October 2016
51
Our photometer returns a value directly proportional to the light intensity.
We will call : N.
If the photometer was outside the earth atmosphere (1 AU
(5)
of the sun) for
measuring the brightness of the sun, it would give N
0
value.
(5) Astronomical Unit. It is equal to the average Earth-Sun distance (150 million kilometers).
N
=
N
0
⋅
exp
(−
m
(τ
a
+
a
R
⋅
p
p
0
+ τ
o
3
))
We will introduce a correction term taking into account the Earth-Sun distance
varies depending on the day of the year.
N
=
N
0
⋅[
r
0
r
]
2
⋅
exp
(−
m
(τ
a
+
a
R
⋅
p
p
0
+ τ
o
3
))
With
r
0
, the distance of 1 AU and
r
then Sun-Earth distance at measure date (in
AU).
r
=
[
1
−
e
2
]
[
1
+
e
cos
(
2
π
n
365
)]
Avec
e
= 0.0167
We now express
τ
a
,
optical thickness due to aerosols, according to the other
terms
ln
(
N
)−
ln
(
N
0
⋅[
r
0
r
]
2
)=−
m
(τ
a
+
a
R
⋅
p
p
0
+ τ
o3
)
τ
a
=
[
ln
(
N
0
⋅[
r
0
r
]
2
)−
ln
(
N
)]
m
−
a
R
⋅
p
p
0
−τ
o
3
[2]
The Aerosol Optical thickness is denoted AOT.
The part of this thickness created by aerosol is called Aerosol Optical Depth noted
AOD
Calibration parameters
•
N
0
parameters are determined by calibration (No_465 for blue, No_540 for green and
No_619 for red)
•
a
R
is calculated :
For CALITOO, this parameters are :
•
τ
o
3
: is supplied by le LOA - Aeronet
0.465
0.19490
0.540
0.10637
0.619
0.06119
WaveLength (µm)
a
R
Summary of Contents for CALITOO
Page 49: ...October 2016 49 4 Appendix...
