Photo chemical process in radiography
Successive stages of silver halide photographic process on gelatin layers, They are essentially common to both negative, and for the positive process. Therefore, almost all of the following for a negative process applies to positive. The photochemical process includes the following steps: manifestation, intermediate rinsing, clamping, intermediate rinsing (subject to collect for extraction of silver), Final rinse. known, photochemical reaction that occurs under the action of light in a photosensitive emulsion, as a result of which a latent image is formed in the centers of sensitivity.
Manifestation Manifestation – this process, wherein the latent image, obtained by shooting, amplified in the millions and billions of times and becomes visible. In the brightest areas of the photographic object is restored greatest amount of silver, and in the dark – least. transitional tone (halftone) will be darker or lighter, depending on the amount of reflected light and the feature being, Consequently, under reduced manifestation of metallic silver. The image quality depends not only on the amount of light, has entered the photosensitive layer, but also on the properties of the developing solution. Consider the basic properties of the developing solution. Selective ability of the developer is in its ability to recover metallic silver image is proportional to the light to act. The more light fell on the photosensitive layer, the faster the recovery process. In areas, where the light did not work, metallic silver is recovered at the end of the process in small amounts, forming the so-called veil. The more selective ability of the developer, the greater the gap in time between the display of the latent image and the appearance of the veil, Consequently, the higher the selective ability of the developer, the lower the veil. Developer action rate is characterized by the time symptoms, during which the desired image contrast is reached. This property depends on the components, a part of the solution, and the temperature of the solution. Time, elapsed since dipping the exposed photographic material in the developer before the first trace image, It called induction period, the magnitude of which depends not only on the speed of action of the developer, but also amounts podeystvovavshego world. By the induction period can be judged on the correct exposure time and the extent of exhaustion of the developer. The maximum contrast of the image, created by the developer, It depends both on the composition of the developing solution, and on the processed light-sensitive material, as well as the development time. If the X-ray process, made under the same conditions, at the same time, but exhibit various solutions, we get a different contrast ratio, but, changing the development time, we can get the same contrast ratio. Consequently, for obtaining high contrast, some developers require less time, other more, t. it is. contrast – a function of the developer speed, which is indicative of the contrast as a property developer. Applying fine-grained developer with fenidonom, You can increase the sensitivity to 4-6 time, changing the processing time, but it increases image contrast. Effect of the developer on the graininess depends on the amount of silver grains halogen, the value of which in turn depends on the sensitivity of the emulsion. But during processing may to some extent reduce the size of these grains. The main substance, influencing the grain size during development, It is sodium sulfite, which has a solvent action on the silver grains halogen. Hence, a large amount of sodium sulfite in the developer fine. Fine-grained developers are also characterized by a low content of alkali, thereby increasing the development time, which has a positive effect on leveling property developer. Processing more deteriorates the image quality of the photographic material, because as the display is changed photographic qualitative and quantitative composition of the solution, ie. pH of the solution is changed, due to reduced concentration of alkali accumulation of oxidation products, bromidov and t.d. To increase the stability of the developing solution and in order to save consumption of chemicals introduced therein so-called reinforcing additives, which is the problem, to maintain the same level and the concentration of developing agents pH, which significantly increases the service life of the solutions and their ability to handle a greater number of photographs. For this developer solution, not going to use, They should be stored in closed vessels, and you must, that between the surface of the solution and the lid was minimal air. For these purposes, use tanks with floating lids, which are in contact with the surface of the solution, regardless of the volume of solution in the tank. Knowing exhibiting basic properties of solutions, You can operate on them, focusing on a particular property (strengthening or weakening it) to produce an image with predetermined parameters.
manifestations rate depends on the solution temperature: it increases with increasing temperature and decreases with decreasing. But it is necessary to take into account, that the change in the speed display in the areas of the emulsion, receive different exposure value, different, and it changes the nature of the image. Therefore, one of the basic conditions of a normal process is the temperature stability of solutions to meet specified tolerances for the developer. Different in character action developers have different rates of action to achieve the desired contrast ratio and the maximum density of blackening. But in all their actions speed solutions for the entire process is different. Increasing in the first, a so-called induction, period, manifestations velocity reaches a maximum in the second period – posleinduktsionny. Then manifestations speed gradually decreases. Consequently, with an increase in development time of maximum density of blackening, and the contrast ratio are increased to a certain limit, at which the maximum increase in density ceases, but the minimum density and fog density continues to increase, and the contrast ratio starts to decrease. There are two main methods of processing photographic negative: time processing and visual inspection.
INTERMEDIATE WASHING To extend the life of the fixing solution processed material after development is necessary to subject the intermediate washing of the emulsion to remove the developing solution. Lack intermediate washing is, process manifestations in the treated material will continue after washing, that can help to increase the density of the processing speed of the developer materials in. If necessary, stop the development process quickly, should drastically decrease the pH in the photographic layer. For this purpose shown photographic material to be processed in solution, having acidic.
