BASIC PRINCIPLES AND DESIGN 



Sources of Ultravaiolet Energy. The 



noisy and odorous cadmium spark, originally 

 used as the source for ultraviolet, has been 

 replaced by the quartz- jacketed mercury 

 arc. Combination glass and liquid chemical 

 filters have been replaced by the grating 

 monochromator, which provides simple and 

 convenient means for changing and select- 

 ing desired ultraviolet wavelength regions. 



Viewing Systems. Perhaps the most diffi- 

 cult technical problem in the design of the 

 ultraviolet microscope has been that of 

 achieving a receptor to permit seeing the 

 image. All of the early work used photog- 

 raphy with trial and error search for best 

 focus. Later the fluorescent screen was em- 

 ployed, but the image was generally too 

 grainy and dim for much useful work. 



The advent of high-speed processing of 

 photographic film led to the possibility of 

 only a very short time delay between ex- 

 posure of the film and seeing the image. The 

 "color translating microscope" developed 

 by the Polaroid Corporation utilized this 

 technique. Three adjacent frames of ultra- 

 violet sensitive film were exposed rapidly 

 in sequence to three pre-selected wave- 

 lengths in the ultraviolet. The film was 

 rapidly developed, fixed, and projected onto 

 a viewing screen. Each frame was projected 

 through a primary color filter, and the 

 images, optically superimposed on the 

 screen, formed a composite three-color 

 image. This instrument was, of course, ex- 

 tremely expensive, and its use, accordingly, 

 severely restricted. 



Television image scanning techniques 

 have also provided means for seeing ultra- 

 violet images. Two basically different ap- 

 proaches have been used in applying tele- 

 vision apparatus and methods. In one, called 

 the "flying spot microscope" (q.v.) an ultra- 

 violet-emitting cathode ray tube face is 

 located in what is normally the image plane 

 of a photomicrographic set-up. The micro- 

 scope, acting in reverse, forms a greatly 

 reduced image of the cathode ray tube face 



Fig. 1. The 53X, 0.72 N.A. Catadioptric objec- 

 tive for ultraviolet, designed by D. S. Grey and 

 manufactured by Bausch & Lomb. Refracting ele- 

 ments are of fluorite and fused quartz. 



on the specimen. An ultraviolet sensitive 

 photomultiplier tube picks up the signal 

 transmitted by the specimen, as it is scanned, 

 and the energy signal from the photo- 

 multiplier is converted to a visual image on 

 a conventional cathode ray tube, which is 

 tuned to the same sweep frequency as the 

 illuminating cathode ray tube. The particu- 

 lar virtue of this "flying spot" system is the 

 low dosage of ultraviolet concentrated on 

 the specimen, since prolonged exposure to 

 ultraviolet is lethal to most living specimens. 

 Its disadvantage is the technical difficulty 

 of attaining a satisfactory means of varying 

 the wavelength of the ultraviolet emitted 

 from the cathode ray tube. 



Another television type solution to the 

 problem is to use a conventional ultraviolet 

 source and monochromator to illuminate 

 the microscope, and to pick up the image on 

 an ultraviolet sensitive image-orthicon, and 

 by means of a closed circuit television send- 

 ing and receiving set-up, view the image 

 on a conventional television screen. 



Both of these television systems share the 

 basic problems of the color translating 

 microscope, that is, they are extremely costly 

 and elaborate to build, and require con- 

 siderable training and skill to keep in good 

 operating condition. Because of the com- 

 plexity of these systems, their use has been 



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