In eons long past, the inhabitants of the planet Tellus experienced an amazing discovery. With the aid of a prism, the light of the sun was separated into the colors of the rainbow. A Wise One named Hershel assumed that one of the colors carried the heat of the sun, and moving a thermometer from color to color he tried to determine which one. None of the colors made much impact, but when he placed the thermometer into the darkened void adjacent to the color red, he finally discovered the heat source; located within the darkened void he could not see with his eyes! We know i t today as infrared (which means “below red”).
A few years later another Wise One named Ritter theorized that there might also be an invisible light adjacent to the blue violet color. On the theory that chemical oxidation could also be caused by the invisible light he exposed silver chloride, a chemical known to oxidize in the presence of blue and violet light, into the darkened area next to the blue and violet light and observed that oxidation was much faster. We know this invisible light today as ultraviolet (which means “above violet”). The Society of Wise Ones proclaimed that sunlight consisted of visible light and invisible light, and that the two types of invisible light had different powers. One seemed to warm the planet, and one seemed to cause chemical reactions. It was not long before they also realized that light was also the catalyst for biological response for every living being on their planet.
Thus began a period of great enlightenment in which other Wise Ones like Maxwell and Hertz theorized an entire electromagnetic spectrum that included the three types of light and other phenomena. Soon all three types of natural light could be artificially created by electricity and managed using principles of optics. Thus started an era of scientific and business in which the Wise Ones became increasingly knowledgeable and worked for companies to make lighting products.
Members of one such company theorized that they could create an indoor appliance known as the Triple Sun which produced a combination of visible, UV and IR light with the promise of improving the health and wellbeing of their fellow Tellurions that recreated the light they presumed to be found in nature. But with the rush to bring “indoor triple source natural light” to market, they failed to fully research the mutual reciprocity and complexities of the natural combination of the three natural light energies. Eventually, medical researchers discovered that a specialized connective tissue cell, known as a fibroblast, was damaged and rendered dysfunctional by the artificially combined light. Fibroblast is vitally important for cellular regeneration and plays a major role in wound healing and stopping the progression of cancer, pulmonary disease and renal disease. The Wise Ones finally realized that there was more to the challenge of creating a “healthy light” indoors than what they assumed.
The Potential of “Natural Light”
Natural light on the surface of the blue planet is plainly essential to life and health. Having the ability to control the spectrum of solar light and to generate a very specific intensity and spectrum of light, from IR to visible and UV, mankind now possesses tools to potentially do it right only presumed to be safe with judicious research. In horticulture, for example, by converting sunlight to electricity, vertical indoor farming can increase crop density, substantially reduce water use and eliminate pests and pesticides from the food chain while eliminating carbon emissions to the atmosphere. Emulated natural light combining factors of the earths atmosphere and the solar spectrum can replicate the photosynthetic conditions matching natural light. Similar emulations of natural light are used daily in a variety of critical environments, such as the International Space Station, where 24-hour periods of light and darkness are critical in maintaining the health of the crew. As our society presses towards 24-hour life cycles contrary to nature, better tools will be needed to ensure a person’s daily cycle of wake and sleep are synchronized to their individual cycle.
Natural light has been studied carefully for these and many other reasons. Sunlight travels through outer space (yellow); it is filtered by the atmostphere as water and oxygen molecules in air absorb some of the solar radiation in specific wavelengths, and the ozone layer of the lower stratosphere interconverts UV into thermal energy, heating the atmosphere and preventing the most potentially harmful UV wavelengths from reaching the planet’s surface. Of the solar energy making it to the planet’s surface on a clear day, about 45% is visible light, 50% IR, and 5% UV1. These percentages vary depending on time of day, season, and orbit variations. Most importantly, the intensity and distribution of daylight varies constantly with the weather. For horticulture, all of these factors play a major role in the photosynthetic process necessary for plants to thrive, produce and feed other living beings.
Figure 1 Solar Radiation Spectrum. Source: Wikipedia Commons
IR: The Invisible Light Source
Infrared light (IR) is the portion of the solar spectrum with wavelengths between 780 nanometers and 1,000,000 nanometers (1 millimeter). It is categorized as IR-A, also known as near-IR (780 nm-1400 nm), IR-B (1400-3000 nm) and IR-C, also known as far-IR (3,000 -1,000,000 nm). Most recognize IR as radiant heat, often observed in conjunction with long wave red light. It has many modern uses ranging from night vision and motion sensors to medical therapy, digital communications and weapons. LiFi, for example, employs light (typically IR) to provide high-speed data communications, but it is limited to line-of-sight because it is after all, light! Specialty medical therapies and procedures employ man-made IR and NIR, and there are numerous over-the-counter consumer products that use IR to apply soothing heat to a host of ailments from aching joints to receding hairlines.
