Lighting with LEDs – More than just Illuminating Objects, by Bartenbach Lichtlabor

How can the needs of people be served by new LED-lightings? LEDs as a digital light source offer new technical possibilities for fulfilling special visual, biological and emotional requirements. At the same time our knowledge about how lighting affects people has increased significantly. This article gives an overview of experiences with LED lighting up until now as well as the latest findings in lighting effects. Recommendations will be given to improve future LED-lighting.

Lighting technology is in a time of ground-breaking changes that can be symbolically described by two milestones. One of them being the progression of the LED from a weakly glowing signal lamp to a highly efficient point light source, and the other being the discovery of the so called “third receptor” in the human eye. It is a photoreceptor containing melanopsin that regulates the discharge of melatonin in the brain and thus has an indirect impact on the circadian (day and night) rhythm of humans. Both milestones can be dated around the year 2001.

The LED as Illuminant of the Future

The LED as a highly efficient and digital light source has triggered a rapid change both in the lighting industry and also in the implementation of illumination. With a common light yield around 120-140lm/W in realistic operation the LED already surpasses all other high-grade luminaires in general lighting. And the end is not in sight! In the coming years the light yield will increase up to around 200 lm/W. At the same time the life span of 50,000 hours outperforms conventional luminants by far (for example a light bulb has approx. 1,000 hours, an incandescent halogen light bulb approx. 3,000 hours, a fluorescent tube approx. 20,000 hours). Therefore although the initial costs (investment) for LED lighting systems are higher, due to the longer life time, the profitability outperforms that of most customary systems.

Figure 1: Efficacy forecast of white LEDs, U.S. Department of Energy [1]

The characteristic of LEDs to respond to control signals instantaneously, meaning without time delay, is ideal for usage in communication technology (digital light source = electronic unit). The combination of LEDs with light- and movement-sensors, interconnected with complex control and regulation algorithms, turn the lighting into a smart and adaptive system that can react to alterations in its environment (time, weather, etc.) in a flexible way.

A future vision is an autonomously acting “thinking” LED-luminaire, that is integrated into a higher-ranking information and communication network over the “internet of things”.

Figure 2: LED, optics and luminaire

Merging several color LEDs to a single spectrum, while each LED is dimmable, allows the luminous perceived color to be set in an almost user-defined manner. In so doing, lighting systems can be fashioned with which the perceived color, the intensity and the light distribution can be dynamically modified for each use case or individual requirement.

Newest Findings in Light Impact Research

Third receptor inside the eye
With the discovery of the so called photosensitive retinal ganglion cells in 2001, a new chapter in lighting technology was opened: This so-called “third receptor” (beside rods for scotopic and cones for photopic vision) at adequate intensity, blocks the nightly discharge of the sleeping hormone melatonin by the pineal gland. In experiments it could be shown that this discharge can be abruptly stopped inside a few minutes by illuminating the retina with light of appropriate wavelength and intensity. While light constrains the flow of the sleeping hormone melatonin during the night it stimulates the production of the hormone serotonin (an endorphin) during the day. This way light triggers the day-and night-time rhythm of humans and significantly contributes to well-being and health.

This discovery is seen as proof that light is not only needed to see with but also has an impact on physiological processes and directly affects human health. Thus light will, in future, not only be important for the visual environment of people.

Non-visual lighting impacts
Innumerable studies [2, 3] indicate that light does not serve man for seeing (visual perception) only. Light acutely influences cognitive processes [4] and has a strong influence on various circadian physiological rhythms [5]. In addition, the effect of improving your mood due to bright light in the early morning hours is uncontested [6] and is extensively employed in the treatment of people with seasonal affective depression.

Figure 3: The new R&D office at Bartenbach

About 15 years ago the research into non-visual effects of light gained momentum due to the publication of groundbreaking studies regarding the acute nightly suppression of Melatonin [7, 8]. Based on this research, a range of activities regarding non-visual effects of light was determined [9]. As an addition, design recommendations for lighting design taking into account biological aspects were published in 2013 [11].

At the same time, further studies were conducted in order to learn more about the active factors directly related to light (e.g.: brightness, spectrum, point in time and duration of exposure to light). These studies were mainly conducted in laboratories at night with well-selected test person populations. Regrettably research aiming at quantifying non-visual effects of light on humans in their natural living environment during the day has only produced a few explicit results. This fact put a considerable damper on the euphoria regarding this topic.

