illumination requirements are achieved. The LEED 2009 documentation is based upon the daylight factor calculation. The daylight factor calculation is based on uniform overcast skies. It is most applicable in Northern Europe and parts of North America. Daylight factor is “the ratio of the illuminance at a point on a plane, generally the horizontal work plane, produced by the luminous flux received directly or indirectly at that point from a sky whose luminance distribution is known, to the illuminance on a horizontal plane produced by an unobstructed hemisphere of this same sky." LEED v4 daylighting standards are the most current as of 2014. The new standards are similar to the old standards, but also intend to “reinforce circadian rhythms, and reduce the use of electrical lighting by introducing daylight in the space. Two options exist for achieving the maximum value of these two most recent points. One option is to use a computer simulation to demonstrate that a spatial daylight autonomy of 300 lux for at least 50% of the time, and an annual sunlight exposure of 1,000 lux for 250 occupied hours per year, exists in the space. Another option is to show that illuminance levels are between 300 lux and 3,000 lux between 9:00 a.m. and 3:00 p.m. on a clear day at the equinox for 75% or 90% of the floor area in the space. The overall goal of the LEED v4 daylighting metrics is to analyze both the quantity and quality of the light, as well as to balance the use of glazing to ensure more light and less cooling load. See also Architectural glass Circadian rhythm Daylight Daylight harvesting Daylight factor Deck prism Glass brick Healthy building Heliostat Low-energy house Passive daylighting Passive house Passive solar Passive solar building design Solar tracker Skylight Sun path Sustainability Sustainable architecture Transom (architectural) References External links U.S. Department of energy page on passive daylighting Eneref Institute Right to DaylightCompanies or products that use active daylighting systems include: Solatube Daylighting Systems Himawari Daylight Collector Heliostats Sunflower Corporation Ciralight SunTrackers Natural Lighting Co.Int. SunCentral Inc. Parans Solar Lighting Energy-saving lighting Lighting Solar architecture Sustainable building Windows pt:Claraboia