Visual and Non-Visual Effects of Light

Working Environment and Well-Being

Agnieszka Wolska, Dariusz Sawicki, Małgorzata Tafil-Klawe

• Editorial: CRC Press
• Año: 2020
• ISBN:
  9781000168105
• Edición: 1
• Categoría: Tecnología e ingeniería
• Categoría: Seguridad y salud industriales

The introduction of artificial lighting extends the time of wakefulness after dark and enables work at night, thus disturbing the human circadian rhythm. The understanding of the physiological mechanisms of visual and non-visual systems may be important for the development and use of proper light infrastructure and light interventions for different workplace settings, especially for shift work conditions.

Visual and Non-Visual Effects of Light: Working Environment and Well-Being presents the impact of lighting in the working environment on human health, well-being and visual performance. The physiological explanation of the visual and non-visual effects of light on humans which discusses the biological bases of image and non-image forming vision at the cellular level may be of particular interest to any professional in the field of medicine, physiology, and biology. It is one of the intentions of this book to put forward some recommendations and examples of lighting design which take into account both the visual and non-visual effects of light on humans. These may be of particular interest to any professional in the field of lighting, occupational safety and health, and interior design.

"What effects on health can a light 'overdose' or light deficiency have? What is bad light? The authors of the monograph provide answers to these questions. Just as for a physicist, the dual nature of light comprises an electromagnetic wave and a photon, the duality of light for a physician comprises visual and non-visual effects."

--------------------------------------------------------Prof Jacek Przybylski, Medical University of Warsaw

"This is a unique publication in the field of lighting technology. The authors have skillfully combined both the technical and biomedical aspects involved, which is unprecedented in the literature available. As a result, an important study has been created for many professional groups, with a significant impact on the assessment of risks associated with LED sources."

--------------------------------------------Prof Andrzej Zaj?c, Military University of Technology, Warsaw

1 Introduction

Light Light. The visible reminder of Invisible Light.

T.S. Eliot

1.1 Common Definitions of Light

Light is an important regulator of physiology and behavior in all living entities. Its significance is enhanced by the fact that life on Earth is subject to alternating cycles of day and night (light and darkness) imposed by the rotation of our planet. For humans, the sense of vision plays a central role when interacting with the environment. Consequently, most definitions of light are related to one’s visual response to this phenomenon. For example, according to the Merriam-Webster Dictionary [Webster 2019], “light” (as a noun) has three definitions:

• something that makes vision possible,
• the sensation aroused by stimulation of the visual receptors,
• electromagnetic radiation of any wavelength that travels in a vacuum with a speed of 299,792,458 m (about 186,000 miles) per second, specifically; such radiation that is visible to the human eye.

These three definitions do not relate to the non-visual response to light. Why?

Although for a long time it has been widely known that sunlight is one of the most important regulators of human physiological functions related to circadian rhythm, in-depth research into the biological mechanism of the non-visual effect of light began in the late 20th and early 21st centuries, with the discovery of a new photoreceptor on the retina called the intrinsically photosensitive retinal ganglion cell (ipRGC). A rapid development of research related to the human non-visual response to light was launched. It has been proved that light has the ability to change circadian rhythms, i.e. to change the time periods in circadian cycles, which may result in shifting the phases of physiological cycles. Light also affects a number of physiological reactions, such as the regulation of hormone secretion (e.g. it can inhibit the pineal hormone responsible for melatonin secretion at night), affects the level of body temperature, induces the pupillary reflex, raises subjective alertness, and, under certain conditions, changes the bioelectrical brain activity indicating sleepiness or alertness. That is why the non-visual effects of light are so important for human functioning.

