Light tubes (also known as solar pipes, tubular skylights or sun tunnels) are structures that transmit or distribute natural or artificial light for the purpose of illumination and are examples of optical waveguides.
In their application to daylighting, they are also often called tubular daylighting devices, sun pipes, sun scopes, or daylight pipes. They can be divided into two broad categories: hollow structures that contain the light with reflective surfaces; and transparent solids that contain the light by total internal reflection. Principles of nonimaging optics govern the flow of light through them.
Types
IR light tubes Manufacturing custom designed infrared light pipes, hollow waveguides and homogenizers is non-trivial. This is because these are tubes lined with a highly polished infrared reflective coating of gold, which can be applied thick enough to permit these tubes to be used in highly corrosive atmospheres. Carbon black can be applied to certain parts of light pipes to absorb IR light (see photonics). This is done to limit IR light to only certain areas of the pipe.
While most light pipes are produced with a round cross-section, light pipes are not limited to this geometry. Square and hexagonal cross-sections are used in special applications. Hexagonal pipes tend to produce the most homogenized type of IR Light. The pipes do not need to be straight. Bends in the pipe have little effect on efficiency.
Light tube with reflective material
The first commercial reflector systems were patented and marketed in the 1850s by Paul Emile Chappuis in London, using various forms of angled mirror designs. Chappuis Ltd's reflectors were in continuous production until the factory was destroyed in 1943. The concept was rediscovered and patented in 1986 by Solatube International of Australia. This system has been marketed for widespread residential and commercial use. Other daylighting products are on the market under various generic names, such as "SunScope", "solar pipe", "light pipe", "light tube", and "tubular skylight".
A tube lined with highly reflective material leads the light rays through a building, starting from an entrance-point located on its roof or one of its outer walls. A light tube is not intended for imaging (in contrast to a periscope, for example); thus image distortions pose no problem and are in many ways encouraged due to the reduction of "directional" light.
The entrance point usually comprises a dome (cupola), which has the function of collecting and reflecting as much sunlight as possible into the tube. Many units also have directional "collectors", "reflectors", or even Fresnel lens devices that assist in collecting additional directional light down the tube.
In 1994, the Windows and Daylighting Group at Lawrence Berkeley National Laboratory (LBNL) developed a series of horizontal light pipe prototypes to increase daylight illuminance at distances of 4.6-9.1 m, to improve the uniformity of daylight distribution and luminance gradient across the room under variable sun and sky conditions throughout the year. The light pipes were designed to passively transport daylighting through relatively small inlet glazing areas by reflecting sunlight to depths greater than conventional sidelight windows or skylights.LBNL: The Design and Evaluation of Three Advanced Daylighting Systems: Light Shelves, Light Pipes and Skylights
A set-up in which a laser cut acrylic panel is arranged to redirect sunlight into a horizontally or vertically orientated mirrored pipe, combined with a light spreading system with a triangular arrangement of laser cut panels that spread the light into the room, was developed at the Queensland University of Technology in Brisbane.Ken Yeang: Light Pipes: An Innovative Design Device for Bringing Natural Daylight and Illumination into Buildings with Deep Floor Plan , Nomination for the Far East Economic Review Asian Innovation Awards 2003 In 2003, Veronica Garcia Hansen, Ken Yeang, and Ian Edmonds were awarded the Far East Economic Review Innovation Award in bronze for this development.Lighting up your workplace — Queensland student pipes light to your office cubicle , May 9, 2005Kenneth Yeang , World Cities Summit 2008, June 23–25, 2008, Singapore
Light transmission efficiency is greatest if the tube is short and straight. In longer, angled, or flexible tubes, part of the light intensity is lost. To minimize losses, a high reflectivity of the tube lining is crucial; manufacturers claim reflectivities of their materials, in the visible range, of up to almost 99.5 percent. Tube de Lumière
At the end point (the point of use), a diffuser spreads the light into the room.
The first full-scale passive horizontal light pipes were built at the Daylight Lab at Texas A&M University, where the annual daylight performance was thoroughly evaluated in a 360° rotating 6 m wide by 10 m deep room. The pipe is coated with a 99.3% specular reflective film and the distribution element at the end of the light pipe consists of a 4.6 m long diffusing radial film with an 87% visible transmittance. The light pipe introduces consistently illuminance levels ranging between 300 and 2,500 lux throughout the year at distances between 7.6 m to 10 m.
To further optimize the use of solar light, a heliostat can be installed which tracks the movement of the sun, thereby directing sunlight into the light tube at all times of the day as far as the surroundings' limitations allow, possibly with additional mirrors or other reflective elements that influence the light path. The heliostat can be set to capture moonlight at night.
