ANSI IT2.37-2001 pdf download Print Grain Index – Assessment of Print Graininess from Color Negative Film
4 Computational steps A number of physical and perceptual factors affect the perceived graininess of the final viewed print. The process of printing a negative film and the resulting graininess of the print can be modeled as a series of transformations, each of which encompasses several physical and/or perceptual factors.
4.1 Conversion of color negative film rms-granularity to color print rms- granularity This transformation models the physically measurable print rms-granularity in terms of the physically measurable film rms-granularity and the parameters of the optical printing system, print material, and measurement instrumentation.
4.1.1 Relationship of print granularity to film granularity The first, and most complex, transformation models the printing of the film grain pattern from the color negative onto the photographic paper. An approximate relationship between the granularity of the print and the granularity of the film negative is derived in annex A. Based on this derivation, the following equation gives the relationship between the film rms-granularity and the print rms- granularity,
where:
i is an index pertaining to the channel of interest (i = R, G,B);
σ p,i is the rms-granularity of the print;
i n,σ is the matte-adjusted, exposure-averaged rms-granularity of the film sample;
All of these factors are discussed sequentially, and values for each are subsequently specified.
4.1.2 Matte-adjusted, exposure-averaged film rms-granularity For the calculation of the PGI, the rms-granularity shall be measured. All recommended procedures for sample preparation and measurement given in ANSI/I3A IT2.40 shall be followed. Two adjustments shall be made to these measurements before they are used to compute the PGI. The first adjustment pertains to the effect of matte particles on the measured film rms-granularity. Color negative photographic films commonly incorporate matte particles in the film overcoat to protect against ferrotyping. These particles are transparent, but they protrude from the surface and cause light scattering. This light scattering causes a microdensity variation when the rms-granularity is measured with a microdensitometer. Tests have shown that while the diffuse illumination of most photographic printers lessens the contribution of this component to the resulting print granularity, it is not completely negligible [1].