2. Computing Absorbance

Computing absorbance for photometry
28may17

Absorbance A is related to Transmittance T  – Standard expressions below:
It is intensity light exiting cuvette
Io, intensity of the light entering the cuvette
Transmittance (T) = It / Io  ==  read/blank
A = – log10 T  or   A = log10 (1/T)
A = 2 – log10 %T

Programming Language Expression

C++
    A_1 = log((float)x1a/(float)x2a)/(log(10));  //calculate the absorbance  green
A_2 = log((float)y1a/(float)y2a)/(log(10));  //calculate the absorbance  blue

C++,   log()  is the natural log function.  division by log(10)  converts A to a log base 10

This code from b.y. inverts transmittance to get a positive Absorbance number but uses the natural log function.
log10(1/T)  would do the same thing.

Python

greenAbs = math.log (( greenBlank/greenRead))/(math.log(10))  # green
blueAbs = math.log (( blueBlank/blueRead))/(math.log(10))  # blue

A =  math.log10(blank/read)   #  replaces  the above

New in version 2.7.
math.log(x[, base])  With one argument, return the natural logarithm of x (to base e).With two arguments, return the logarithm of x to the given base, calculated as log(x)/log(base).  Changed in version 2.3: base argument added.

math.log1p(x)  Return the natural logarithm of 1+x (base e). The result is calculated in a way which is accurate for x near zero.

New in version 2.6.
math.log10(x)  Return the base-10 logarithm of x. This is usually more accurate than log(x, 10).

Excel
T = blank/read
A = log10(1/T)
several log functions are available in excel

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