Optical fiber Optical fibers can also be used for daylighting. A solar lighting system based on plastic optical fibers was in development at Oak Ridge National Laboratory in 2004.Article on Hybrid Solar Lighting "Let the Sun Shine in", Discover Magazine, Vol. 25, No. 07, July 2004 ORNL - Solar Technologies Program The system was installed at the American Museum of Science and Energy, Tennessee, USA, in 2005,HSL Featured in Popular Science's What's New Section June 2005, Page 28 and brought to market the same year by the company Sunlight Direct.Oak Ridge National Laboratory – New Oak Ridge company putting hybrid solar lighting on map Sunlight Direct- Architectural Design Information However, this system was taken off the market in 2009.
In view of the usually small diameter of the fibers, an efficient daylighting set-up requires a parabolic collector to track the sun and concentrate its light. Optical fibers intended for light transport need to propagate as much light as possible within the core; in contrast, optical fibers intended for light distribution are designed to let part of the light leak through their cladding.Use Of Diffusive Optical Fibers For Plant Lighting
Optical fibers are also used in the Bjork system sold by Parans Solar Lighting AB.Parans Bjork Parans Bjork system review by Inhabitat The optic fibers in this system are made of PMMA (PolyMethyl MethAcrylate) and sheathed with Megolon, a halogen-free thermoplastic resin. A system such as this, however, is quite expensive.Typical system starting at $10,000
The Parans system consists of three parts. A collector, fiber optic cables, and luminaires spreading the light indoors. One or more collectors are placed on or near the building in a place where they will have good access to direct sunlight. The collector consists of lenses mounted in aluminum profiles with a covering glass as protection. These lenses concentrate sunlight down in the fiber optic cables.
The collectors are modular, which means they come with either 4,6,8,12 or 20 cables depending on the need. Every cable can have an individual length. The fiber optic cables transport the natural light 100 meters (30 floors) in and through the property while retaining both a high level of light quality and light intensity. Examples of implementations are Kastrup Airport, University of Arizona and Stockholm University.
A similar system, but using optical fibers of glass, had earlier been under study in Japan.Hybrid Solar Lighting: Bringing a little sunshine into our lives, NBC News, March 2005
Corning Inc. makes Fibrance Light-Diffusing Fiber. Fibrance works by shining a laser through a light-diffusing fiber optic cable. The cable gives off a lighted glow.Corning Fibrance official website
Optical fibers are used in fiberscopes for imaging applications.
Hybrid solar lighting
Transparent hollow light guides A prism light guide was developed in 1981 by Lorne Whitehead, a physics professor at the University of British Columbia,Switch off the lights, here comes the sun Toronto Globe and Mail, 2012 January 28Use Of Prismatic Films To Control Light Distribution and has been used in solar lighting for both the transport and distribution of light.Solar Canopy Illumination: Solar Lighting at UBC research frame A large solar pipe based on the same principle was set up in the narrow courtyard of a 14-floor building of a Washington, D.C. law firm in 2001,Solar Light Pipe in Washington, D.C. IDOnline.com - The International Design Magazine - Graphic Design, Product Design, Architecture "Solar Light Pipe in Washington, D.C.", DETAIL 4/2004, Building with light and a similar proposal has been made for London.Apple London – Special Ceiling A further system has been installed in Berlin. "Tageslicht aus der Tube", Faktor Licht, Nr. 4, 2003 (with a description of the light pipe on Potsdamer Platz, Berlin)
The 3M company developed a system based on optical lighting filmHeliobus with 3M Optical Lighting Film (OLF) and developed the 3M light pipe,3M Light Management Solutions (US) which is a light guide designed to distribute light uniformly over its length, with a thin film incorporating microscopic prisms, which has been marketed in connection with artificial light sources, e.g. sulfur lamps.
In contrast to an optical fiber which has a solid core, a prism light guide leads the light through air and is therefore referred to as a hollow light guide.
The project ARTHELIO, partially funded by the European Commission, was an investigation in years 1998 to 2000 into a system for adaptive mixing of solar and artificial light, and which includes a sulfur lamp, a heliostat, and hollow light guides for light transport and distribution.
Disney has experimented with using 3D printing to print internal light guides for illuminated toys.
Fluorescence based system In a system developed by Fluorosolar and the University of Technology, Sydney, two fluorescent polymer layers in a flat panel capture short wave sunlight, particularly ultraviolet light, generating red and green light, respectively, which is guided into the interior of a building. There, the red and green light is mixed with artificial blue light to yield white light, without infrared or ultraviolet. This system, which collects light without requiring mobile parts such as a heliostat or a parabolic collector, is intended to transfer light to any place within a building. Fluorosolar FluoroSolar - Bringing the Sunshine Inside , Treehugger, February 5, 2006 (retrieved on January 13, 2007)Video on fluorescence based system By capturing ultraviolet, the system can be especially effective on bright but overcast days; this is since ultraviolet is diminished less by cloud cover than are the visible components of sunlight.
Properties and applications
Solar and hybrid lighting systems
Solar light pipes, compared to conventional skylights and other windows, offer better heat insulation properties and more flexibility for use in inner rooms, but less visual contact with the external environment.