RECORDING Fixing – transfer into soluble compounds during development unreduced silver halogen, and silver salts Ag4[Fе(СN)6]. Great impact on the speed of fixation has the diffusion rate of the fixing solution layer. Most of the observed rate of diffusion boundary layer, concentration value which should be sufficient. However, since the boundary layer capacitance is small and the concentration of the fixing solution therein is quickly exhausted, requires a constant supply of fresh solution, which is achieved by stirring the fixing solution or the movement of the processed photographic material with respect to the solution. Besides, the diffusion rate increases with increasing temperature of the solution. By fixing the length and composition of the fixer, and the quality depends afterflush. End lock can not be considered negative in the bleaching solution, since there are still a layer insoluble silver salts, which process while continuing to react with sodium thiosulfate, forming a water-soluble salt of. Therefore, fixing the duration determined by a triple double-lightening time depending on the material being processed. fixation reaction, like any other, It is a change in the concentration of substances involved in the process. In the fixing process decreases the concentration outside the fixer material and increases the concentration of substances, formed by the reaction of, And of course, what is a qualitative change in the fixer composition significantly affects the speed and quality of fixation. When machining materials, where there are several of fixing tanks and is a constant circulation of the solutions, apply counter-fixing, solution moves toward the moving film. In this way, fresh solution processes the film in the last stage. fixatives three types are used for processing photographic materials: simple, sour and tanning. simple fixatives, which include only sodium thiosulfate, have a pH of the order 8 and require careful washing after development, to prevent the developer from entering in the fixing solution. Otherwise passing into fixer silver may partially recover. With vigorous developer metallic silver forms a dichroic veil, and oxidation products of the developing agent gelatin is stained in yellow. To reduce intermediate washing in this case must be applied intermediate acidic bath. Acidic fixers not require the use of acidic baths and intermediate, since they do not form a veil or dichroic dye gelatin. In an acidic environment, pH of which has a value of 4 to 6, manifestation immediately terminated. Unlike simple acidic fixer have a greater ability to dissolve the metallic silver, wherein the dissolution rate depends on the pH. At pH 5 the dissolution of metallic silver becomes so significant, it is necessary to consider the impact of this on the image density, because in addition to a halogen silver in this environment begins to dissolve and metallic silver. Acidic tanning fixatives used if necessary zadublivaniya photolayer. The treated film in such a solution becomes more resistant to elevated temperatures, the emulsion hardness increases, gelatin swelling during washing is reduced, helping to accelerate the drying of the negative.
From the quality of the final wash, final rinse will determine the future preservation of photographic materials. The washing process is to remove the emulsion of sodium thiosulfate and the reaction products, absorbed by the emulsion in the process of chemical-physical treatment. Physically, the washing process is diffusion of solutes from the emulsion in the wash water passes, and a two-step:
1) diffusion of substances from the photographic layer;
2) removing the diffusing substances replaceable water.
There are several ways photographic washing.
1. Changing or water transfer photographic material from one bath to another while still water, thus it is necessary to perform for one hour 5-6 water changes.
2. Threaded method, when rinsing baths arranged staggered and fresh water flowing into the upper bath, where photographic tested last washing stage. In the lower water bath already supplied with a small concentration of thiosulfate, it produced the first washing step. As washing washable Photomaterial shift from the lower bath to the upper. The cascade process is a countercurrent, as the promotion of the emulsion takes place against the movement of water. It is economical, but slower, The more intense. 3. intensive method, wherein the tank is constantly supplied fresh water, deleted after use.
4. shower method, in which most washing speed is achieved by disrupting the boundary layer water jets.
Washing the photographic speed is also dependent on the water temperature, from which in turn depends on the rate of diffusion and swelling of the gelatin emulsion. The best wash rate nezadublennyh or weakly zadublennyh layers is achieved at a temperature of 14-20 ° C. Increasing the temperature to 20 ° C and above causes excessive swelling of the gelatin. Although the diffusion coefficient increases with increasing temperature, but substantial gain in speed washing gives, as it increases the path of the diffusing particles. Therefore, the best washing regime considered above temperature range.
The quality of washing is best determined by alkaline potassium permanganate solution with the following composition: potassium permanganate, city- 1 potash (or soda), city- 1 distilled water, l.- 1 For this purpose, two beakers poured on 250 ml of water from the water, then taken from the last washing it with film and allowed to drain solution in one of the glasses during 30 with. The second glass is used for monitoring. Then, in both glasses added 1 ml of the above solution. In the presence of sodium thiosulfate violet washing water passes in orange for about 30 with, and at higher concentration yellowing or completely decolorized. accuracy: 10 mg of tiosulyfata 1 liters of water.
