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Going beyond what's recommended: two 'benefit metrics' show a better way than design governed by footcandles.

Every lighting project should begin and end with client-based objectives. Sometimes the potential benefits offered to clients by lighting have been previously rationalized into formal recommendations. For example, if the client wants staff to see fine spatial detail, high photopic illuminance levels are needed on the task to process the visual information quickly and accurately. (1) After many years of research (2) and practical feedback, the current lighting recommendations can meet this client-based objective quite well. But what does a specifier do if the client wants to make people feel safe and secure in a parking lot? Or how do we light a healthcare facility for seniors with dementia to help them sleep better at night?

Many lighting specifiers would try to meet these design objectives by looking for a recommended footcandle level in an application guide. But this is like trying to put a square peg in a round hole. Recommended photopic illuminance levels are functionally based on foveal visual performance; (1) they are not directly relevant for helping people feel safe in a parking lot or helping seniors sleep better. New metrics, unavailable in current recommended practices, are needed to meet these client-based objectives.


Consider the example of a par king lot where the primary design objective for the client is to ensure that people feel safe and secure as they leave and return to their parked cars. First, it is important to know that research has shown that a person's perception of the over all brightness of a par king lot is closely tied to that person's sense of safety and security in that parking lot-parking lots that appear brighter are perceived as safer. (3,4) Perception of brightness is, of course, affected by light level, but it is also dependent upon the spectral composition of the light source illuminating the parking lot. At the same photopic illuminance level, a parking lot illuminated by cool sources will appear brighter than one illuminated by warm sources. (4) This happens because the photopic luminous efficiency curve, the spectral weighting function for all current recommended light levels, is a long-wavelength biased representation of how we actually perceive the spectral composition of t he light illuminating a parking lot. (5)

It is possible to calculate the added benefit and/or the reduced cost of illuminating a parking lot based upon perceived brightness, for example, to 57 lux with either an HPS or with a 6500K LED lighting system; 57 lux was chosen because in a recent study this was the average illuminance needed in a university parking lot for pedestrians to feel safe and secure under an HPS lighting system. (6)

Table 1 compares three scenarios, (a) a base case where both light sources are needed to meet 57 lux, (b) a second scenario where power density is fixed at 0.59 watts per sq meter and (c) a third, hybrid design where brightness, and thus the impressions of safety and security, is fixed for both sources. For the same photopic illuminance level (57 lux), the parking lot illuminated by the 6500K source will be perceived (on a linear scale) 85 percent brighter, and safer, but the power density will be 20 percent greater. For the same power density (0.59 watts per sq meter), the parking lot illuminated by the 6500K source will be (again, on a linear scale) 54 percent brighter. For the same brightness illuminance, the power density of the 6500K source will be 35 percent lower. All three examples are valid de sign solutions, depending upon the client's objectives.

The important point of this example is that lighting specifiers can respond meaningfully and differently to the design objective of people feeling safe and secure in a par king lot at night without reference to current recommended practices. If one were to simply follow current recommended footcandle levels, either the needs of the client would not be met or lighting energy would be wasted.


A similar analysis can be performed for lighting a senior facility aimed at improving residents' sleep efficiency and sleep consolidation, but a different benefit metric is needed because the eye responds differently to light for the circadian system than it does for brightness or for visual performance. Using the spectral sensitivity and an overall response function for the human circadian system, (7,8) it is possible to calculate for any spectral power distribution (SPD) a circadian stimulus (CS) value. CS represents the effectiveness of a given SPD to stimulate the human circadian system from threshold (CS = 0.0) to saturation (CS = 0.7). Our research shows that exposure to a CS of 0.4 for at least one hour at the cornea in the morning (a) is effective for improving sleep and reducing agitation and depression in persons with Alzheimer's disease. (9)

Table 2 compares a warm white (2700K LED) with a cool white (6500K LED) light source for three scenarios. For the prescribed CS value of 0.4, it takes 567 lux (at the cornea) from the warm source but only 375 lux from the cool source, and the power density for the cool source is roughly half (54 percent) that required for the warm source. For the same power density (4.7 watts per sq meter), the warm source does not meet the specification. Finally, at the same photopic illuminance (567 lux) needed by the warm source to meet the specification of CS = 0.4, t he cool source is in excess of the specification, unnecessarily wasting more energy than required. As with the parking lot example, the client's objectives can be readily met in several ways with the tools provided by this research, but they are not yet part of recommended practices.


It is not news to anyone reading this article that over the past 15 years, lighting technology has changed dramatically. Unquestionably, solid-state lighting (SSL) systems can provide better spatial, temporal and spectral control than ever before, but we are not going to realize the full potential of SSL unless we focus on meeting the client's objectives before we focus on the technology. We must begin to embrace benefit metrics that meaningfully represent those client needs, and some are becoming available. (5) By changing to a "brightness metric" that better represents judgments of safety and security, we can tailor the spatial and spectral composition of parking lot lighting. (6,10,11) By changing to a "circadian stimulus metric" that better represents the growing demand for healthy lighting, we can tailor the temporal and spectral composition of ambient lighting, so that seniors in long-term care facilities and living at home can sleep better and longer at night. (9,12) And now that SSL prices are beginning to plummet, there is no longer a cost barrier to accomplishing these new and much better value propositions for clients.

