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.
Microdensitometry is being standardized through the activities of the American National Standards Institute. Terminology and notation have been agreed upon. The fundamental problem of nonlinearity, which arose in connection with this standards activity, has been studied and resolved.
Tradeoff studies were made and tests were performed on the optical scanning system of a linear microdensitometer. The final system includes a laser source, an illuminator system capable of producing a 1 m spot on the film, a wide anglecollection system (NA > 1.0), and a photodiode detector. Performance of the optical system was verified by breadboard tests. The test results showed a square wave response of 99 percent at 200 kp/mm and greater than 90 percent at 500 Qp/mm. Tests performed with the 1µm scanning spot and NA 1.0 collection indicate that the optical system was linear (as measured by nonresponse to a phase target) for object frequencies up to 500 Qp/mm. The photometric precision was +0.01 D from 0 D to 2 D and +0.02 from 2 D to 4 D. These precisions were obtained at scanning speeds up to 8000 samples/sec.
Tradeoff studies and tests on the various subsystems of a microdensitometer have resulted in a recommended design configuration for a Linear Microdensitometer. The photometric subsystem includes laser sources suitable for color operation, an illumination system capable of producing a 1 um spot on the film, a wide angle collection system (NA = 1.0), a photodiode detector, and digital density conversion. Performance of the photometric system was verified by breadboard tests. The results include an MTF (square wave response) of 99 percent at 200 Qp/mm and 90 percent at 500 kp/mm, and linearity (object independence) to greater than 400 Qp/mm. The photometric precision is ±0.01 D from 0 D to 2 D and ±0.02 D from 2 D to 4 D. These specifications are attained at scanning speeds up to 8000 samples per second. A two-beam optical system and automatic focus control ensure photometric repeatability. The two-axis stage is driven by phase-locked loop servos incorporating interferometric position sensors. The servos permit scanning at skew angles, obviating stage rotation. The stage sub-system is precise to within ±0.5 pm over 5 inches of travel. Stage speed is variable from 0.4 mm/sec to 40 mm/sec. Human factors features include rapid sample acquisition by virtue of a viewing system with magnification variable from 16X to 200X and simplified operator control through a CRT graphic terminal.
The increased resolution capability of modern photographic emulsions and optics has created a need for increasingly precise and automated microdensitometry. This article describes a laser interferometer controlled microdensitometer with greatly improved positional accuracy of 0.5 micrometer.
This paper presents the results of a design and experimental evaluation of two potential diffuse efflux collectors for microdensitometers designed to obtain linearity at high sample spatial frequencies. Use of a large numerical aperture condenser and an electroformed reflector were examined. The phase edge response, a generalized Collier's Q-factor, and susceptibility to stray light noise were measured. Either collector offers improved linearity, however, better rejection of stray light noise will be required for their practical use.
The photometric fidelity of the optical system of microdensitometers is compared to that of flying-spot scanning devices. Microdensitometers, which were originally developed to study fine structure of silver halide photographic images, move the object plane through a fixed, Koehler-illuminated dual projection optical system that controls scattered light and ensures uniform photometric response everywhere on the scanned area. Flying-spot and video-based scanners now being applied to photographic photometry are optical reciprocals of one another. They operate on a critically-illuminated image of the film similar to that produced by a camera, and therefore require precautionary measures to limit errors from flare light and their inherent lack of spatial stationarity. The low inertia of such systems permits high data-sampling speeds with the possibility of preprogrammed or adaptive scans under computer control. Advances in stage design, however, now allow microdensitometer optical systems also to sample at photon noise limited rates. One such microdensitometer, a spiral-scanning EDP Scanning Microscope, is described and examples of its real-time-processed density displays ("isophote plots") and computer interfaces are given.