Laboratory calibration of Beckman-Whitley's long wave radiometer was carried out by using two radiators of the Dines type, as shown in Fig, 1. Of these radiators, the surface temperature of one is near the room temperature, about 10°C, the other about 20-45°C. Let the temperature of the surface be T1 and T2, the electric motiv force V and V′, then have
(1-ε)⋅κ⋅a⋅σ(T14-T24)=k⋅(V-V′),
where ε is an absorptivity of water vapour and CO2 in the wooden box, a is blackness of radiator, κ is a constant depending upon the solid angle of the radiator seen from the element. k is the calibration constant, and obtained from Fig. 2.
These value of k is significant for the column radiation from the upward. However, a value for the total radiation from all parts on the hemisphere is desired in the measurement of atmospheric radiation. Assuming the relative value of absorptivity of the receiver surface depended on the direction of incident radiation and intensity of the atmospheric radiation from each direction, we can obtain a value required, that is larger than the value obtained from Fig. 2.