A New Type of the Spatial Filter for Detecting the Sharpness of Images

Abstract

A description is given of experiments carried out using pulsed ruby and argon lasers to investigate the possibility of producing pulses of high electron emission current densities from tantalum, tungsten and lanthanum hexaboride cathodes heated by a focused laser beam. Current densities as high as lo4 A cm-2 were recorded using the ruby laser but subsequently it was shown, using the repetitively pulsed argon laser, that electron current densities of this order were obtained only when the cathode material was being melted. Lanthanum hexaboride was found to undergo changes on the surface when repetitively pulsed and was rejected in favour of the pure metals tungsten and tantalum. From these metals stable emission currents up to 1 mA peak were recorded from spots approximately 15 pm in diameter (i.e. 570 A cm-2) for several hours without incurring surface damage. Current density values of this order are near to those calculated from the Richardson equation for temperatures just below their melting points.Experimental results have confirmed theoretical predictions regarding the delayed onset of emission, the delay in the peak emission relative to the peak light intensity and the extremely rapid termination of the emission pulse.