Instructions

HD2402 - 8 - V1.3
2 RADIOMETRY OVERVIEW
The foregoing exposition is far from being exhaustive, but will be useful to make the
HD2402 unit operation known and to understand the approximations made by the in-
strument for the calculation of the various radiometric quantities requested by L.D. No
81/08.
2.1 IRRADIANCE
Irradiance E
i
at a point of a surface is defined as the density of energetic flux d Φ
I
that
flows through the unit area dA (Figure 1):
dA
d
E
i
i
Φ
=
dA
d


Figure 1: irradiance
The irradiance measurement unit is [W/m
2
]; when calculated at different wavelengths,
we have spectral irradiance [W/(m
2
nm)].
From the definition of irradiance we obtain some properties useful for understanding
the light behavior:
1. We have maximum irradiance on a surface when the incident light beam is per-
pendicular to the surface and decreases with the incident angle according to the
cosine formula (Law of Cosine):
)cos(
0_
θ
⋅
=
ii
EE
where:
E
i
is the irradiance on the surface taken into consideration,
E
i—0
is the irradiance when the surface is perpendicular to the flux,
θ is the angle between the surface and the flux direction.
2. As to a point light source (or to be considered as such) that radiates light uni-
formly in all directions (isotrope), the irradiance produced on a surface decreases
with the square of the distance.
d
Φ
i