LASER MEASUREMENT TECHNIQUES AND DATA ANALYSIS

The optical configurations for interferometry, schlieren and shadowgraph as well as data retrieval from optical images are discussed in the following sections.

OPTICAL ARRANGEMENT

For measurements, a continuous wave helium-neon laser has been used as the coherent light source. A monochrome CCD camera (Sony, Model: XC-ST50) of spatial resolution of 768 X 574 pixels was used to record the convective field. The camera was interfaced with a personal computer (HCL, 256 MB RAM, 866 MHz) through an 8-bit A/D card. Interferograms, schlieren and shadowgraph images presented in this work have been subjected to image processing operations to improve contrast; the data analysis for recovering the field concentration and temperature are, however, based on the intensity changes in the original images.The layout for each of the three imaging techniques used in the present work is shown in Figure 5.15.

Figure 5.15: Optical layout of (a) Mach-Zender interferometer, (b) Schlieren and (c) Shadowgraph

The M-Z interferometer has two mirrors and two 50% beam splitters of 150 mm diameter. The mirrors are coated with 99.9% silver and employ a silicon dioxide layer as a protective layer. The interferometer floats on pneumatic legs to isolate the optics from external vibrations. Experiments have been carried out in the infinite as well as the wedge fringe setting. In the infinite fringe setting, the optical path difference between the test and the reference beams is initially zero. When a density disturbance is introduced in the path of the test beam, it is seen as a set of fringes over which the depth-averaged concentration or temprerature field is a constant. In the wedge fringe setting, the optics is slightly misaligned to produce a set of straight fringes. When exposed to a thermal or concentration field, the fringes are displaced to an extent depending on the change in temperature or concentration. The fringes in the wedge fringe setting of the interferometer are thus representative of the temperature variation in the rectangular cavity and concentration profile in the crystal growth chamber. In the present work, the infinite fringe setting initially produced an image of uniform brightness indicating true constructive interference between the test and the reference beams. In the wedge fringe setting, the fringes were seen to be straight and horizontal.