In recent years two out-of-focus te

In recent years two out-of-focus techniques have been introduced for sizing spherical particles within a two- or three-dimensional domain. The first of these, interferometric particle imaging, has recently been commercialized under various names. The second technique, global phase Doppler, employs two illuminating light sheets and is closely related to the well-known phase Doppler technique.Each of these techniques results in an image in which the spacing or number of interference fringes must be related to the particle size through a conversion relationship. The derivation of these relationships has not been well documented to date and in some cases falsely reported in the literature. The present paper provides these derivations, their range of validity and some additional results for specific situations.