|
|
It amazes me that parabolic lighting is still so popular and widely used. It produces arguably the most undesirable work environment imaginable and has no discernible function in today’s workplace. The parabolic luminaire represented an attempt to cut down glare at approaching angles from a recessed luminaire. In the 1980s, glare on computer screens was often deemed a distraction to workers. While the parabolic products were successful in addressing this issue, today’s modern work environment doesn’t require the mitigation of glare on computer screens, as will be discussed later in this article.
Generally speaking, there are five major methods for illuminating work environments: parabolic troffers, lensed troffers, linear indirect, casework task lighting and the newer “volumetric”-type lighting. This article will focus on three of these techniques—parabolic troffers, lensed troffers and volumetric lighting—and compare their performance based on four criteria. Since the most commonly used parabolic luminaire today is the 3-in. louver with 12 parabolic cells for two-lamp units and 18 parabolic cells for three-lamp units, the 3-in. louver parabolic will be used for all comparisons.
Over the years, I have done several iterations of research comparing parabolic to these other systems, in the hope of identifying appropriate applications for them. This article will address the most recent evaluation based on calculations using Lithonia’s Lighting Visual program, as well as market data and more subjective assessments of the three systems in question.
A few clarifications before discussing the specific findings. First, for the purposes of this article, volumetric fixtures refer directly to the new shallow luminaires with dedicated curved lenses for the lamps (typically two lamps). These products have gained steam since their release and have become an alternative to the recessed perforated basket-type luminaires that suffered from very poor efficiency. Because perforated basket luminaires without louvers cut into them are so inefficient, they have been omitted from this article.
What follows is a look at the four criteria and how each of the three luminaire types stacked up.
1. Light Levels (work surface-horizontal illumination). Horizontal or work surface footcandle measurements are perhaps the most popular, but sometimes overrated, measurement used by engineers. While horizontal levels are critical, they’re not the “end-all” measurement. Horizontal footcandle calculations are often used as an easy way to describe to building owners how “bright” their building will be. This is not true: High horizontal footcandle levels do not make for a bright space, rather the reflection of light off of surfaces, both horizontal and more importantly vertical, creates the “brightness” of a space. While working environments Lamp & Fixtures in the Field such as schools, offices, laboratories, etc., demand good horizontal footcandles on work surfaces, this measurement typically does not tell the whole story.
Which of the three luminaire types succeeded in providing the best horizontal footcandle levels? When it comes to lumens per watt and fixture efficacy, the lensed troffer is at the head of the class. The effectiveness of a prismatic lens to deliver usable light from linear fluorescent lamps was unmatched. Using the modeling software, a basic 10-ft by 12-ft office with (2) two-lamp 2-ft by 4-ft lensed troffers produced an average of 39.6 fc vs. only 38.5 fc from the volumetric type product. The parabolic finished last at 34 fc*. Light levels were computed on a horizontal surface at a 2½-ft work plane.
Horizontal illumination was the category where I expected the parabolic to have an advantage due to its design intent of driving light directly down to the work surface. So why does it fall short of the others? Due to its depth and the finish, the parabolic reflector absorbs too much of the light. While it is able to cut off glare from angles outside the primary dispersion pattern, this function drastically reduces the efficacy of the fixture, as well as its ability to produce good vertical light levels. While the parabolic cuts off glare on vertical surfaces, it creates a very large amount of glare on horizontal surfaces, particularly those right below it. This is due to the bare fluorescent lamps being focused by the louver.
2. Light Levels (wall surface-vertical illumination). This measurement is rarely used in design by engineers, but in most applications it’s a critical component. Many applications require good vertical footcandle levels to create a balanced environment. High horizontal footcandle levels and low vertical footcandle levels often create a cavernous space. Horizontal and vertical maximum and minimum levels should be compared to ensure a balanced environment. One thing worth noting is that vertical footcandles can sometimes be perceived as glare in products that cannot control the light properly. This must also be considered.
In a close contest, the top performers among the recessed products for vertical footcandles were the volumetric and lensed troffers depending on the application. Parabolic troffers clearly performed the worst. Due to the nature of their design, parabolic luminaires reduce vertical footcandles by “focusing” the light directly down to the horizontal work surface. In the same 10-ft by 12-ft office, wall illumination with (2) two-lamp 2-ft by 4-ft lensed troffers averaged 26.5 fc with a max/min ratio of 4.3:1, while the volumetric troffers with the same lamping had an average of 28.4 and a max/min ratio of 5:1. In contrast the two-lamp parabolic troffers averaged only 24.8 fc with a much higher max/min ratio of 6.9:1*. Light levels were computed on one wall surface.