DRYING negative for removing excess moisture from the emulsion and the substrate film is dried in a dry clean room with temperature and humidity of the room or in drying ovens, where clean air is supplied, having a certain temperature and humidity. In the first case, the drying time depends on the temperature and humidity of the environment (from 5 to 14 no), second – the temperature and humidity of air supplied. When natural drying at film can get different particles, reducing its quality; when in closets drying is possible, as the feed air first passes through special filters. drying modes affect the state of the substrate and the image quality. At high temperatures, the drying air may increase the contrast and density of the negative image, emulsion layer structure with foodstuff dries out too acquires, received for the grain. Besides, film overdrying can cause significant substrate warpage and shrinkage. The residual humidity of the substrate should be at least 15%, since at 10% residual moisture content, the film becomes brittle. In addition to the automatic photographic method the undoubted convenience in work, automatic way medical X-ray photographic films ensures high stability of the results. In proyavochnыh machines, basically, place the same processes, and that in manual photoprocessing, but, at substantially higher values of the developer and fixer temperature (not lower than 25 ° C) and less time processing. full cycle time from the time of receipt of the film to the developing machine until dry radiographs (“dry to dry”) no more than a few minutes. The most widely used in medicine got Developing machines processed roll type.
In processing the general purpose radiographic films are usually used the first two process, which it is a modern rapid process, which for 1,5-2 minutes get ready radiograph. In the third process, the film is subjected to the maximum rigid treatment, to give the desired, eg, mammography, high contrast image. The fourth process requires special reagents and is less common yet. When processing fluorography films in the developing machines processed roll type should take into account the fact, that the roll films are produced on a thin base, than leaf. In order to ensure their safe passage through the developing machine to the top of the roll you want to attach the so-called “leader” format not less than 13x13 cm. The leader can be used as a sheet of radiographic film, designed for automatic processing. Developing all types of processed roll machines are designed, basically, equally. To provide workers in the tanks processing machines stable photographic process is automatically added (proportional to the amount of processed film) regenerators developer and fixer. Norma regeneration fixer is usually larger because, that the car is difficult to implement an effective intermediate rinsing, and fixer together with the film regularly gets a certain amount of the developer. By regularly adding regenerators developing machines can be operated for a long time without a complete replacement of the working solutions. However, the waste solutions in any case should not fall into the tank for fresh regenerators developer and fixer. Only in this case, it provided the necessary quality of radiographs. Due to the high temperature and humidity in the developing machines is created a very aggressive medium, so the machine parts are subject to increased wear. To lengthen the life of the developing machines must be regularly (at least once a month) carry out preventive measures in accordance with the instruction manual specific machine. for darkroom Photographic Equipment must be equipped with running water, sewerage, general and special (working) light and have a device for chemical treatment of the photographic films. Manual processing of radiographic films is usually carried out in the tanks, the tanks with the use of special frames for fixing films, allowing to carry out their processing in a vertical position. Modern devices for manual radiographic photographic films are made of plastic materials, not corrodible, and equipped with a unit for temperature control and timer developer solution. It should be emphasized, that the processing sheet films in cells not recommended due to the instability of the results. For manual processing fluorography films are best used opaque cylindrical tanks, which have the coil to secure the film rolls in a fixed position in the form of a spiral. Fluorography film may also be processed in conventional tanks, the tanks, pre-wrapping it around the frame, for treating a radiographic film sheet. When this film emulsion must be turned outwards. Otherwise, the light stripes are formed at the points of contact of the emulsion with a frame of the film, resulting in a loss of information in the image. Modern way of medical X-ray photographic films is the use of the developing machine processed roll type. Besides the unmistakable convenience Developing machines provide high process stability photoprocessing. For working light darkroom lights are used with different light filters. When working with a blue-sensitive films are recommended (the number of produced in Russia) yellow-green filter № 117 or red filters №№ 104 and 107, with Silver halide film - only red filters. film, is sensitive to red light , You should be handled in total darkness. The photo lab lamp is allowed to use incandescent lamps with an output of not more than 25 Watt. The distance from the lamp to the desktop surface should be at least 50 cm for yellow-green color filter № 117 and not less than 75 See for red filters №№ 104 and 107. If necessary, use a lamp power 40 Watts should either increase the distance, or in any way increase the density of the filter. However, in this case the lamp is best used for indirect lighting darkroom, eg, directing light lamp on the ceiling. Installation in a photo lab light bulb with more power is not allowed. Before working with every type of X-ray film, check darkroom safelight. For this purpose, in the dark box removed from the unexposed film sheet and place it on the desktop space, covering approximately half of opaque material, eg, a piece of cardboard. Then include a lantern and were exposed underneath the film for 3 minutes, then in total darkness spend her photographic mode, which will be used in further work. If exposed film area observed clearly noticeable blackening, that the darkroom is not suitable for lighting work with this film. According to the current rate of coverage is considered safelight, If the density is less than the increase of the veil 0,1 B.