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Radiant Flux Density
Irradiance
Radiant exitance
Radiosity is an old term for radiant exitance.
Radiant emittance is an old term for radiant exitance.
(Terms of radiometry)
The radiant flux per unit area at a point on a surface,
where the surface can be real or imaginary (a
mathematical plane). An imaginary surface can for example
be used to calculate or measure radiant flux density
anywhere in space.
There are two cases:
Irradiance
(usually Ei in formulas) is the total
amount of radiative flux incident upon a point on a surface
from all directions above the surface.
Irradiance weighted with the response curve of the
human eye gives Illuminance.
Radiant exitance
is the total amount of radiative flux leaving a point on
a surface into all directions above the surface.
Radiant exitance weighted with the response curve of the
human eye gives Luminous Exitance.
Standard unit of all forms of radiant flux density
is watts per square meter (W/m2).
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