N = D / 2K0 (3.3 ) 
where o0 1s the standard deviation in measured density for a number of 
density levels, 0 the density range of the emulsion and K the number 
of standard deviations by which adjacent density levels should be 
separated. Thus K is a quantity related to the confidence of 
distinguishing between adjacent levels. 60 = 2Ko will correspond to AD 
i formula ( 3.4 ) and we get: 
Mr x 6D / Y (03.6) 
Here 60 is no longer the minimum density difference required to re- 
solve a target of a certain contrast ( modulation ) but the density 
interval represented by one level according to ( 3.5 ). The evaluation 
is made for a certain emulsion without considering the properties of 
the target or the human observer, focusing on the effect of film 
granularity. 
4. NOISE IN THE PHOTOGRAPHIC IMAGE 
The photographic image 1s degraded by noise. The main source is the 
grain noise. The grain in the photographic image varies in size and is 
unevenly distributed in the emulsion. This causes variations in 
density even if the imaged object is homogeneous. 
The noise affects the digitizing in two ways. It is the limiting fac- 
tor of both spatial resolution and grey level resolution. 
4.1 Granularity 
Granularity is an objective measure of the grain noise in the image. 
The grain noise sets a limit to resolution and to modulation and also 
influences the quantization. Granularity is usally expressed in den- 
sity units and in terms of the root mean square (rms) value of the 
statistical fluctuations in density, o.. The measurements are made in 
a microdensitometer with an aperture of 48 um at a density level D=z1. 
These conditions are used for example by Kodak when they supply 
granularity data for their emulsions. In the data sheets the measured 
O0 1s multiplied by 1000. 
Investigations have been .made on the relation between granularity and 
the area A of the aperture of the measuring device. As a result it was 
been found that o JA = constant is a valid relation (Billingsley, 
1375). This makes it possible to convert granularity data supplied by 
manufacturers to fit the aperture of the actual digitizer used. 
Measurements of grain noise are generally expressed in terms of 
density. The sensor in the digitizing system is sensitive to light, 
‘which has a linear relation to film transmittance rather than density. 
For this reason transmittance is the important parameter and noise 
considerations should be in terms of transmittance noise rather than 
density noise. Density noise on the other hand is important when we 
consider the visual effects. 
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