# How many lumens does the sun produce

The ancient Greeks developed theories about this ubiquitous natural phenomenon. Aristotle believed that light moves in a similar way to waves of water; the Pythagorean school made every visible object emit particles of light. The doctrinal dispute lasted thousands of years. Today we know:
The phenomenon "light" or "radiation" is best described by the dualism of wave and corpuscle. The propagation of light can be illustrated by a wave model (Christiaan Huygens) and the photo effect, i.e. the triggering of photoelectrons from irradiated substances, by a corpuscle model (Isaac Newton). The two models can be used side by side without any logical contradictions.

The wave nature of light was modified at the beginning of the 20th century by Einstein's theory of light quanta. According to this, light is a wave that is sent out in small bursts (the quanta).

These light quanta or photons move in a straight line away from the light source at the speed of light. They spread "spherically" in all directions or "in the full solid angle".
Maxwell developed the theory of electromagnetic waves ("Maxwell's equations"). In it he was the first to classify light into electromagnetic waves.
Heinrich Hertz proved in 1886 that electromagnetic waves have the properties of light waves. The unit of measurement for frequency (Hz) was named after him - the number of oscillations of an electromagnetic wave per second.

Electromagnetic waves differ in their wavelength and frequency. Wavelength is the distance from wave crest to wave crest, and frequency is the number of oscillations per second. All electromagnetic waves propagate at the speed of light (300,000 km / sec).

The sun, our energy supplier!

The light and radiation source for the daylight arriving on earth is known to be the sun. Bare numbers about the largest lamp in our solar system here!

The sun's energy spectrum covers a wavelength range of 10-15 m ≤ λ ≤ 105 m,
the upper limit of which is radio waves several kilometers long and the lower limit of which is the
Billiardths of a meter are short waves of cosmic rays. One refers to the outside of the
Radiance of all wavelengths measured in the earth's atmosphere at an average distance between earth and sun as solar constant I.Owho started today with I.O = 1361 ± 7 W / m2 (according to DIN 5034-2: IO = 1.37 kW / m2) is specified.

The area carrying the total energy is around:
0.2 x 10-6 m ≤ λ ≤ 3.0 x 10-6 m

The relevant spectral range can be divided into smaller steps:

On closer inspection, our precious daylight turns out to be a “wave dwarf”: a full fourteen thousandths of a millimeter are allocated to it on the cosmic energy wave scale.

Before solar radiation reaches the earth's surface, it is weakened by various influences. To simplify matters, the earth's atmosphere is initially assumed to be cloudless; clouds also reduce the proportion of radiation reaching the earth.
The weakening occurs through scattering on air molecules and vapor droplets as well as through absorption in atmospheric gases.

scattering

In the case of scattering (Rayleigh scattering), the light rays are deflected without energy being converted or a change in wavelength occurring.
In addition to the air molecules, there are a number of other aerosol particles in the atmosphere, also known as haze, which can scatter sunlight. Conversions in the form of heat only occur when these particles can absorb.
As experience shows, the haze content of the atmosphere depends on the weather and is subject to seasonal fluctuations. In general, because of the particularly strong scattering of short-wave light, the relative UV component of daylight increases with decreasing irradiance due to the growing influence of scattered radiation.

absorption

Except with air molecules and aerosol particles, the atmosphere is with gases like O.3, CO2 and H2O filled. In certain areas of the radiation spectrum, the so-called absorption bands, these gases absorb the radiation while converting it into other forms of energy. For the UV range, the ozone band named after Hartley (O3) is important, because at 290 nm it delimits the radiation spectrum downwards. This means that high-energy and harmful radiation is blocked by ozone. In this respect, the protective ozone layer is so important for the earth, and measures against the growing "ozone hole" are vital.

Cloudiness

Due to the simultaneous presence of air molecules, aerosol particles and gases, the processes described above occur simultaneously and fluctuatingly, and this effect is referred to as the clouding of the atmosphere.
Linke tried to summarize the influence of water vapor and haze to form a cloud factor. The Turbidity factor "T" indicates how many haze-free and absorption-free Rayleigh atmospheres would have to be layered on top of each other in order to allow the same radiation intensity integrated over the wavelength l to arrive on the ground as the real atmosphere allows.
The Turbidity factor fluctuates with the day and the season, but allows different measurements to be compared at different locations The turbidity acts z. B. on the UV radiation in such a way that a strong decrease in UV intensity occurs long before noon.

Natural colors - it's all in the mix!

The sun sends us the cocktail of rays of red, orange, yellow, green, blue, indigo and violet evenly mixed. None of the rainbow colors color the sunlight. The sum of this great mixture of colors is a neutral white light that we cannot see ourselves, but which lets the objects appear in natural colors to our eyes.

Natural colors are relative. Basically, no object has natural colors, but only those which the current lighting dictates. We only see the colors that are reflected by the objects under a certain lighting situation (e.g. changing colors of the sea).

All objects in our environment are preprogrammed for a certain color reflection due to their properties. They only reflect light of a certain wavelength. Whether they can do this depends on the waves offered by the lighting. If they are not given their reflective color by the lighting, they no longer reflect anything. For example, in the range of electric light bulbs on offer, red and yellow outweigh blue by far. This means that objects appear different in color under incandescent or neon light than under daylight.
 Spectrum daylight Spectrum fluorescent lamp daylight white Spectrum energy saving lamp Spectrum light bulb

Daylight with its even color mixture is unsurpassed in any case: every object can choose its reflection color from the well-sorted spectrum. Seen in this light, it is fair to say that daylight produces natural colors.

