spectral-total directional emissivity

[2.13.8.1] ε(θ,φ,T), (at a point on the surface of a thermal radiator and in a given direction)
The ratio of the radiance of the thermal radiator at temperature T, at the point and in the given direction, to that of a blackbody (bb) at the same temperature T.

epsilon left ( x,y,theta ,phi ,T right )=Lleft ( x,y,theta ,phi ,T right )/L_{bb}left ( T right )
where:
x,y = the coordinates of the point

θ,φ defines the direction

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