Kipp&Zonen BSRN Scientific Solar Monitoring System - Manual (Page 136)

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C 3. Annex 2 to the Diffuse Geometry WG Report: Optimization of Diffusometers to Pyrheliometers
Prepared by G. Major and M. Putsay,Hungarian Meteorological Service
C 3.1 Introduction
The instrum ent m ost com m only used to m easure global radiation is the pyranom eter. The sensitivity
of pyranom eters depend on several factors. To avoid several accuracy decreasing effects the following
suggestion arises: m easure the global radiation as sum of the diffuse radiation and of the vertical
com ponent of the direct radiation. The m easured direct radiation always includes and the m easured
diffuse radiation always excludes som e circum solar radiation. To m inim ize the bias in the global radiation
values, these two kind of circum solar radiation should be as close to each other as is possible. The
full equivalence can not be achieved (Major 1992). In this work geom etrical param eters of diffusom eters
are derived to fit them to som e selected pyrheliom eters.
C 3.2 Basic considerations
The direct + diffuse m ethod of m easuring of global radiation is based on the following equation:
G = I sin(h) + D (1)
where I = the direct radiation com ing from the solar disc,
h = the solar elevation angle,
D = the diffuse radiation, defined by (1).
The pyrheliom eter m easured direct radiation:
p d ir
I = I + C (2)
d ir
where C = the circum solar radiation received by the pyrheliom etric sensor.
The diffusom eter m easured diffuse radiation:
d d if
D = G – (I + C ) sin(h) (3)
d if
where C = the circum solar radiation excluded by the diffusom eter.
Our purpose is the m inim ization of the following difference:
d p d if d ir
DE = G – (D + I sin(h)) = sin(h) (C – C ) (4)
Taking into account the form ulae of calculating the circum solar radiation (Major 1994, p. 34), the difference
is:
d if p yr
DE = sin(h) B I L(z) sin(2z) (F – F ) dz, (5)
where z = the angle m easured from the solar center,
the I extends from the sm aller of the slope angles to the larger of the
lim it angles of the two instrum ents,
L(z) = the radiance distribution along the circum solar sky (sky function),
F(z) = the penum bra function for the diffusom eter and pyrheliom eter respectively.
p yr d if
For any pyrheliom eter F is a definite function of its geom etry, while in the case of diffusom eters F
is a function of the geom etry and of the solar elevation as well. L(z) sin(2z) plays the role of a weighting
function in integrating the difference of the penum bra functions. It weights the “circum stances” of the