Granularity, or RMS granularity, is a numerical quantification of film-grain noise, equal to the root-mean-square (rms) fluctuations in optical density, measured with a microdensitometer with a 0.048 mm (48-micrometre) diameter circular aperture, on a film area that has been exposed and normally developed to a mean density of 1.0 D (that is, it transmits 10% of light incident on it).
Granularity is sometimes quoted as "diffuse RMS granularity times 1000", so that a film with granularity 10 means an rms density fluctuation of 0.010 in the standard aperture area.
When the particles of silver are small, the standard aperture area measures an average of many particles, so the granularity is small. When the particles are large, fewer are averaged in the standard area, so there is a larger random fluctuation, and a higher granularity number.
The standard 0.048 mm aperture size derives from a drill bit used by an employee of Kodak.
Read more about this topic: Film Grain