The diagram below illustrates the meanings of these different terms: luminance (measured in nits, candela/m 2), luminous intensity, luminous flux (measured in lumens), and illuminance (measured in lumens/m 2, aka lux). Lux is an essential consideration for interior lighting design for spaces where humans live and work. Illuminance refers to the amount of light that shines onto a surface, measured in lumens per square meter (lm/m 2), also called lux. Luminous intensity is a measure of the light that shines from the source in a given direction. We quantify luminous flux in units of lumens (lm), a photometric unit of measurement. Luminous flux is a measurement of the total amount of light a light source emits, integrated over the entire angular span of the light. The amount of light emitted from or reflected off an object is called its luminance-commonly referred to as brightness. They are described by their energy or, inversely, by their wavelength. The radiation of light is made up of sub-atomic particles (photons), which have no mass but are simply carriers of energy. b. Electromagnetic radiation of any frequency or wavelength.īeyond this basic definition, we have tools and language to precisely describe, quantify, and analyze the electromagnetic waves (visible and invisible) that are all around us, emanating from the sun, from flames, and from artificial sources such as light bulbs. a. Electromagnetic radiation that is visible, perceivable by the normal human eye as colors between red and violet, having frequencies between 400 terahertz and 790 terahertz and wavelengths between 750 nanometers and 380 nanometers. Also called visible light. Starlight, overcast moonless night sky įor a table of approximate daylight intensity in the Solar System, see sunlight.The American Heritage Dictionary tells us: Starlight, clear moonless night sky, excluding airglow Starlight, clear moonless night sky, including airglow Sunrise or sunset on a clear day (ambient illumination)Įxtreme of thickest storm clouds, sunset/riseįor comparison, nighttime illuminance levels are: Shade illuminated by entire clear blue sky, midday Intensity in different conditions IlluminanceĪM 1.5 global solar spectrum sunlight (= 1,000.4 W/m 2) It may be darker under unusual circumstances like a solar eclipse or very high levels of atmospheric particulates, which include smoke (see New England's Dark Day), dust, and volcanic ash. However, the outdoor illuminance can vary from 120,000 lux for direct sunlight at noon, which may cause eye pain, to less than 5 lux for thick storm clouds with the Sun at the horizon (even <1 lux for the most extreme case), which may make shadows from distant street lights visible. For an explanation of why it is not exactly half, see here). (This is true for slightly more than 50% of the Earth at any given time. Therefore, daylight excludes moonlight, despite it being reflected indirect sunlight.ĭaylight is present at a particular location, to some degree, whenever the Sun is above the local horizon. In practice, the image of the lamp filament is focused onto the front focal plane of the condenser while the diffuser glass is temporarily removed from the light path. Sunlight scattered or reflected by astronomical objects is generally not considered daylight. Often, a sintered or frosted glass filter is placed between the lamp and the collector lens to diffuse the light and ensure an even intensity of illumination. This includes direct sunlight, diffuse sky radiation, and (often) both of these reflected by Earth and terrestrial objects, like landforms and buildings. World map showing the areas of Earth receiving daylight around 13:00 UTC in Aprilĭaylight is the combination of all direct and indirect sunlight during the daytime. For other uses, see Daylight (disambiguation).
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