Corrected conversion from transmittance to absorbance.

Relationship between transmittance (T) and absorbance (A) is:
A = -log10(T) = log10(1 / T)
rather than A = 1.0 - T, as previously used, according to
http://en.wikipedia.org/wiki/Absorbance, or
http://books.google.com/books?id=BNqp0RO7DXcC&lpg=PP1&pg=PA27#v=onepage&q&f=false