Research of the automatic imaging focusing measurement for off-axis Fresnel digital holography

Digital holography technology, which adopt CCD digital camera to record holograph, can get amplitude and phase information of the sample tested respectively through processing the image. Re-focus of the sample image can be realized by calculating complex amplitude information obtained. However, digital holograph reproduction system can't offer any criterion which determine whether reproduction image achieve fixed-focus state when reproduction image stands in the optimum position. Digital holography reproduction process need add an exterior criterion function, which implement automatic focus as normal imaging system. Aimed to digital imaging automatic focusing problem of off-axis Fresnel holography, image gradient energy method, image Laplace energy method, image shade of gray variance method are compared from two points of theory and practice view, and present the advanced measurement method based on spectrum band-stop filter. Experiment shows that the algorithm has some adaptability, especially image Laplace energy method based on spectrum band-stop filter has non-skewness, monotonicity, unimodality, kurtosis and robust.