Introduction

Spectroscopic techniques are an excellent tool for thermodynamic measurements. Typically they do not require a direct contact with the tested medium and are therefore considered nonintrusive. This is a significant advantage over measurements that require physical probes, particularly when the environment at the point of measurement is hostile owing to such extreme conditions as high temperature, aggressive chemical reaction, or fast flows. In addition, many spectroscopic techniques can be adapted to image the two- or three-dimensional distribution of select properties. With the advent of electronic cameras, images can be digitized and stored almost instantaneously by computers. Exposure times of as short as 5 ns are possible with the use of CCD (charge-couple device) intensified cameras. Recording rates of up to 12 kHz can be achieved with image converter cameras, and storage of hundreds of images is possible on most desktop computers. These device characteristics enable such fast phenomena as explosions or detonation waves to be recorded electronically; depending on the selected spectroscopic technique, images of the distribution of thermodynamic parameters of such events can be obtained.

Spectroscopic techniques can be classified in several ways. A simplistic but useful classification includes only two groups: passive techniques, which depend on spontaneous emission by the tested object; and active techniques, which require an excitation of the tested object to induce detectable emission.