@prof misrolav balda
here is my question in mathlab answer
http://www.mathworks.com/matlabcentral/answers/65291-imaginary-value-from-solution-nonlinier-least-square
check it there please

@ prof Misrolav Balda
actually my data consist from so many wavelength but this i give you from two wavelength
for 690nm
F = [0.779543143 0.624771688 0.387776261 0.266986051 0.174732672 0.130313689 0.110540421 0.051412959]

a is in range = 0.09 - 0.8
b is in range = 7 - 15

for c = [0.8 0.9 1 1.1 1.2 1.3 1.4 1.6]

i always get the imaginary solution with your code, please help me