Radiographic density

Radiographic density is an optical property of processed film. A densitometer sends visible light through a selected area and measures how much reaches the detector. Because the formula uses a base-ten logarithm, each increase of one density unit represents ten times less viewer light transmitted through that area. The quantity is dimensionless.

Section 1.3.2.1 of the IAEA report Design, Development and Optimization of a Low Cost System for Digital Industrial Radiology expresses optical film density as this ratio of viewer luminance measured without and through the film. ISO 11699-1 covers classification and performance of industrial radiographic film systems. Neither source turns density alone into a weld acceptance result; the applicable technique, product and acceptance documents must be read for the job’s required image conditions and disposition criteria.

Worked distinction: density and contrast

Suppose a densitometer receives 100 units of viewer light before the film and 1 unit after the light passes through a selected film area. The density is log10(100 / 1) = 2. That calculation describes how dark the measured area is.

Now measure an adjacent area and obtain 2.3. The density of each area is its individual darkness. The difference between the readings contributes to radiographic contrast. A film can therefore have suitable overall darkness yet show too little density difference between an indication and its surroundings for reliable interpretation. Density is not a synonym for contrast, defect sizing or sensitivity.

The film qualification also does not transfer unchanged to digital radiography. Digital detectors use signal, noise and display controls rather than processed-film blackening. When an image reveals porosity or lack of fusion, the governing quality level and inspection procedure decide the result. Radiographic density only establishes one condition of the film image used for that interpretation.