The outdoor light level is approximately 10000 lux on a clear day. In a building in the area closest to the windows the light level may be reduced to approximately 1000 lux. In the middle area it may be as low as 25 - 50 lux. Additional lighting is often necessary to compensate low levels.
According EN 12464 Light and lighting - Lighting of workplaces -Indoor work places, the minimum illuminance is 50 lx for walls and 30 lx for ceilings. Earlier it was common with light levels in the range 100 - 300 lux for normal activities. Today the light level is more common in the range 500 - 1000 lux - depending on activity. For precision and detailed works the light level may even approach 1500 - 2000 lux.
In architectural lighting, light intensity or light output is measured to understand whether a particular light source provides enough light for an intended application. The lighting industry has well-established light level recommendations for a wide range of applications and space types. It is especially useful to understand light intensity in order to properly evaluate whether or not a space has adequate lighting conditions.
While these are useful to lighting experts, how do these terms relate to the real world We need a little context. A typical classroom, for example, is recommended to have a light level of around 30-50 footcandles or 300-500 lux. Compare this to a professional laboratory which the lighting standards recommend have a light level of 75-120 footcandles or 750-1200 lux. The differences in recommended light levels are published by the IESNA (Illuminating Engineers Society of North America). The recommendations are based on years of visual testing to determine how much light the human eye needs to properly see different tasks with varying levels of detail. You can see from this example how specific environments have very different light level requirements.
Lighting professionals use a light meter (also called an illuminance meter or lux meter) to measure the amount of light in a space/on a particular work surface. The light meter has a sensor that measures the light falling on it and provides the user with a measurable illuminance reading.
These handheld devices are commonly used by photographers to help calculate proper light exposure. However, they are also an essential tool that are used to measure and verify light levels in the built environment. Light meters are an especially useful tool if you are measuring light for safety or over-illumination, which cause eye strain and waste energy.
This is why lux meters are configured to CIE standard illuminant A. A standard lux meter is essential to measure incandescent lighting, but what about LED lighting To measure light intensity from LED lighting, you would use an LED light meter.
Take a typical office environment, the recommended light level for the open office is around 30 footcandles (average) or 300 lux (average). However, it does not make sense, nor is it comfortable, to have the same light intensity level everywhere.
Another environment for which interior light intensity is an important factor is classrooms. Learning is a highly visual experience, so appropriate light solutions should work in-line with the physical environment. We must consider horizontal tasks (the amount of light needed on desks) and vertical tasks (the amount of light needed to see writing on whiteboards). In general, 30 footcandles (300 lux) in the horizontal plane is recommended for a typical classroom.
In a school environment we also want to consider methods for reducing glare while maintaining consistent light levels so all students can see. In addition, research has shown that childrend and adolescents who receive proper morning light signals have improved performance, alertness, and reduced hyperactivity.
If we focus on patient rooms, providing a healthy, restful environment is important for patient recovery. Generally, 10 footcandles (100 lux) is a comfortable and lower light intensity for resting.
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Recent epidemiological evidence in children indicates that time spent outdoors is protective against myopia. Studies in animal models (chick, macaque, tree shrew) have found that light levels (similar to being in the shade outdoors) that are mildly elevated compared to indoor levels, slow form-deprivation myopia and (in chick and tree shrew) lens-induced myopia. Normal chicks raised in low light levels (50 lux) with a circadian light on/off cycle often develop spontaneous myopia. We propose a model in which the ambient illuminance levels produce a continuum of effects on normal refractive development and the response to myopiagenic stimuli such that low light levels favor myopia development and elevated levels are protective. Among possible mechanisms, elevation of retinal dopamine activity seems the most likely. Inputs from intrinsically-photosensitive retinal ganglion cells (ipRGCs) at elevated light levels may be involved, providing additional activation of retinal dopaminergic pathways.
The lux (symbol: lx) is the unit of illuminance, or luminous flux per unit area, in the International System of Units (SI). It is equal to one lumen per square metre. In photometry, this is used as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. It is analogous to the radiometric unit watt per square metre, but with the power at each wavelength weighted according to the luminosity function, a model of human visual brightness perception, standardized by the CIE and ISO. In English, \"lux\" is used as both the singular and plural form.
Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (with the unit lumen) as a measure of the total \"amount\" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance is inversely proportional to area when the luminous flux is held constant.
A flux of 1000 lumens, spread uniformly over an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux. However, the same 1000 lumens spread out over 10 square metres produces a dimmer illuminance of only 100 lux.
Achieving an illuminance of 500 lx might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same illuminance (lux) requires a greater luminous flux (lumen).
The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of apparent magnitude 0 provides 2.08 microlux (μlx) at the Earth's surface. A barely perceptible magnitude 6 star provides 8 nanolux (nlx). The unobscured Sun provides an illumination of up to 100 kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions. This direct normal illuminance is related to the solar illuminance constant Esc, equal to 128000 lux (see Sunlight and Solar constant).
The illuminance on a surface depends on how the surface is tilted with respect to the source. For example, a pocket flashlight aimed at a wall will produce a given level of illumination if aimed perpendicular to the wall, but if the flashlight is aimed at increasing angles to the perpendicular (maintaining the same distance), the illuminated spot becomes larger and so is less highly illuminated. When a surface is tilted at an angle to a source, the illumination provided on the surface is reduced because the tilted surface subtends a smaller solid angle from the source, and therefore it receives less light. For a point source, the illumination on the tilted surface is reduced by a factor equal to the cosine of the angle between a ray coming from the source and the normal to the surface. In practical lighting problems, given information on the way light is emitted from each source and the distance and geometry of the lighted area, a numerical calculation can be made of the illumination on a surface by adding the contributions of every point on every light source.
The lux is one lumen per square metre (lm/m2), and the corresponding radiometric unit, which measures irradiance, is the watt per square metre (W/m2). There is no single conversion factor between lux and W/m2; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light.
The peak of the luminosity function is at 555 nm (green); the eye's image-forming visual system is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the amount of illuminance for a given amount of irradiance is maximum: 683.002 lx per 1 W/m2; the irradiance needed to make 1 lx at this wavelength is about 1.464 mW/m2. Other wavelengths of visible light produce fewer lux per watt-per-meter-squared. The luminosity function falls t