T.O. 33B-1-1 6-26 Table 6-5.  Relative Speeds of X-ray Films Exposed at 100 kVp. Film Designation Relative Film Speed* Agfa D8 D7 D6R D5 D4 D3 D3 (single coat) D2 3.7 2.7 1.5 1.7 1.0 0.75 0.28 0.28 Kodak AA T B M R R (single coat) 3.1 2.07 2.0 1.0 0.4 0.2 Fuji IX150 IX100 IX80 IX59 IX50 IX29 IX25 3.6 2.0 1.0 .8 0.5 0.4 0.36 *  Film speed numbers should be compared only within a single manufacturer. 6.4.1.4 Film Density. The characteristic curves (see Paragraph 6.4.1.6) 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: D I I_t = log⁰ Where: D = film density. I₀ = original light intensity falling upon one surface of film. I_t = light intensity transmitted through the film.