"Equation (2) can be further expressed as follows: S = ln ??? ( I I BL ) = - d ??? ( ?? 1 ??? ?? ??? c 1 + ?? 2 ??? ?? ??? c 2 ) ( 3 ) where: I BL = I o ??? ( e - ??? j = 1 a ??? ?? j ??? d j ??? c j ) = baseline s is a value obtained by measuring I with the photodetector and calculating the ratio of I to IBL after taking the natural log." . . . .