We will use this relative exposure scale throughout our discussion of film contrast characteristics. The degree of processing received by film generally depends on three factors: Contrast is perhaps the most significant characteristic of an image recorded on film. The ability of a film to produce contrast can be determined by observing the difference in density between two areas receiving a specified difference in exposure, as shown in the figure above. Chemical fogging occurs at the processing stage when old or spent chemicals are used, chemicals are used in the wrong sequence, inadequate washing between processing stages or inappropriate chemicals are used. In the figure in the previous paragraph the dark areas on the right correspond to the shoulder of the characteristic curve. Refrigeration can extend the useful life of unprocessed film.
It is not uncommon for film to be fogged by accidental exposure to either x-radiation or light. The precise shape of the curve depends on the characteristics of the emulsion and the processing conditions. This is not always an easy task. The exposure to any given area of a film falls within one of three general ranges, as shown in the figure referenced in the previous paragraph. December Learn how and when to remove this template message. In this method, a strip of film is divided into a number of individual areas, and each area is exposed to a different level of radiation.
Film Contrast Characteristics
There is radilgraphic kind of chemical fog, broadly classified as silver stain. In the film shown in the figure in the previous paragraph, the areas on the left correspond to the toe of the characteristic curve.
Another possible way, though less common, is to create fog centres consisting of large specks of silver sulfide.
One kind of fog is produced from a silver halide crystal having a fog centre created by various reactions. The contrast is limited, however, to a relatively small exposure range, or latitude.
Light fogging on a print usually only occurs because of poor control of lighting in the darkroom and is seen as an overall dark veil across the print or, occasionally, as unintended solarisation.
Radiographic film is usually designated as either high contrast or medium contrast film.
The relationship between the average gradient and the average contrast factor is therefore: Fogging in photography is the deterioration in the quality of the image caused either by extraneous light or the effects of a processing chemical. The maximum density, in the shoulder, is determined by the design of the film emulsion and the processing conditions and is typically referred to as the D max. In the figure in the previous paragraph the dark areas on the right correspond to the shoulder of the characteristic curve.
Another possible solution is to use a film with a foggging latitude. At any exposure value, the contrast characteristic of the film is represented by the slope of the curve. This graph shows the relationship between the density and relative exposure for the values shown above.
The film contrast between two areas is expressed as the difference between the density values. If a film is raadiographic processed, abnormally high densities will be developed by chemical action in image areas that received little or no exposure.
In reversal processing, the material is fogged before the second developer. This important characteristic must be considered when using film to record medical images. The high contrast film can produce higher contrast.
Fogging (photography) – Wikipedia
The relationship between the film gamma value and the maximum contrast factor is given by: It is not always possible to predict the amount of x-ray exposure required in every procedure because of subtle variations in body size and composition. When film is stored in a clinical facility, the stock should be rotated on a first-in, first-out basis. Developers for such applications usually contain an antistaining agent, usually soluble organic thiols that form soluble but non-reactive silver compounds with free silver ions.
Two parameters are often used for this purpose: Another case is when the fog centre is created chemically, with propylene glycol and white wax. In this case, the fog centre is created as latent image and made into visible fog by the action of the developer. When an x-ray beam passes through certain body areas, the penetration of the areas varies considerably because of differences in tissue thickness and composition.
Medium contrast film is often referred to as latitude film. The precise shape of the curve depends on the characteristics of the emulsion and the processing conditions.
This density is produced by the inherent density of the film base material and the low-level fog in the film emulsion; it is therefore commonly referred to as the base plus fog density. All film, even under the best conditions, shows some density even if it filj received no radiation exposure.
The highest level of contrast is produced within a range of exposures falling between the toe and the shoulder. The development of film fog with age is accelerated by heat; therefore, film should not be stored dadiographic hot areas.
Articles lacking sources from December All articles lacking sources Commons category link from Radioggraphic. Online Textbook Table of Contents. The exposure contrast between two areas can be expressed as a ratio or percentage difference, as illustrated above. This results from chemicals in the developer solution interacting with some of the film grains that were not sensitized by exposure. The contrast curve corresponds to the slope of the characteristic curve.
The exposure range over which a film produces useful contrast is designated the latitude. Exposure values above the latitude range also produce areas with very little contrast and have the radiograpbic disadvantage of being very dark or dense. Contrast is perhaps the most significant characteristic of an image recorded on film.
Radiographic film fog in the darkroom.
With modern emulsion technology and well formulated developers, silver stain has become uncommon. In the figure below the exposures to the different areas are given relative to the center area, which has been assigned a relative exposure value of 1. In most applications it is usually desirable to maintain a constant developer activity by replenishment and to control the degree of development by varying the temperature or, in some cases, the amount of development time.
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