ULTKAMOLET AIlCHOSCOrY 



IMAGE FORMATION BY A FRESNEL 

 ZONE PLATE* 



This article describes experiments in the 

 use of a new type of Fresnel zone plate for 

 image formation with visible light and ultra- 

 violet radiation. Tests with visible light and 

 ultra\'iolet down to 2537 A are described 

 below followed by a description of our plans 

 to extend these tests down to about 100 A. 

 There is good reason to believe that the same 

 zone plate will operate over a large range of 

 wavelengths from the soft x-ray region 

 through the infrared because its transparent 

 zones are completely open. Therefore, it is 

 opportune to speculate on the possibility 

 of using the zone plate to focus even other 

 radiations and particles. First, however, let 

 us consider the motives that prompted us to 

 construct this zone plate for soft x-ray and 

 extreme ultraviolet radiation. 



Early research in the means of focusing 

 light was motivated in part by interest in 

 the construction of microscopes and tele- 

 scopes. These instruments were, after all, 

 the earliest "space probes" into both the 

 intermolecular and extragalactic worlds. 

 Later, the properties of the electromagnetic 

 spectrum outside of the visible region 

 prompted natural extensions of image-form- 

 ing devices into the near infrared and the 

 ultraviolet. 



Tranmissivity (I/Io) of Various Transparent 

 Substances 



Fig. 1. The transmissivity of several materials 

 in the ultraviolet. The crystalline materials that 

 might serve for lenses cease to show any apprecia- 

 ble transmission below 1000 A. The thin metal films 

 could serve as filters. 



* See also "X-Ray Telescope for 1-100 A Re- 

 gion," in "Encyclopedia of Spectroscopy," p. 778. 



Today similar motives exist for the con- 

 struction of microscopes and telescopes to 

 work in the soft x-ray and extreme ultra- 

 violet (hereafter referred to as euv) region. 

 While no definite bounds exist for the euv 

 region we shall limit our study to wave- 

 lengths from 10 A to about 1000 A. Al- 

 though the developments in x-ray micros- 

 copy and microradiography have already 

 been responsible for two international 

 congresses (1, 2), the new interest in tele- 

 scopes that can operate in the soft x-ray and 

 euv regions stems chiefly from discoveries 

 in astrophysics. 



Rocket experiments have already demon- 

 strated that solar, stellar and interstellar 

 sources of euv exist (3), but the earth's 

 atmosphere prevents most of the radiations 

 shorter than 3000 A from reaching its surface. 

 The advent of rocket and satellite-borne 

 experiments justifies serious consideration 

 of telescopes and spectrographs sensitive to 

 wavelengths between 1 A and 3000 A. 



We distinguish between instruments, such 

 as lenses and mirrors, whose main function 

 is to form optical images and those such as 

 prisms and gratings, whose purpose is to 

 produce dispersion. Our interest for the 

 present is primarily in image formation. The 

 classical means of focusing visible light for 

 image formation involve reflection (mirrors) 

 and refraction (lenses). Proper choice of 

 materials allows the use of reflectors and 

 refractors in the ultraviolet from 3000 A to 

 about 1000 A. Between 1000 A and 10 A 

 refraction is out of the question because 

 most materials are opaque to radiation in 

 this region. Special types of glass (4) have 

 been developed which can transmit about 

 70 per cent at 2200 A through a thickness of 

 1 mm. But this is exceptional since the 

 transmission of most t3T3es of glass drops 

 essentially to zero below 3000 A. Although 

 transmission of wavelengths below 10 A 

 does occur, the index of refraction is so close 

 to unity that refraction is impractical. 



The transmission of several materials is 

 shown in Fig. 1 (5). Except for thin fihns 



552 



