A Masking Technique for Contrast Control in Electron Micrographs

The technique of "dodging" photographic prints for electron microscopy is almost universally practiced. One merely interposes a hand or some suitable object in the light path during enlargement in order to produce prints of even density from negatives of uneven density. There are occasions when the intricate shape of areas to be dodged render such a simple procedure inadequate. This is especially true of negatives of tissues in which irregular patches of very dense material are adjacent to structures of moderate or low density. Dense areas may black out completely in prints made to obtain optimal tonal separation in light areas. Conversely, areas of low density often become "washed out" when details in dense areas are desired. If one attempts to correct excessive large-area contrast by using low contrast paper, the resulting fine-detail contrast is often too low, especially for photomechanical reproduction. A printing instrument that uses a scanning light source has been described (1). I t automatically corrects for excessive large-area contrast while retaining, or enhancing, fine-detail contrast. Light from the flying spot is monitored by a photomultiplier tube after it has passed through the negative. By means of negative feed-back, the spot is dimmed as it passes over light areas in the negative. The amplitude of the feed-back signal determines the degree of contrast correction in the final print. The size of the flying spot determines the min imum size of areas, the contrasts of which will be altered. Strictly photographic methods for dealing with this problem have been known for a long time. The reader is referred to Yule (2) for an introduction to the pertinent literature. Aerial photographs (3) and roentgenograms (4) have been printed by exposing prints through an unsharp positive transparency (mask) superimposed in register on the negative. This produces effects on prints similar to those produced by the electronic instrument described above. Making the mask unsharp is like defocusing the flying spot; increasing the contrast of the mask has the same effect as increasing the amplitude of the negative feed-back signal. A method for visualizing fine details in large areas of radically different densities on the same negative is an important aid in obtaining maxim u m information from a given negative. I t is also desirable to minimize unevenness in the nega-