Secondary electron emission from oxide-coated cathodes

The secondary electron emission from alkaline-earth oxide-coated cathodes has been investigated under both continuous and pulsed bombardment. Various factors affecting the yield, such as dependence upon primary voltage, collecting voltage, temperature, time, and angle of incidence, are noted, and the present state of the theory is discussed.Experiments have been performed with three types of apparatus. Yield vs. Energy data reveal values of δ of 4–7 at room temperature, with a more or less flat maximum at approximately 1,000 volts primary energy.The yield increases with temperature in an exponential manner, and plots of log Δδ (i.e. δ_K° ? δ_300°K) vs. 1/T give straight lines. Values of Q₁ between 0.9 – 1.5 eV. are generally indicated, and from extrapolation of these curves, yields exceeding 100 at 850° C. are deduced. The secondary emission depends upon the degree of activation, and increases with enhancement of the thermionic emission characteristics. Short-time effects such as growth or decay of secondary current after the onset of primary bombardment or persistence after the cessation of bombardment have not been observed, and values of yield obtained by pulsed methods are in accord with those obtained under D.C. conditions. Tail phenomena reported by J. B. Johnson and interpreted as “enhanced thermionic emission” from oxidecoated cathodes become manifest only under experimental conditions characterized by certain space-charge effects, and have been effectively simulated by bombarding a tantalum target adjacent to an electron-emitting tungsten filament. Various measurements of the energy distribution of secondary electrons as a function of primary voltage and temperature have been obtained. It was observed that the average energy of the secondary electrons decreases with temperature at a rate which more than compensates for the increase in the number of secondaries emitted per incident primary. The mechanism of the observed dependence of yield upon temperature is not well understood. Various alternative explanations are discussed and, in the light of the present state of our knowledge, regarded as untenable.     

Studies in the sensitivity of photographic materials

The sensitivity of photographic emulsions is not measured by the exposure necessary to produce an arbitrarily chosen density. It is measured by a definite value of DID…, which is given by the inflection point of the characteristic curve.For simple characteristic curves, the inertia point is quantitatively related to the inflection point.The most complete measurements of sensitivity are obtainable by a quantic analysis of the shape of the characteristic curve.     

Theory of the halftone process. III. Dot size and halftone density

The theoretical and experimental quantitative relationships between the dot diameter, dot area, and density of a halftone are given. Experimental results do not agree exactly with the theoretical relationships except for screen negatives and positives with sharp dots.For accurate rendering, highlight dots in a reproduction must increase extremely rapidly in size for small density increments in the highlights of an original. A “emi-dark-field” densitometer is described which eliminates the effect of dot fringe in measuring densities of screen positives. The relationship between density of screen positive and density of proof is briefly discussed.     

Theory of the halftone process. II. The diffraction theory-calculation of the light distribution

An approximate method of calculating the light distribution in the dots formed behind a halftone screen from the Fresnel diffraction theory is described, and in a typical case, the distribution is found to check closely with that determined experimentally, being very different from that calculated from geometrical optics.