The External Climate - Greenhouse Technology and Management - page 38

The two most usual types of sensors are
photoelectric and thermoelectric. The most-
used photoelectric sensor is a photodiode
sensor (photovoltaic). When exposed to
radiation within its range of sensitivity, it
generates a potential difference propor-
tional to the radiation flux received.
The thermoelectric sensor uses a black
body that absorbs the radiation impacting on
the sensor and measures the resulting heat.
To avoid temperature alterations due to air
convective flows, they are covered with a
plastic or glass dome, as a filter, whose trans-
mission to different wavelengths limits the
range of radiation to be measured.
There are spherical sensors (net radiom-
eters) which measure the incident radiation
(on their upper part) and the reflected radi-
ation (albedo) on their lower part. The differ-
ence between them is the net radiation.
The difficulty in accurately measuring
solar radiation, especially in the past, due to
the high cost of the sensors, necessitated the
estimation of global radiation by means of
measurement of the number of sun hours,

which varies throughout the year for each lati-
tude (Table 2.1) and also depends on local
cloudiness. The insolation or duration of sun
hours during the months of autumn and winter
is, in the absence of reliable data on solar radi-
ation, used as an estimation of the aptitude
index of a given area for protected cultivation
(Nisen et al ., 1988).
Procedures to estimate radiation,
when local measurements are not avail-
able, are described in detail in Appendix 1
section A.1.2.

2.5 The Earth's Radiation

Anybody whose temperature is above
−273°C (i.e. absolute zero, on the Kelvin
scale) emits radiation. This radiation will
have a wavelength dependent on its tem-
perature (see Chapter 5). The Earth's sur-
face, therefore, emits long-wave radiation
(far IR), which varies according to its tem-
perature which in turn is influenced by the
type of soil or vegetation (Fig. 2.14).

8

7

6

5

4

3

2

1

0

10 20 30 40 50

Wavelength ( m m)

Fig. 2.14. The earth's radiation emission according to the wavelength at the approximate earth temperature
(300 K, equivalent to 27°C). The hatched area below the curved line represents the radiation absorbed by the
earth's atmosphere and the clear area represents the remainder of the radiation that is lost into outer space
under clear sky conditions. This latter part is known as the 'atmospheric window' (adapted from Rose, 1979).