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[S1 + S2 + S3 + S4 ]
           p
I0 =
Signal Sum
S1
S4
S2
S3
S1
Time
Intensity
[S1 - S2 -S3 + S4 ]
           2 I0
a =
[S1 + S2 - S3 - S4 ]
           2 I0
b =