In addition, the early initiatives for establishing standards for non-visual effects of light were strongly questioned by scientists. Currently the DIN standard committee NA 058-00-27 AA (Effects of light on man) is discussing the question, if the action spectrum for non visual effects of light previously based on the acute nightly melatonin suppression (with a maximum spectral sensitivity at 450-460nm – Figure 4), should be replaced by the spectral sensitivity of the melanopic ganglion cells in the human eye (with a maximum spectral sensitivity at 480-490nm – Figure 4). This would represent a significant paradigm shift in the evaluation of the spectral distribution of light.

Furthermore the scientific evidence for the published design recommendations regarding non-visual effects of light is quite weak [12].

For generating specific non-visual effects of light on man a deep understanding and knowledge is necessary, that is currently not in existence, and that could only be gained by future research with representative random samples in naturalistic settings und long periods of observation. Currently it is not known if it will be possible in future to influence man´s cognitive, endocrine and autonomous physiological processes with the aid of lighting based on scientific findings and thus to assign a new and sustainable added value to the factor “light”.

First Experiences with LED Applications

The lighting industry is competing in terms of efficiency and new records are set frequently. The inconsiderate usage of the technological possibilities involves the danger of creating useless or very bad solutions that in the end raises questions about the complete technology.

For example, the combined spectra of several LEDs entail the risk of being perceived as bad color quality, or specific surfaces may seem unnatural. The combining of different color LEDs can lead to irritating color shadows, or the commonly used pulse width modulation (PWM) for dimming the LEDs can generate disturbing flicker effects with moving parts. The extremely high light density of high power LEDs (>50 Mio cd/m.) can produce uncomfortable light pressure and strong glare in the radiation field.

Figure 4: Spectral sensitivities of the human eye

The quality criteria stated in relevant harmonized standards that are used in lighting engineering, are not suited for this kind of LED illumination.

The LED as a digital light source (and electronic component) has made the standards and interface definitions that have established over the decades invalid: Where it was self-evident to replace a lamp, no matter from which manufacturer or nationality (even if the installation was 20 years old), the built in components and LEDs nowadays are often no longer available or cannot be dismantled. At best, the lighting component can be replaced together with the electronics (as one element) and often is deliverable solely by the original vendor. The LED is, at present, a product that is outperformed by a better LED within months. But at the same time it has a life span of several decades. All this generates large uncertainty with handlers and consumers and makes long term planning difficult.

To use the potential of LED-technology in a positive way for the end-consumer, new rules (technical but also qualitative standards) are needed. The requirement, therefore, is, amongst other things, the establishment of new quality criteria to quantify these benefits.

Color Rendition
Color rendition (color rendering index, CRI) is an important spectral quality criterion of a light source. This index tells us to what degree colors are rendered correctly in comparison to a reference light source (daylight or thermal radiator) when illuminated by an artificial light source.

Figure 5: Color rendition (source: http:// www.ledigma.lt/technology/)

The early LED generations often had CRI values of less than 80 and thus did not meet the requirements stipulated in the standards for interior lighting. At the same time, it has been noted that the color saturation of objects illuminated by LEDs, at least in part, appeared to be considerably stronger and also more appealing. This fact and the knowledge regarding the outdated elements in the calculation of the CRI determination method [13] motivated many research institutions across the world, to research color rendition once again.

Currently some studies [14, 15] indicate that there are two diametrical aspects are contained in the theoretical construct of color rendition, namely color fidelity and color preference. Regarding the latter, the distortion of color impressions leaning towards an increased color saturation play a particular role (due to the fact that more saturated colors are often preferred) and thus complements the concept of color fidelity.

This finding is, amongst others, the reason why the International Commission on Illumination (CIE) formed the new Technical Committees TC 1-90 (“Color Fidelity Index”) and TC 1-91 (“New Methods for evaluating the color quality of white light sources”). They have been tasked to develop updated color fidelity metric.

Currently various updated color fidelity metrics [16, 17] are discussed to become the replacement for the hitherto existing calculation regulation. They show that particularly light sources with a color temperature below 4000 Kelvin and tri-phosphor fluorescent lamps with a previous CRI value between 80 and 85 should often have a lower color fidelity value and thus would actually not meet the lighting standards. By contrast, light sources with a CRI value of greater than 90 according to the hitherto existing calculation rules continue to have updated CRI values of the same magnitude.

Currently the color rendition of saturated test colors is often cited as a quality criterion for the spectral quality of light. Updated calculation regulations show, for instance, much higher color rendition values for a test color with a saturated shade of red (R9 test color) as determined with the previous calculation method. Consequently the color rendition of the test color R9 does not represent a valid quality criterion for the spectral quality of a light source. For other saturated colors the results are not pointing clearly into one direction.

