Future Direction for LED Lighting

P. Mullins   October 2013
Can the Industry maximize the perceptual response per watt of optical radiation ?

Human vision hasn′t changed due to technology or national economy, we still need what we always did. 

Early LED marketing brought on a blue light frenzy, hoping to claim lumens-per-Watt (L/W) competition for conventional lighting.  The frenzy began near year 2000, about the time when LEDs became real power devices.  They gained large chips, thermal conductivity and consistent photometrics available with a lens.  Unfortunately they gained lots more.

One attention getter was price related as Lumens-per-Dollar (L/D) but that was too low.  At the time LEDs had low L/W compared to conventional, so marketing came to scotopic lumens, scotopic luminance level, scotopically enriched, and other jargon.  Coincidentally our scotopic receptor, the rod, just happened to be in the cool-white band.  Next they claimed "scotopically enriched" sensitivity was achieved while in photopic levels by merely adding blue light.  It never slowed, and that path has interfered with marketing even a decade later. 

Effectiveness of luminance is based on our visual sensitivity.  The colors of three cones (Red-Green-Blue) allow the entire visible spectrum, known as Light, at luminance of 0.6 cd/m2 or higher.  That′s photopic vision and our rods are not active.  Scotopic vision with rods is single color (blue) and visual sensitivity about 0.01 cd/m2 and lower.  Scotopic luminance level is compared to a Full Moon with no other lighting nearby. 

There are thousands of Internet pages misrepresenting LEDs, from claiming extreme scotopic efficacy, photopic peak color stated as orange, showing night time photos with full R-G-B color spectrum while claiming Scotopia.  LED lighting has a reputation, and with the following test results you will see how that may be resolved. 

Human vision for reading, driving, or any activity needing visual detail and focus is called Foveal Vision.   This relies upon those retinal cones directly inline with the Iris and lens.  Foveal Vision does not use rods, and sensitivity is therefore, photopic.  Between the scotopic and photopic sensitivity is a reception range called mesopic.  Luminance transitions covering the mesopic range have an asymmetrical time delay.  Our vision going from mesopic to photopic sensitivity levels needs seconds to adapt;  we may need tens (10s) of seconds going from photopic to mesopic. 

That length of time relates to the luminance range of change.  Aged visual conditions, and alcohol, drugs or other things can modify this timing, but the mesopic up vs down difference still exists.  With periodic photopic light sources in some area, the ambient lighting level must be at least photopic minimum, otherwise objects can′t be seen.  Scotopic luminance is not from lighting at all, and is pedestrian dangerous.  Lumen maintenance is serious there, so LED lifetime hours must radiate photopic levels, or get replaced.

In 2009 a group called Project Candle supported doctoral investigation and lab testing for LED lighting called:
A test of the S/P (scotopic/photopic) ratio as a correlate for brightness perception... 

Following are some overview statements from the Project Candle test:
These color responses are measured in nanometers (nm), and it has been demonstrated that the spectral regions at or around 450, 530, and 610 nm are uniquely efficient at stimulating the visual system when targeting increased brightness and color perception per watt.  The intermediate regions that are centered near 490 and 570 nm are less effective at eliciting perceptions of brightness.  Thornton has called the 450-530-610 nm spectral regions the "prime color" regions and the 490 and 570 nm regions the "anti-prime" [Thornton 1992a].  (These are shown later as 500 and 580 nm.)
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When considered in the context of working interiors, Berman′s assertions have often been simplified to the recommendation of employing lamps with higher Correlated Color Temperature (CCT) since such sources tend to have higher S/P ratios.  Despite the fact that there are many studies in the peer-reviewed literature that conflict with Berman′s conclusions [for example, Smith & Rea 1979, Vrabel and others, 1995, Boyce and others, 2003, Houser and others, 2004, Hu and others, 2006], high CCT lamps are nonetheless being pressed into use because of a desire to reduce energy consumption.

To expand on one of the studies cited above, Houser and his colleagues developed prototype fluorescent lamps based on Thornton′s prime color criteria [Houser and others, 2004].  By adjusting the magnitude of optical radiation within the prime-color and anti-prime spectral regions, they created lamps with low and high CCT and lamps that varied in their trichromatic potential.  Their experiment demonstrated: 1) a pair of lamps with similar S/P ratios that elicited statistically different perceptions of spatial brightness, and 2) a pair of lamps with very different S/P ratios that elicited equivalent perceptions of spatial brightness.  These results cast doubt on the use of the S/P ratio and related measures such as [P.sup.*][(S/P).sup.0.5] as proxies for brightness perception and called into question the practice of using high S/P lamps as an energy saving strategy.  Houser and his colleagues suggested that the perception of brightness is more dependent upon the placement of optical radiation within key spectral regions than on the absolute amount of radiation within each of these regions.

