Published on April 28th, 2018 | by James Ayre0
Light Tubes, Prism Lighting, & Other Daylighting Solutions
April 28th, 2018 by James Ayre
While LED lighting is arguably far “greener” than incandescent lighting is, both lighting modalities are easily trumped by daylighting solutions, where and when that is applicable.
While daylighting solutions won’t replace dark-hour lighting needs, they can easily displace artificial lighting needs during transitional near-dawn and dusk periods of time — as well as improve interior lighting (and thus mood and health in many people), and provide daylighting needs in areas that otherwise wouldn’t have access (basements, far-interior rooms, etc.).
With that in mind, I’m going to provide an overview here of various underutilized daylighting solutions and technologies. I’ll be ignoring the option of conventional skylights due to the many problems inherent to them relating to heat/cold loss and gain during exactly the “wrong” times of year, and the potential for roof-cut leakage. There are other, better, more-interesting options out there — some of which have been in use for quite some time, and could potentially be of use in new applications.
Light Tubes — Tubular Daylighting Solutions With Good Thermal Performance & Underground Effectiveness
I’ll start the discussion here with light tubes — a technology seemingly invented in Old Kingdom Egypt (though possibly a legacy technology inherited from elsewhere), that utilizes tubes lined with reflective material that’s arranged in such a way as to transmit large amounts of external light deep into a structure.
Generally speaking, these tubular lighting solutions have been and are used as an alternative to conventional skylights or deck prisms, and are rather better at limiting unwanted heat gain and/or loss. As such, they can make far more sense in passive solar buildings or homes than conventional skylights do, or even than deck prism style solutions.
To simplify light tubes (light pipes; sun pipes; daylight pipes), they work by transmitting light indirectly down long (well insulated or completely solid) tubes. As such, they can be quite effective at bringing large amounts of light into deeply recessed areas — with other options as well (as regarding the type of radiation transmitted).
As noted above, based on the type of design in question, the tubes can be either hollow systems or solid systems — with designs based around mirrors or reflective coatings, fiber optic materials, and light diffusers/spreaders, being the most common, amongst others.
Modern commercial style light tube systems first came to prominence through the work of a Paul Emile Chappuis, based out of London in the 1850s. These light tubes remained in use until World War 2 when the manufacturing facility was destroyed. Beginning in the 1980s or so light tubes made a come back though, on the back of the appropriate tech movement.
Most systems are based around just a couple of basic components: a structure meant to collect and reflect light downward located on the roof or external wall (often a dome or lens); an internal body/design that redirects and focuses light downward; and a light spreader or diffuser located at the intersection with the interior of the building. Systems relying upon optical fibers can be set up a bit differently, though, it should be noted.
Prism Lighting, Deck Lights, Vault Lights, Pavement Lights, Etc.
Prism lighting is a solution to daylighting that relies upon the use of prism-induced light refraction and reflection — in other words, prisms are used to bend light and redirect it. Use of such systems allows for the redirection and/or diffusion of light coming through windows so as to increase total room-cover, so as to make the room seem brighter, in particular in the otherwise darkened corners and back-end. Prism lighting solutions are a form of so-called anidolic lighting.
The primary aim of such a solution is to redistribute and spread the light entering a room out to such a degree that the whole room appears brighter (as a result of the brightening of the darkest portions). Human vision and interpretation being what it is, such an approach makes a room as a whole seem much brighter, despite the fact that the total light entering the room has remained roughly the same (and is simply now less concentrated).
Prism lighting has been in use in various forms for quite some time via so-called deck prisms — prisms of glass or similar material laid into the deck of a ship or watercraft and designed in such a way as to provide diffuse daylight to large internal areas despite the limited (and thus safe) size of the apertures, which are regularly walked upon by deck-hands.
Despite these (possibly ancient) origins, prism lighting didn’t (apparently) see its heyday until the late 1800s and early 1900s — when prism lighting was widespread throughout many parts of the world, before affordable electric lighting became common.
While some old-style prism lights remain in use in many cities throughout the world (sidewalk lights, vault lights, store/transom lights, etc.), most current prism lighting solutions rely upon lightweight acrylic panels or sheets of thin adhesive-films that can be applied to existing windows.
With regard to the sheet films — there are two basic types: refractive and reflective; which possess somewhat different light-bending qualities. Prism shape will determine the exact light-bending qualities in question.
As regarding deck prisms themselves, they were in common use to provide daylight below the decks of ships for a long time for good reason — they function better than open apertures (and are safer as well), and are considerably safer than candles or lamps, especially when it came to ships that were sealed with bitumen or something similar, or were transporting flammable cargo.
Essentially such deck prisms are simply refractive prisms (usually made of glass, clear stone, or acrylic) set flush into the deck of a ship and hanging into the internal space below — thereby supplementing or replacing the light from side hatches. (As a side note, the purple colors sometimes seen in historical specimens are simply due to long periods of exposure to EV radiation — such prisms were colorless originally).
Large Windows & White Walls — Pros & Cons of This Simple Approach
Writing an article about daylighting solutions without mentioning the basic strategy of increasing window-area and utilizing whitewashed walls probably wouldn’t make sense, so I’ll provide a basic overview here.
As we noted in our earlier article about passive solar building design, window sizing and placement obviously determines the extent and intensity of the solar radiation that can penetrate a structure — as well as determining the times of day and year when light (and thus heat gain potentials) are at their peak.
What we also noted in that article is that well insulated windows (double or triple glazed) when paired with good weatherstripping will allow for buildings to meet most of their heating needs simply through solar gain in many regions, provided that thermal mass is located in the right places.
That reality brings up one of the weak points when it comes to relying upon large windows for daylighting solutions — when using uninsulated windows, much heat or cold could be gained/lost through them (thus driving up energy costs), and when using well-insulated windows, excess heat gain is easily a possibility (especially when thermal mass levels aren’t well chosen). Excess heat loss is possible at night even when using well-insulated windows as well. This can be limited to some degree by utilizing external shutters or sun shades during the day and/or window quilts at night. Obviously, though, utilizing external sun shades during the day limits the amount of daylight entering the structure.
Alternately, windows can simply be sited so as to avoid direct exposure (and thus large amounts of heat gain when using well insulated and weather-sealed windows) but to still provide light, but doing so still results in a somewhat lower thermal performance for the building.
Utilizing light-colored walls and window sills can allow for the wide diffusion of limited quantities of light that are entering a building (such as a small window), but such an approach has its limits.
Light reflectors are another option to increase/modify window performance when it comes to light and/or heat gain (think of the way that ground-snow reflects light during the winter), but they are not much in use nowadays.
So, the pros of this approach is that it is in some ways simple and that it allows for an open feel to a building, and the primary cons are that thermal performance is reduced somewhat and that excess heat gain during the warmer months (the furnace effect) is possible.