TWI Frequently asked questions
by Ivan Pinson
Radiographic quality is assessed using the four criteria shown below:
- Density
- Contrast
- Definition
- Sensitivity
Density: The log of the ratio of the intensity of light incident on the film to the intensity of light transmitted through the film defines density:
Density =
The technician reads the calculated density of the radiograph on an instrument called a densitometer, which provides a numerical value of the amount of light transmitted.
Contrast: Degree of density difference between adjacent areas on a radiograph.
Low contrast Definition: Sharpness of the dividing line between different density fields also referred to as Geometric Unsharpness (Ug) or Penumbra,
f = size of the radiation source ofd = the thickness of the specimen being radiographed fod = distance from the radiation source to the item being radiographed
Sensitivity: Objective measurement of radiographic quality.
An object referred to as a penetrameter is included within the radiographic image. The penetrameter contains either holes or wires of known dimensions and sensitivity is determined by relating the thickness of the thinnest wire or hole present to the thickness of the specimen being radiographed as shown below. The lower the value obtained the better.
%Sensitivity = 