ANSI IT2 37:2001 pdf free 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,
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].
4.1.3 Spectral
correction factor Ideally, the microdensitometer used to measure the rms-granularity of the color negative film would have a spectral response matching that of the color photographic paper on which the print is to be made. In practice, an exact match is not possible, particularly in the red region of the spectrum. The effect of this spectral mismatch on the measured rms-granularity can be accounted for by a spectral correction factor (SCF) derived for each color channel. Topfer [4] has shown that the correction factor for each color channel can be experimentally determined by comparing the reflection densities obtained by printing a series of unmodulated, neutral exposures of the color negative film onto the color photographic paper with the reflection densities obtained from a standard neutral sensitometric exposure of the color photographic paper. Assuming a linear relationship between the transmission densities of the color negative and the log exposure seen by the photographic paper leads directly to the spectral correction factors. In general, the SCF will depend on the exact characteristics of the color negative film, the print material, and the microdensitometer response. In practice, it has been found that film and paper characteristics are sufficiently consistent across the industry so that an average set of characteristics suffices [1].