The application of microphotographic methods for the processing and storage of information has grown steadily during the last two decades. The microphotographic system, through which the information must pass at a minimum is as follows: Step 1: Photographing of the original document at a large reduction onto a silver halide microfilm. Step 2: Duplicating through contact printing onto a non-silver photographic material. Step 3: Second generation duplication onto a nonsilver photographic material (This image may be used on a microfilm reader). Step 4: Enlargement of the second generation duplicate to give a hard copy print. One of the main objectives of this system is the production of a legible image at the end of the process. As the steps mentioned above indicate, the creation of the original negative at a large reduction is the first major step and critical to the success of future steps in the process. A mistake in camera exposure at this point may well cause an illegible image to be produced at the end of the system. A technique long utilized by the microfilm industry for the measurement of small scale image quality of greatly reduced images is resolution in lines per millimeter. The purpose of this experiment is to determine the relationship between resolution and legibility for similarly produced original negatives. The methodology of the experiment consisted of producing original negatives in a conventional microphotographic system at 24x reduction of two types of resolution test targets. The NBS 10-10 (100:1 contrast) test chart and the R.I.T. alphanumeric (100:1 and 8:1 contrast) test objects v/ere the three targets used. The three targets were photographed at eight equally spaced exposure levels with a total ratio of 10:1. Replicate images were made at each exposure level. The resolution measurements were read from an 80x microscope and from a microfilm reader at 24x by this author. A series of standard documents in two different type faces (ten point serif and sanserif) were exposed and processed in the same manner as the resolution targets. Replicate sets were made at each exposure level. The standard documents consisted of meaningful, continuous, textual material. The original negatives of these standard documents were evaluated for their legibility by placing them in a microfilm reader (24x) and determining the distance at which the images became legible. Fifteen observers were employed. The average distance measurements were used to determine the relative legibility of the original negatives. A two- factor ANOVA at an alpha risk of 0.05 indicated that type face did not affect the relative legibility of the negatives. Camera exposure level was found to be significant, which means that images of varying legibility were produced. The functional relationships between resolution from the three targets versus camera exposure level and between relative legibility versus camera exposure level were determined and found to fit second-order models. The optimum camera exposure for both measures of image quality were less than one-third of a stop apart. The correlation between resolution and relative legibility was high in all cases (minimum of .86) and significant at the 0.01 level. A linear model best fits these functional relationships. The R.I.T. 8:1 contrast target provided the highest correlation with r equal to O.96. The NBS and R.I.T. test charts were very similar in the resolution measurements obtained from each. However, when a secondary observer's measurements were compared to this author's the between observer differences in resolution values for the NBS target were nearly four times as large as the differences on the R.I.T. target. In summary, the camera exposure of the original negative that provides the highest resolution in a microfilm system will also be very close to the exposure that yields the optimum legibility for commonly employed type styles and sizes. The resolution measurements can be obtained directly from the microfilm reader that is a part of the system. If the R.I .T. 8:1 contrast target is used the legibility predictions will have a high level of accuracy and excel lent precision.
Library of Congress Subject Headings
Microphotography; Legibility (Printing)
Department, Program, or Center
School of Media Sciences (CIAS)
Compton, John, "The Relationship Between the Relative Legibility of Textual Material and Resolving Power in a Microphotographic System" (1975). Thesis. Rochester Institute of Technology. Accessed from
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