Although the radiation created in the solar core consists mostly of x rays, internal absorption and thermalization convert these super-high-energy photons to lower-energy photons before they reach the Sun's surface and are emitted out into space. The Sun emits EM radiation across most of the electromagnetic spectrum. The spectrum of the Sun's solar radiation can be compared to that of a black body with a temperature of about 5,800 K (see graph). ![]() See also: Ultraviolet, Infrared, and Light (However, the radiance at the center of the sun's disk is somewhat higher than the average over the whole disk due to limb darkening.) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m 2. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation).ĭividing the irradiance of 1,050 W/m 2 by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. ![]() Multiplying the figure of 1,050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides an illuminance of approximately 98,000 lux ( lumens per square meter) on a perpendicular surface at sea level. This is higher than the efficacy (of source) of artificial lighting other than LEDs, which means using sunlight for illumination heats up a room less than fluorescent or incandescent lighting. ![]() ĭirect sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux. At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV), with most of the extra UV consisting of biologically damaging short-wave ultraviolet. ![]() In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm). If the extraterrestrial solar radiation is 1,367 watts per square meter (the value when the Earth–Sun distance is 1 astronomical unit), then the direct sunlight at Earth's surface when the Sun is at the zenith is about 1,050 W/m 2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1,120 W/m 2. It is about 3.3% higher than average in January and 3.3% lower in July (see below). The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. The atmospheric extinction brings the number of lux down to around 100,000 lux. Where c is the atmospheric extinction and m is the relative optical airmass. The extraterrestrial solar illuminance ( E ext), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by E e x t = E s c ⋅ ( 1 + 0.033412 ⋅ cos ( 2 π d n − 3 365 ) ), To calculate the amount of sunlight reaching the ground, both the eccentricity of Earth's elliptic orbit and the attenuation by Earth's atmosphere have to be taken into account. Researchers can measure the intensity of sunlight using a sunshine recorder, pyranometer, or pyrheliometer. Solar irradiance is the solar energy available from sunlight. Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to synthesize carbohydrates and to fuel the organisms' activities.ĭaylighting is the natural lighting of interior spaces by admitting sunlight. A photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface. Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D 3 synthesis and a mutagen. Sources estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance. When blocked by clouds or reflected off other objects, sunlight is diffused. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. Sunrise over the Gulf of Mexico and Florida.
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