Residential Lighting - Illuminating Engineering Society

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was in the level of luminance value beyond which subjects felt discomfort for all angles. The 1999 study with lighting professionals reported a value of 9000 cd/m 2 while the 2000 study showed a 16,000 cd/m 2 value for the naïve subjects. This suggests that lighting designers are more sensitive to discomfort glare than naïve subjects. While there may be technical debate on this discrepancy, this much is clear: for practical applications in longterm work environments, we need to consider a level of overhead source luminance that is much lower than the BCD values determined in the studies. This is because we do not want to design a lighting system in which only 50 percent of the people are satisfied. Moreover, this BCD level is for luminance of the glare source at 85 degrees. For angles below 85 degrees, say 55 and 65 degrees, the lamp luminance values should be much lower. There was another interesting finding from this research: the pattern of the results are exactly what would be expected from the fundamental formulae on which the conventional discomfort glare prediction systems, such as VCP, Glare index and the UGR system are based. As a matter of fact, the 2000 study shows that the approximate level of discomfort produced by a glare source between 55 degrees from line of sight and the edge of the visual field of view can be predicted using the Unified Glare Rating system. Is overhead glare a different kind of glare No. The research results imply that overhead glare is simply an extension of discomfort glare and not an entirely separate phenomenon. It confirmed that discomfort glare does not cease at 55 degrees from line of sight, but continues until the glare source passes well outside the field of view. The data does show that the luminance required to produce discomfort glare at very high angles, i.e., when the it is overhead, is much higher than is required at lower angles i.e., when it is closer to the line of sight. There is no doubt such luminances are well within the range of our present day light sources and luminaires. When is overhead glare a concern for lighting designers and engineers Well, the short answer is that whenever visual comfort is an important issue. Take for example, lighting for school classrooms. Most of the time, students’ attention will be on the teachers or the the pattern of the results are exactly what would be expected from the fundamental formulae on which the conventional discomfort glare prediction systems are based chalkboards. If the luminaires on the ceiling direct most of the light downward, it can create overhead glare and can create a very uncomfortable visual condition for the occupants of the classrooms. This is especially true for luminaires with high lumen and high brightness sources such as HID, compact fluorescent or the linear T5 and T5HO lamps where the bare lamps are visible. One can experience similar overhead glare in offices, conference rooms, libraries, hospitals, and courtrooms just to name a few. An adverse by-product of overhead glare is veiling reflection. When most of the intense brightness of the luminaire is directly overhead, veiling reflection is most prominent. As we stated earlier, in long-term work environments, we need to consider a level of overhead source luminance that is much lower than the BCD (9000 - 16,000 cd/m 2 ) level. There are still some aspects of overhead glare that need further explorations, such as the relationship between glare source size and visual comfort. However, at this time we recommend the maximum luminance of the luminaires should be no more than 10,000 cd/m 2 . Those who feel “more comfortable” with a lower value should feel free to reduce the luminance. After all, 9000 cd/m 2 represents only a 50 percent satisfaction level for lighting professional. References: Illuminating Engineering Society of North America, IESNA Lighting Handbook, 9th Edition, 2000. Commission Internationale de I” Eclairage, CIE Publication 117, Discomfort Glare in Interior Lighting, 1995. Lukiesh, M., and Guth, S.K. Brightness in Visual Field at Borderline between Comfort and Discomfort (BCD). Illuminating Engineering. pp. 650-670. 1949. Hopkinson, R.G., Architectual Physics: Lighting, Her Majesty’s Stationary Office, London, 1963. Fry, G.A., A Simplified Formula for Discomfort Glare, JIES, 8, pp 10- 20, 1976. Akashi, Y., Muramatsu, R., and Kanaya, S., Unified Glare Ratings (UGR) and Subjective Appraisal of Discomfort Glare, Lighting Research and Technology,28, pp 199- 206, 1996. Mistrick, R.G., and Choi, A., A comparison of the Visual Comfort Probability and Unified Glare Rating Systems, JIES, 28, pp 94-101, 1999. Sheedy, J. E., and Bailey, I.L., Symptoms and Reading Performance with Peripheral Glare Sources, Work with Display Units 94, Eds. A Grieco, G. Molteni, B. Piccoli, and E. Occhipinti, Elsevier Science, Amsterdam, 1995. Ngai, P., and Boyce, P.R., The Effect of Overhead Glare on Visual Discomfort, JIES, 29, 29 – 38, 2000. Boyce, P.R., Hunter, C.M., and Inclan, C., Overhead Glare and Visual Discomfort, Conference Proceedings, The Illuminating Engineering Society of North America, pp. 43 – 64, 2002. Peter Ngai, LC, PE, FIESNA, is vice-president, engineering, Peerless Lighting, Berkeley, CA. 12 LD+A/February 2003 www.iesna.org
