Imaging Media, Detectors and Film Processing
Film
Radiographic film has a polyester base coated with silver halide emulsion in gelatin. Smaller grains give higher resolution and slower speed; larger grains give faster film and grainier images. Film systems are graded by ISO 11699-1 into classes C1 to C6, and the technique standard selects the class with the required gradient and granularity. Lead intensifying screens shorten exposure through electron emission and filter scatter; fluorescent screens suit lower-energy X-rays. Density is D = log10(I0/It), so a hundredfold attenuation of viewing light gives D = 2.
Processing
Manual processing: developer at about 20 °C for the specified time with agitation, stop bath, fixer, wash, dry. Automatic processors need correct chemistry temperature, replenishment and clean rollers. Underdevelopment gives low density and low contrast; overdevelopment raises density and fog. Safelights must match the film sensitivity and be at the correct distance and wattage. Common artifacts: pressure and kink marks, static, water spots, chemical streaks, screen marks and light fog.
Digital detectors
Computed radiography uses photostimulable phosphor plates read by a laser scanner; digital detector arrays use amorphous silicon or selenium. Both are characterised by basic spatial resolution, grey value, SNR and normalized SNR, and ISO 17636-2 sets minimum values.
Traps
Assuming higher density always means better image quality; confusing film speed with system class; and blaming the radiographer for artifacts caused by screens or chemistry.
Worked example
Incident light 1000 units, transmitted 10 units: D = log10 100 = 2.0. To raise density from 2.0 to 2.6 requires about four times the exposure, since 10^0.6 ≈ 4.