Architecturally, IR presents challenges. As a heat source, IR can be useful, especially for warming building occupants during winter months. But IR and visible light easily pass through ordinary window glass, and although the visible light is generally useful, the IR contributes to heating interior spaces that most of year need air conditioning. Modern building glass called low-emissivity (“low-e”) employs coatings that reflect the IR energy back towards the sun and sky; by removing most of the infrared energy and limiting the amount of entering light, buildings become much more thermally efficient. Low-e coatings on window glass reduce solar heat gain as much as almost 75% with little impact on visible light. In winter, the same window layers reduce radiant heat transfer out of the building. Choosing and specifying glazing, especially with consideration of its spectral transmission characteristics, is a critical important part of daylighting design and overall building energy efficiency.
Photo Credit: hunterpic2013
UV: The Invisible Reactive Source
In nature, the human exposure to UV is limited by 5 variables, including time, atmosphere, latitude, altitude, and clothing. Mountain climbers and skiers know that altitude increases solar and UV exposure, and swimmers and boaters know the sunburn risk that is increased by reflected light off the water on sunny days.
UV light is the portion of the spectrum of wavelengths classified in 1958 to range from 400 nm to 100 nm. Historically divided into three bands: UVA (315-400 nm) UVB (280-315 nm) UVC (100-280 nm) medical researchers are now beginning to consider expanding the boundaries to include 405 given the noted biological response in this region. As an example, 95% of UV at the planet’s surface is UVA, which was once thought to be minimally impactful to humans but is now recognized as a major contributor to skin cancer development and other negative health impacts. Since UV-B is mostly filtered out by the atmosphere, it can have serious impacts including skin cancer and corneal damage if skin barrier protection methods are not properly applied. UV-C does not penetrate the atmosphere, but it can be generated using certain electrical techniques like xenon and carbon arc lamps and welding arcs. It is seriously damaging to most life forms and can kill organisms in water and in the air. Accidental exposure to UVC risks serious skin and eye damage, and long-term exposure for indirect exposure has not been fully researched.
In occupied buildings UV exposure is rare and typically caused in equipment for which safely controls are bypassed or not working properly. Because of this danger, UV has largely been relegated to specific uses that are closely regulated for safety such as UV purification and sterilization. Fluorescent lamps, for example, employ a mercury arc discharge that creates UV, but the glass tube of the lamp prevents UV radiation into the environment. Purification and sterilization equipment is required to have interlocks that prevent operation of the UV source unless the equipment enclosure is sealed to prevent exposure.
In science and modern practice, UV has been used for decades but only in controlled applications to prevent visual exposure. Blacklight UV lamps are typically fluorescent or LED lamps employing a special type of cover that transmits visible light and UV-A. Sterilization and purification light sources, on the other hand, generate broadband UV but are only allowed to be used in interlocked enclosures. But the Covid-19 pandemic changed the stakes. The comparative ease of constructing new LED luminaires encouraged the application of a new generation of UVLEDs with different wavelengths from UV-A to UV-C, each having been tested for its ability to kill viruses with specific nanometer radiation with minimum side effects when concealed from visual exposure. In most applications lighting controls are used to automatically detect human presence and ensure that UV is turned off. Research findings, marketing and intellectual property protection have caused an interesting marketplace of creative solutions.
Back to the Future
In terms of scientific discovery, the understanding of the connection between biology and modern lighting is still in its infancy. Although hundreds of theories have since been presented and conferences held to attempt to capitalize on these discoveries, many challenges remain. But are we rushing too fast before understanding the full depth of the biological consequences? For instance, in a leading-edge 2020 study of viable human skin biopsies 2researchers used a solar simulator to recreate a combined UV, IR and visible light (“Vis”) dose equivalent to a 2- and 7- hour Mediterranean sun exposures, respectively. They found that the artificially combined light reacted differently than that combined in nature. Their research demonstrated that skin exposed to the artificially produced combined light had increased reactive oxygen species (ROS), increased DNA damage, and more importantly, the engineered combined light caused greater damage to the biologically critical fibroblasts than UV exposure alone. These groundbreaking findings run contrary to previously held expectations of the lighting industry currently dependent upon single light source studies examining UV, Vis, and IR exposure.
Given the results of this study, there remains a great many questions to be addressed and medical research conducted to be completed done before our industry assumes we can recreate the perfect combined light – visible and invisible – for indoor spaces. Paraphrasing the researchers of this seminal paper, “Solar radiation is polychromatic and its effects on the body are not only the result of the separate actions of each wavelength on human biology but rather the result of the interaction of the numerous wavelengths combined”. By rushing headlong into developing products for occupied spaces which combine light sources, like the Tellurions, we will risk discovering that duplicating what nature provides is not that simple as it appears.
2 Hudson L, Birch-Machin MA. (2020) Individual And Combined Effects Of The Infrared, Visible, And Ultraviolet Light Components Of Solar Radiation On Damage Biomarkers In Human Skin Cells. FASEB Journal: 34:3874–3883. https ://doi.org/10.1096/fj.20190 2351R
This article was originally featured in the October issue of designing lighting (dl).