It is easy to see that in these circumstances a very short exposure, which gives a perfect result with a film used between intensifying screens, would give almost no trace of image at all by direct action on the emulsion. Images, therefore, which are obtained in this manner, should not really be called radiographic images, but photographs obtained by the action of fluorescence.
932. Intensifying Screens. Intensifying screens, the use of which has been described in the preceding paragraph, are generally prepared in two varieties ; one with a matt unprotected film, and the other with a glossy film obtained by a washable coating of varnish.
While matt screens, the fluorescent material of which is in effective contact with the emulsion, sometimes give images which are sharper than those given by the washable screens, in which the fluorescent salt is separated from the emulsion by the thickness of the varnish, they have the drawbacks of being exceedingly fragile and causing stains, due to contact with fingers and to splashes from solutions in a badly-organized photographic The washable screens, on the other hand, can be washed with soap and water as frequently as is necessary, and can thus be kept in a condition of perfect cleanliness..
If the two intensifying screens are not iden tical, the one with the thinnest coating of fluorescent salt should he placed on the side of the film nearest to the generator.
When filling the plate holder, care should be taken to avoid specks of dust between the intensifying screen and the film ; these would show as shadows on the film.
It has been pointed out that certain intensi fying screens acquire after a time a lasting phosphorescence ; in this case, some time should be allowed between two successive uses, to avoid the formation of an image of the previous exposure on an unexposed film.
933. Elimination of Diffused and Secondary Radiation. It is necessary, in the first place, to reduce to a minimum the production of diffused and secondary radiations, and, secondly, to absorb as completely as possible any rays, the occurrence of which has not been prevented, before they reach the film.
In the absence of any precautions, diffused radiation may have from four to ten times as much effect in the radiography of thick parts of the human body as the direct radiation (R. B.
Wilsey, 1921). This explains how it is that a radiographic image may be deficient in detail and contrast, in consequence of the general fog from these rays.
The production of diffused rays in the body which is being radiographed is limited not only by the diaphragm, which is generally placed near the generator, but also by a localizer of lead, which is placed close to the subject and which allows the X-rays to have access only to the parts of the body which it is intended to examine.
The rays diffused by the irradiated portion of the subject can be absorbed by an anti diffusing grid of the Potter-Bucky type, which consists of a series of thin shallow strips of lead. held in position by strips of a material trans parent to X-rays (e.g. strips of wood). When the grid is in the working position, all the lamellae are oriented towards the source of the X-rays ; during exposure, the complete diaphragm is moved in a cylindrical path concentric with the source of the rays. Direct rays can thus pass between the lamellae, which, owing to their movement, are not themselves registered on the film, while the diffused radiation, with the exception of that portion of it which is propagated in the same direction as the direct, is arrested.
A well-made anti-diffusing grid (very thin, flat, and perfectly convergent lead strips) which is properly centred approximately doubles the time of exposure ; bad centring results in a much longer time of exposure, as the shadows of the strips become much wider, particularly at the edges of the field.
In the radiography of pieces of metal (radio metallography), and particularly of thick pieces, thick masks of lead, adjusted exactly to the shape of the metal to be examined, are used as well as the localizing screens when a series of identical pieces of regular profile have to be examined. Irregularly-shaped pieces are placed in pure beeswax, which exactly takes their shape, and which is then cut to the shape of the cone bounded by the extreme rays arising from the source and by the outline of the object ; the whole is then placed in a cardboard or aluminium box, which is filled with lead 2 shot.