Understanding NRT- Reading 1- 2 of 2- Radiogaphic Testing A

This is true above 4.0 H&D, however as density reaches 4.0 H&D an
extremely bright viewing light is necessary for evaluation.
Density, technically should be stated "Transmitted Density" when associated
with transparent-base film. This density is the log of the intensity of light
incident on the film to the intensity of light transmitted through the film. A
density reading of 2.0 H&D is the result of only 1 percent of the transmitted
light reaching the sensor. At 4.0 H&D only 0.01% of transmitted light reaches
the far side of the film.
Charlie Chong/ Fion Zhang