Sizing of bubbles by incoherent imaging: defocus bias

Digitized images of millimeter bubbles are obtained with backlighting from a diffuse source. In order to investigate how the measured and real sizes of a bubble in or out of focus relate, a Ray Tracing Model of image formation, based on geometrical optics and photometric laws is developed. It emphasizes the contribution of direct, reflected and refracted rays. The model shows that the size of the source has a major role in the image formation. Moreover no reliable information on the defocus sign can be found in the image. A method for sizing blurred bubbles is suggested.