instrument has a particular filter, such as a lux meter, its readings are restricted to be used
for a special purpose If universal comparison among different radiation sources, such as
radiation from the sun and from various artificial light sources is necessary and
photosynthesis is not a consideration, then an energy unit such as W/m2 would be better,
even for the visible region. The results should clearly indicate the wavelength measured.
The irradiance measured outside of the earth's atmospheric layer on a surface held
perpendicular to the solar beam is called the solar constant. Its mean value is 1373 w/m*m.
The solar energy flux density measured at sea level cannot exceed this value due to
attenuation by the atmosphere. A maximum of approximately 75% of the solar constant or
1000 W/m*m can be measured at the Earth's surface.
near noon on a clear day the average photon flux density on the ground in PAR can be
calculated; assuming a measured total solar flux density of 500 W/m*m, then dividing by
will provide approximately 2000 mol photon/m*m/s PAR flux density
from artificial light sources such as high pressure sodium lamps and fluorescent lamp,
commonly used in practice, varies from 200 to 1000 p.1T10l photon/m*m/s.
5. 3 Solar radiation properties of covering materials
The spectral distributions of transmissivity for three types of films are shown in Fig. 5.1.
Spectral distribution of transmissivity is important, but reflectivity and absorptivity should
also be noted. All three properties are expressed in either decimal fraction or percentages;
their total is 1, or 100%. If expressed in decimal fractions.