We like to rely on recommended practices. Those documents work well for lighting design objectives from earlier eras, but lighting recommendations have not kept pace with the evolving demands of society, thus limiting our ability to capitalize on the potential of SSL technology. Benefit metrics like the two described here pave the way for new recommendations, but specifiers and clients should not have to wait. Hopefully the examples given here illustrate how new lighting value propositions can be implemented easily and immediately.

In closing, I want to thank Paul Tarricone for providing an o pportunity to me and to my co lleagues Peter Boyce, John Bullough, Mark Fairchild, Mariana Figueiro, Dan Frering and Nadarajah Narendran to communicate our views on new lighting value propositions through LD+A.

Editor's Note: This issue marks the last "Value Proposition" article from the Lighting Research Center. Over the past few years, this column has made a persuasive case for new metrics not based on energy and/or cost. Many thanks to Mark Rea and the LRC team for their submissions. While the "Value Proposition" column has ended, LRC's contributions to LD+A will not. Keep an eye out for new topics in a new format in 2016.

Mark S. Rea, Ph.D., is a professor and director of the Lighting Research Center at Rensselaer Polytechnic Institute.


(1.) Rea MS (ed.) IESNA Lighting Handbook: Reference and Application. 9th ed. New York, NY: Illuminating Engineering Society of North America, 2000.

(2.) Rea MS and Ouellette MJ. Relative visual performance: A basis for application. Light Res Technol. 1991; 23: 135-44.

(3.) Boyce PR, Eklund NH, Hamilton BJ and Bruno LD. Perceptions of safety at night in different lighting conditions. Light Res Technol. 2000; 32: 79-91.

(4.) Rea MS, Bullough JD and Akashi Y. Several views of metal halide and high pressure sodium lighting for outdoor applications. Light Res Technol. 2009; 41: 297-320.

(5.) Rea MS. Value Metrics for Better Lighting. Bellingham, WA: SPIE, 2013.

(6.) Rea MS, Bullough JD and Brons JA. Parking lot lighting based upon predictions of scene brightness and personal safety. Light Res Technol. 2015; in press.

(7.) Rea MS, Figueiro MG, Bullough JD and Bierman A. A model of phototransduction by the human circadian system. Brain Res Rev. 2005; 50: 213-28.

(8.) Rea MS, Figueiro MG, Bierman A and Hamner R. Modelling the spectral sensitivity of the human circadian system. Light Res Technol. 2012; 44: 386-96.

(9.) Figueiro MG, Plitnick B, Lok A, Jones G, Higgins P, Hornick T a nd Rea MS. Tailored lighting intervention improves measures of sleep, depression and agitation in persons with Alzheimer's disease and related dementia living in long-term care facilities. Clin Interv Aging. 2014; 9: 1527-37.

(10.) Rea MS, Bullough JD and Brons JA. Spectral considerations for outdoor lighting: Designing for perceived scene brightness. Light Res Technol. 2014; published online before print.

(11.) Narendran N, Freyssinier J a nd Zhu Y. Energy and user acceptability benefits of improved illuminance uniformity in parking lot illumination. Light Res Technol. 2015; published online before print.

(12.) Figueiro MG, Hunter CM, Higgins P, Hornick T, Jones GE, Plitnick B, Brons JA and Rea MS. Tailored Lighting Intervention for Persons with Dementia and Caregivers Living at Home. 2015; submitted.


(a.) Don't do this in the evening or at night!
Table 1: Lighting value propositions based upon brightness

                 Light    Photopic    Relative     Photopic
                 Source   Efficacy   Brightness   Illuminance
                           (Im/W)     Efficacy       (lx)

Equal Photopic    HPS        96         1.0           57
Illuminance       LED        80         0.65          57
Equal Power       HPS        96         1.0           57
Density           LED        80         0.65          47
Equal Brightness  HPS        96         1.0           57
Illuminance       LED        80         0.65          30

                  Relative         Power
                 Brightness       Density
                 Illuminance   (W/[m.sup.2])

Equal Photopic       1.0           0.59
Illuminance         1.85           0.71
Equal Power          1.0           0.59
Density             1.54           0.59
Equal Brightness     1.0           0.59
Illuminance          1.0           0.38

Table 2: Lighting value propositions based upon circadian system

                  LED     Photopic   Relative     Photopic
                 Light    Efficacy   Circadian   Illuminance
                 Source    (Im/W)    Efficacy       (lx)

Equal Photopic   2700 K      65         1.0          567
Illuminance      6500 K      80        0.52          567
Equal Power      2700 K      65         1.0          305
Density          6500 K      80        0.52          375
Equal Circadian  2700 K      65         1.0          567
Stimulus         6500 K      80        0.52          375

                 Circadian       Power
                 Stimulus       Density

Equal Photopic      0.4           8.7
Illuminance        0.48           7.1
Equal Power        0.28           4.7
Density             0.4           4.7
Equal Circadian     0.4           8.7
Stimulus            0.4           4.7
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Author:Rea, Mark S.
Publication:LD+A Magazine
Date:Nov 1, 2015
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