Promising candidates for a color preference metric, such as the gamut area index [18] or the CQSg [19], are currently suffering from the problem that there are no concrete recommendations for the characteristic of the calculated values, and, that it is not yet clear in terms of specific applications which balance would represent an optimum between the values of color fidelity and color preference.

Figure 6: Dynamic illumination of a hospital ward and a domestic dining room

Currently there is a deep disagreement regarding the question of if the hitherto existing color rendition computation method should be updated at all. This is shown by the contradicting position papers authored by the European and American Illuminating Engineering Societies. Thus Lighting Europe [20] is in favor of retaining the hitherto CRI calculation regulation. Contrary to this the Illumination Engineering Society IES [21] advocates an update of the color fidelity computation and for the introduction of an additional calculation of the color saturation potential of a light source.

Despite these disagreements, it has a lot to commend an updated calculation regulation for the determination of color fidelity of a light source.

In conclusion, it shall be mentioned that due to the rapid development of the LED technology during recent years the color rendition of LEDs has greatly improved – quantified by both, as per the traditional method as well as per the updated computation models.

Dynamic artificial lighting
Generally dynamic lighting varies the lighting level, the distribution, and the color temperature of an interior space. The transition of the lighting parameters occurs slow enough to be below the awareness threshold (subliminal) mostly lasting more than 15 minutes.

The primary objective of dynamic lighting is either controlling artificial lighting in the interior subject to the time of day in order to facilitate certain specific non visual effects of light on mood, sleep, cognition and physical activity or supplementing missing natural light with artificial light with the aid of sensors in order to increase visual comfort and to establish an energy efficient means of adding artificial lighting.

Currently the scientific evidence regarding the non-visual effects of dynamic light is quite limited. Today there are only a few well managed studies (De Kort, 2010 [21]; Barkmann, 2012 [22]; Boyce, 2000 [23]; Hoffmann, 2008 [24]; Barrick 2010 [25]; Canazei, 2013 [26]; SSL-erate Deliverable 3.2., 2015 [27]) indicating that dynamic lighting in offices, class rooms, in homes for elderly people and for shift workers some positive effects can be achieved. It can be assumed that in these studies the interior lighting plays an important role and had its positive non-visual effects particularly during the early morning hours (up to two hours of getting up), in the late evening (up to two hours prior to going to bed) and during the night.

Figure 7: Computation of the monthly energy saving potential of dynamic LED lighting (Source: dalec.net)

On the other hand, it is uncontested that the dynamic addition of artificial lighting as a supplement for natural light can reduce the energy consumption of artificial lighting considerably.

Figure 7, for instance, shows that – based on the simulation software DALEC – the energy consumption of the lighting of an office in Innsbruck (Austria) with a south-facing facade and in compliance with the standards can potentially be reduced by 27 – 54%.

From a technical point of view, dynamic interior lighting can either be realized by mixing different color LEDs with / without a white LED (RGB- respectively RGBW mixing) or by mixing white LEDs of different color temperatures. Although the chromaticity coordinates of the resulting spectral distributions can be kept close to the black body radiation (Planck´s law), this method of mixing light does not represent a quality criterion for dynamic lighting per se. It is recommended to also spectrally evaluate the mixed spectrum by means of updated color rendering computation methods (e.g. CRI 2012 [28]).

Flicker
Flicker is the temporal modulation of the luminous flux of a light source and can be visible or invisible. As is currently known, human ERG (electroretinogram) signals at light frequencies up to 162Hz [30].

Flicker can induce epileptic seizures, headaches, fatigue, eyestrain, blurred vision, migraines, and distraction, reduce visual performance and altered perception of moving objects [29].

Figure 8: Temporal modulation of light sources [31]; cited flicker indices see IES [34]

Figure 9: Temporal modulation of LEDs, examples [35]

Flickering light matters in applications with fast moving objects, where eyes have to move quickly (e.g. while reading or searching) and where video cameras are used.

From a technical perspective flicker can be described in terms of its modulation frequency, modulation amplitude, DC-component, duty cycle and modulation waveform. Figure 8 shows examples of flickering light sources.

The quantity of flicker for LEDs strongly depends on the used electric components and on the operating conditions (e.g. dimming, see Figure 9). At the moment these details are not specified neither for LEDs and ballasts nor for LED luminaires and thus it is difficult to decide on the quality of products concerning flicker.

Application-related factors which modulate the impact of flicker are the duration of exposure, stimulated retinal area, location in the visual field, brightness of the flickering light source, and local luminance contest of the light source to its surrounding.

