This integrated solar irradiance is called solar irradiation, solar exposure, solar insolation, or insolation. Solar luminosity is related to solar irradiance (the solar constant). Variability in irradiance influences conditions in the ionosphere and thermosphere. Measurements were generally expressed as scattered intensities or scattered irradiance. Solar irradiance variability assessments are useful for solar power applications. Total solar irradiance (TSI) changes slowly on decadal and longer timescales. One of its programs was the measurement of the variability of solar irradiance. Energy changes in UV irradiance involved in production and loss of ozone have atmospheric effects. For Irradiance cosine correcting optics are required. The SI unit for spectral irradiance is W/m. Such reconstructions show that solar irradiance varies with distinct periodicities. There are several measured types of solar irradiance. Actual efficiency is influenced by temperature, irradiance and spectrum. A pyrheliometer is an instrument for measurement of direct beam solar irradiance. The locations with highest annual solar irradiance lie in the arid tropics and subtropics.