Light allows us to see, but its effect is far from over. Already in his habilitation thesis in 1948, Dr. Hollwich, a well-known German ophthalmologist and author of textbooks on ophthalmology, distinguished between the visual and the energetic (non-visual) transmission path of light. [1]

The visual effects of light are well known, there are three types of cones for colour day vision: blue (S) - cyanolab, green (M) - chlorolab, red (L) - erythrolab and rods for non-coloured night vision (R) - rhodopsin. The absorption of a photon in the dye causes a biochemical reaction that produces an electrical charge, which is processed into an excitation that is transmitted to the nervous system. See Figure 2 for wavelengths and spectral efficiencies.

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The hypothesis of another photoreceptor on the retina dates back to 1923, when its presence was used to explain the photoreaction of pupils in mice that did not have functional rods or cones. However, it was not until 1991 that the light-sensitive ganglion cells (iPRGCs) in the mouse retina were discovered. These are neurons equipped with melanopsin, another type of eye dye with a specific spectral sensitivity (curve C in Figure 2) with a maximum in the blue region: 450-482 nm. In 2007, Professor Russell G. Foster's team discovered them in the human retina. A year later, Prof. Foster was elected a Fellow of the Royal Society.

Functions of light-sensitive ganglion cells of the retina:
▪ synchronization of the central biological clock
▪ control of melatonin levels in the blood
▪ photoreaction of the pupils
▪ possibly contributing to visual perception

The central biological clock is located in the mesenchyme, specifically in the hypothalamus above the optic nerve junction (suprachiasmatic nuclei, SCN). It is a cluster of about twenty thousand neurons that make up the mammalian central biological clock. In the young human, their free period (i.e., without synchronization) is slightly more than 24 hours. Their rhythm is referred to as approximately diurnal, i.e., from the Latin circa dies, circadian. With age, this cycle shortens (with great individual differences). Their beating is manifested by the switching on and off of certain genes. According to this central clock, the cellular clocks of each organ in the body synchronize to their daily cycles. The synchronization of the central clock, i.e. the setting of the beginning of the subjective day, occurs mainly through the action of morning light, but food intake or social contact also has an effect. It is therefore not only a biological but also a social synchronisation.

Time order and cycles in the living realm are the subject of chronobiology, a science co-founded in the 1950s by Prof. Franz Halberg (1919-2013), an American scientist of Romanian origin. Among Czech chronobiologists, the most famous is Prof. Helena Illnerová, who with her team was the first in the world to discover that melatonin production in the pineal gland is controlled by the biological clock in the brain.

With insufficient morning light stimulus, the subjective day is delayed and morning malaise shifts into the morning hours and is accompanied by a decline in performance and alertness. Bright morning light, preferably sunlight when walking in nature, but also artificial light (in winter, when the sky is overcast) helps people to step into the day in an active and alert state. Bright light is of great importance in the treatment of depression, not only seasonal (SAD) but also non-seasonal. According to research [2], the effect of light is fully comparable to that of antidepressants. According to research [3], in many workplaces, work productivity can be increased and error, accident and sickness rates reduced by increasing the intensity of lighting.

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Shift workers usually have this circadian rhythm disrupted. The International Agency for Research on Cancer (IARC) has classified shift work with disrupted circadian rhythms as Group 2A, i.e. probably carcinogenic to humans. The importance of regular sleep cycles and alternating light intensities is probably still not fully appreciated in our society.

Light also acts through the retinohypothalamic tract (RHT) on melatonin levels in the blood. This is a kind of optic nerve hotline, independent of visual perception. Melatonin is a sleep hormone, helps in the daily regeneration of the body, scavenges free radicals and even counteracts degenerating cells. In the evening (in low light), its levels rise, peak after midnight and gradually fall by morning, see Figure 1. In plenty of light, its levels are minimal and it swaps its role with cortisol, the hormone of activity and stress. The alternation of these two hormones corresponds to the individual biological day and night. Thus, it is advisable to have plenty of light during the day (remember our ancestors who worked mainly outdoors) and, on the contrary, to have good quality darkness at night without disturbing light.

