How does film work?
The photographic emulsion on x-ray film contains light-sensitive silver halide crystals, called “grains“.
- Photons of light activate the silver grains, and create a latent image. During film processing, activated silver grains are reduced to black metallic silver and create a visible image.
- Optical density (OD) is the concentration of metallic silver that remains on the developed film.
Photographic emulsions have limitations that may affect experimental results.
The response of film to light is non-linear.
- Film response is proportional to light intensity and exposure time (the Reciprocity Law). This law applies across a limited linear response range.
- Strong and faint signals display “reciprocity failure“, where film response is no longer proportional to exposure.
Low-intensity and high-intensity signals are not accurately recorded.
Saturation of strong signals.
Strong signals cause film response to plateau and saturate.
- Strong signals will saturate the darkening capacity of film. Saturation occurs when all silver grains have been activated, and no more signal can be recorded.
- Film response begins to plateau well before saturation is reached. As more silver grains are activated, each new photon of light is statistically less likely to strike an unactivated grain.
- Strong signals are under-represented, and often appear similarly dark.
Film is less responsive to low-intensity signals.
Unresponsiveness to faint signals.
- Each silver grain must absorb multiple photons of light to form a latent image. Faint bands may be unable to form a stable latent image, even with long exposures.
- In a dilution series, this causes faint signals to drop off abruptly rather than decreasing gradually.
Image clarity and band resolution may be compromised by the photographic emulsion.
Spreading of strong bands.
- On film, strong signals blur and spread — obscuring adjacent bands. “Blow-out“ of strong bands makes it difficult to detect both strong and faint signals on the same blot.
- Parallax effects (caused by double-emulsion film) also reduce image clarity and sharpness.
- Scratching and static electricity may cause image artifacts.