You can consider using AWS D1.1 as a reference.
The IQI is a means to determine whether the technique employed is adequate to produce a radiograph that has the required clarity and sensitivity. A number of welding standards use 2% sensitivity as a quantifier that the quality of the radiograph is acceptable to "read". The darker the film, i.e., the higher the density, the more information the film contains, but the brighter illumination must be used to see that information. The densitometer measures the film density to verify the density is between a high and low value as specified by the governing standard.
Best regards - Al