From the divergence of the V(λ) and C(λ) curves in Figure 2, it is clear that different light sources can have different proportions of the circadian (blue) activation component at the same intensity. Several quantities have been proposed for comparison of the lights, which have different pitfalls. Therefore, the author proposed the quantity Ac, [4] which allows comparing lights not only with each other but also with daylight. The definition of Ac also accounts for the future update of the waveform C(λ). The quantity Ac compares a given light with daylight D65 in terms of circadian effect.

Thus, according to Table 1, incandescent light provides only about one third of the activating component compared to daylight at the same illuminance. Incandescent bulbs and other sources of warm light are suitable for illumination in the evening because they do not interfere with the onset of melatonin. Warm light is (according to the Kruithof diagram) pleasant at low illuminances.

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For workplace illumination, on the other hand, a high proportion of the activating component (Ac → 100) is an advantage. Here, however, there is a marked difference between the different sources. Fluorescent lamps with a neutral white tone (marked 840, Cool White) provide only about 55 % activating light at the same illuminance compared to cool daylight. Fluorescent lamps with a cool daylight white tint (denoted 865, Daylight) are better off, where the proportion of the activating component is close to daylight. However, neither of these variants complies with the standard [5] for lighting in dental premises.

In surgeries and other designated areas in the healthcare sector, we (unfortunately often) encounter fluorescent lamps with Ra ≈ 80 (840/865). The standard [5] prescribes light sources with Ra > 90 for surgeries. Such fluorescent lamps are marked e.g. 965 or full spectrum. The colour rendering, especially of deep reds and yellows, is essential for diagnostics. In daylight or in the light of sources with Ra > 90, for example, a patient can be safely diagnosed with cyanosis or jaundice, whereas in the light of fluorescent lamps with Ra ≈ 80, this is virtually impossible, as the relevant wavelengths (certain shades of yellow and red) are essentially absent from their spectrum.

The eye is equipped with an automatic white balance function - similar to a digital camera. This function gives us a natural colour perception under different lighting (chromatic) conditions, but it also allows us to adapt to less suitable light, which after a while we find acceptable. To test the light in the surgery, simply adapt to it (at least 10 minutes, shading the light from the outside as best you can) and then look out of the window at the daylight. If it looks too blue to you, you are probably using light sources with a low chromaticity temperature and therefore low Ac. Ideally, you won't notice any difference. You need to give a first impression when the difference is most noticeable.

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Light and dark are polarities that were here long before us and that we have literally encoded in our genes. With inadequate or inappropriate light during the day and distracting light at night, we disrupt these polarities. What degree of distortion is still acceptable and what is already disturbing is a matter for research and debate, in which economic factors will certainly enter. Yet it only takes a little, for example, to increase lighting in the workplace or improve shading in the bedroom, and we can get these polarities back on our side.

Literature:

[1] HOLLWICH, Fritz. The Influence of Ocular Light Perception on Metabolism in Man and in Animal. New York: Springer-Verlag, 1979, 129 p. ISBN 0387903151.

[2] LAM, R. W. et al: The Can-SAD Study. Am J Psychiatry, 2006, no. 163, 805-812.

[3] BOMMEL VAN, W. J. M. - BELD VAN DEN, G. J. - OOYEN VAN, M. H. F.: Industrial lighting and productivity. Philips Lighting. August 2002.

[4] FUKSA, A.: Light and the biological clock. Light, 2010, no. 6, pp. 56-58.

[5] EN 12464-1. Lighting and illumination - Lighting of work areas: Part 1: Indoor work areas. 2012 edition. Prague: ÚNMZ, 2012.

Author. Antonín Fuksa
Published in Era21 3/2014