1.2 Visual and Non-Visual Response to Light

Retinal photoreceptors make it possible to gather information passed from the eye to the visual parts of the brain, which analyze and modify this information to form a representation of objects, working in a system of conventional image-forming vision (visual responses to light). Light enters the eye through the cornea, and then through the pupil, whose diameter is controlled by the muscles of the iris. Behind the iris lies the lens, which in conjunction with the cornea focuses the incoming light on the back of the eye, i.e. on the retina, which contains light-sensitive neuron photoreceptors: rods and cones. Photoreceptors are responsible for the process of phototransduction: the conversion of the energy of the sensory stimulus, the photons of light, into an electrical signal – action potential, transmitted and analyzed by the cells of the nervous system. Five general classes of neuron make up the retina. One class, retinal ganglion cells, uses its axons to form the optic nerve, which carries visual information from the eye to the visual brain centers.

However, the form of photodetection involved in the synchronization of biological processes with the dark/light cycle, i.e. non-image forming vision (non-visual responses to light), in which the basic information is light or darkness over time, seems to be far more ancient than image-forming vision, and has been widely discussed in recent years.

Although it was originally believed that all light signals for image- and non-image-forming vision began with rods and cones, the first suggestion that these classical photoreceptors do not account for the spectral sensitivity of the pupillary light reflex can be found as early as 1923. In 1980 it was reported that light regulated the levels of the neurotransmitter dopamine in rats’ retinas with degenerated rods and cones. Over the subsequent years, several studies showed a shift of circadian rhythms according to the external light/dark cycle in rodless/coneless mice. At the end of the 20th century similar observations were made in humans: light was effective in entraining the circadian clock in blind people without impinging on their conscious perception. The next step was a discovery of a new photopigment, melanopsin, localized to a small subset of retinal ganglion cells which project to the hypothalamus, and in particular to the suprachiasmatic nucleus (SCN), the master circadian pacemaker and biological clock, as well as to other brain regions serving non-image-forming vision. Subsequent important studies described intrinsically photosensitive retinal ganglion cells (ipRGCs), whose cells and the melanopsin-expressing retinal ganglion cells were shown to be one and the same. Both of these differ from other retinal output neurons; they show autonomous phototransduction, mediated by the photopigment melanopsin. They are diverse and are now thought to comprise five types, with different physiological functions. Generally, these cells are typically associated with non-image-forming functions: circadian photoentrainment and the pupillary light reflex, but some subtypes also influence the visual function, suggesting interaction between image- and non-image-forming vision. Recent studies provide evidence that ipRGCs can contribute to our awareness of external light, pointing to an important functional overlap of the ipRGCs and conventional rod/cone systems.

Thus, it is not a surprise that the complex morphological and functional retinal organization makes it possible to consider the dual nature of human responses to light and lighting: visual and non-visual. These responses are especially important in the context of human exposure to artificial light at night and during the day or in shift work conditions.

1.3 Light and Circadian Rhythm

Day–night cycles regulated by daylight control the human internal biological clock known as the circadian rhythm. The name comes from the Latin phrase circa diem, which is translated as “around a day”. It lasts for about 24 hours and is a continuous rhythm based on our body’s reaction to the presence or absence of daylight. The common presence of artificial light of different spectra, both during the day and at night, has a significant influence on our physiology, including circadian rhythm disruption.

The circadian rhythm of human physiological functions is controlled by two clusters of neurons called the suprachiasmatic nuclei (SCN), which are located at the base of the hypothalamus in close proximity to the intersection of the optic nerves (hence the name). Information on the alternation of day and night reaches the SCN via a visual tract through photosensitive retinal ganglion cells. In response to light, melanopsin is activated and information is transmitted, thanks to which SCN cells begin “measuring” the next day. In the suprachiasmatic nucleus, the path to sympathetic centers in the thoracic spinal cord begins. From here, further fibers exit into the pineal gland, which secretes melatonin. The lack of light is a signal for the pineal gland to produce this hormone, and thus prepares the human body for the sleep phase. In contrast, the presence of bright white light or monochromatic light with specific wavelengths in the range between 420 and 550 nm inhibits the secretion of this hormone and puts the body in a state of readiness (wakefulness). It has been proven that light with a length perceived as blue (between 450 and 490 nm) is most responsible for the direct non-visual effect.

