Density - TM-1-1500-335-23

T.O. 33B-1-1 6-92 6.8.5 Density. Radiographic density is the blackening or darkening produced on the radiograph resulting from the metallic silver deposits remaining on the film after exposure and processing. Density is measured in terms of visible light transmission. The accepted scale of density measurement is the logarithm of the ratio of incident light to transmitted light as given by the following equation: D = log I I o E Where: D = density Io = intensity of incident light It = intensity of transmitted light 6.8.5.1 Measurement of radiographic density SHALL be done with electronic direct-reading type densitometers. The electronic direct-reading type densitometer is more accurate than the visual type. This densitometer SHALL be capable of measuring the light transmitted through a radiograph with a film density up to 4.0 with a density unit resolution of 0.02. When film densities greater than 4.0 are required to perform a radiographic inspection a densitometer applicable to film densities up to maximum density is necessary. A photographic or radiographic-calibrated reference density strip, traceable to the National Institute of Standards and Technology (NIST), SHALL be used to calibrate the densitometer prior to determining the density of a radiograph. These calibrated density strips shall be replaced whenever they are physically damaged (i.e., scratched, crimped, or become wet by any fluid) to such an extent that it may influence their effectiveness. The carbon, dot printed, etc. density strips SHALL NOT be used even though they may be NIST traceable. These strips are not able to correlate the densitometer directly to Air Force radiographic needs. Each type of calibrated reference density strip will calibrate the densitometer to a different standard level. The restrictive use of only the photographic or radiographic calibration reference density strip will better enable the standardization of all densitometers to a single calibration value establishing a common (H and D units) density for a given radiographic inspection. The aperture of the densitometer SHALL be black in color. If it is not, it may be darkened with a black magic marker or other indelible ink. 6.8.5.1.1 While performing the densitometer calibration procedure, the following SHALL apply: a. Follow manufacturer's instructions, substituting the calibration strip supplied with the instrument with the NIST traceable radiographic calibration reference density strip. b. The calibration reference density strip SHALL be removed from its protective cover during the calibration procedure and maintained in its protective cover when not in use. c. The calibration reference density strip SHALL NOT be pulled or slid when it is between the aperture and stage diffuser. The aperture SHALL be raised so that it is not in contact with the density strip when the strip is being repositioned or removed from the densitometer. d. Calibrated reference density strip measurements SHALL be determined from the center of the steps that are used for the calibration procedure. e. While reading the density of a radiograph, DO NOT pull or slide it between the aperture and stage diffuser. When repositioning or removing the radiograph from the densitometer the aperture SHALL be raised. 6.8.5.1.1.1 The density of a radiograph is important. Densities less than 0.5 show very little of the object due to three factors: (1) the density of the emulsion base, (2) the basic "fog" of the film and (3) the lack of uniform response of the film at low