title: Wiener spectrum of radiographic systems: comparison of different evaluation methods.
abstract: The noise power spectrum, or Wiener spectrum, of the radiographic mottle is a fundamental quantity in film-screen image quality evaluation. In this paper, using a high-quality computerized microdensitometer, two different acquisition and calculation methods for noise evaluation are compared. The first one is the classic (unidimensional) method used in film noise evaluation: a long and narrow slit (10 x 400 microns2) is used to delimit the microdensitometer light beam and the transmission data are collected by scanning the sample in a rectilinear pattern. A section of the two-dimensional Wiener spectrum is thus obtained. The second (two-dimensional) method is similar to that used in digital image noise evaluation: a square slit is used on the microdensitometer window and data are collected by scanning the sample on a square pattern. To evaluate the effect of different sampling frequencies, our data were acquired both selecting a 50 x 50 microns2 square slit and a 20 x 20 microns2 square slit. The two-dimensional Wiener spectrum thus obtained is then reduced to a unidimensional function. The measurements were made on two different films (Kodak Ortho G e Kodak T-MAT G) exposed with the same screen (Kodak Lanex Regular). These films have the same sensitivity but a different emulsion structure. One film (Ortho G) is made of irregular halide silver grains and the other (T-MAT G) of tabular grains. A satisfactory agreement between the two procedures was found which makes the comparison of data from the laboratories using microdensitometers and those using TV-grabbing system for film-screen evaluation meaningful.
authors.affiliation: Servizio di Fisica Sanitaria, A.S.S.N. 1, Trieste.
authors.forename: P
authors.initials: P
authors.lastname: Bregant
authors.forename: M
authors.initials: M
authors.lastname: De Denaro
authors.forename: F
authors.initials: F
authors.lastname: De Guarrini
authors.forename: G
authors.initials: G
authors.lastname: Borasi
mesh_terms: D055585:Physical Phenomena; D010825:Physics; D011859:Radiography / Q000379:methods*