"Alternatively, [we can] suppose that the line is scanned by a spectrophotometer with a Gaussian sensitivity function" . Then, in this experimental framework, we have the convolution of the line with the instrumental function profile. Let us remember that "the general expression that takes account of all the instrumentally induced distortion of the true band shape can be called the instrument function" . It is also known as the "instrumental transfer function" . As told by S.G. : "Each monochromatic component 𝜑(𝑥)𝑑𝑥 of the true radiation is replaced by the apparatus [instrument] function, as a result of which, at some arbitrary point 𝑥′, there is created an illumination (or current) 𝑎(𝑥′-𝑥)𝜑(𝑥)𝑑𝑥. Other monochromatic components of the true distribution also make a corresponding contribution to the illumination at the point 𝑥′, and as a result the observed distribution 𝑓(𝑥′) will be expressed by the following integral":