Based on the luminaire’s design, the parabolic was unable to get as much light onto the wall as its lensed and volumetric counterparts. Also note that at the top corner, where the wall meets the ceiling, the light level from the parabolic was about half that of the others. The vertical footcandle level is often a good representation of perceived light because it describes the illumination we look at directly. High vertical levels of well-controlled light are highly desired in almost every application, in particular work environments.
3. Price. In today’s marketplace, particularly in the U.S., price drives almost every decision when constructing commercial buildings. While lighting is typically a very small portion of a project’s budget, it often is the first to get value engineered. All three troffer types in this analysis tend to be high-value products or those that would be used in a budget-conscious project.
The least expensive of the three products is the lensed troffer. Due to the fact that lensed troffers are sold in such volume, use less materials to construct and require uncomplicated assembly, manufacturers are able to crank them out at very low margins and highly competitive prices. The general rule of thumb (based on my experience as a manufacturer’s representative as well as feedback from local sales outlets) regarding pricing is as follows: lensed troffers, $50; parabolic troffers, $75; and volumetric troffers, $100. This provides lighting specifiers a nice range to select products that fit their design intent and budget.
4. Aesthetics. This is a very subjective criterion, and everyone will have their own opinion. One thing I assume most specifiers will agree on is that the volumetric option is the most aesthetically pleasing. This may be because it is new, because it does not have as sterile a look as the parabolic and lensed versions, or simply because it is better looking. The lensed troffers and parabolic troffers are typically considered the least aesthetically desired lighting products with perhaps the exception of strip lights and wrap-type luminaires.
In my view, the lensed troffer provides a cleaner more streamlined look and eliminates direct viewing of the fluorescent lamps. There are several inherent problems with installed parabolic systems, one is that the louvers tend to carry fingerprints very easily. Also, the louvers are very delicate and can be dented or scratched without much contact. Another issue with the louvers is they collect dust like crazy. No product is perfect, but the inherent problems with parabolic luminaires certainly should be considered prior to specifying.
WHY IS IT SPECIFIED?
Parabolic troffers cost 50 percent more than lensed troffers, yet lensed troffers perform better in both horizontal and vertical footcandle measurements. So, the question is, where and why does the parabolic still get specified? One reason may be that parabolic troffers became very popular back in the ‘80s due to their ability to reduce glare on “bubble-vision” computer monitors. (You remember those things; they weighed about 100 pounds, had a terrible picture, got really dusty and took up half your desk space.) Because these tube-type monitors had a curved front glass, they were highly susceptible to reflecting vertical illumination directly back into the user’s eyes. Well, those days are now long behind us, and we now have glare-resistant flat panel LCDs on our desktops. These types of monitors all but eliminate the need to reduce vertical glare.
As an engineer I spend a lot of time on my computer but I also spend a bit of time working with plans laid flat on my desk. Plotter paper tends to reflect light very well, so the bare fluorescent lamps in parabolic troffers create a tremendous amount of eye-tiring glare.
Have you ever walked down a hallway with parabolic troffers in it? It was probably designed to a specific horizontal footcandle level, but no care was taken to ensure the walls were properly illuminated. Without properly illuminated walls, the corridor appears to be cavernous and much narrower than it actually is. An equivalent horizontal footcandle level with lensed or volumetric troffers in the same corridor would be perceived by almost everyone to be brighter. Getting light on the walls creates a perception of a more spacious environment. While a corridor is the easiest space to demonstrate these principles, they can also be attributed to office space lighting. Creating a comfortable, open and inviting office space greatly increases worker productivity and even reduces illness.
A study performed by several professors at Cornell University back in 1989 provides proof as to the effect of lighting on employees. The study compared lensed indirect lighting to parabolic troffers in a controlled environment. The results were very interesting: Employees working under parabolic lighting reported more than twice as many eye-strain issues. And during the study, over half of the parabolic luminaires were altered by employees to try and create a more comfortable environment. The study also reported that of those working under parabolic lighting, 75 percent would prefer to work under lensed indirect. In contrast, only 12 percent of those working under lensed indirect said they would prefer to work under parabolic (http://ergo.human.cornell.edu/lighting/lilstudy/lilstudy.htm).
This brings us back to the original question: Why are we still specifying parabolic luminaires? Are we using boilerplate specification from 1990? Are we uninformed as to the advantages and disadvantages of various products? Do we actually think parabolic troffers are an aesthetically pleasing luminaire? Do we know that lensed troffers outperform the parabolics and cost less? The findings presented here should provide the answer—like other relics of the ‘80s the parabolic troffer should be stashed away, only to be used in very specialized applications.
*Calculations were preformed in Lithonia Lighting’s Visual program. Parameters used were: 10 ft by 12 ft by 9 ft with 80/50/20 reflectances, two-lamp T8 2,850 lumen lamps, and a .95 LLF for all luminaires.
September 2009 |
