IMAGE FORMATION BY A FRESNEL ZONE PLATE 



60 



400 800 1200 1600 2000 2400 2800 3200 3600 \ A, 



Reflectivities op Evaporated Metal Film 



Fig. 2. Normal incidence reflectivities of various materials in the ultraviolet. Aluminum overcoated 

 ^•ith magnesium fluoride gives useful reflectivity even below 1000 A. Below 400 A there is a dearth of 

 experimental information on reflectivity. 



of aluminum, tin, indium, and bismuth, as 

 shown, (22) the materials of which lenses 

 might be made cannot transmit below 1000 

 A. Below this wavelength, then, lenses are 

 apparently not worth considering. 



How about mirrors? Anastigmatic image 

 formation with a single mirror requires good 

 reflectivity at normal or near-normal inci- 

 dence. Recently Hass (6) and his co-workers 

 have developed coatings of evaporated metal 

 films which, when deposited on glass or 

 other materials, produce high reflectivities 

 down to fairly short wavelengths. Fig. 2 (5) 

 summarizes the characteristics of some of the 

 best coatings. The data show trends rather 

 than authoritative values. For these the 

 reader is referred to the modern litera- 

 ture.^- -^ For wavelengths shorter than 585 A 

 the Fresnel zone plate may find applica- 

 tion as a recurring device. Nevertheless, 

 one must not exclude the possibility of using 

 very large mirrors with very low reflec- 

 tivities. 



For angles of incidence close to 90° it 

 becomes convenient to speak of the "grazing 

 incidence angle," that is, the complement of 

 the usual angle of incidence, which is the 

 angle subtended by the incident ray and the 

 normal to the reflecting surface. For x-rays, 

 the phenomenon of total reflection at graz- 

 ing incidence is well known (7, 8). The 

 reflectivity is 100 per cent because the index 

 of refraction for most materials is less than 



unity. Point-to-point grazing-incidence sys- 

 tems for forming images have been built by 

 Kirkpatrick and his students (9, 10, 11), but 

 reflection from a single curved mirror at 

 grazing incidence is highly astigmatic and 

 can be corrected only by the difficult tech- 

 nique of crossed mirrors. Systems of this 

 type have not been developed for wave- 

 lengths as long as 100 A but they may have 

 to be considered. 



If w^e restrict ourselves to the less formid- 

 able process of image formation by normal 

 incidence reflection at a single surface, we 

 can probably conclude that a region exists, 

 somewhere between 10 A and 1000 A, where 

 image formation by reflection or refraction 

 is either difficult or impossible. 



Zone Plate for Focusing Extreme Ultra- 

 violet and Soft X-Radiation 



Diffraction offers still another way of 

 bending light to produce focusing. Several 

 such methods have been suggested (12, 13) 

 but even the simplest, the Fresnel zone 

 plate (14, 15) has not received serious use 

 in any region of the spectrimi, let alone that 

 between 10 and 1000 A. Several writers have 

 noted that Fresnel zone plates might be used 

 for image formation in this difficult region, 

 but no successful attempts to build a zone 

 plate for x-rays or euv have come to this 

 author's attention. 



This paper describes the construction and 



553 



