set terminal postscript eps color enhanced
set output 'plot.eps'

set fit errorvariables

I(x)=bg+(I0*exp(-od*(36.784/((4*(x-x0)**2)+36.784))))
bg=0;
I0=1;
x0=135;
od=10;

fit I(x) filename u 1:(($2-16)/($3-16)) via od,I0,x0
print od,od_err
plot filename u 1:(($2-16)/($3-16)), I(x)