1.4 LED Lighting and Potential Health Hazard

In the last ten years light sources and technology have experienced a revolution. The new generation of light sources – light-emitting diodes (LEDs) – have become widely used in industrial and commercial environments, but also in non-industrial applications: in TVs, computers, smartphones, and tablets. The most common white LEDs used for illumination are phosphor-based ones. Usually, blue light LEDs (450 nm) are covered by phosphor material which converts monochromatic blue light into broad-band white light. But regardless of the phosphor used, there is a visible peak of around 450 nm blue light in the white light spectrum, which could suppress melatonin secretion during the night, both at work and at home. Even a smartphone LED may be a source of artificial light at night, which influences the circadian regulation of the sleep–wake cycle, suppresses melatonin secretion, alters mood and cognitive functions, and evokes fatigue. Although the light emitted by LEDs appears white, it has peak emission in the blue light range.

Exposure to high intensity blue light can affect many physiological functions, and can even induce retina injuries or damage. This is why the blue light hazard arising from artificial sources must be evaluated and exposure levels at workers’ eye positions must be compared with exposure limit values for blue light hazard established by the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and Directive 2006/25/EC of the European Parliament and of the Council. However, present-day knowledge about blue light can also be used to treat sleep disorders. In modern society, the use of blue light is becoming increasingly prominent. A large world population is exposed to artificial light at unusual times of the day or late at night. Light has a cumulative effect of many different characteristics (wavelength, intensity, duration of exposure, time of day). It is important to consider the spectral output of light sources for the improvement of alertness during night shifts, but also to minimize the danger associated with blue light and artificial light exposure.

1.5 New Idea of Lighting Design – Human-Centric Lighting (HCL)

So far, lighting has often been designed only to ensure safety and appropriate conditions for performing visual work, while maintaining the greatest possible comfort of vision and human well-being. Usually, the non-visual mechanisms of light’s impact on the human body were not taken into account. This approach has been changing since the discovery of ipRGCs and the role of the non-visual effects of light. The point-of-view on lighting design changed in the early 2000s, when the innovative idea of dynamic lighting design was born. Moreover, a few years ago, together with the development of knowledge about the impact of light on the human circadian system and new circadian metrics, a new stage began in the design of lighting, focused on human health and well-being, which is called human-centric lighting (HCL) or integrative lighting.

The aim of human-centric lighting is to benefit human health and well-being in various ways resulting both from the effects of light on the visual and non-visual system. Figure 1.1 shows that the same aspects (features) of light, i.e. amount (intensity), spectrum (SPD), spatial distribution, timing, and duration, impact both the visual and non-visual systems, but they have different effects.

FIGURE 1.1 Visual and non-visual systems are all, in different ways, impacted by lighting.

The effects of light can be assigned to three basic groups: visual, biological, and psychological (emotional). The visual group mainly takes into consideration lighting parameters which influence acuity, visual performance, appearance, and safety. The parameters from this group are supposed to provide high-quality lighting for human vision. The biological group mainly looks at circadian/melanopic metrics of light, which influence our circadian performance (phase-shift and alertness). The parameters from this group are supposed to provide high-quality lighting for the human circadian system. The psychological (emotional) group is concerned with the users’ mood, behavior, and comfort, which are affected both by visual and biological parameters. All three groups together focus on human individual needs, performance, health, well-being, satisfaction, and comfort.

Nevertheless, there is no consensus in the scientific community on the circadian metric which should be taken for designing light. Hence, various metrics are used for this purpose, of which the two most commonly used ones are circadian stimulus (CS) and equivalent melanopic lux (EML). How to design lighting so as to take into account the non-visual effect of light is also controversial. Moreover, there is a huge problem with what kind of lighting to design for night shift work.

1.6 What Is This Book About?

The book presents a physiological explanation of the visual and non-visual effects of light on humans. It includes two chapters (Chapters 2 and 3), which discuss the biological bases of image- and non-image-forming vision at the cellular level. These may be of particular interest to doctors and medical students, as they provide a thorough overview of the latest discoveries in these fields.

Since the circadian system of shift workers is permanently disturbed, the chapters devoted to the new approach to lighting design at workplaces focus on adopting lighting parameters that could fulfill the needs of both visual and...