Table 6-5. Relative Speeds of X-ray Films Exposed at
Relative Film Speed*
D3 (single coat)
R (single coat)
* Film speed numbers should be compared only within a
The characteristic curves (see Paragraph 184.108.40.206) are used to relate the action of exposure to radiation on a film, which
becomes apparent in varying degrees of blackening in the processed film. In photographic usage, density is a measure
of the degree of blackening of the processed film caused by exposure to radiation. The term exposure as used in this
section refers to the amount of radiation energy reaching a particular area of the film. It could be expressed as ergs per
square-centimeter, but is more convenient for practical use when expressed in terms of dimensionless relative units, one
particular exposure value being used as a reference for other exposures. Film density is the logarithm of the reciprocal
of the fraction of light transmitted through the film with respect to the light incident on the film:
original light intensity falling upon one surface of film.
light intensity transmitted through the film.