Direct sunlight has a luminous efficiency of about 93 lumens per watt of radiant flux, which includes infrared, visible, and ultra-violet light. Bright sunlight provides luminance of approximately 100,000 candela per square meter at the Earth's surface.

Sunlight is a key factor in the process of photosynthesis, crucially important for life on Earth.

Calculation

To calculate the amount of sunlight reaching the ground, both the elliptical orbit of the Earth and the attenuation by the Earth's atmosphere have to be taken into account. The extraterrestrial solar illuminance (E_ext), corrected for the elliptical orbit by using the day number of the year (dn), is:

$E\_\{\backslash rm\; ext\}=E\_\{\backslash rm\; sc\}\backslash left\backslash cdot\; \backslash cos\backslash left(2\backslash pi\backslash frac\{\{\backslash rm\; dn\}-3\}\{365\}\backslash right)\backslash right,$

where dn=1 on January 1; dn=2 on January 2; dn=32 on February 1, etc. In this formula dn−3 is used, because in the modern times Earth's perihelion, the closest approach to the Sun and therefore the maximum E_ext, occurs around January 3 each year.

The solar illuminance constant (E_sc), is equal to 128×10³ lx. The direct normal illuminance (E_dn), corrected for the attenuating effects of the atmosphere is given by:

$E\_\{\backslash rm\; dn\}=E\_\{\backslash rm\; ext\}\backslash ,e^\{-cm\},$

where c is the atmospheric extinction coefficient and m is the relative optical airmass.

Solar constant

The solar constant is the amount of incoming solar electromagnetic radiation per unit area, measured on the outer surface of Earth's atmosphere in a plane perpendicular to the rays. The solar constant includes all types of solar radiation, not just the visible light. It is measured by satellite to be roughly 1,366 watts per square meter (W/m²), though this fluctuates by about 6.9% during a year (from 1,412 W/m² in early January to 1,321 W/m² in early July) due to the Earth's varying distance from the Sun, and typically by much less than one part per thousand from day to day. Thus, for the whole Earth (which has a cross section of 127,400,000 km²), the power is 1.740×10¹⁷ W, plus or minus 3.5%. The solar constant does not remain constant over long periods of time (see Solar variation). The approximate average value cited, 1,366 W/m², is equivalent to 1.96 calories per minute per square centimeter, or 1.96 langleys (Ly) per minute.

The Earth receives a total amount of radiation determined by its cross section (π·R_E²), but as it rotates this energy is distributed across the entire surface area (4·π·R_E²). Hence the average incoming solar radiation (sometimes called the solar irradiance), taking into account the angle at which the rays strike and that at any one moment half the planet does not receive any solar radiation, is one-fourth the solar constant (approximately 342 W/m²). At any given moment, the amount of Solar radiation received at a location on the Earth's surface depends on the state of the atmosphere and the location's latitude.