Introduction

Optical methods of measurement are known to have specific advantages in terms of spanning a field-of-view and being inertia-free. Though in use for over half a century, optical methods have seen a resurgence over the past decade. The main factors responsible are the twin developments in the availability of cost-effective lasers along with high performance computers. Laser measurements in thermal sciences have been facilitated additionally by the fact that fluid media are transparent and heat transfer applications in fluids are abundant. Whole-field laser measurements of flow and heat transfer in fluids can be carried out with a variety of configurations: shadowgraph, schlieren, interferometry, speckle and PIV, to name a few. In the present module, temperature field measurements in fluids by laser interferometry has been addressed.

The ability to record interferograms on a PC using CCD cameras has greatly simplified image analysis. It is possible to enhance image quality and perform operations such as edge detection and fringe thinning by manipulating the numbers representing the image. Image analysis techniques have also been discussed in the present module. When combined with holography, laser interferometry can be extended to map three dimensional fields. Holographic interferometry can be cumbersome in some applications due to the need of holographic plates, particularly when large regions have to be scanned. This difficulty is circumvented by using an analytical technique called tomography. Here the interferograms are viewed as projection data of the thermal field. The three dimensional field is then reconstructed by suitable algorithms. In principle, tomography can be applied to a set of projection data recorded by shadowgraph, schlieren, interferometry or any of the other configurations. The present module covers tomography applied to interferograms recorded with a Mach-Zehnder interferometer. These comments carryover to schlieren and shadowgraph methods as well.