One-image depth-from-focus for concentration measurements

A one-image depth-from-focus technique for the measurement of the concentration and size distribution of small particles is described. The technique is based on a precise knowledge of the 3-D point spread function and requires objects of uniform brightness and simple shapes. The true area of the object and the distance from the focal plane is obtained from parameters such as the apparent (blurred) area of the object and the mean brightness in this area. The technique has been applied to measure the size distribution of bubbles submerged by breaking waves. A depth criterion is used to define a virtual measuring volume that is roughly proportional to the size of the bubbles.