Currently recommendations to mitigate health and safety-related effects of flickering light are mainly based on studies dealing with stroboscopic and phantom array effects [32]. Research on physiological effects of flickering LED lights after hours of exposure are still needed to quantify the long-term impact of invisible flicker.

IEEE recently recommends [33] modulation amplitude (max. - min. light output / max. + min. light output) smaller than 8% of the flicker frequency for the normal population and modulation amplitude smaller than 3.33% of the flicker frequency for the flicker-sensitive sub-population. Additionally, flicker should be below 10% modulation for flicker frequencies below 100 Hz.

Figure 10: Phantom array effect

Glare
Vision is strongly influenced by the luminance distribution in the field of view. Especially in sceneries with objects much brighter than the adaptation level of the eye these potential sources of glare can decrease visual comfort as well as visual performance. In a working environment this reduces productivity and in other applications like traffic it might even be a threat to human life. Fortunately there are well-established metrics for glare that are commonly used all over the world. Sticking to threshold values defined in standards should prevent unpleasant or dangerous consequences of artificial lighting systems. Widespread examples are the CIE Unified Glare Rating UGR [36] for discomfort glare and the Threshold Increment TI [37] for disability glare.

Standards specify e.g. UGR ≤ 19 for office lighting [38] or TI ≤ 15% for certain situations in road lighting [39]. Is this really the whole story and thus glare a topic completely under control?

The Unified Glare Rating UGR is intended to describe discomfort glare in interior space caused by artificial lighting devices. According to the “International Lighting Vocabulary” discomfort glare is defined as “glare which causes discomfort without necessarily impairing the vision of objects” [40]. Despite criticism that UGR is too complicated and difficult to apply in practice there have been many studies and publications [41, 42, 43] indicating the adequacy and applicability of this method.

When applying UGR calculation in daily practice an attentive designer faces several problems:

• Different light planning programs compute different UGR-tables for the same luminaire. [44]

• Even within one software program the UGR-table depends on default settings that vary from country to country (Spacing-to-Height-Ratio might be 0.25 or 1.00).

• Typically there is no check if the dimensions of the luminous area are within the range specified by the CIE: Its solid angle has to be larger than 0.0003 steradian, which is a cone with an aperture 2 x 0.56°, (compare with average sun angle 2 x 0.27°). And it has to be smaller than 0.1 steradian corresponding to a cone with an aperture 2 x 10°. As a consequence the UGR-values for LED points calculated by standard software are ways too high.

Generally the undiscerning use of UGR-tables is questionable. These values are based on a regular and closely spaced luminaire arrangement. For a ceiling 1.6 m above the user’s eye the luminaires are placed at a distance of 0.4 m (Spacing-to-Height-Ratio 0.25) and thus the whole ceiling is paved with luminaires. When using real world arrangements and calculating UGR-values for certain positions in the room the values can be much higher than the values in the UGR-tables. But there can even be no glare at all because there is no luminaire in the field of vision. In reality the UGR-value depends not only on the luminaire data, but also on the observer’s position, his viewing direction and the positions of the luminaires. Such exact calculations for a real project can be performed in software based on UGR formula and it is astonishing to note that the UGR-values specified e.g. in EN 12464 [38] explicitly reference the use of the tabular method stating that the threshold values when using the UGR formula are to be discussed.

Figure 11: Light spot decomposition, free form facets for outdoor and indoor applications

For both methods to calculate UGR-values, the tabular method and the usage of the UGR formula to make more detailed predictions, the restrictions concerning the solid angle of the luminous areas of the luminaires are a crucial problem. These difficulties are currently ignored by the light planning software available on the market. Especially when using LED luminaires the luminous areas of the single optics frequently are far below the minimal solid angle of 0.0003 steradian. In many, but not in all cases, it is arguable to average over larger areas with dark regions in between. Sometimes even the dimensions of the housing are used as luminous area that can dramatically reduce the UGR values of the luminaire. The luminaire data provided by light fixture manufacturers e.g. in EULUMDAT-or IES-format contain only very simple models of the luminous area (rectangle or disk) and thus each software relying on these data is forced to average the luminance over the whole specified area. The detailed guidelines on how to handle luminous area and luminance for certain types of luminaires in CIE 117, Appendix C “Luminaire data” [36] are widely ignored by manufacturers as well as software engineers of light planning programs.

