of 
ber 
be 
of 
AD 
re- 
Lty 
Lon 
of 
L1m 
the 
is 
in 
je. 
| SO 
Nn - 
he 
in 
ly 
‘ed 
ind 
vas 
y, 
by 
of 
it, 
‚se 
an 
Enlarged section of "uniform" film 
  
Scanning 
aperture 
Typical Average 
microdensitometer density 
trace 
Fig 4.1 
The grain structure of the emulsion under magnification. Scanning 
aperture (d) and resulting microdensitometer trace ( after Billings- 
ley, 13975 |. 
This is no problem because conversion between the two can be made 
using the formula 
0, - 0.4313 0, / T Us.) 
D T 
Formula ( 4.1 ) is derived from the definition of density D in terms 
of film transmittance T. 
0: log ( 17 T) 
From a theoretical model of the film grain ( O'Neill, 1963 ) it can be 
shown that the noise at any transmittance value T may be derived from 
a known value of the noise at a given transmittance T1 according to 
the formula: 
Or Or, AIT IT 1-117) («4.2 ] 
4.2 Spectral Characteristics of Grain Noise 
Another interesting property of the grain noise is the frequency 
distribution. When no specific information is available the best that 
can be done is to treat the noise as white. A consequence is that the 
noise has a greater band width than the signal. 
Investigations on the spectral characteristics of grain noise show 
that for practical purposes it can be modelled as white (Castleman, 
1919. ). 
145