Figure 8 below shows the construction of the retransmitting section of the system with retransmitting array, cylindrical lens and image medium. The design parameters were chosen to image a section of steel of depth up to 40 cm, which is well in excess of typical applications.

Figure 9 show the trolley-mounted 1st prototype system designed in accordance with the simplified block diagram shown in Figure 5. The optical assembly is equipped with an ultrashort stroboscopic light source [16] mounted on the right and a video camera for capturing

*3.2.3. System operation*

*3.2.4. Results from the first prototype*

as expected from the system design.

**Figure 10.** Images of 2mm side drilled holes.

relationship correctly maintained.

*Image linearity and Isochronicity*

best focus.

When testing, the receiving probe array is coupled to the test object medium with a liquid couplant such as oil or jelly. The test object is insonified with a short pulse of ultrasound and the delay of the stroboscopic light source is set appropriately such that the acoustic images are optically frozen in time within the optical imaging medium (schlieren cell) at the instant of

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The resulting optical image of the whole object field can be seen either by naked eye or can be captured with a video camera. Since the time required to form a complete image field is now only limited by the time of flight of the acoustic signals the frame rate could reach the theo‐

Although the system was designed with 30 channels, only 15 were used because of the difficulty of making elements of the arrays with sufficiently close characteristics as required for this imaging topology. However, as can be seen from the images, they are exceptionally of high quality in terms of image clarity and resolution. Figure 10 shows a test block with side drilled holes and the resulting image, clearly demonstrating image linearity and isochronicity

Notice the axial magnification as depicted in equation 10 in appendix. The targets central to the array obviously have stronger return echoes, so they are somewhat saturated due to relatively low dynamic range of the channel amplifiers used. Figure 11 below show a test object, again with side drilled holes and the respective image with lateral and axial magnifications equalised. This image demonstrates exceptional image quality with object-to-image spatial

retically possible maximum limits (e.g. in excess of 1000 frames per second).

**Figure 8.** Sonoptical assembly including the imaging cell

images on the left along the optical axis. For experimentation, only 15 of the 30 available channels were used in this prototype.

**Figure 9.** The active acoustooptical imaging system (1st prototype)