The CIE Technical Report CIE 147-2002 “Glare from Small, Large and Complex Sources” [45] proposes adaptations of the UGR calculation to overcome some of the problems but these suggestions have not been realized yet. As there seem to be some inconsistencies in these adaptations other algorithms have to be proposed in future [55]. At the moment there is a lot of research on discomfort glare that is driven by the problems of the UGR concerning LED luminaires [46, 47, 48].

A method to reduce glare to a minimum called “light spot decomposition” was proposed many years ago [49, 50, 51]. This method has been patented in many countries all over the world [52] and has been realized in projects and products. The idea is to split up the light source into many small light points that can be separately observed from any point of place within the illuminated area.

When comparing such a light spot decomposition with a conventional light source with the same luminance concentrated on a compact luminous area the advantages of the decomposition is quite obvious. It is much more interesting to match against a conventional lighting device with the same luminous area with a more or less constant luminance. Here the luminance of the light spots is far higher than the luminance of the conventional device. Perception Studies in the framework of the Competence Center Light [53] suggest that in this case the light spot decomposition might induce similar or even worse discomfort glare but it seems to be superior concerning disability glare.

In the “International Lighting Vocabulary” disability glare is defined as “glare that impairs the vision of objects without necessarily causing discomfort” [54]. As the visibility of objects on the street largely determines security in road lighting it is of vital interest to concentrate on disability glare in these standards [39]. The established metric for this glare is the Threshold Increment TI [%] based on the veiling luminance in the human eye created by sources of glare [37]. The TI-value is calculated from the vertical illuminance at the observer’s eye (illuminance perpendicular to the line of sight). As the luminaires are at large distance the luminous intensity of the luminaires in the direction of the observer’s eye are the crucial factors for a given luminaire arrangement. Thus the threshold increment does not reflect positive effects of light spot decomposition on disability glare. Additionally, all these calculations are based on photopic vision that seems to be odd for road lighting at nighttime. During the last years mesopic vision, in general, and the visibility and the visual performance in mesopic conditions, in special, are in the focus of Technical Committees (TC) of the CIE [56, 57] and of researchers all over the world [58, 59, 60, 61].

Another problem is that state-of-the-art LED street luminaires has acceptance problems in the population due to the high luminance of today’s high-power LEDs. This high luminance might not pose a security risk or even influence disability glare but it can be seen in the context of discomfort glare in outdoor lighting. These topics attract more and more interest and some CIE technical committees are engaged in finding new answers [62], [63], [64] that also might be incorporated in future standards in road lighting. Like different developer of high-end LED street luminaires, Bartenbach, with a 10 year history of studies in visual perception, currently focuses on new ideas for tunnel-lighting [65] as well as research on street lights with minimized glare [66].

Stable perception
Even if the luminance is within a range where no glare is to be expected the visual performance and especially the contrast sensitivity LB / ΔL (LB is the environment or background luminance and ΔL = L – LB is the difference of the task luminance L and the background luminance LB) according to [67] depends on the task luminance L as well as on the luminance of the environment LB (Figure 12). This so-called “model of stable perception” has been adopted and additional studies have been performed during the last decades. It means that you need balanced luminance distributions in the field of view to reach a stable perception. Ideally the infield L is brighter than the environment LB. Nowadays such considerations are even taken into account by standard planning software due to a differentiation in requirements in standards as EN 12464-1 [38]: The area of the visual task, the direct environment and the background are examined separately with different demands.

Energetic and Ecologic Aspects

Worldwide, about 20% of electrical power is used for lighting. In industrial countries that is approx. 1,000 kWh of final energy per person per year. The bulk of this energy is gained from fossil fuels, whereby about two thirds of this obtained energy is lost on its way to the end-consumer due to thermal losses (primary energy factor). The approximate 200-300 liters of oil that are therefore consumed per person and year are a substantial part of the economic footprint.

In some buildings, for example in administration buildings, the percentage of electrical energy for lighting amounts to 50%. In that case the illumination becomes an important factor on the way to zero- or low-energy houses. LED technology offers the possibility to meet these energetic requirements in future.

Figure 12: Contrast sensitivity dependent on luminance of infield and environment, data based on Schumacher R. O. [67]

Design Aspects

Lighting design is more than the planning of stipulated light intensities and luminance levels given by normative guidelines. Lighting design means the creation of an appearance (e.g. of a room), which complies not only with the technical requirements but also with the visual and non-visual (biological) as well as the emotional and aesthetic requirements of the users. Adequate illuminance levels horizontal and vertical (regarding room utilization, visual tasks, etc.), proper balanced luminances in the field of view, control of direct and reflected glare, and adequate color rendering are only minimal conditions for good lighting.

