en- Biodynamic Lighting

An adjective composed of the Greek words βίος (bios: life, aliveness) and δύναμις (dynamis: force, potential, variability) refers not only to artificial lighting, whose variability imitates daylight, but also to the action of external forces on living organisms (adjacent to biomechanics) or to Rudolf Steiner's method of farming (adjacent to organic farming) or, for example, to signals from the universe that would indicate the presence of life.

en- The attribute consisting of the Greek words βίος (bios: life, liveliness) and δύναμις (dynamis: strength, potential, variability) is used not only for artificial light that imitates the daylight with its variability, but also for the effect of exterior elements on live organisms (related to biomechanics) or a form of farming according to Rudolf Steiner (related to environment friendly farming) or for example signals from outer space that would be evidence of the presence of life.

LIGHT FROM THE SKY

The variability of daylight is due to the Earth's orbit around the Sun, its rotation around its own tilted axis and the presence of an atmosphere. Sunlight is partially scattered as it passes through the atmosphere, and the direct and sky components recombine for the observer depending on the atmospheric conditions of the moment.

Fig. 1 Azimuth and altitude of the sun during the day and year at 50°N and 15°E [5].

LIGHT IN INTERIORS

Due to its variability, daylighting alone cannot in most cases provide the continuous illumination necessary for the performance of visual tasks. Today's typical artificial lighting is fluorescent and static. It can only be switched on and off - often in sections - and rarely dimmed. The chromaticity temperature of typical artificial light is fixed.

THE NEED FOR LIGHTING DIVERSITY

People are aware and subconsciously feel the need for different lighting conditions during different activities and at different times of the day. According to statistics, for office work, we would prefer around 2,000 lx of neutral or cool light (the standard [1] requires a minimum of 500 lx of medium maintained illuminance). In the evening, on the other hand, we relax with tens of lux of warm light. We thus try to emulate the variability of daylight within the means available.

NON-VISUAL EFFECTS OF LIGHT

Light affects us in other ways than sight - it stimulates our nervous system and synchronizes our internal biological clock. The light-sensitive retinal ganglion cells (ipRGCs) that mediate this effect are most sensitive to the blue component of light and predominate in the lower part of the retina, where the sky is projected externally. This signal adjusts our central clock to the start of the day. It controls the wake-sleep cycle, hormone levels, body temperature and synchronizes the internal clocks of the various organs. The length of the day serves as a luminous calendar for photoperiodic animals, which governs their annual life cycles, whether it be hibernation, fur replacement, or even fertility. Seasonal mood swings, a greater need for light in winter, or a surge of vigour in the pre-spring suggest that humans also have an annual photoperiod. The standard [2] foresees consideration of the extra-visual effects of light in a future revision of the standard [1] for artificial lighting, and a proposal for such an extension is described in the standard [3].

"ARTIFICIAL" VERSUS "NATURAL"

Daylight is inherently irreplaceable and we can only approach it in various ways by technical means. We react differently to artificial light than to daylight. We can distinguish ordinary artificial light indoors from daylight without looking for the source. Subconscious criteria may include the following:

• illuminance - is it appropriate to the expected level?
• contrast - is the brightness distribution within an acceptable range?
• chromaticity - does it correspond to this part of the day?
• colour rendering - do the memory colours (skin, familiar objects) match?
• rendering of shapes - are the shadows of objects natural (or too sharp/soft or multiple)?
• direction of shadows - can the angle of incidence match the actual position of the sun?

If most of the answers are no, it is hard to believe that the space is lit by daylight. However, sometimes it does happen that people feel that there is no light in an artificially lit space and only daylight penetrates the space. So perhaps there is some level of approximation from which we would consider artificial light to be daylight.

ZOOM I - INTENSITY

Noon outdoor illuminance can be approximately 100,000 lx on a clear summer day. In contrast, indoor artificial lighting is two to three orders of magnitude lower. Manufacturers now offer most luminaires in dimmable versions. Such luminaires can be connected to a specific communication network, which users can control using buttons, dimmers, scene selectors, control panels or from an app on a computer or smartphone. They can thus set up lighting conditions in advance for different activities or times of the day, or set the desired lighting intensity individually. In addition to luminaires, occupancy detectors, light sensors or shading elements can also be connected to such a network. Automatic lighting control can be a source of considerable energy savings [2]. DALI and KNX systems are very widespread. Biodynamic lighting can be provided by a controller or computer with a program that adjusts the intensity of the artificial lighting throughout the day. The algorithm calculates the theoretical horizontal outdoor illuminance for the current time and date, and can also take into account the geometric layout and orientation of the space, the working hours of the users, the settings of controls and sensors, or add a variable component from a pseudo-random number generator (cloud simulation). It then sends the calculated intensity request to the luminaire network.

