The introduction of computers in the workplace has dramatically changed not only how we work but also how we illuminate our offices. In the 1980s and 1990s, issues pertaining to reflections and reflected glare on computer monitor displays prompted the development of dedicated lighting recommendations by the Illuminating Engineering Society (IES), such as RP-24-89: Lighting Offices Containing Computer Visual Display Terminals. This guide remained unchanged for quite some time. (An update to RP-24, American National Standard Practices for Office Lighting was issued in 2004, RP-1-04, and another was issued more recently, in 2012, RP-1-12.) But as the nature of our work has changed, to become more focused on computers and screen-based tasks coupled with newer display technologies and today’s energy-efficiency criteria, what was once standard practice for office lighting has also evolved.
Though first noted in cathode ray tube (CRT) monitors, reflections on video display terminals (VDTs) can occur on any type of specular screen and are detrimental to the computer user. They impair visual acuity by competing with computer-generated text and images for the eye’s attention, requiring the eye to adjust focus. Reflected glare, or veiling reflection, caused by daylight or electric sources, is particularly problematic, as it severely reduces the visibility of the screen. Left unmanaged, screen reflections can lead to eyestrain and headaches, common complaints with CRTs.
While reflections are not exclusive to CRTs, the technology is inherently reflective. Named for their bulky glass tube, CRT monitors generate images by using an electron gun to shoot streams of electrons across a glass surface coated with millions of phosphor dots. When struck, the dots emit light. In color monitors, three guns and three phosphor compositions produce red, green, and blue light, whose intensities are varied and combine to render images in a wide range of colors.
These phosphors give CRTs their inherent reflectance, says Raymond Soneira, president of DisplayMate Technologies, a test lab for display products. Although the light generated is colored, the phosphors are, in fact, white and therefore very reflective. “As a result, essentially all CRTs have some form of anti-reflection treatment,” which typically involves adding a layer of dark or etched glass to the front of the monitor and applying an anti-reflection coating, notes Soneira.
Other factors compound the reflectance of CRT monitors. Early models displayed negative contrast—colored text against a black background—and were highly prone to reflections. CRTs have limited brightness—100 to 250 candelas per square meter—making them incapable of standing up to high luminance and color contrasts in their surroundings. In addition, their convex screens mirror a wider swath of their surroundings than do flat displays.
Few CRT monitors are still in use today, having been replaced by brighter and less-reflective technologies. Of these technologies, LCD (liquid crystal display) is the most widely used in desktop monitors, laptops, tablets, and cellphones. These displays also tend to be the brightest, achieving luminances of 300 to 450 candelas per square meter, compared to 100 to 250 candelas per square meter for both CRTs and plasma screens. LCDs owe their high luminance to an independent backlight—cold cathode fluorescent or LED—“that can be more easily engineered to be brighter,” says Soneira.
Apple’s retina screens are also LCDs. The difference is their high pixel density, which makes individual pixels undetectable by the human eye—or retina—at a viewing distance typical for the size of the device. According to Soneira, “retina screen” is more a marketing term than a specific technology. Additionally, 1920x1080 HDTVs are technically “already retina displays,” as larger LCDs require fewer pixels per inch to provide the same visual experience.
LCDs render images in a variety of ways, the most popular being twisted nematic. In this process, liquid crystal molecules, sandwiched in glass between two polarizers (set 90 degrees to each other), modulate light by twisting to allow the light to pass through and untwisting to block it. Adjusting the current varies the intensity of the light, to which color is added via a filter of colored pixels, each comprising three subpixels in red, blue, and green.
LCDs are inherently less reflective than CRT displays, notes Soneira. The front polarizer absorbs and blocks ambient light, and “the liquid crystal itself is a clear liquid, rather than a highly reflective white phosphor,” he says. While LCDs can be treated with an anti-reflectance coating, which lends a matte appearance, those touted for their clarity and vibrant colors tend to have a glossy finish—as do tablets and smartphones, to enhance their sensitivity to touch. According to Soneira, a simple way to gauge reflectance is to examine computer displays in their off state: Monitors with less reflectance will look darker.