Visual perception is, first of all, a mental procedure and not only a pure sensation (like e.g. a thermal sensation, which causes feelings of cold or heat). To see means to receive information about our surroundings, about distances, surfaces, textures, what happens around us, etc. And all this information arouses emotions. Our perception is very selective, prejudiced by our personal experience, and is influenced also by our actual mental state, history and expectations.

The perception of the visual environment cannot be measured quantitatively, and therefore cannot be mathematically planned or converted. Visualizations by computer simulations (renderings) or scaled models are just aids, ultimately the true effects can only be experienced in the real situation.

Figure 13: Vineyard, comparison of two different lighting atmospheres (© Peter Bartenbach)

The pictures of figure 13 show the creation of two milieus only by changing the light color and light distribution.

From an architectural point of view, lighting is a means to express and underline the desired character of the building or room, which may be defined by an overall design style of the architect, by the use of a special room etc. From a functional point of view we have to take care that basic visual requirements have to be fulfilled to serve for an appropriate visual perception, dependent on the application. For example, in administrative buildings (offices, etc.) functionality is the key element (to satisfy ergonomic, safety and communication requirements), whereas in residence and tourism aspects like comfort, aesthetics, value, and social status are in the foreground.

Conclusion

LED-technology in cooperation with communication technology offers new possibilities for high grade and individual lighting solutions. To create a sustainable benefit for the users, generally acknowledged criteria must be set up, with which the new qualities of light can be assessed and measured. Furthermore, the non-visual effects of light on the human being must be better understood to be able to create “healthy” light.

The lighting community is facing the unique challenge of changing the value of lighting in society from an understated and cheap topic to an important and well-noticed issue regarding life quality, health and life style.

And, without any doubt the LED-technology provides the potential to cover the increasing need for light while at the same time reducing energy consumption.

References 
[1] U.S . Department of Energy: Solid-State Lighting Research and Development - Multi-Year Program Plan, 2014. http://energy.gov/ eere/ssl/technology-roadmaps

Non-visual light impacts:
[2] CIE x027 (2004): Proceedings of the CIE Symposium 2004 on Light and Health: Non-Visual Effects. 30 Sep. - 2 Oct. 2004, Vienna, Austria.

[3] CIE x031 (2006): Proceedings of the 2nd CIE Expert Symposium “Lighting and Health”. 7-8 September 2006, Ottawa, Ontario, Canada.

[4] Cajochen C (2007): Alerting effects of light. Sleep Medicine Reviews, 11(6): 453-464.

[5] Duffy J et al. (2009): Effect of light on human circadian physiology. Sleep Med Clin., 2009, 4(2): 165-177.

[6] Lam R et al. (1999): Clinical guidelines for the treatment of seasonal affective disorder. Clinical & Academic Publishing, Vancouver, BC, Canada. ISBN 0-9685874-1-0.

[7] Brainard GC, et al. (2001): Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. J. Neurosci. 2001, 21:6405-6412.

[8] Thapan, K, et al. (2001): An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans. J. Physiol. 2001, 535: 261-267.

[9] DIN V 5031-100 (2009): Strahlungsphysik im optischen Bereich und Lichttechnik – Teil 100 :

[10] Über das Auge vermittelt, nichtvisuelle Wirkung des Lichts auf den Menschen – Größen, Formelzeichen und Wirkungsspektren. Beuth-Verlag, 2009.

[11] DIN SPEC 67600 (2013): Biologisch wirksame Beleuchtung – Planungsempfehlungen. Beuth-Verlag, 2013.

[12] Lucas RJ, et al. (2014): Measuring and using light in the melanopsin age. Trends in Neuroscience, 2014, 37(1).

Color Rendition:
[13] CIE 13.3 (1995): Method of measuring and specifying colour rendering properties of light sources. Commission Internationale de l`Eclairage, Publ. CIE 13.3-1995.

[14] Houser K et al. (2013): Review of measures for light-source color rendition and considerations for a two-measure system for characterizing color rendition. Optics Express, 2013, 21(8): 10393-10411.

[15] Rea M et al. (2008): Color rendering: a tale of two metrics. Color Research Application, 2008, 33(3): 192-202.

[16] Smet K et al. (2013): CRI2012: A proposal for updating the CIE colour rendering index. Lighting Research and Technology 2013, 45(6): 689-709.

[17] David A et al. (2015): Development of the IES method for evaluating the color rendition of light sources. Optics Express, 2015, published 8 June.

[18] Judd D (1967): A flattery index for artificial illuminants. Illum. Eng. 1967, 62, 593–598.

