US2129296A - Screen for x-ray photography - Google Patents

Description

Sept. 6, 1938. K7 WURSTLIN' 2,129,296

SCREEN FOR X-RAY PHOTOGRAPHY Filed May '7, 1936 Karl W6 rsHM I INVENTOR BY m ATTORNEY Patented Sept. 6, 1938 UNITED STATES PATENT OFFICE Karl Wiirstlin, Berlin, Germany, assignor to Kali- Chemie Aktiengesellschaft, Berlin, Germany Application May 7, 1936, Serial No. 78,397 In Germany May 10, 1935 4 Claims.

This invention relates to screens for X-ray (Rontgen ray) photography. As is well known, screens are used in making X-ray pictures in order to reduce the time of exposure. The

5 screens generally consist of a sheet of cardboard or Celluloid to the surface of which a fluorescent substance such as calcium tungstate is applied in crystal form and as uniformly as possible by means of a binding agent. The so-called carrierless screens have also been employed, which consist only of a layer of gelatine containing calcium tungstate.

An object of the invention is to provide a screen with which X-ray photographs of much better sharpness can be obtained than with the known screens, the time of exposure being the same. A further object of the invention is to provide a screen which contains the active layer of calcium tungstate in the form of a mixture of coarse grains and finer grains.

A further object of the invention is to provide a screen having a layer containing fluorescent material in which there are arranged in this active layer between coarser grains of the fiuores cent material grains of the same material of considerably smaller diameter and in quantity such that the intermediate spaces between the coarser grains are filled for the most part by the material of finer grain.

Further important features and advantages of the invention will be clear from the following description.

It is known that screens containing a layer of calcium tungstate reduce the time of exposure.

The use of these screens has, however, the disadvantage that they have an unfavourable effect upon the sharpness of the photographs owing to the occurrence of reflected rays. If the attempt be made to obviate this disadvantage by treating the layer containing the calcium tungstate with a dyestuif or with a light-absorbing medium or by arranging behind the active layer a layer which does not substantially reflect the light emerging from the active layer or by considerably reducing the thickness of the active layer, then, it is true, an improvement in sharpness is obtained; this improvement is, however, only obtained at the expense of an increase in the time of exposure, which may be considerable.

It has now been found that the sharpness obtained with the use of a screen can be considerably increased without the time of exposure being simultaneously prolonged by adding to the calcium tungstate used for the manufacture of the known screens or foils a larger or smaller quantity of a calcium tungstate of very fine grain. The very fine-grained calcium tungstate fills the spaces between the coarser crystals. It is clear that in this way the occurrence of reflected rays is greatly suppressed so that sharper pictures are 5 obtained.

As regards the size of the crystals of the coarsegrained tungstate, they may vary within wide limits. Thus, for example, material of from 4 to 5 or 7 to 8 or 10 can be used. The fine- 10 grained tungstate must possess in comparison with the coarse-grained tungstate a size of grain such that it is adapted substantially to fill up the intervening spaces between the coarser grains. The best results are obtained when the size of 15 grain of the very fine-grained tungstate does not substantially exceed 1,11,.

The amount of fine-grained tungstate which is added can vary within considerable limits, for example it may amount to about 20 to 25% of the 20 total amount of tungstate. The percentage of very fine-grained tungstate can, however, be considerably higher, but care should be taken if the best results are to be obtained that so much fine-grained tungstate of a size of grain which 25 preferably does not considerably exceed 1 is present that the spaces between the larger crystals are filled as completely as possible. What proportion is most favourable when using a coarser material of a definite size of grain can 30 easily be determined by making a number of comparative exposures.

The invention is illustrated by way of example. in the accompanying drawing which shows an elevated view of a screen according to the present 35 invention.

In the drawing I is a carrier sheet consisting of cardboard, Celluloid or the like. 2 designates an active layer of calcium tungstate in crystal form glued tothe carrier sheet by means of a 40 transparent acetyl cellulose lacquer. According to the present invention the active layer 2 contains calcium tungstate.

The active layer of a screen contains calcium tungstate of a grain size of from 4 to 5 and of a 45 grain size of about lit in the ratio of 1:1. When a screen of this kind is used for X-ray exposures, photographs are obtained which are in no way inferior as regards sharpness to the pictures obtained with the use of a screen having a dyed 50 active layer. At the same time, however, the time required for exposure is considerably less and amounts to about half the time required when dyed screens are employed.

The symbol ,u or micron used in the fore- 55 going specification and in the following claims to designate the magnitude of the grain size, signifies 0.001 mm.

I claim:-

1. A screen for Rontgen ray exposures in which the active layer contains 50% calcium tungstate having a size of grain of from 4 to 5; and 50% calcium tungstate consisting of grains of about and not exceeding 1 in size.

2. A screen for Rontgen ray photographic purposes having a layer of fluorescent material,

said layer containing 50 to 80% of the fluorescent material in the form of grains having a size of at" least 4 and the remaining portion of fluorescent.

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