Rob wrote:

According to what I know and read on the web, global radiation in the sum of
"direct radiation" and "diffuse radiation".

That's true, though this terminology is only relevant for solar radiation,
because there is a direct component (from the sun) and a diffuse (scattered)
component.

Though the term is rarely used, I suppose there is a "global terrestrial
radiation" also, which would refer to the downward terrestrial (longwave)
amount incident to horizontal. But for the idea of direct and diffuse
components is obviously meaningless.

Thus the term "global radiation" could refer to both solar and terrestrial -
essentially the sum of ALL wavelengths, downward and incident to horizontal.
Yet again, that's probably not what you want. Above, you speak of the sum of
a direct and diffuse component, which as I said is only relevant for solar.
This is why I asked what you are trying to calculate - i.e., ultimately what
are you using this for ? That'd clarify which "global radiation" you really
want.

Though the basic ideas are simple, see how confusing the terminology can
become ?