This is an important reason why, despite all the advances in artificial lighting, sufficient daylight has to get into living and working spaces.

Effect of daylight on people

Daylight has a variety of effects on living beings. It also controls the circadian rhythm of humans. Every day it "resets" our internal clock. A video from the ZDF-heute editorial team, which you can watch here, reports on the functioning of the internal clock and the serious effects it has on people.

Further extensive information on the effects of light on people is available on our page From the perspective of the medical practitioner.

Lumens and Lux

The two most important terms in daylight technology are the luminous flux F and the illuminance.

The luminous flux Φ

measured in Lumen (lm), is the radiation power emitted by a light source and perceived by the eye in the visible range of the entire spectrum.

Examples:
100 W light bulb - approx. 1,400 lumens
40 W fluorescent lamp - approx. 3,000 lumens

The illuminance E

measured in lux (lx), the ratio of luminous flux Φ to illuminated area is A.

In the professional world, the interior illuminance Ei and the outside illuminance Ea differentiated. To get an idea of ​​the illuminance E, here are two examples of the external illuminance E that prevails in our open aira:

Cloudless summer day (July):
E.a = approx. 100,000 lx

Gloomy winter afternoon (December):
E.a = approx. 3,000 lx

This means that there can be a lux difference of 30: 1 between the bright July sun and December gray-on-gray in our latitudes.
The outside illuminance or also "horizontal illuminance Ea"In the open air can be calculated with the following equation according to DIN 5034-2:

E.a = (300 + 21000 sin γs) × lx,
where γs is the height of the sun.

Standard value: 5000 lux according to DIN 5034-6

For the daylight technician, the day begins and ends when the outside illuminance is 5000 lux, everything below that is twilight. 5000 lux corresponds roughly to the outside illuminance at Kassel with an evenly overcast sky on December 10th around 10 a.m.
The actual outside illuminance, however, depends on the degree of latitude.
These differences can be taken into account for precise light value calculations.

Recommended illuminance levels

As you know, the natural lighting conditions change daily and seasonal. In addition, meteorological events such as clouds and fog or precipitation such as rain and snow impair continuous lighting with a constant illuminance throughout the day. For this reason, only artificial lighting technicians have so far made specifications for the illuminance levels to be planned for the additional electrical lighting to be provided.
In the table “Lighting requirement” we have an overview of the lighting requirement for the maintenance value of the illuminance compiled depending on usage.
In the table "Daylight technical lighting requirements", we have compiled an overview of the proportion of the necessary roof surface area in relation to the room floor area, corresponding to DIN EN 12464-1, depending on the various requirement levels and uses. Further information can be found in DIN standard series 5035 "Lighting with artificial light" or in DIN EN 12464-1 "Light and lighting - Lighting of workplaces - Part 1: Workplaces in interior spaces; German version EN 12464-1: 08-2011 ".

Daylight quotient "D"

In contrast to an artificial light source, the sky above us shines quite differently, it is subject to fluctuations. In order to obtain a constant basis for daylight calculations, the ratio of illuminance E is usedp at one point in the interior to exterior illuminance Ea. This quotient is called daylight quotient “D” after the English “daylight factor”.

The daylight quotient states what percentage of the exterior illuminance reaches an interior through openings. Due to the proportionality of the illuminance Ep and Ea and due to the rotationally symmetrical luminance distribution of the covered sky, regardless of the time of day and the season as well as the horizontal orientation of the daylight openings and thus of the building.

Every point in a room has its individual daylight quotient. If we imagine a given room without a window only with an opening of a certain size in the ceiling, then it is brightest at the height of the useful plane exactly under the opening in the room. The brightness decreases towards the side walls. The daylight quotient follows the course in about a bell curve. We therefore get the maximum daylight quotient D under the openingMax, at the edge the minimum daylight quotient Dmin. If you now determine the daylight quotients at several points, e.g. B. in a grid of 2 meters, adding them up and dividing them by the number of measurements, you get the mean daylight quotient . If we increase the distance between the ceiling and the floor in the example above, the overall brightness in the room is reduced.
If we now enlarge the room to the sides while maintaining the same height, the brightness drops more sharply at the edge.
If we change the size of the skylight opening or its properties with regard to the glazing, increase or decrease the brightness in the interior.
If we now paint the previously light walls and the shaft black, it would also be darker in the room.

We recognize that the daylight quotient is essentially a geometric parameter, because there are only dependencies on:

• Room proportions (height, length, width),
• Skylight geometry (area, arrangement, frame and bars, shaft shape),
• Glazing material (transmittance, pollution) and the
• Degree of reflection of the room boundary surfaces and the shafts.

Recommended mean values ​​for the daylight quotient can be found here.

uniformity

The daylight quotient runs in waves under light openings.The wave crests lie vertically below the openings (it is lighter there, the curve rises), between the openings are the wave troughs (here it is less bright, the curve sinks). The greater the difference between mountains and valleys, the more uneven the lighting.
Since strong light-dark contrasts are harmful to the human eye and can cause accidents, rooms must appear uniformly bright.
Poorly distributed individual light surfaces create hard changes between light and dark zones, only insufficiently illuminate a room and lead to strong contrasts and glare.
Therefore the evenness should g1 the lighting in the usable level in rooms lit exclusively by skylights must be at least 1: 2.
You can find the definition of evenness here.

The center distance of the individual light surfaces and the room height are decisive for the uniformity of the lighting. Low rooms require more, smaller openings for light. Higher rooms can handle larger openings with larger distances.

Rules of thumb:

• The width of a light strip should not be greater than half the height of the room.
• The distances between the roof light strips should be at least twice the light strip width.