What influcences the speed of film in radiography?

In radiography the relationship between exposure (in C/kg) and resulting density is commonly referred to as film speed. Other than in normal photography where film speed is indicated by a DIN or ASA number, films for industrial radiography do not carry an internationally recognized speed number.

The generally accepted method of measuring the film speed of radiographic films is to measure the exposure required to achieve a density of 2.0 above base and fog, using a specific processing technique. The various relative exposure values are shown in table 1-8.

What does the characteristic curve or density curve indicate?