25. INSTRUMENTS AND METHODS FOR THE GROUND-LEVEL
REFERENCE MEASUREMENT OF SOLAR RADIATION, ALBEDO
AND NET RADIATION
255
between the surface of the cavity and the direction is constant and equals
We have chosen this shape of radiators as a result of the following
considerations.
It is known (Blokh, 1962, Drummond, 1970) that consideration of the
non-spherical indicatrix of radiation in the calculations of blackbody
emissivity is rather complicated. To simplify the calculations, we have made
an attempt to find a configuration of the cavity which allows the application
of spherical indicatrixes without considerable decrease in the accuracy of
calculations. This may be done if the emissivity of the surface element
in
the direction of the opening does not differ essentially from the emissivity
of
which is determined from the integral flux, radiated by the surface
element into the hemisphere. In view of identity the indicatrixes of radiation
and absorption, the experimental data, necessary for the determination of the
shape of the black body, can be obtained from angular characteristics of
radiation detectors, which are covered by the Parson varnish. These studies,
as well as the results published in (Harris, 1967, Kmito, 1976) have shown
that, for the geometry of the cavities presented in Fig. 8, the requirement
mentioned above is fulfilled with sufficient accuracy, and that the use of
spherical indicatrixes cannot cause considerable errors.
The resulting reflectivity of this radiator is calculated at
and
this result is expressed as
after considering the
approximation of neglecting the reflections of higher orders, and estimating
the errors. This value of determines the self-radiation of the blackbody
with a high enough accuracy, but it does not permit to consider the increase
of the outgoing radiation flux caused by the reflection of external fluxes
from an insufficiently black cavity. This phenomenon is more essential for
low-temperature radiators, and calls for a correction of reflectivity according
to the formula
where is the corrected reflectivity, is
the radiation of external sources entering into the cavity of the blackbody, T
the temperature of the blackbody. The external flux
must be estimated
separately in every specific case. In our experiments, was up to 50% of the
flux of the radiator. The accuracy of the calculations of the radiation of the
blackbody exceeds the accuracy of modern net radiometers several tens of
times. When calibrating modern net radiometers, special attention should be
paid to methodical errors, but errors in the determination of radiation fluxes
can usually be neglected. To eliminate the reason for the main methodical
error, the non-radiative heat transfer, a method of artificial ventilation is
devised.
It can be concluded that this method of calibration of miniature net
radiometers in the diffuse flux of infrared radiation enables to determine the
REFERENCE MEASUREMENT OF SOLAR RADIATION, ALBEDO
AND NET RADIATION
255
between the surface of the cavity and the direction is constant and equals
We have chosen this shape of radiators as a result of the following
considerations.
It is known (Blokh, 1962, Drummond, 1970) that consideration of the
non-spherical indicatrix of radiation in the calculations of blackbody
emissivity is rather complicated. To simplify the calculations, we have made
an attempt to find a configuration of the cavity which allows the application
of spherical indicatrixes without considerable decrease in the accuracy of
calculations. This may be done if the emissivity of the surface element
in
the direction of the opening does not differ essentially from the emissivity
of
which is determined from the integral flux, radiated by the surface
element into the hemisphere. In view of identity the indicatrixes of radiation
and absorption, the experimental data, necessary for the determination of the
shape of the black body, can be obtained from angular characteristics of
radiation detectors, which are covered by the Parson varnish. These studies,
as well as the results published in (Harris, 1967, Kmito, 1976) have shown
that, for the geometry of the cavities presented in Fig. 8, the requirement
mentioned above is fulfilled with sufficient accuracy, and that the use of
spherical indicatrixes cannot cause considerable errors.
The resulting reflectivity of this radiator is calculated at
and
this result is expressed as
after considering the
approximation of neglecting the reflections of higher orders, and estimating
the errors. This value of determines the self-radiation of the blackbody
with a high enough accuracy, but it does not permit to consider the increase
of the outgoing radiation flux caused by the reflection of external fluxes
from an insufficiently black cavity. This phenomenon is more essential for
low-temperature radiators, and calls for a correction of reflectivity according
to the formula
where is the corrected reflectivity, is
the radiation of external sources entering into the cavity of the blackbody, T
the temperature of the blackbody. The external flux
must be estimated
separately in every specific case. In our experiments, was up to 50% of the
flux of the radiator. The accuracy of the calculations of the radiation of the
blackbody exceeds the accuracy of modern net radiometers several tens of
times. When calibrating modern net radiometers, special attention should be
paid to methodical errors, but errors in the determination of radiation fluxes
can usually be neglected. To eliminate the reason for the main methodical
error, the non-radiative heat transfer, a method of artificial ventilation is
devised.
It can be concluded that this method of calibration of miniature net
radiometers in the diffuse flux of infrared radiation enables to determine the