Components of the Film–Screen System

A. Intensifying Screens

• Purpose: amplify X-ray signal by converting X-rays into visible light photons.
• Each absorbed X-ray photon produces thousands of light photons, which expose the film more efficiently, greatly reducing required patient dose.

Typical structure:

1. Base: polyester support for stability.
2. Reflective layer: improves light collection efficiency.
3. Phosphor layer: active layer that emits light (Calcium tungstate – CaWO₄ and later rare earth phosphors like Gadolinium oxysulfide – Gd₂O₂S:Tb).
4. Protective coat: thin transparent layer to prevent damage.

B. Radiographic Film

• Purpose: record the light image created by the intensifying screen.
• Structure:
  1. Base: transparent plastic (polyester).
  2. Emulsion layer: silver halide (AgBr) microcrystals suspended in gelatin.
  3. Adhesive and protective layers.

Film may be single- or double-sided depending on use (e.g. mammography uses single-sided for higher resolution).

Mechanism of Image Formation

This is largely here for you information. The specifics make up a very small part of the exam if they do come up. For those looking to be one of the top students, it’s worth going over the fundamentals here.

1. X-ray Exposure and Light Conversion

a. X-ray photon absorption by the screen

• When the X-ray beam exits the patient, photons strike the intensifying screen inside the cassette.
• The phosphor crystals in the screen (e.g. calcium tungstate or rare-earth phosphors like Gd₂O₂S:Tb) absorb X-ray energy through photoelectric interactions.

b. Light photon emission (fluorescence)

• The absorbed X-ray energy excites the phosphor atoms, raising electrons to higher energy levels.
• As these electrons return to their ground state, they emit visible light photons, a process called fluorescence.
• Each absorbed X-ray photon produces ~1,000–5,000 visible light photons.

c. Light transmission to film

• The emitted light photons travel toward the radiographic film, which lies in close contact with the screen.
• Some light reflects internally or is scattered, but most reaches the film’s emulsion layer.
• The result is a spatially varying pattern of light intensity that corresponds to the varying X-ray intensity exiting the patient (the latent image precursor).

2. Film Exposure (Latent Image Formation)

The radiographic film consists of a gelatin emulsion containing silver halide crystals (typically AgBr) coated on a transparent base. When the film is exposed to the screen light (and a small amount of direct X-rays), the following sequence occurs within the emulsion:

a. Photon interaction

• Light photons from the screen (or X-ray photons directly) interact with the silver bromide (AgBr) crystals.
• This excites an electron from a bromide ion (Br⁻), which becomes a free electron within the crystal lattice.

b. Electron trapping at sensitivity specks

• Each silver halide crystal contains small imperfections or “sensitivity specks”, typically silver sulfide (Ag₂S) sites introduced during manufacturing.
• The free electron migrates through the crystal and is captured at one of these sensitivity specks.

c. Silver ion reduction

• The trapped electron attracts a positively charged silver ion (Ag⁺) from the crystal lattice.
• The silver ion is reduced to metallic silver (Ag⁰) at the speck: Ag⁺ + e⁻ → Ag⁰
• This process repeats for a few electrons, forming a tiny cluster of metallic silver atoms.

d. Formation of the latent image

• These microscopic silver clusters are too small to be seen. This is called the latent (invisible) image.
• The number of sensitised crystals (those containing silver specks) is proportional to the local exposure.
• Areas of the film that received more X-rays (and therefore more light from the screens) will have more sensitised grains.

3. Chemical Development (Latent → Visible Image)

During chemical processing, the latent image becomes visible through selective reduction:

a. Development

• The film is immersed in a developer solution (e.g. hydroquinone, phenidone).
• These chemicals reduce exposed silver halide crystals (those with latent image centres) to black metallic silver.
• Unexposed crystals (those without silver specks) remain unchanged.
• The process is self-reinforcing: once reduction begins, it accelerates within sensitised grains.

AgBr + reducing agent → Ag⁰ + Br⁻ + oxidised developer

b. Fixing

• The film is then placed in a fixer solution (e.g. ammonium thiosulfate), which dissolves and removes all unexposed silver halide crystals.
• This leaves behind only the developed metallic silver, which forms the visible black image.

c. Washing and drying

• The film is washed to remove residual chemicals and dried to stabilise the emulsion.

4. Visible Image Characteristics

After development:

• Darker regions on the film correspond to areas of high X-ray exposure (e.g. lungs), because more silver was reduced.
• Lighter regions correspond to low X-ray exposure (e.g. bone or metal), where fewer grains were exposed.

This inverse relationship (more exposure → darker film) is the optical density–exposure relationship described by the Hurter–Driffield curve (also known as the Characteristic curve).

To summarise

Stage	Process	Energy Conversion/ Process
1. X-ray absorption	X-rays absorbed by phosphor	X-ray → electron excitation
2. Fluorescence	Phosphor emits visible light	Electron relaxation → light photon
3. Film exposure	Light photons expose silver halide	Light photon → electron in AgBr
4. Latent image formation	Electrons trapped at sensitivity specks → silver atoms	Charge trapping and reduction
5. Development	Chemical reduction of sensitised grains	Ag⁺ → Ag⁰ (metallic silver)
6. Fixing	Removal of unexposed AgBr	Chemical dissolution
7. Visible image	Pattern of metallic silver densities	Latent image → visible image

The relationship between optical density and film exposure is described by the Film characteristic curve (Hurter-Driffield Curve)

Film Characteristic Curve (Hurter–Driffield Curve)

This curve plots optical density (OD) vs log exposure, showing how film darkens with increasing exposure. Think of this as a measure of how see-through the film is at changing film exposures.

Regions of the curve:

1. Toe: low exposure region (underexposed, low density, see through film).
2. Linear region: proportional response, diagnostic range, film optical density changes change linearly with exposure, highlights contrast between structures well at these exposure levels..
3. Shoulder: high exposure region (saturation, at these exposure levels the film is opaque, unable to differentiate subtle contrast differences).

Key terms:

• Film contrast (γ): slope of the straight-line portion. Steeper slope = higher contrast.
• Film speed: exposure required to produce a given OD.
• Latitude: range of exposures producing diagnostically useful densities (inversely related to contrast).

Advantages and Limitations

Advantages

• High spatial resolution.
• Permanent, stable record.
• No electronic artefacts.

Limitations

• Narrow exposure latitude (limited dynamic range).
• No post-processing capability.
• Chemical processing required (time, maintenance, waste).
• Dose higher than modern digital systems at equivalent image quality.
• Physical storage and degradation issues.

Key Takeaways and Exam Tips:

• Film–screen systems used phosphor screens to convert X-rays to light, reducing dose.
• Film recorded a latent image, developed chemically to yield visible silver density.
• Hurter–Driffield curve: relates optical density to exposure; slope = contrast, latitude = exposure/dynamic range.
• Faster screens reduce dose but lower spatial resolution.
• Limited exposure latitude and manual processing variability led to replacement by digital imaging.
• (Not so) common exam question: “Describe the principles of film–screen radiography and explain how intensifying screens improve efficiency.”

Up Next

Next, we’ll cover Digital Radiography Overview, introducing computed (CR) and direct digital (DR) systems. These technologies replaced film–screen imaging, forming the basis of all modern radiographic detection.