Kipp&Zonen CM 121 B/C Shadow Ring - Manual (Page 20)

5. MEASUREMENT OF THE DIFFUSE SKY RADIATION
Instruction manual CM 121 19
5. MEASUREMENT OF THE DIFFUSE SKY RADIATION
5.1. The shadow ring correction
A pyranometer equipped with a shadow ring is measuring the downward diffuse solar radiation as
received by a horizontal surface from a solid angle of 2
π with the exception of the solid angle
subtended by the shadow ring.
To obtain an estimation of the radiation that would be received from the whole hemisphere if the
ring were not present, a correction must be introduced. Such a correction factor is a function of the
solid angle subtended by the ring part and the sliding bars of the CM 121B and their altitude above
the pyranometer horizon.
The special U-profile of the shadow ring offers the advantage that the intercepted circumsolar part
of the sky is far more constant during the year compared to an I-profile (flat) shadow band.
(Compare formula 6.8 and 6.9).
The correction presented here is for the case of uniform sky radiation. The intercepted part can
easily be calculated. A list with correction factors is supplied. See table 1 in 5.2.
It is possible to use more refined models, assuming a more realistic distribution of the radiation
across the sky, however it is difficult to indicate how much improvement can be attained doing this
as these models tend to depend on empirically obtained data. A brief discussion concerning this
problem is found on page 429 of the International Geophysical Year instruction manual 1958,
Pergamon Press, London, Paris, New York.
The table is arranged so that the correction factors for particular latitude are in one row. The factors
are computed for declinations of - 24
0 to + 24 0 with intervals of two degrees.
The declination and the date are related to each other. On the top and the bottom of the table there
is a row with the corresponding date intervals, the upper row for increasing declination, the lower
one for decreasing declination.