Editing Sunlight (section)

==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 [[orbital eccentricity|eccentricity]] of Earth's [[elliptic orbit]] and the [[Extinction (astronomy)#Atmospheric extinction|attenuation]] by [[Earth's atmosphere]] have to be taken into account. The extraterrestrial solar illuminance ({{math|''E''<sub>ext</sub>}}), corrected for the elliptic orbit by using the day number of the year (dn), is given to a good approximation by<ref>{{cite journal|author=C. KANDILLI|author2=K. ULGEN|name-list-style=amp|title=Solar Illumination and Estimating Daylight Availability of Global Solar Irradiance|journal=Energy Sources}}</ref>
:<math>E_{\rm ext}= E_{\rm sc} \cdot \left(1+0.033412 \cdot \cos\left(2\pi\frac{{\rm dn}-3}{365}\right)\right),</math>
where dn=1 on January 1; dn=32 on February 1; dn=59 on March 1 (except on leap years, where dn=60), etc. In this formula dn–3 is used, because in modern times [[Apsis#Earth perihelion and aphelion|Earth's perihelion]], the closest approach to the Sun and, therefore, the maximum {{math|''E''<sub>ext</sub>}} occurs around January 3 each year. The value of 0.033412 is determined knowing that the ratio between the perihelion (0.98328989&nbsp;AU) squared and the aphelion (1.01671033&nbsp;AU) squared should be approximately 0.935338.

The solar illuminance constant ({{math|''E''<sub>sc</sub>}}), is equal to 128×10<sup>3</sup>&nbsp;[[lux]]. The direct normal illuminance ({{math|''E''<sub>dn</sub>}}), corrected for the attenuating effects of the atmosphere is given by:
:<math>E_{\rm dn}=E_{\rm ext}\,e^{-cm},</math>
where {{mvar|c}} is the [[atmospheric extinction]] and {{mvar|m}} 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 (see below). 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<sup>2</sup>, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1,120 W/m<sup>2</sup>.<ref name="Solar constant at ground level">{{cite web|title=Introduction to Solar Radiation|url= http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|publisher= Newport Corporation|url-status= live|archive-url= https://web.archive.org/web/20131029234117/http://www.newport.com/Introduction-to-Solar-Radiation/411919/1033/content.aspx|archive-date=October 29, 2013}}</ref> In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 [[nanometre|nm]]), 42 to 43 percent visible (400 to 700&nbsp;nm), and 3 to 5 percent ultraviolet (below 400&nbsp;nm).<ref>Calculated from data in {{cite web|url=https://www.nrel.gov/grid/solar-resource/spectra.html |title=Reference Solar Spectral Irradiance: Air Mass 1.5|access-date=2009-11-12|url-status=live|archive-url=https://web.archive.org/web/20130928011257/http://rredc.nrel.gov/solar/spectra/am1.5/ASTMG173/ASTMG173.xls|archive-date=September 28, 2013|publisher=National Renewable Energy Laboratory}}<br />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/m<sup>2</sup> respectively. It would be good to have more direct figures from a good source, rather than summing thousands of numbers in a database.</ref> At the top of the atmosphere, sunlight is about 30% more intense, having about 8% [[ultraviolet]] (UV),<ref>Calculated from the ASTM spectrum cited above.</ref> with most of the extra UV consisting of biologically damaging short-wave ultraviolet.<ref name="Solar radiation">
{{cite book |last=Qiang |first=Fu |chapter=Radiation (Solar) |chapter-url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |editor1-last=Holton |editor1-first=James R. |title=Encyclopedia of atmospheric sciences |volume=5 |publisher=Academic Press |location=Amsterdam |date=2003 |pages=1859–1863 |oclc=249246073 |isbn=978-0-12-227095-6 |url-status=live |archive-url=https://web.archive.org/web/20121101070344/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |archive-date=2012-11-01 }}
</ref>

{{vanchor|Direct sunlight}} has a [[luminous efficacy]] of about 93&nbsp;[[lumen (unit)|lumens]] per watt of [[radiant flux]]. This is higher than the efficacy (of source) of [[artificial lighting]] other than [[Light-emitting diode|LED]]s, which means using sunlight for illumination heats up a room less than fluorescent or incandescent lighting.<!--This depends on the efficacy of source, not of radiation, for the artificial lighting. For the Sun, LER=LES, so there's no need to specify which one the 93 is. (User:Eric Kvaalen)--> 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]] ([[lumen (unit)|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]]).

Dividing the [[irradiance]] of 1,050&nbsp;W/m<sup>2</sup> by the size of the Sun's disk in [[steradian]]s gives an average [[radiance]] of 15.4&nbsp;MW per square metre per steradian. (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&nbsp;MW/m<sup>2</sup>.<ref>{{cite book | title=Introduction to Optics | publisher=[[Prentice Hall]] | last=Pedrotti & Pedrotti | date=1993 | isbn=0135015456 | url-access=registration | url=https://archive.org/details/introductiontoop00pedr }}</ref>