Early VDT Lighting
Given how reflective CRT monitors are, office illumination designed in the 1980s and 1990s prioritized minimizing glare and ensuring visual performance, says Mitchell Kohn, president of Chicago-based Mitchell B. Kohn Lighting Design. Illuminance levels for screens were kept between 5 and 10 footcandles for optimal visibility. Lamps were shielded and luminance limits were established by the IES for direct lighting at high angles between 50 and 90 degrees above nadir, typically called “the glare zone.” Compliance involved the use of low-glare lenses or louvers, which “created a cutoff at angles that commonly caused reflections,” Kohn says.
The sharp cutoff, however, resulted in a cave effect and a gloomy office environment. “Because the luminaires were so controlled, there really wasn’t any light on the walls,” says David Pfund, president of West Haven, Conn.–based Tambient, a division of lighting manufacturer The Lighting Quotient. Although light on the horizontal surface was plentiful, the upper regions of a space were darker, and scalloping was visible on the walls. To address the problem, designers supplemented with wallwashing and accent lighting on perimeter walls. In workstations, the strong directional lighting required tasklighting to eliminate shadows under shelves.
Indirect lighting offered another option for eliminating direct and reflected glare. The light reflected off the ceiling was softer, less bright and therefore more accommodating of VDTs. Fixture spacing was optimized to ensure uniform illumination and avoid high-luminance contrasts in the ceiling that could reflect on the CRT displays. The dark undersides of the fixtures, however, were sometimes visible on the screens. A screen-friendly alternative was a direct–indirect fixture, but, again, the direct component required proper shielding. Moreover, with ceilings lower than 9 feet, both fixture types became visual obstructions or required closer spacing for uniform illumination. Fortunately, screen reflections tend to be less of a problem with modern display technologies. Anti-reflectance treatments have proven effective in eliminating reflected imaging, and higher internal brightness and increased contrast have improved screen visibility, even in elevated ambient light situations. More importantly, irrespective of finish, today’s computer screens are often adjustable and can be tilted to optimize viewing, or they’re portable, as in the case of laptops and tablets, which allows users to move away from sources of glare.
Such developments, as well as the ubiquity of workplace computers, have pushed the IES office lighting guidelines to evolve. Current recommendations, such as American National Standard Practices for Office Lighting RP-1-12 not only specify different requirements for four levels of VDT usage but also distinguish between matte and glossy displays. Not surprisingly, luminaire and ceiling luminance limits for the former are more relaxed than for the latter. These changes point to “the proliferation of VDTs in every type of business and acknowledge the growing prevalence of new technology, highly specular flat-screen monitors,” says Pfund.
In general, advances in computer displays have given designers a “freer hand” in devising appropriate office lighting solutions, says Dan Frering, director of education at the Lighting Research Center in Troy, N.Y. No longer confined to fixtures with low brightness or indirect lighting, the designers “are able to design different types of lighting,” he notes, and in the process, alleviate some of the earlier gloominess associated with office lighting design. The forgiving nature of modern VDTs “allows us to create brighter spaces,” says Stephen Margulies, a partner at New York–based lighting design firm One Lux Studio. “The walls can tolerate more brightness; the fixtures can tolerate more brightness.”
In fact, Gary Woodall, senior designer at Gary Steffy Lighting Design in Ann Arbor, Mich., and chairman of the IES Office Lighting Committee, has noticed a return to the use of white or opal lenses in direct lighting fixtures. Woodall says the trend is driven by the need “to make offices less expensive, more flexible, and easier to install.” The lenses not only diffuse the light but also “bounce” it to walls, eliminating dark shadows, adding brightness to the space and enhancing comfort. However, unlike louvers, they require no supplemental “lighting hardware,” and thus reduce cost. “So your laptop is bright enough to see,” Woodall says, “but if it’s too bright where you’re sitting, you change the angle of your screen.”
