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":