A method for increasing depth of field during droplet imaging

The measurement of water drops using direct optical imaging involves a tradeoff between the camera depth of field and the accuracy of the measured droplet size. A large depth of field increases the length of that portion of the optical axis that is in focus. However, since drops closer to the camera appear larger than those farther away, increasing the depth of field also increases errors in the measured drop size unless a priori knowledge exists concerning the location of the drop along the optical axis. Herein a method is presented for ascertaining drop location using a single camera. The method uses a characteristic of the droplet image which is observed when droplets are illuminated from behind. Once the location of the droplet is obtained, the appropriate magnification ratio is applied, permitting an accurate droplet size measurement. This method has been tested on glass spheres of various size. The relevance of this work to precipitation science is discussed.