These lenses, however, have a downside. Aside from softer illumination, they’re “not really controlling the output,” Kohn says. The high-angle brightness emitted may make a space seem more inviting but, without proper management—and depending on the ceiling height and room size—it can produce direct glare and reflected glare if glossier screens are being used. Moreover, the lenses create brightness patterns on the ceiling that again can impair screen visibility and be visually distracting.
Addressing glare and reflected glare is particularly critical in open-plan offices and as more office work becomes computer-intensive, notes Kohn. Whereas paper tasks are done with one’s head in a downward position, computer tasks tend to be horizontally oriented, putting the ceiling and luminaires within workers’ field of view. How much glare a fixture appears to emit, however, does depend, in part, on its surroundings. The same light fixture may be more tolerable in a space with light finishes than one with dark, where the contrast is more pronounced and uncomfortable.
But the largest source of reflected glare in VDTs, of course, is windows. The solution can be as simple as positioning the computer display perpendicular to the window or employing window coverings. “We are spending a lot of energy, believe it or not, on trying to convince our clients on a lot of projects to use automated shading systems,” says Margulies. With daylight increasingly seen as key to sustainable and productive office environments, such systems are indispensable. Margulies notes that they are even willing to re-examine the lighting budget so that monies originally allocated for electric lighting may be spent on motorized shade systems.
As it relates to luminaire design, however, interest in glare control has waned, according to Kohn—and not because of improvements in VDT technology. Instead, concern for visual performance in office lighting has taken a back seat to energy codes and energy performance. Fixtures that prevent glare or light indirectly are less energy-efficient and, consequently, selection is more limited now than it was before. LEDs further complicate the issue, because “when you try to crank that much light out of a small light fixture, you get lots of glare,” Margulies says.
Regardless of display technology, other aspects of office lighting remain essential to ensuring comfortable VDT tasking, such as contrast control, says Margulies. Although high contrast ratios between text and background enhance visual acuity and reading, they are less desirable for illuminating the surfaces adjacent to a computer screen. Instead, IES guidelines restrict light and dark contrasts between those surfaces to a ratio of 3:1. Cubicle walls and the top of a desk, for example, should be no more than three times lighter or darker than the computer display.
For more remote surfaces, keeping contrast ratios below 10:1 is ideal. This applies to the luminance difference between the computer screen and, for example, a far wall, as well as that between the darkest and lightest remote surfaces within the field of view. To avoid distracting brightness patterns in the ceiling that could reflect in a glossy display, differences in ceiling luminances also should not exceed a ratio of 10:1.
Higher contrasts than those recommended require the eye to constantly adapt, potentially leading to eye fatigue. This is also why computer tasking with a window in direct view is not advisable. The contrast ratio between the screen and window can be as high as 1,000:1. Similarly, direct lighting fixtures with white lenses may produce contrast ratios in excess of 100:1 or 200:1, notes Woodall. “We’re functioning OK,” he says, but “we’re not particularly comfortable.”
Comfort may not be too far off as new products begin to address both energy usage and visual performance. Some see promise in lighting systems that offer more individualized control, be it through furniture-integrated fixtures or wireless LED technology. First and foremost, a shift in attitude is needed. As Woodall notes, “Until people decide that lighting quality is equally important to the energy used, then we’ll not see a lot of movement.”
A list of reference sources that address office lighting issues.
RP-1-12: American National Standard Practices for Office Lighting, Illuminating Engineering Society, January 2012.
David L. DiLaura, Kevin W. Houser, Richard G. Mistrick, and Gary R. Steffy, The Lighting Handbook, Tenth Edition: Reference and Application (Chapter 32: Lighting for Offices), Illuminating Engineering Society, 2011.
Corky Binggeli and Patricia Greichen, Interior Graphics Standards, Second Edition (Chapter 5, Services: Lighting), John Wiley & Sons, 2011.