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Radiographic density (AKA optical, photographic, or film density) is a measure of the degree of film darkening.  Technically it should be called "transmitted density" when associated with transparent-base film since it is a measure of the light transmitted through the film. Radiographic density is the logarithm of two measurements: the intensity of light incident on the film (I0) and the intensity of light transmitted through the film (It). This ratio is the inverse of transmittance.

Similar to the decibel, using the log of the ratio allows ratios of significantly different sizes to be described using easy to work with numbers. The following table shows the relationship between the amount of transmitted light and the calculated film density.

 Transmittance (It/I0) Percent Transmittance Inverse of Transmittance (I0/It) Film Density (Log(I 0/I t)) 1.0 100% 1 0 0.1 10% 10 1 0.01 1% 100 2 0.001 0.1% 1000 3 0.0001 0.01% 10000 4 0.00001 0.001% 100000 5 0.000001 0.0001% 1000000 6 0.0000001 0.00001% 10000000 7

From this table, it can be seen that a density reading of 2.0 is the result of only one percent of the incident light making it through the film. At a density of 4.0 only 0.01% of transmitted light reaches the far side of the film. Industrial codes and standards typically require a radiograph to have a density between 2.0 and 4.0 for acceptable viewing with common film viewers. Above 4.0, extremely bright viewing lights is necessary for evaluation. Contrast within a film increases with increasing density, so in general, the higher the density the better. When radiographs will be digitized, densities above 4.0 are often used since digitization systems can capture and redisplay for easy viewing information from densities up to 6.0.

Film density is measured with a densitometer. A densitometer simply has a photoelectric sensor that measures the amount of light transmitted through a piece of film. The film is placed between the light source and the sensor and a density reading is produced by the instrument.