Measurement of total and spectral solar irradiance: Overview of existing research

The quantitative assessments of the solar radiation flux and the variations of its spectral distribution in the visible and near-infrared ranges of the electromagnetic spectrum are of great interest in studying solar-terrestrial influences. The reason is that the main part of the solar radiation energy is concentrated in that range and it determines the thermal equilibrium of the earth's atmosphere. This paper provides an overview of spectral global solar irradiance observations and of broadband solar irradiance observations from the ultraviolet to the near infrared. Measurements of the spectral solar irradiance in the near UV, visible and near IR were carried out by many researchers in two types of measurements; spectral global solar irradiance and broadband solar irradiance. The results from this study show that the measurement of the spectral solar radiation in the near UV, visible and near IR ranges can be made either by high precision and expensive instruments or by aid of rather simple, less precise and comparatively inexpensive broadband instrument - pyrheliometers or pyranometers in combination with glass filters. Selected narrow waveband instruments are characterized by simpler, less expensive and easy to maintain and calibrate compared to high-resolution scanning instruments.