年間 6 号発行
ISSN 印刷: 2150-766X
ISSN オンライン: 2150-7678
Indexed in
MEASUREMENT OF THREE-DIMENSIONAL TEMPERATURE FIELDS BY HETERODYNE HOLOGRAPHIC INTERFEROMETRY
要約
Holographic interferometry is a powerful tool to measure the deviation between two wavefields within a fraction of the wavelength of the coherent monochromatic light source. In the reconstruction of the double-exposure hologram, the phase difference of the wavefronts due to a different spatial refractive index distribution of the two object states shows up as an intensity modulation, the resulting interference fringe pattern. Starting at a point of the field where the temperature remained constant, the temperature at any point in the field can be obtained by detecting the total phase difference at this position.
However, with classical holographic interferometry, quantitative information on the interference phase is only reliable in the minima and maxima of the fringes, corresponding to multiples of 180° or π. The interpolation between the fringes is difficult and not very accurate. Furthermore, in regimes of higher fringe concentration, for example in the boundary layer near the wall, background illumination and image noise due to the laser speckles reduce the fringe contrast.
Based on the idea of heterodyne holographic interferometry, we introduce a small frequency shift between the optical frequencies of two reference wave fields. This results in an intensity modulation at the beat frequency of approx. 100 kHz of the two light fields for any given point in the interference pattern. Due to the fact that this method works fine for phase objects like a hot gas, it becomes an important approach to visualize and analyze heat transfer in boundary layers.
The experimental verification of heterodyne interferometry for heat transfer measurements will be presented, and the properties of this technique will be discussed.