"vector sum" refers to the total or combined effect of adding multiple vectors together. It involves both the amount or magnitude of each vector and their direction. The
vector sum represents the result of adding these vectors together as if they were arrows pointing in different directions.
Full definition
It depends on a major error by Houghton who used the Schuster - Schwarzchild «two - stream approximation» which is used in astrophysics but breaches the most basic of radiation physics, Poynting's Theorem, which states that
the vector sum of all arriving vectors at a point.
The only way the momentum can balance in gases is by collisions with other gases, and
the vector sum of all these collisions will have to represent the original momentum of the impinging wave.
Poynting's Theorem states that the net power is determined by
the vector sum of all the PVs at a point.
Only
the vector sum of Emittances predicts real net IR flux, from Maxwell's Equations.