We have already reported the correction of a spectrum observed by a monochromator by means of the optical transfer functions (OTF). In these reports, the grain of a film produced a noise which could be contained in the line profile of a corrected spectrum and the line spread function was used to determine OTF. Therefore, in this study the process of smoothing noise by means of the method of least squares was performed, and then the correction of the spectrum was made. We already knew that the signals and the grains make products which represent the intensity of the light which transmits through films. After considering and examining these relationships, we conclude that the correction of a spectral image by means of OTF is difficult to make, unless the process of smoothing is performed on the noise included in signals.