[19] Davis W et al. (2010): Color quality scale. Opt. Eng., 2010, 49(3), 033602.

[20] LightingEurope (2014): LightingEurope position paper on color quality. October 2014.

[21] IES-PS-8-14 (2015): Color Rendering Index (CRI).

Dynamic artificial lighting:
[21] De Kort Y et al. (2010): Effects of dynamic lighting on office workers: first results of a field study with monthly alternating settings. Lighting Research and Technology, 2010, 42: 345-360.

[22] Barkmann C et al. (2012): Applicability and Efficacy of Variable Light in Schools. Physiol Behav, 105 (2012), 621-7.

[23] Boyce P et al. (2000): Individual lighting control: Task performance, mood and illuminance. Journal of the Illuminating Engineering Society, 29, 131-142.

[24] Hoffmann G et al. (2008): Effects of variable lighting intensities and colour temperatures on sulphatoxymelatonin and subjective mood in an experimental office workplace. Applied Ergonomics, 39, 719–728.

[25] Barrick A et al. (2010): “Impact of ambient bright light on agitation in dementia.” Int J Geriatr Psychiatry 25(10): 1013-1021.

[26] Canazei et al. (2013): Effects of dynamic ambient lighting on female permanent morning shift workers. Lighting Research and Technology, 2013.

[27] Deliverable 3.2 and 3.4 (2015): Lighting for health and well-being in education, work places, nursing homes, domestic application, and smart cities. EU-funded project “SSL-erate” under FP7-ICT-2013_11-619249.

[28] Smet K et al. (2013): CRI2012: A proposal for updating the CIE colour rendering index. Lighting Research and Technology 2013, 45(6): 689-709.

Flicker:
[29] Wilkins A. et al. (2010): LED lighting flicker and potential health concerns: IEEE standard PAR1789 update. Conference: Energy Concersion Congress and Exposition (ECCE), 2010.

[30] Berman S et al. (1991) Human electroretinogram responses to video displays, fluorescent lighting, and other high frequency sources. Optom Vis Sci., 68(8),645-62.

[31] US Department of Energy (2013): Fact sheet on flicker. (http:// apps1.eere.energy.gov/buildings/publications/pdfs/ssl/flicker_ fact-sheet.pdf - 16 June, 2015)

[32] Lehman B et al. (2014): Designing to mitigate the effects of flicker in LED lighting. Reducing risks to health and safety. IEEE Power Electronics Magazine, September 2014.

[33] IEEE 1789-2015: IEEE PAR1789 Recommended Practice for Modulating Current in High-Brightness LEDs for Mitigating Health Risks to Viewers. ISBN: 9780738196442, June 2015.

[34] IES (2011): The Lighting Handbook. Tenth Edition: Reference and Application. Illuminating Engineering Society, ISBN 978-087995-241-9

[35] Tschager M.: Pulsweitenmodulierte LED-Dimmung. Master Thesis, “Master of Light and Lighting (MLL)“. Leopold-Franzens- Universität Innsbruck / Lichtakademie Bartenbach, 2011. http:// www.lichtakademie.at/

Glare:
[36] CIE 117-1995: Discomfort Glare in Interior Lighting.

[37] CIE 031-1976: Glare and Uniformity in Road Lighting Installations.

[38] EN 12464-1:2011: Light and lighting - Lighting of work places - Part 1: Indoor work places.

[39] EN 13201-2:2003: Road lighting - Part 2: Performance requirements.

[40] CIE 17.4-1987: International Lighting Vocabulary.

http://www.electropedia.org/iev/iev.nsf/display?openform&ievr ef=845-02-56 (Electropedia of International Electrotechnical Commission IEC, “IEV Online”)

[41] LiTG 20 (2003): Das UGR - Verfahren zur Bewertung der Direktblendung der künstlichen Beleuchtung in Innenräumen.

[42] Hesse J., Müller T., Stolzenberg K.: Blendungsbewertungsverfahren und UGR-System. Schriftenreihe der Bundesanstalt für Arbeitsschutz und Arbeitsmedizin: Dortmund, Berlin, 1998.

[43] Akashi Y., Muramatsu R., Kanaya S.: Unified Glare Rating (UGR) and subjective appraisal of discomfort glare. Lighting Research & Technology, Vol. 28, No. 4, 1996.

[44] Son A-R., Kim I.-T., Choi A.-S., Sung M.-K.: Analysis of UGR Values and Results of UGR Calculations in Commercial Lighting Simulation Software. LEUKOS Vol. 11, No. 3, 2015.

