219 
r< 
(2) 
H \ -1 
Sin Ji = Sin J} = 
nVf 
+ Pr 
=f ■> 
where n - is refractive index for water 
The intensity of parallel light beam in a mass of 
water is reduced according to rule 
T = Uf cr , 
where T - is regular transmission factor; V - is 
thickness of water layer; c - is total weakening fac 
tor equal to the sum of refractive index and absorption 
index and related to relative transparency of water 0 
by ratio 
c = - In 0 
Therefore, when observed from an aircraft at an 
gle J3 from vertical, the brightness of underwater ob 
ject B Q g (1) will be lower than brightness of the same 
object B 0 g ( h ) when observed in nadir: 
B 0 g(U = 10 ct 
BoSOh) 10~ ch 
or 
Boi(l) = BoS(hVlcf c(t " h) (3) 
The brightness of underwater object is overlaid 
with brightness Bg , i.e. brightness of light ref 
lected by sea surface and scattered throughout the water. 
The aeronegatives allow measuring of différencies 
between optical densities AI) of images of underwa 
ter object I) and sea surface Dg (near the object)