# Development of Simultaneous Dual-Resolution Digital Holography System
## 3 Experiments and Results  
Figure 4 shows a simultaneous dual-spatial-resolution digital holography measurement system. This system utilizes a Verdi G5 laser as well as a 2453 \(\times\) 2056-pixel grayscale CCD camera. The pixel sizes of the CCD camera in the horizontal direction and the vertical directions are both 3.45 \(\upmu\)m. L1, L2, L3, and L4 are the lenses. The focal lengths of the imaging lens, L1, is 35 mm (f1 = 35 mm). The focal lengths of L2 and L3, placed on the two sides of the beam splitter, are 100 mm (f2 = f3 = 100 mm), and the focal length of L4 is 150 mm (f4 = 150 mm). The normal beam splitter and polarized beam splitter both have split ratios of 50:50. The single-mode optical coupler splitter is 50:50.  
Figure 3: Sketch of the Fourier spectrum.  
The measurement procedure is as follows: (1) to take an image before the loading, (2) to load the object, and (3) to take an image again after the loading, then, to conduct an evaluation of the measured result by Fourier and inverse Fourier transforms. The time for the measurements is determined by steps 1 to 3. Taking two images requires just dozens of milliseconds and the loading can be conducted within dozens of seconds, therefore, the whole measurement can be performed within one minute.  
A 100 \(\times\) 100 mm square aluminum plate was tested to verify the feasibility of the method. The plate was fixed along its four edges. A screw micrometer was used to apply a load to the center of the plate, and images before and after loading were captured for evaluation. A spectrum distribution was obtained, as shown in Figure 5a, by applying a FT. Then two holograms of two resolutions, as shown in Figure 5b,c, were obtained by using an IFT to obtain the phase information and subtracting the phase value before and after loading.  
Figure 4: Experiment setup.  
Figure 5: Schematic of spectrum of speckle pattern image: (**a**) is the spectrum, (**b**) is the hologram of large field of view, and (**c**) is the hologram of small field of view.  
To verify the functionality of the digital holography system presented in this paper, an NDT application was performed on a round aluminum plate with a small-sized defect inside. The round plate had a thickness of 10 mm and was fixed on a 150 mm diameter sealed chamber. The sealed chamber was connected to a vacuum to load the round plate. The defect was a small circle with an 8 mm diameter and 0.6 mm of thickness to the surface. A 10 KPa pressure difference was applied to detect the defect, the result of which is shown in Figure 6. From the image, we can find that the whole area can be observed using the large field of view, but, as shown in Figure 6a, the small-sized defect may be ignored. However, the small-sized defects can be found under the small field of view, as shown as Figure 6b.  
Another deformation test was completed using this system and a defect-free square aluminum plate. The plate was fixed all around with screws, and the plate size was 100 \(\times\) 100 mm. Figure 7 shows the phase map for the large field of view and small field of view, respectively. A load was applied at the center of the plate by a screw micrometer. Under the large field of view, the phase map, shown in Figure 7a, was very unclear because the large magnitude of the load caused the fringes to be too dense; as a result, it cannot be used to calculate the deformation. However, under the small field of view, the phase map, shown in Figure 7b, was clearer and could be used to calculate the deformation.