Lighting Design + Application • October 2003 - Illuminating ...

RESEARCH MATTERSEfficient Lighting Through a Self-luminous Mode ApproachYukio Akashi, Ph.D.Lighting ResearchCenterSUCCESSFUL LIGHTING INretail settings helps capture customers’attention and focus it on themerchandise for sale. To do this, displayaccent lighting uses high illuminanceratios, in the range of 15:1 to30:1, between an object and its background(Philips Lighting, 1991). IESNA(2001) suggests using accent ratios of5:1 (for low theatrical effect), 10:1 (fordoubling the perceived brightness)and 15:1 (for theatrical effect). Upscaledepartment stores might usetwo or three 60-W PAR lamps toachieve an accent ratio of 5:1 for a display.This can result in lighting powerdensities that often exceed five wattsper sq ft, while the energy efficiencycode of the American Society of Heating,Refrigeration, and Air ConditioningEngineers, ASHRAE/IESNA 90.1-1999, requires less than 3.3 watts persq ft for retail lighting. Using a newapproach to display lighting, however,it might be possible to reduce thislarge amount of electricity consumptionfor accent lighting, perhaps by 40percent.Another way to create a self-luminousmode is to match the beam patternof accent lighting precisely withthe outline of an object as shown inFigure 1.How Does Self-luminousMode Work?While many of us might havenoticed how self-luminous colorsattract our attention, few empiricaldata support our perceptual experience.Some research does, however,exist. Uchikawa, et al. (1989) used aset of colored chips to look into theeffects of viewing mode on colorappearance and brightness perception.Subjects compared the colorand brightness appearance of chipsseen in self-luminous mode and ofchips seen in surface mode, the usualway of illuminating objects.The chipsappeared up to 25 percent brighterwhen perceived in the self-luminousmode.Another approach to estimatingthe brightness enhancement effect ofthe self-luminous mode is to evaluatethe “luminosity threshold” of anobject—the threshold when anobject surface begins to appear selfluminous.Several studies (Wallach,1948; Lie, 1977; Bonato and Gilchrist,1994) used a simple visual stimulus -a disk separately illuminated from itsbackground - to examine the luminositythreshold.Bonato and Gilchrist(1994) analyzed these studies andfound a simple rule that the luminositythreshold occurs at a luminancevalue 1.7 times higher than luminanceof a surface that would appear whitein the same context.How Much Energy Can TheSelf-luminous ApproachSave In Accent Lighting?To interpret the simple rule givenabove in terms of accent lighting, let’sassume the reflectance values of anobject surface and a white wall behindthe object are 40 percent and 70 percent,respectively. In this case, theobject should be lighted to an illuminancethree times higher than thewhite background to follow the rule,What Is The NewApproach?Using colored light to illuminatethe background of objects has alreadybeen distinguished as one possibleapproach to reducing energy consumptionby accent lighting.A recentstudy found that blue- or red-coloredlight used in display backgroundsenhanced visual appeal and attentiongettingcompared to traditional“white” display background lighting(Simeonova, et al., 2003). In fact, withthe colored background lighting, theamount of accent lighting on featuredobjects could be reduced by halfwithout compromising visual appealand attention-getting properties.Another approach is to make thebest use of what is called the “selfluminousmode,” whereby objectsare much higher in luminance thantheir background and actually appearto glow. Colors appearing self-luminousoften stand out and seem toattract our attention almost irresistibly(Bonato and Gilchrist, 1994,1999).One way to achieve a self-luminousmode is to put a light sourcebehind a translucent colored screen.Exit signs use this method for makingthe sign letters more conspicuous.Figure 1—Pictures seen in self-luminous mode(Courtesy of WENDELIGHTING). . . . . . . . . . . . . . . . . . . November . . . . . 2003 . . . LD+A . . . . 10. . www.iesna.org. . . . . . . . . . . . . . . . . . . . . . . . . . . . .