There is little disagreement about the opportunity to reduce energy consumption by better aligning the radiant output of electric light sources with the spectral regions that yield the most beneficial visual response.  There is not agreement about how such tuning should occur.  In North America, many lighting design professionals and building occupants prefer environments to be illuminated with warmer light sources that have lower S/P ratios [Houser and others, 2004].  It has also been shown, both theoretically and experimentally, that higher CCT light sources should not be expected to appear brighter [Hu and others, 2006].  The work presented here provides additional clarity on the best path forward.  As will be shown, in a direct test of the S/P ratio using methodologies comparable to those used by Berman in his original work, the S/P ratio was found to relate poorly to the perception of brightness. 
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In this experiment the same three spectral primaries, which were generated using narrow emitting LEDs (described below), were used to create light spectra with two different CCTs and, correspondingly, two different S/P ratios.  This method minimized the confounding that is characteristic of other work.  Berman and his colleagues radically modified the wavelength components in order to generate light of the same chromaticity but with different S/P ratios [Berman and others, 1990].  In such situations, attributing the visual response (brightness perception) to the derived metric S/P ratio is an act of faith.  The psychophysical response attributed to the S/P ratio of the illuminant could just as plausibly be explained by changes in the wavelengths of the spectral components that combined to form the composite spectra that were evaluated by the subjects.
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The 1990 work of Berman and his colleagues [Berman and others, 1990] made use of lighting conditions that were approximately metameric.  In the years since, their results have been generalized and applied to nonmetameric situations.  Where there is a change in CCT, there is a corresponding hue shift and metamerism no longer exists.  It is important to lighting practice and to the spectral design of lamplight that the conclusions of Berman and his colleagues about the correlation between brightness perception and [P.sup.*][(S/P).sup.0.5] may only apply at constant chromaticity.  We don't dispute the results of their original work, but the data presented herein makes it clear that [P.sup.*][(S/P).sup.0.5] does not generalize to situations where the illumination is not metameric.
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The S/P ratio and CCT were not found to be important predictors of brightness perception.  The light settings evaluated were selected to present the practical extremities of CCT (2900K vs. 7200K) and a large difference in S/P ratio (1.7 vs. 2.6). These very large differences did not lead to statistically different perceptions of brightness at equal luminance with either the rapid-sequential or side-by-side methodologies.  Further, when the ratio [P.sup.*][(S/P).sup.0.5] was made equal, by making luminance unequal, subjects overwhelmingly selected the room with the higher luminance as brighter.  When considered along with other work, we conclude that brightness perceptions at photopic light levels are unrelated to the S/P ratio of the illuminant.

Within this experiment, brightness perception was not perceived to be unequal when the following two conditions were met: 1) luminance was equal; 2) the spectral lights that combined to form the SPD were altered in relative magnitude, but not wavelength.  This provides indirect evidence that spatial brightness perception is dependent upon the placement of optical radiation within key spectral regions, more so than on the magnitude of the radiation within those regions.  Because the visual system is fundamentally a three-channel system, light sources have the potential to be most effective if they are designed to harmoniously stimulate the three visual channels that underlie normal human vision.  Thornton has demonstrated that these peak spectral sensitivities are near 450-530-610 nm, with minima sensitivities near 500 and 580 nm and at the limits of the visual spectrum [Thornton 1992a, 1992b, 1992c].  Light sources tuned to the visual needs of people will almost certainly exploit these underlying characteristics of normal human vision. 

Dr. Kevin W Houser is Editor-in-Chief of LEUKOS and a full member of IESNA.  Some of his findings in this test are considered by those he mentioned to be erroneous, but some findings clearly illustrate the S/P Ratio cannot improve efficacy ratings to the degree once believed.  Dr. Houser is not alone.  My findings, and others′ are that "scotopically enriched" is especially a hopeful act of faith. 

Further, significant results of the test are to actually align the optical radiation where it is useful, and lower it between our receptor peaks where less functional.  Cool, neutral and warm whites should change the radiation levels and keep the nanometers right where they belong.  This can help LED development improve and unify CRI, reduce energy consumption, and raise those Lumens per Dollar.  See full report here.

The above test results AND the S/P ratio ineffectiveness has been known for 5 years.  The IESNA has not accepted S/P ratio as a lighting measurement, therefore, photometric design layouts using IES files have been and likely always will be photopic levels.  Consequently, due to current LED Lumens per Dollar, construction and lighting contractors have been unable to competitively bid LED lighting devices where illuminance is specified and to be certified by licensed professional.

Eventually the solution may be for white LED phosphor color to be aligned with the cones′ reception ("prime-color"), and to reduce output at the minima sensitivity zones ("anti-prime") between the cone peaks.  This method can possibly improve Lumen per Watt ratings by 20% and the cost of LED manufacture will be quickly repaid.  This will raise the Lumens per Dollar that now affects LED lighting market penetration.

When you′ve gathered the facts on human vision, some of the Internet material will be strange and comical.  Be aware that many, many consultants are in business to help clients grow business, thus sales assistance won′t always be truthful.  With a client already given an untruthful path, how can they change?  Real human vision and Laws of Physics have long-term business potential for LED lighting.

This project was made possible by Project CANDLE partners: Cooper Lighting, Erco Lighting, Fisher Marantz Stone, Gabriel Mackinnon, Horton Lees Brogden Lighting Design, I2 Illuminations, IALD Education Trust, Lighting Design Alliance, Litecontrol Corporation, Lutron Electronics, Naomi Miller Lighting Design, Office for Visual Interaction Inc, Penn State University, Philips Lighting Company, Philips SSL Solutions, Randy Burkett Lighting Design, Schuler Shook, and the US Department of Energy (under PNNL Contract Number 79894). Litecontrol is gratefully acknowledged for the donation of the luminaires, Lumileds for the donation of the LEDs, and Lighting Science Group Corporation for the development of the control system hardware and software. Many thanks to Jamie Devenger, Luke Renwick, and Dan Moynagh for assisting with the apparatus and for their help with running the subjects.

IESNA scotopic notice 
Recent court case to resolve mis-advertised lumens see pages 21+

Human vision sensitivities are defined with luminance while light measurement is illuminance.  Illuminance rates the light source in lux or footcandles, and luminance is based upon surface area and its reflectance.  Typical 15% reflectance may be carpeted or garage floors, concrete sidewalks, and many every day surfaces.  With a constant reflectance of 15%, conversion to illiminance is a 2× multiplier. 
With 15% reflectance, 0.6 cd/m2 = 1.2 footcandles (2×)           With 30% reflectance, 0.6 cd/m2 = 0.6 footcandles (1×)
          Footcandles to Lux is about 10× so photopic needs about 12 Lux with that 15% reflectance. 

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