Optical breakdown in water induced by 1064 nm Nd:YAG laser was visualized and measured by applying shadow and Schlieren optical diagnostic techniques. The research was focused on the earlier stages of the breakdown process, just after the end of the heating laser pulse. The spatial characteristics of laser spark columns were measured and compared to numerical analysis of cavities arising over inclusions. Anomalously long laser spark columns were observed. The distribution of cavity dimensions and energies absorbed in the spark were measured along the laser spark column. A novel non-probability based criterion for laser breakdown threshold was suggested, namely, the pulse energy which generates a spark of negligibly small length. (C) 2009 Elsevier B. V. All rights reserved.