A review of the current developments in microdensitometry is made, with emphasis on the investigations leading to the current level of understanding of optical performance. The classical microdensitometer is then analyzed according to the principles of the theory of partial coherence. Conditions for the insuring of linear operation are derived, and the idea of effective incoherence at the source aperture is presented with a discussion of the implications. The various microdensitometer configurations are subjected to analysis, and the four possible variations (viz., overfilling, underfilling, with two possible locations for the sampling aperture) are thoroughly evaluated. The new concept of linear microdensitometry is discussed and summarized briefly. The current concerns of microdensitometry are then presented and considered. The restrictions on maximum sample frequency as a result of the partial coherence in the illumination is a major concern of this paper, and tables are presented for typical microdensitometer configurations that delineate the kinds of limitations that can be expected.