IMAGE FORMATION BY A FRESNEL ZONE PLATE 



0.02lo\/7i, 71 = 1, 2 • • • 38. For a zone plate 

 with a large number of rings the more pre- 

 cise formula for r„ would be needed, 



r„ = VfnX Vl + nX/if. 



(5) 



The narrowest band was designed to meas- 

 ure 0.0017 cm in width. Actually it varied 

 between 0.0010 and 0.0020 cm. The zone 

 plate was made for us by the Buckbee Mears 

 Company of St. Paul, IMinnesota. As far as 

 we know this is the first zone plate with trans- 

 parent regions that are completely open 

 (except for the supporting radial struts). 

 The images formed are very sharp, compar- 

 ing favorably with those made by lenses of 

 similar focal length and aperture (see Fig. 

 9). We may suggest one explanation for the 

 good quality obtained with these zone 

 plates as compared with that of the photo- 

 graphically based zone plates. The phase 

 condition that requires an increment of 

 exactly one wavelength between successive 

 transparent bands must be difficult to meet 

 with a film base many microns thick that is 

 not held to optical flatness. This condition 

 is probably less difficult to meet with the 

 new zone plate that has no film base at all. 



Since the transparent regions of the zone 

 plate are completeh" clear they should 

 transmit radiation of all wavelengths. The 

 gold bands are practically opaque to all 

 radiations between 10 and 1000 A; there- 

 fore, the zone plate should be able to focus 

 soft x-rays and euv in this region. Of course, 

 it can also operate in the visible and infrared 

 regions. It should also work with particles 

 having wave-like characteristics of the 

 proper wavelength, but our present interest 

 is in the soft x-ray and euv region. 



Our first zone plate, with which all the 

 tests reported in this article were made, was 

 designed to have a focal length of about -400 

 cm at 100 A. At 4000 A its focal length is 

 10 cm; it thus lent itself very conveniently 

 to tests with visible light. All the tests de- 

 scribed below with the exception of those at 

 2537 A were made with visible light. We 



Fig. 3. An enlarged photograph of the self -sup- 

 ported gold zone plate used in these experiments. 

 The diameter of the outer circle is 0.2596 ± 0.0002 

 cm. The central circle has a diameter of 0.0426 ± 

 0.0002 cm. The thickness of the gold is estimated 

 as 10 microns. The white bands representing the 

 transparent regions are completely open and hence 

 will transmit electromagnetic radiation of all 

 wavelengths. 



plan to continue tests at 100 A and 1000 A 

 with newly acquired sources. 



Resolution 



Theoretically, the smallest angular separa- 

 tion, 0min , between two monochromatic 

 point sources at infinity that can just be 

 resolved when imaged by a zone plate, can 

 be shown (14) to obey the Rayleigh criterion 

 for a lens: 



sin 0nnn = 1-22 (X/D) . 



(6) 



Here X is the wavelength and D the 

 diameter of the outermost circle of the zone 

 plate. To test this relation we used fine mesh 

 screens as objects, illuminating them by 

 transmission as shown in Fig. 4. In Eq. 6 

 we could vary X, the wavelength being used, 

 but not D, as all our zone plates had the 

 same diameter, approximately 0.26 cm. A 

 zone plate is a highly chromatic device, 

 having a focal length that is inversely 

 proportional to the wavelength. We used 

 filters to monochromatize the radiation or 



555 



