The characteristic curve, also known as the H and D curve, is a representation of the response of a screen film radiograph to light. The characteristic curve represents the change in optical density (OD) of the screen film in response to changing exposures (incident x-rays on the screen film).
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Image formation
Incident x-rays reaching the screen film are converted to light by the scintillator (commonly Gd2O2S crystals) within the intensifying screen, a process called fluorescence. X-ray conversion to light is directly proportional.
Light from the intensifying screen interacts with the silver halide molecules in the emulsion that coats the screen film base. The silver (Ag+) ions undergo reduction when exposed to light. This gain of an electron converts Ag+ into a stable uncharged Ag atom. The deposition of silver atoms onto the film represents the latent image.
The film is then processed. The stable silver atoms act as a catalyst for surrounding Ag ions to be reduced. Silver deposition onto the film represents dark regions on the film. More silver deposition on the film results in less light passing through that region when placed on a light box.
Optical density
The amount of light permitted through the film is known as the transmittance value. Transmittance (T) is calculated as follows 4:
T = It/I0
It = intensity of light transmitted through the screen film
I0 = Intensity of light from the light box
Meanwhile, I0/It measures the opacity of the film or ability of the film to stop light 4. The transmittance value can be converted to a unit known as optical density (OD) 4.
OD = -log10(T) or log10(I0/It)
A change in OD of 1 represents a 10-fold change in the transmittance of light.
OD never reaches 0. A new film will have a minium OD of base + fog of 0.12 4.
At the "toe" and "shoulder" regions of the characteristic curve, a large change in relative exposure does not produce a significant change in OD 4.
Relative exposure
Exposure of a radiographic film is measured in mAs and can be adjusted by varying the time of exposure. Relative exposure is the amount of exposure increase needed to produce a change in optical density. Doubling the amount of exposure will double the change in optical density. The absolute exposure (measured in mAs or mR or number of photons per square mm) is not needed to understand the relationships between exposures 4. Using logarithmic scale of relative exposure has several adavantages, namely: wide range of exposures can be represented on a single graph and exposures can still be represented on a linear scale, making curve analysis easier 4.
An increase in log of exposure of 0.3 represents the doubling of relative exposure 4.
Latitude
Latitude is the range of logarithmic exposures that produces useful OD (usually OD of between 0.25 to 2.0) 4.
The characteristic curve is specific for each screen film. The linear part of the characteristic curve represents the exposure range at which OD will change linearly. This exposure range is equivalent to the dynamic range/latitude of the screen film 5,6. The steeper the linear region, the less dynamic range the film has, and the more contrast the radiograph will have 4.
Gamma
The slope or gradient of the linear portion of the curve is known as film gamma, given by the formula below 4:
Gamma = (D2-D1) / (Log E2 - Log E1)
where D2-D1 is the difference between the optical densities of the steepest part of the curve while Log E2 - Log E1 is the difference between the exposures 4. A gamma of one represents the original subject contrast. Meanwhile a gamma of more than one exaggerates the subject's contrast and a gamma of less than one decreases the subject's contrast 4.
A film-screen system will always have higher contrast than films that exposed to X-rays directly. This is because an intensifying screen always have higher sensitivity towards X-rays. An intensifying screen would require 1 mR of exposure to produce a density of 1.0 while exposing films directly to X-rays would require 30 mR of exposure 4.
Speed
Film speed or sensitivity is the exposure required (in roentgens) to produce a density of 1.0 above base + fog density. It can be defined as follows 4:
Speed = 1/roentgens
The difference between speed and gamma is that: gamma affects the film contrast and the shape of the characteristic curve. Meanwhile, the speed affects the location of the curve along the logarithmic exposure scale 4:
A film that requires higher exposures to change OD values (a right shift of the characteristic curve) is said to be a 'slower' or 'less sensitive' film 3. Increasing the development time or temperature increases the film speed/sensitivity, film gamma and fogging because less exposure is needed to produce a specific change in OD values 3,4.
Higher speed film tends to produce more noise 2.
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