A method for increasing the photographic contrast of electron micrographs

Emulsion-developer combinations used regularly in conventional electron micrography yield an approximately linear relationship between density (D) and exposure (E) up to quite high densities (>2.0). This is due to the fact that over the corresponding exposure range the density yield per electron is approximately constant. A practical consequence is that the slope of the D/log(10) E curve, termed the contrast, is a function of D only, i.e. it is independent of the emulsion-developer combination. To achieve higher than normal contrast requires that the density yield per electron should increase with exposure over some exposure range, at least. This effect may be obtained by using what are known commercially as 'lith' films and a system of infectious development. The high contrast of such films results from an acceleration of development rate when the developer oxidation products reach a critical concentration. Sensitometric data are presented for electron exposures which show that for average densities of similar to 1.0 in a micrograph a six-fold increase of contrast can be obtained with a 'lith' system. It is also shown that though the special development effect gives higher granularity than that obtained at the same exposure with a conventional system, the signal/noise ratio is not adversely affected. Practical demonstrations of the effectiveness of the 'lith' system are presented.