디지털 홀로그래피를 이용한 원공시험편의 표면조도 측정

Abstract

Holography was invented in 1948 as a method for recording and reconstructing amplitude and phase of a wave field by Dennis Gabor. Hologram image is recorded interference pattern between a wave field scattered from the object and a coherent background named reference wave. It is contains the information about the entire three-dimensional wave field and the object wave can be reconstructed by illuminating the hologram with the reference wave again. An observer recognizes a three-dimensional image with all effects of perspective and depth of focus. Holography was developed to Digital Holography with development of Computer technology and CCD’s (Charged Coupled Devices). This method enables full digital recording and processing of holograms, without any photographic recording as intermediate step. Hologram that was recorded by digital holography is reconstructed with scalar diffraction theory. There are three main algorithms most widely used for reconstruction of digital holograms (Fresnel transform method, Angular spectrum method, Convolution Method). Each method has different properties with respect to available reconstruction distance, resolution of reconstructed image and computational load. It is very important for reliable reconstruction to choose the appropriate method in a given situation. In this thesis, measurement zone is very small and it need a method can correctly reconstruct in near wavefields so, angular spectrum method is choose for reconstruction algorithm. Recently, Holography is used widely in several areas. For examples, measure of height and movement in a cell, X-ray application, measurement for the determination of material quantities, evaluation of the refractive index, distance shape control. However, the application in the mechanical industrial field is deficient. Surface roughness means the degree of the minute unevenness happened when processing the product. Generally it is the numerical value which shows the precision of the structure surface. Surface roughness have repercussions on abrasion, durability, precision of product and lifetime, friction of machine parts and it is important to know the value of surface roughness for safety and hygiene. There are two methods for Surface roughness measurement. Firstly comparative measurement method is compares the standard sample and the objective surface to the naked eye. And the other is method to use the surface roughness tester. Comparative measurement method’s precision is low because it depends upon individual eyesight and a sense of touch. And the method using surface roughness tester has a demerit that is contact the surface of material. Therefore, in this thesis, Surface roughness is measured by Digital holography that has merits of non-destructive and non-intrusive in order to increase of digital holography’s application in industrial field. For this, surface roughness of circle holed specimen that was stretched in fixed length is measured using digital holography and calculated a surface roughness value and compared. As a result of conduct the experiment, surface roughness can be measured by digital holography interferometry in high resolution. Also the experiment proves that when the surface roughness value closed to the closed circle holed area and the stretched length had increased, the surface roughness had been roughed by external conditions.