Direct sunlight gives about 93 lumens of illumination per watt of electromagnetic power, including infrared, visible, and ultra-violet.

Bright sunlight provides illumination of approximately 100,000 candella per square meter at the Earth's surface.

Sunlight is a key factor in the process of photosynthesis.

Calculation

To calculate the amount of sunlight reaching the ground both the elliptical orbit of the earth and 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, known as the Julian date (Jd), is: E_ext=E_sc(1+0.034 * cos(2pi(Jd-2)/365))

The solar illuminance constant (E_sc), is equal to 128 Klux. The direct normal illuminance, (E_dn), corrected for the attenuating effects of the atmosphere is given by: E_dn=E_ext*e^-cm

Where c is the atmospheric extinction coefficient and m is the relative optical air mass.

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 1366 watts per square meter (W/m²), though this fluctuates by about 6.9% during a year (from 1412 W/m² in early January to 1321 W/m² in early July) due to the earth's varying distance from the Sun, and by a few parts 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, 1366 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.