Published on 05-Jul-2022

Types of radiographic films

Types of radiographic films

Have you ever wondered what Radiography means and how it works? 

Here in this article, we will dive deep into Radiography, Radiography films, Radiographic film function, its parts and a lot more. Let us start with the beginning – the history of Radiography.

History of Radiography

Professor Wilhelm Conrad Roentgen (1845-1923), a professor at Wuerzburg University in Germany, discovered X-rays in 1895. While working in his laboratory, Roentgen noticed a luminous glow of crystals on a table near his cathode-ray tube. Roentgen's tube was made up of a glass envelope (bulb) encasing positive and negative electrodes. When the tube was evacuated of air, it created a fluorescent glow when applied a high voltage. Roentgen used heavy black paper to protect the tube and noticed a green-coloured fluorescent light coming from a material a few feet away.

He concluded that the tube was emitting a new form of ray. This ray penetrated the thick paper covering and exited the room's phosphorescent components. He discovered that the new beam could penetrate through most materials, casting solid object shadows. According to Roentgen, the radiation could likewise pass through human tissue, but not bones or metal objects. A film of his wife, Bertha's hand, was one of Roentgen's initial tests in late 1895. Roentgen generated a radiograph of weights set in a box to show his colleagues, indicating that the earliest use of X-rays was for an industrial (rather than medical) application.

Producing a Radiograph

A radiograph is a photographic image created by passing radiation through a subject and onto film. Some kind of recording system is required. The film used to be the most popular technology, but digital detector arrays have mostly supplanted film in most industries. A radiograph is a photographic recording created by passing radiation through a subject and onto a detector, resulting in a latent image of the subject.

A latent image is an image formed on a detector due to the interaction of radiation with the detector's substance. This latent image will not be apparent to the naked eye until it has been processed further. X-ray film is exposed to chemicals similar to those used to prepare photographic film to make the latent image visible. X-ray photons are counted in an array of pixels to generate an image in digital detectors. So the Radiographic film function is to make the latent image visible.

What is a radiographic film?

An emulsion-gelatin containing radiation-sensitive silver halide crystals, such as silver chloride or silver bromide, and a transparent, flexible, blue-tinted base make up X-ray films for general radiography. To account for the specific features of gamma rays and x-rays, the emulsion is different from those used in other types of photography films, yet X-ray films are light sensitive. Typically, the emulsion is coated in layers of roughly 0.0005-inch thickness on both sides of the base. The amount of radiation-sensitive silver halide doubles when the emulsion is applied on both sides of the base, increasing the film speed. Because the emulsion layers are thin enough, developing, fixing, and drying may be done quickly. Because the emulsion layers are thin enough, developing, fixing, and drying may be done promptly.

A few radiography films only have emulsion on one side, resulting in the image's most detail.

Some Br- ions are liberated and trapped by the Ag+ ions when gamma rays, x-rays, or light strike the grains of the sensitive silver halide in the emulsion. A "latent (hidden) image" is a change that is so subtle that it cannot be detected using regular physical procedures. When exposed grains are exposed to a chemical solution (developer), they become more sensitive to the reduction process, resulting in the creation of black, metallic silver. The image is created by the silver, which is suspended in the gelatin on both sides of the base...

Developing Film

To comprehend how a radiograph image is created, we must first examine the Parts of radiographic film. There are three major Radiographic film layers - base, emulsion, and protective coating are among them.

The base

Every radiographic film has a base to which the additional elements adhere. The film foundation is commonly made of cellulose acetate, which is clear, flexible plastic. This plastic looks like what you'd find in a wallet for storing photos. The base's primary purpose is to give support to the emulsion. It is not radiation-sensitive and cannot record images.

The film base's clarity or transparency is an important feature. The light-transmitting radiographic film is required. After a film has been chemically treated, it can be interpreted. An illuminating film device, which is usually a high-intensity light source, is commonly used for this.

The emulsion

The other two components are the film emulsion and protective coating, which are essentially constructed of the same substance. They are frequently put to the film during production and have a pale yellow tone with a glassy aspect. Even though they are constructed of the same material, they provide the film with two different features. The picture layer of the emulsion is separated from the protective layer by these features.

The protective layer

The protective layer is responsible for safeguarding the softer emulsion layers beneath it. It's only a thin gelatin skin that protects the film from scratches while handling. Shrinkage (after drying that generates glassy protective layers) and dissolving in warm water are two highly important qualities for filmmakers. If the layer is dissolved in cold water, it swells and absorbs the water.

"Emulsion” is the scientific term for the softer layers of the gelatin coating. Something is suspended in an emulsion. The latent picture on the film is formed by this substance in suspension that is sensitive to radiation. Silver bromide is added to the dissolved gelatin solution during film production. Silver bromide crystals are kept suspended throughout the emulsion while the gelatin solidifies. The silver bromide crystals in the film become ionized in various degrees when exposed to radiation, generating the latent picture. Every ionized grain or crystal of silver bromide can be reduced or developed into a grain of black metallic silver. This is what appears on the radiograph as a visible image. This visible image is made up of a vast number of silver crystals, each of which is individually exposed to radiation yet works together to generate the image as a whole.

Chemical development is required after a film has been exposed to radiation and has acquired the latent image. The goal of developing the film is to bring out the latent picture so it can be viewed. Three processing solutions must be utilized to transform exposed film into a usable radiograph. The developer, stop bath, and fixer is the three. Each of these options is critical for processing the image so that it may be viewed and saved throughout time.

