A sample test on the tilt angle of object light illumination in the digital holographic 3D surface shape detection

Abstract

This paper discussed the measurement error of digital holographic 3D surface shape caused by imprecise measurement of the incident angle and the tilt angle of object light illumination in theory. The result showed that the error caused by the tilt angle is much greater than the incident angle. Then a method to accurately calculate the object light's tilt angle by detecting the interference field of reference plane is proposed based on the above discussion. Finally, the feasibility of the method is proved through a 3D surface shape detection experiment.Keywords: Noncontact optic detection, Digital holography, 3D surface shape detection 1． INTRODUCTION3D surface shape (surface contouring) measurement of object is very important. It has widespread application prospect in CAD, numerically controlled processing technology, product quality examination, even in the fields of medicine, arts and so on. There are various technologies for 3D surface shape measurement [1][2][3] . With the rapid development of digital holography in recent years, as well as its advantages, such as noncontacting, high precision and high resolution, and also with the use of a variety of high performance devices, digital holography technology has been considered as the most practical method in optic interference measurement. In the 3D surface shape detection studies with the help of digital holography, the optic wave field of object plane with a long wavelength can be formed equivalently through an illumination by two object lights with small difference in wavelength or by changing object light illumination direction.Hence, surface shape detection of scatter object can be realized. When using the measurement technology of changing object light illumination direction, the key to realize accurate measurement is the precise control on tilt angle and the accurate measurement on actual tilt angle. Usually the rotation of object light illumination is completed by a mechanical rotation mirror. Although the mechanical rotation system can be controlled by a stepping motor, the mechanical rotation error is difficult to eliminate. A comparison between two errors caused by the incident angle and the tilt angle of object light illumination has been made based on the basic principle of 3D surface shape measurement. The result showed that the impact of tilt angle measurement error is much greater than the incident angle on the measurement results. To achieve more accurate measurement, a reference plane is placed near by the object. For calculating the tilt angle of object light through the fringe width on reference plane we take the difference between the reconstructed phases of object light field, which are recorded before and after a small tilt angle of object illuminating beam. And then a better result can be obtained.