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Fig. 2 Modelled outdoor illuminance waveform for 21 June; the DALI control code for simulating the indoor waveform is shown in red.

ZOOM II - CHROMATICITY

Depending on the height of the sun above the horizon, sunlight passes through different layers of the atmosphere: thin around midday and thicker low above the horizon. On clusters of molecules (or pressure fluctuations), Rayleigh scattering of light (a special case of Mie scattering) occurs, the effective cross section of which is inversely proportional to the fourth power of the wavelength of the radiation. Violet light of wavelength λ = 380 nm scatters 16 times more than red light of wavelength λ = 760 nm (the edges of the visible spectrum). Due to the combination of the spectrum of sunlight above the atmosphere, scattering and the spectral sensitivity of our vision, we see the sky and the Earth as blue from space. In good weather conditions, the sun appears yellow-white at noon, golden at sunrise or sunset, and even red in polluted air. On a cloudless day, the chromaticity temperature of the direct component varies approximately as shown in Figure 3. The Tc of the northern sky is in the tens of thousands of kelvin and under uniformly cloudy skies the Tc during the day is approximately 6 500 K.

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Fig. 3 Model runs of chromaticity temperature and circadian index of the direct daylight component for 21 June.

The control unit can also control the chromaticity temperature - if the luminaires allow it. For fluorescent lamps, the chromaticity temperature is fixed by the composition of the phosphor and Tc control is only possible by lighting sources with a different chromaticity temperature - either in the same or in a different luminaire. For LEDs the situation is much better: there are modules and strips on the market with light of two different chromaticity temperatures or RGBW or RGBA (Red - red, Green - green, Blue - blue, White - white or Amber - yellow), containing four colour components from which (unlike simple RGB) a high quality white light (Ra > 90) can be composed over a wide range of chromaticity temperatures. Newly developed tunable OLED materials allow simultaneous control of luminous flux and chromaticity temperature.

Pleasing combinations of chromaticity temperature and illuminance are found between the curves of the Kruithof diagram. For the active part of the day, higher illuminances (above 500 lx) are suitable, which correspond to cooler tones (> 5 000 K). Here, artificial lighting mimics bright sunlight. For relaxation and preparation for sleep, on the other hand, warmer light is preferable, which is pleasant at lower illuminances (tens to several hundred lux) and at the same time contains several times less of a biologically activating component than cool light. It therefore mimics the setting sun or firelight.

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Fig. 4 Kruithof diagram.

ZOOM III - LIGHT DIRECTION

Artificial lighting can also mimic the apparent movement of the sun across the sky (see Figure 1). By rotating parts of the luminaires or their optical components, the light can be directed at the necessary angles. It is also possible to use additional luminaires on the walls or as part of the interior. These can have a lower luminous flux compared to ceiling luminaires, but also a lower chromaticity temperature, because the control system activates them only when the sun is low above the horizon (less intense and warmer light). It can be used to simulate real sky events as well as in an "extended day" programme in the office, where biodynamic lighting can act as a light clock to indicate to the user that rest time is approaching.

CONCLUSION

Daylight controls our biological rhythms. Artificial light is mostly static for now. All the elements for implementing biodynamic artificial lighting are available today. We can therefore control artificial light so that it has a similar effect on our biorhythms as daylight.

Graphs: author's archive, [5]

Literature:

[1] EN 12464-1 Light and lighting - Lighting of work areas: Part 1: Indoor work areas. ÚNMZ, 2012.

[2] EN 15193 Energy performance of buildings: energy requirements for lighting. CNI, 2008.

[3] DIN SPEC 67600 Biologisch wirksame Beleuchtung - Planungsempfehlungen (Biologically Efficient Lighting - Design Guidelines), 2013.

[4] COELUX SRL: Experience the sky: Insubria University Spin-off. Online, retrieved March 24, 2015, available from: http://www.coelux.com/.

[5] Sun path chart program. UNIVERSITY OF OREGON: Solar Radiation Monitoring Laboratory. Online, retrieved March 24, 2015, available from: http://solardat.uoregon.edu/SunChartProgram.html.

Author: Antonín Fuksa
Published in ERA21 2/2015