# Development of Simultaneous Dual-Resolution Digital Holography System
## 1 Introduction
(2) The second example is a two-channel system with the same spatial resolution that simultaneously measures the deformation and its first derivative. For these systems, one channel is related to a holographic/electronic speckle pattern interferometry (ESPI) system and the other is related to a shearographic system [16; 17; 18]. Such kinds of systems are used if both the deformation and its derivative are needed at the same time. (3) Our last example is two-channel systems with the same spatial resolution that simultaneously measure both out-of-plane and in-plane strains. In these systems, one channel is used for an out-of-plane shearographic system and the other for an in-plane shearographic system [19; 20]. They are usually utilized for 3D strain measurements. Another application of two-channel systems is to test only one measurement target, but with different spatial resolutions. A Sagnac interferometer-based digital shearography system has been reported by [PERSON] et al. [21]. They used a Sagnac interferometer-based optical layout to enable a dual spatial resolution shearography system to measure the first derivative of deformation. It was the first recorded report of a system that allows two shearograms, one for a large field of view and one for a small field of view, to be obtained simultaneously. The development of this technology advanced digital shearography for NDT. However, a deficiency of the developed technology is the phenomenon of spectrum aliasing because the two shearograms have the same shearing direction, e.g., in the x-shearing direction, and all the spectra lie on the x-axis. In shearography, the orientation and separation of the spectrum are determined by the shearing direction and shearing amounts,respectively. Avoiding the spectrum aliasing of the two shearograms requires careful selection and adjustment of the shearing amount of the two channels, which complicates the measurement. [PERSON] reported an improved method to place one spectrum on the \(x\)-axis and the other on the \(y\)-axis to avoid the overlap [22]. However, this system is a two-target measurement, i.e., one channel measures the x-direction derivative and the other channel measures the y-direction derivative. As described above, almost all of the simultaneous two-channel systems are based on digital shearography and for two target measurements because shearography can easily create two measurement targets, e.g., the first derivatives in the x- and y- directions, respectively. While the measurement of the first derivative of deformation has benefits for NDT, measuring deformation is as important as measuring the first derivative in the area of experimental mechanics, sometimes even more important because deformation is the basic behavior of an object after loading. Measuring a large-sized object with a high spatial resolution is always desired in the areas of experimental mechanics and non-destructive testing. In the past few years, a technology to enhance the spatial resolution by continuous-wave terahertz high-resolution imaging via synthetic technology has been achieved [23; 24; 25]. This technology can reach an extremely high spatial resolution with dozens of micrometers and has been proven to improve the reconstructed image quality and resolving power. Although the proposed technique can be employed in numerous domains, its application to experimental mechanics and non-destructive testing for processing directly recorded images in holographic and speckle interferometry with a close to real-time measurement speed for relatively large objects (one hundred to several hundred millimeters in diameter) is limited due to the complicated synthetic and reconstructing technologies. Another approach to reach a high spatial resolution is to use a dual-spatial-resolution shearographic system based on the Sagnac interferometer to measure the derivative of deformation as already described above.