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Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter. The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and a mutagen. == Summary == Researchers may record sunlight using a sunshine recorder, pyranometer, or pyrheliometer. Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. A photon starting at the centre 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.〔(【引用サイトリンク】title= NASA: The 8-minute travel time to Earth by sunlight hides a thousand-year journey that actually began in the core )〕 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. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1367 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 1050 W/m2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W/m2. 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).〔Calculated from data in The first of each set of two figures is for total solar radiation reaching a panel aimed at the Sun (which is 42° above the horizon), whereas the second figure of each pair is the "direct plus circumsolar" radiation (circumsolar meaning coming from the part of the sky within a couple degrees of the Sun). The totals, from 280 to 4000 nm, are 1000.4 and 900.1 W/m2 respectively. It would be good to have more direct figures from a good source, rather than summing thousands of numbers in a database.〕 At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV),〔Calculated from the ASTM spectrum cited above.〕 with most of the extra UV consisting of biologically damaging short-wave ultraviolet.〔 〕 Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux, higher than most artificial lighting, including fluorescent. Multiplying the figure of 1050 watts per square metre 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. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation). 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 fuel the organisms' activities. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sunlight」の詳細全文を読む スポンサード リンク
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