Refractive index profiling across a candle flame using speckle techniques

Speckle techniques for refractive index measurements are analyzed theoretically. It is demonstrated that the resulting speckle pattern is equivalent to the ordinary pattern convolved with a function representing the effect of light deflection, while the obtained interference pattern is modulated by a background representing the autocorrelation function of the diffuser. The technique is applied to the region above a candle flame. An improved quality of the interference fringes is achieved with a multiple-exposure technique.