VIEWS ON THE VISUAL ENVIRONMENT Louis Erhardt Every space has two sets of requirements for vision: those that are external, properties of the scene; and those that are internal, the viewer’s reaction. The external stimulus is the product of illuminance and reflectance. The internal response is retinal adaptation with a perceptive message conveyed by the retinal image to the brain. One begins with the message “a study in visual communication.” In a brief exposition of interior spaces, the concept of what-you-want-tosee is reduced to identifications of the space and of activities to be performed therein. Examples might be a studio to design automobiles, a factory to assemble them, or any of a myriad of spaces and activities that make up our environment. The stimulus is computed by means of a simple set of factors. Our eyes, when we open them, immediately respond to the prevailing brightness. This response is approximately the logarithm of the stimulus. Almost all visual sensitivities—to brightness, color, size, and contrast—are expressed in the same log units as the adaptation. This compendium of visual abilities is instantly available when an adaptation level is selected. Following are the steps of a procedure: Adaptation – Match the adaptation to fit the requirements of the space and task. The selection should present no problem: 1 cd/m 2 for simple tasks, 10 cd/m 2 for average everyday activities, 100 cd/m 2 for the most severe challenges. Reflectance – Average reflectance for the total of all interior surfaces should be determined. Sometimes such information is not readily available. Since lighting design is as much an art as a science, it is not numerically critical. Therefore, the following averages may be used: 0.16 for dark interiors, 0.32 for average mid-range lightness, 0.64 for light reflectances. The selection should also be easy (Table I). Visual adaptation is set by the average weighted (luminance x area) luminance of the visual field. Illuminance – Is quantified indirectly by selection of an adaptation that quantifies an extensive list of visual sensitivities. Adaptation – Two kinds: the unfocused general adaptation, such as the first appearance of an unfamiliar room; second, the focused attention on a task or other detail. General adaptation is measured by the weighted average of the entire field of view; focused, by the foveal area plus about a 10 degree surround. Vision moves smoothly between the two—possibly a neural transition if the change is momentary, chemical if sustained. Both foveal and peripheral vision seem to be adapted simultaneously at night when attention is focused on the road or a sign, and a side-glance reveals a pedestrian moving to step into the street. Fudge Factor – All of the retinal sensitivities, recorded at different adaptations, are threshold values agreed upon by professional observers. Regrettably, my own recording of those values were the result of a personal test, my observations after reviewing public data. Threshold values are minimal. The amount of increase to fulfill visual requirements and overcome the multitude of deleterious factors has been the subject of great discussion. These factors include: viewer’s age, subnormal vision, moving targets, distance, need for accuracy, and many more. Parry Moon has suggested a safety factor of at least ten. For many years, IESNA has used eight—the visibility reference function—as the safety or “fudge” factor. To compare our Adaptation- Reflectance computations with IESNA findings, we will therefore use eight as the safety factor, but only when making a comparison with calculations requiring a numerical end result. To compare and discover the potential of the several approaches, consider the political phrase that has decisive significance in the comparison of different lighting systems: “What did he know and when” In 1970, Foster Sampson visited schools throughout California, measuring significant lighting details and recording them. He drew conclusions as to their degree of meeting pre-design goals. (Once designed, there was a mandated illuminance on desk surfaces, a desire for freedom from glare, and a general sense of comfort.) Glare and comfort are mental perceptions without numerical measure, although efforts continue to write equations for them. What was known were the dimensions of the space and the finishes of the surfaces. Added to these was the specific illuminance level, in compli- Table I Visual Abilities of the Eye’s Sensitivities at Individual Adaptations Adaptation Reflectance Illuminance Applications 1 cd/m 2 Light .64 1.8 lx Circulation, Med. .32 6.6 Conversation, Dark .16 16.5 Storage. 10 cd/m 2 Light .64 18 lx Reading, Med. .32 66 Coarse Assembly, Dark .16 165 Conference. 100 cd/m 2 Light .64 175 lx Tool-making, Med. .32 660 Fine Assembly, Dark .16 1650 Surgery. (To convert lux to footcandles, multiply by 0.093) 14 LD+A/February 2003 www.iesna.org
Page 1 and 2: Lighting Design + Application Febru
Page 3 and 4: President Randy Reid Past President
Page 5 and 6: Thornton’s research suggests that
Page 7 and 8: Do we need a conference and does it
Page 9: When we are presented with a bright
Page 13 and 14: sion, Brightness. If reflectance is
Page 15 and 16: practical applications where the ci
Page 17 and 18: the electric load, which can easily
Page 19 and 20: • notes on lighting design Sarato
Page 21 and 22: Myron Kahn, Developed Polarized Cei
Page 23 and 24: Lighting Controls Association Publi
Page 25 and 26: The water feature is accented with
Page 27 and 28: Interchangeable IC-rated pinhole an
Page 29 and 30: (left) The 146,000-sq-ft Life Scien
Page 31 and 32: (top) Decorative pendants with meta
Page 33 and 34: can be easily controlled throughout
Page 35 and 36: (left) High-tech kitchen is luminat
Page 37 and 38: Diners deposit their trust in Chef
Page 39 and 40: Most are stainless steel, some also
Page 41 and 42: authentic art glass and the pendent

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