The characteristic curve of an emulsion exposed to X-rays differs from the curve ob tained by exposing the same emulsion to ordi nary light 202). The foot of the curve is very long, and the straight-line portion occurs only in the region of very high density.
Curious antagonistic actions between the photographic behaviour of X-rays and light have been observed. In certain circumstances the latter can destroy the latent image produced by X-rays, and it has thus been possible to produce passable direct positives by taking a photograph on a plate which had previously been fogged by X-rays.
930. Sensitive Material for Radiography. In order to obtain the greatest possible effect from the X-rays it was the custom at first to use plates very thickly coated with emulsion con taining a very high percentage of silver bromide. The development of these thick films was usually incomplete, and fixation was difficult. As a result, at the present time, celluloid films coated on both sides with a layer of emulsion of normal thickness are preferred, and penetration of the various baths is much better. The use of these films has become almost general since the intro duction of intensifying screens (§ Various attempts have been made to incor porate, either in the support, in the emulsion, or in an additional film covering the emul sion, salts of heavy metals or substances which become fluorescent under the action of X-rays in order to increase the effect ; but it has been found more economical and just as convenient to use these substances outside the emulsion in the form of intensifying screens which are placed in contact with the emulsion and which can be used indefinitely.
As a rule, the X-ray negative is examined directly without positives being printed, and various attempts have been made to render the film more easy of examination by coating it on a translucent support, or by adding starch or other inert subs,ances to the emulsion to obtain a matt surface.
93x. Method of Working. When X-rays are used directly to record the image, the plate or film is placed in a black paper envelope, or in a holder' of cardboard, ebonite, wood, or alumin ium. Rays of low penetrative power are usually used, the average wave-length being about 1-5 AU. ; for this, a difference of potential of
from 40 to 6o kilovolts or a spark gap (between. metallic points) of from 2/ in. to in. is re quired. The less penetrating 2 are the rays, the more contrasty will be the image. But, owing to the very small effect of X-rays, which are only slightly absorbed by photographic emul sions, the time of exposure is necessarily very long. In these circumstances there is no advan tage in employing intensifying screens, which are completely ineffective.
When the mean wave-length used is less than I AU., or, better still, if it is less than o-5 AU., the direct action of X-rays on a photographic emulsion becomes almost negligible, at least in a time of exposure which is possible in practice. Calcium tungstate, however, emits an intense actinic light when exposed to such rays, and it is therefore a great advantage to expose the plate in contact with an intensifying screen coated with a uniform layer of this fluorescent salt ; or, better still, a film coated on both sides is sandwiched between two intensifying screens of calcium tungstate having coatings sufficiently thin to avoid complete absorption of the X-rays by the first screen on to which the pencil of rays is incident. It is usually impossible to avoid a certain amount of graininess in the image ; this is caused by the discontinuity of the intensifying material. The use of two screens eliminates this granularity to some extent. Finally, the time of exposure is much reduced, and the influence of secondary radiation becomes less, so that images which are both purer and more contrastv are obtained.
To obtain good results with intensifying screens, the generator should be supplied with a current of from 8o to 'coo kilovolts (length of spark gap should be at least 6 in.). The best effect of an intensifying screen is obtained when the maximum energy of the radiation used corresponds with a wave-length of O.179 (the absorption band of tungsten and this occurs when the difference of potential across the terminals of the generator is 90 kilovolts (equivalent to a spark gap of nearly 7 in.).