As figure 4, but only the wavelength range of 5000–8000 Å is compared and the four types of normalizations are adopted in the optimizations of equation (1). The top left panel is the result for constant, top right for linear, bottom left for quadratic, and bottom right for cubic normalization. The green lines in the figure represent linear correction functions (see sub-subsection 3.1.2), and σ is the standard deviation of the residual of the best-fitting linear function. (Color online)