Methods for the deconvolution of defocused-image pairs recorded separately on two CCD cameras: application to particle sizing

An optoelectronic in-line imaging system for particle sizing is described. The images are scanned by two CCD cameras viewing the same object field that is illuminated with two pulses. One of the cameras is double exposed and the other is activated only during the second pulse. Two successive analogical subtractions between the video output signals gives the sign of the transverse velocity vector. Out-of-focus images are deconvolved with the assumption of a Gaussian point-spread function (PSF) whose spatial parameter ς increases with the defocusing distance z. In the case of low particle density, an algorithm based on the exploitation of the power spectral density is used to estimate the particle diameter. This method can be applied to the case of fast-moving particles (e.g., v < 500 m/s). The accuracy of the size measurement is better than 10% in the diameter range 20–160 μm. The main result is that this accuracy is obtained with an amount of defocusing in the range [−2, 2] mm. Thus, the depth of field is significantly extended in comparison with a conventional microscopic imaging system.