The process of developing film

  1. We initially expose the film to the developer solution to begin the process of transforming the latent picture on the radiograph into a useable image. The developer's mission is to create and reveal the latent image. The film is affected by a specific chemical in the development solution that reduces the exposed silver bromide crystals to black metallic silver. This development procedure is actually a multi-step procedure. Remember the film manufacturing properties we discussed earlier? They'll come in handy later in the development process. The developer must first penetrate the film's protective covering in order to alter the silver crystals. Keep in mind that the film's protective layer is constructed of gelatin and is temperature and water sensitive. The developer solution comprises a mixture of chemicals, including alkali, metal, hydroquinone, and water. The alkali's job is to pierce the protective layer so that the metal can reduce the exposed silver bromide to black metallic oxide.
  2. The stop bath is the second step in the development process. This bath is made of water and glacial acetic acid. It's crucial to understand that alkalis and acids balance each other out. The stop bath's purpose is to negate any excessive silver crystal formation swiftly. The radiography image is practically impossible to interpret due to the overdevelopment of the silver crystals.
  3. The fixer is the third stage of development. Its purpose is to keep the image on the film firmly fixed. This is a multi-step procedure as well. The fixer must first remove any unexposed silver crystals from the emulsion before hardening the remaining crystals. This method is used to keep the radiographic image safe over time.
  4. After the film has been adequately developed, it is cleaned in water and dried before being viewed visually.

Film Selection

When radiographing a specific component, the film to use is determined by various parameters. Some of the things to consider when choosing a film and creating a radiographic procedure are listed below.

  1. The part's composition, shape, size, weight, and placement in some circumstances.
  2. Whether x-rays from an x-ray generator or gamma rays from a radioactive source were utilized.
  3. Kilo voltages accessible with x-ray equipment or the gamma radiation intensity
  4. The relative value of great radiographic detail against speedy and cost-effective outcomes.

Choosing the suitable film and creating the best radiography procedure usually necessitates striking a balance between several competing elements. If overall resolution and contrast sensitivity are important, a slower, finer-grained film should be utilized instead of a quicker film.

Film Packaging

A variety of packing options are available for the radiographic film. Individual sheets in a box are the most basic form. Each sheet must be inserted into a cassette or film holder in the darkroom before use to shield it from light exposure. The sheets come in various sizes and are available with or without interleaving paper. A layer of paper separates each piece of film into interleaved bundles. The interleaving paper is removed before the film is inserted into the film holder. The interleaving paper helps separate the film sheets and provides some protection against scratches and dirt during handling.

Industrial x-ray films can also be purchased with each sheet wrapped in a light-tight envelope. Without removing the film from its protective wrapping, it can be viewed from either side. A rip strip makes it simple to remove the film for radiographic film processing in the darkroom. The task of loading the film holders in the darkroom is eliminated with this type of packing. Until the film is removed from the envelope for processing, it is entirely secured from finger marks and dirt.

Rolls of packaged film are also available, allowing the radiologist to cut the film to the desired length. In the darkroom, the ends of the container are secured using electrical tape. Long lengths of the film offer significant cost savings in applications like circumferential weld radiography and the investigation of long joints on an aeroplane fuselage. The film is stretched around the structure's exterior, while the radiation source is placed on an axis inside, enabling a single exposure to cover a broad area.

The film can be purchased sandwiched between two lead oxide screens in an envelope package. The screens serve as scatter reduction screens at energies below 150 keV and as intensification screens at energies above 150 keV.

Film Handling

To avoid physical strains such as creasing, buckling, friction, and so on, X-ray film should always be handled with care. When loading films in semi-flexible holders with external clamping devices, it's important to ensure the pressure is uniform. When a film holder presses against a few high places on a radiograph, such as an un-ground weld, the pressure can be severe enough to cause desensitized patches. When utilizing envelope-packed films, this precaution is incredibly crucial.

Large films should always be held by the edges and left to hang loose to avoid markings caused by wet or unclean fingers and crimp marks. A supply of clean towels should be placed nearby to encourage dry hands frequently and thoroughly. Many of these issues are avoided by using envelope-packed films until the envelope is opened for radiographic film processing.

Another essential precaution is not to remove the film from cartons, exposure holders, or cassettes too quickly. Static electric discharges can cause circular or treelike black patterns in radiographs, which can be avoided with proper care.

What are the different types of radiographic film?

The different types of radiographic film are as follows -

  • Screen type films: faster when used with intensifying screen
  • These are conventional and orthochromatic (green-sensitive)
  • Direct exposure type
  • These are used for dental exposures

Which type of radiographic film is most commonly used in general radiography?

The radiographic image is shown on the X-ray film, which is made up of a single or double emulsion of silver halide (most often silver bromide (AgBr)), which, when exposed to light, creates a silver ion (Ag+) and an electron.

The silver ion is attracted to the sensitivity specks by the electrons. The silver ions then bond together, forming clusters of metallic silver (black).

Silver can be recovered from old x-ray film using a procedure called silver recovery.


So, here we are. At the conclusion. W.e hope the article was informative and we were able to add value to your life.

Got any queries regarding the types of radiographic films? Drop them in the comments section, and our NDT experts will answer them for you!

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Application Notes