Sunlight


Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun and received by the Earth, in particular the visible light perceptible to the human eye as well as invisible infrared and ultraviolet lights. However, according to the American Meteorological Society, there are "conflicting conventions as to whether all three are referred to as light, or whether that term should only be applied to the visible portion of the spectrum". Upon reaching the Earth, sunlight is scattered and filtered through the Earth's atmosphere as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When blocked by clouds or reflected off other objects, sunlight is diffused. 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.
Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. 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 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 fuel the organisms' activities. The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a requisite for vitamin D3 synthesis and a mutagen.
Daylighting is the natural lighting of interior spaces by admitting sunlight.
Solar irradiance is the rate of solar energy received by a unit area from sunlight.

Measurement

Researchers can measure the intensity of sunlight using a sunshine recorder, pyranometer, or pyrheliometer. 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. The extraterrestrial solar illuminance, corrected for the elliptic orbit by using the day number of the year, is given to a good approximation by
where dn=1 on January 1; dn=32 on February 1; dn=59 on March 1, etc. In this formula dn–3 is used, because in modern times Earth's perihelion, the closest approach to the Sun and, therefore, the maximum occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion squared and the aphelion squared should be approximately 0.935338.
The solar illuminance constant, is equal to 128×103 lux. The direct normal illuminance, corrected for the attenuating effects of the atmosphere is given by:
where is the atmospheric extinction and is the relative optical airmass. The atmospheric extinction brings the number of lux down to around 100,000 lux.
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. If the extraterrestrial solar radiation is 1,367 watts per square meter, then the direct sunlight at Earth's surface when the Sun is at the zenith is about 1,050 W/m2, but the total amount hitting the ground is around 1,120 W/m2. In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared, 42 to 43 percent visible, and 3 to 5 percent ultraviolet. At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet, with most of the extra UV consisting of biologically damaging short-wave ultraviolet.
has a luminous efficacy of about 93 lumens per watt of radiant flux. This is higher than the efficacy of artificial lighting other than LEDs, which means using sunlight for illumination heats up a room less than fluorescent or incandescent lighting. 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 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.
Dividing the irradiance of 1,050 W/m2 by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2.

Composition and power

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. The Sun emits EM radiation across most of the electromagnetic spectrum. 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. As a result, the photosphere of the Sun does not emit much X radiation, although it does emit such "hard radiations" as X-rays and even gamma rays during solar flares. The quiet Sun, including its corona, emits a broad range
of wavelengths: X-rays, ultraviolet, visible light, infrared, and radio waves. Different depths in the photosphere have different temperatures, and this partially explains the deviations from a black-body spectrum.
There is also a flux of gamma rays from the quiescent Sun, obeying a power law between 0.5 and 2.6 TeV. Some gamma rays are caused by cosmic rays interacting with the solar atmosphere, but this does not explain these findings.
The only direct signature of the nuclear processes in the core of the Sun is via the very weakly interacting neutrinos.
Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun. The spectrum of nearly all of the solar electromagnetic radiation striking the Earth's atmosphere spans a range of 200 nm to about 4000 nm. This band of significant radiation power can be divided into five regions in increasing order of wavelengths:
  • Ultraviolet C or range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light. Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation has germicidal properties, as used in germicidal lamps.
  • Ultraviolet B or range spans 280 to 315 nm. It is also greatly absorbed by the Earth's atmosphere, and along with UVC causes the photochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes sunburn. In addition to this short-term effect it enhances skin ageing and significantly promotes the development of skin cancer, but is also required for vitamin D synthesis in the skin of mammals.
  • Ultraviolet A or spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes, and can cause cancer.
  • Visible range or light spans 380 to 700 nm. As the name suggests, this range is visible to the naked eye.
  • Infrared range that spans 700 nm to 1,000,000 nm. It comprises an important part of the electromagnetic radiation that reaches Earth. Scientists divide the infrared range into three types on the basis of wavelength:
  • * Infrared-A: 700 nm to 1,400 nm
  • * Infrared-B: 1,400 nm to 3,000 nm
  • * Infrared-C: 3,000 nm to 1 mm.
The sunlight reaching Earth's surface is 49.4% infrared, 42.3% visible, and 8% ultraviolet.
It is sometimes asserted that the Sun's maximum output is in the visible range. However, this statement is a misconception based on only seeing the solar spectral irradiance plotted on a per-wavelength basis. When plotted that way, the power spectral density of sunlight peaks at a wavelength of about 501 nm, which is in the visible range. However, the solar spectral irradiance can with equal validity be calculated on a per-frequency basis, in which case the maximum is at, corresponding to a wavelength of about 882 nm, which is in the near infrared range. Counterintuitively, it is not meaningful to assert that the solar output is greatest at some precise location in the spectrum.

Published tables

Tables of direct solar radiation on various slopes from 0 to 60 degrees north latitude, in calories per square centimetre, issued in 1972 and published by Pacific Northwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Portland, Oregon, USA, appear on the web.

Intensity in the Solar System

Different bodies of the Solar System receive light of an intensity inversely proportional to the square of their distance from the Sun.
A table comparing the amount of solar radiation received by each planet in the Solar System at the top of its atmosphere:
The actual brightness of sunlight that would be observed at the surface also depends on the presence and composition of an atmosphere. For example, Venus's thick atmosphere reflects more than 60% of the solar light it receives. The actual illumination of the surface is about 14,000 lux, comparable to that on Earth "in the daytime with overcast clouds".
Sunlight on Mars would be more or less like daylight on Earth during a slightly overcast day, and, as can be seen in the pictures taken by the rovers, there is enough diffuse sky radiation that shadows would not seem particularly dark. Thus, it would give perceptions and "feel" very much like Earth daylight. The spectrum on the surface is slightly redder than that on Earth, due to scattering by reddish dust in the Martian atmosphere.
For comparison, sunlight on Saturn is slightly brighter than Earth sunlight at the average sunset or sunrise. Even on Pluto, the sunlight would still be bright enough to almost match the average living room. To see sunlight as dim as full moonlight on Earth, a distance of about 500 AU is needed; only a handful of objects in the Solar System have been discovered that are known to orbit farther than such a distance, among them 90377 Sedna and.