# Development of Simultaneous Dual-Resolution Digital Holography System
## 1 Introduction  
The digital holography interferometry technique is a non-contact, full-field, and highly sensitive optical measurement method. It is widely used in a variety of industrial areas, including the automotive and aerospace industries, for deformation/strain measurements and non-destructive testing (NDT) [1; 2; 3; 4; 5; 6]. For this technique, one laser beam is split into two beams. One, called the object beam, illuminates the object's surface, while the other, called a reference beam, goes to a reference object, such as a mirror. These two beams meet at the CCD and interfere with each other on the CCD plane to produce an interferogram. The intensities of the interferograms can be recorded by a digital camera before and after loading. A visible fringe pattern, also called a hologram, that includes the object deformation information can be obtained to subtract the different intensity information and obtain deformation due to the applied load [7; 8; 9; 10]. Currently, most optical inspection systems use a single camera with a fixed field of view to record images, resulting in only one resolution being obtained per measurement. However, some small defects may also be found without increasing the spatial resolution by improving the stability and the contrast of phase maps and avoiding air disturbances and environmental vibrations by a real-time phase processing method with high-frequency synchronizing trigger technology [11]. A high-spatial-resolution setup in combination with this technology would be even better for measuring smaller defects. Usually, one-resolution systems suffer from some disadvantages: (1) when measuring the deformation of an object with a strain concentration in some locations, the fringe pattern may be too dense to be resolved by the measurement system, which can cause the measurement to fail; (2) when conducting non-destructive testing, small-sized inner defects may be ignored due to a low spatial resolution. Measurements using different spatial resolutions would be necessary to solve these issues. Usually, separate multiple tests are used to observe different-sized fields of view. However, this will not only increase the labor costs and testing time, but also complicate the measurement operation. Furthermore, multiple measurements may not be achievable in some cases. Therefore, if a measurement is not repeatable, a simultaneous dual-resolution measurement system with different fields of view is helpful. To reach this goal, two optical channels with large and small fields of view are required to achieve a simultaneous dual spatial resolution measurement using one experimental setup. Research on simultaneous measurement with two-channel systems has been reported, but almost all of the existing methods use the same spatial resolution, i.e., the same field of view, to measure two targets. Examples of these are listed as follows: (1) Two-channel systems with the same spatial resolution which measure the first derivative in the x- and y-directions. In these systems, the two channels correspond to two shearographic systems, one in the x-shearing direction to measure the first derivative of deformation in the x-direction and one in the y-shearing direction to measure the first derivative of deformation in the y-direction. Three methods have been reported to separate the information from these two channels in the final results. The first one is to use a two-camera system to record the information of the two channels separately [12]. The second method is to observe different channels on one color camera using RGB channels in combination with different laser wavelengths [13]. The last method is to use one black/white camera based on the polarization technology to avoid cross-interference [14; 15]. These types of two-channel digital shearographic systems can be useful for NDT because some defects are sensitive to different directions of the first derivative of deformation.