[45] CIE 147-2002: Glare from Small, Large and Complex Sources (part of CIE Collection on Glare 2002).

[46] Geerdinck L. M., Van Gheluwe J. R., Vissenberg M. C. J. M.: Discomfort Glare Perception of Non-Uniform Light Sources in an Office Setting. LED professional Review LpR Issue 44, 2014.

[47] Takahashi H., Kobayashi Y., Onda S., Irikura T.: Position Index for the Matrix Light Source. Journal of Light & Visual Environment JLVE, Vol. 31, No. 3, 2007.

[48] Tashiro T., Kawanobe S., Kimura-Minoda T., Kohko S., Ishikawa T., Ayama M.: Discomfort glare for white LED light sources with different spatial arrangements. Lighting Research & Technology, Vol. 47, No. 3, 2015.

[49] Pohl W.: Der Einsatz von neuartigen Spiegel- und Werfersystemen für Beleuchtungszwecke. LICHT 1994 (Proceedings LiTG).

[50] Pohl W., Anselm C., Knoflach C., Timinger A. L., Muschaweck J. A. , Ries H.: Complex 3D-Tailored Facets for Optimal Lighting of Facades and Public Places. Proceedings SPIE 5186, Design of Efficient Illumination Systems, 2003.

[51] Pohl W., Anselm C., Knoflach C.: Complex systems for daylighting and artificial lighting. Proceedings SPIE 5186, Design of Efficient Illumination Systems, 2003.

[52] United States Patent 5 219 445: Illuminating Apparatus. Bartenbach, 1993.

[53] Kompetenzzentrum Licht (2002 – 2015): Project 26 „Lichtpunktzerlegung“, 2002 – 2005. http://www.k-licht.at/

[54] CIE 17.4-1987: International Lighting Vocabulary.

http://www.electropedia.org/iev/iev.nsf/display?openform&ievr ef=845-02-57 (Electropedia of International Electrotechnical Commission IEC, “IEV Online”)

[55] JTC 7 Joint Technical Committee of the CIE (Division 3: Interior Environment and Lighting Design + Division 1: Vision and Colour): Discomfort caused by glare from luminaires with a non-uniform source luminance. Chair: Naoya Hara (JP).

[56] CIE 191-2010: Recommended System for Mesopic Photometry Based on Visual Performance.

[57] JTC 1 Joint Technical Committees of the CIE (D1/D2/D4/D5): Implementation of CIE 191:2010 Mesopic Photometry in Outdoor Lighting.

[58] Uchida T., Ohno Y.: Effect of High Luminance Sources to Peripheral Adaptation State in Mesopic Range. CIE x038:2013, Centenary Conference »Towards a New Century of Light«, 2013.

[59] Kostic A.M., Kremic M.M., Djokic L.S., Kostic M.B.: Light-emitting diodes in street and roadway lighting - a case study involving mesopic effects. Lighting Research & Technology, Vol. 45, No. 2, 2013.

[60] Vidovszky-Nemeth A.,Schanda J.: Road-Lighting and Mesopic Photometry. Light & Engineering, Vol. 21, No. 1, 2013.

[61] Akashi Y., Kanaya S., Ishikura C.: Interference between Foveal and Peripheral Visual Tasks, Potentially Affecting Mesopic Photometry. Journal of Light & Visual Environment, Vol. 38, 2014.

[62] Technical Committee of the CIE TC 4-15: Road Lighting Calculations. Chair: Sermin Onaygil (TR). http://div4.cie. co.at/?i_ca_id=585&pubid=280

[63] Technical Committee of the CIE TC 4-33: Discomfort Glare in Road Lighting. Chair: Ron Gibbons (US). http://div4.cie. co.at/?i_ca_id=585&pubid=286

[64] Technical Committee of the CIE TC 4-36: Visibility Design for Roadway Lighting. Chair: Vacancy. http://div4.cie. co.at/?i_ca_id=585&pubid=287

[65] Canazei M., Staggl S., Pohl W.: The influence of tunnel lighting on cognitive and visual performance parameters - a field experiment. European Association for Accident Research and Analysis, EVU Congress, 2011.

[66] Cooperative R&D-project founded by FFG: UrbanLight. Bartenbach GmbH, research & development with AIT Austrian Institute of Technology GmbH. 2015 – 2017.

Stable Perception:
[67] Schumacher R. O.: Die Unterschiedsempfindlichkeit des helladaptierten menschlichen Auges. Dissertation TH Berlin, Verlag Ez & Rudolph. Frankfurt/Main 1940.