About White Light, Color and Illuminance Variability

Posted: January 29, 2009 in General Commentary
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While I appreciate and am an enthusiast for SSL, my personal interest extends beyond the white light universe. I personally belive that in the very long haul, fixed white light will eventually give way to blended color, specifically R+G+B+W. Actually, more like Red Orange + Green + Cyan + 6,500K White . In either case, this approach has the potential of producing the complete white light tonal range, pastel soft light colors, interior color tuning, and exterior vision enhancement. Above all, these combination create the potential for the deployment of “Visual Efficacy” we have not yet had an opportunity to approach.


The Kruithoff Study in 1941 for Philips, suggests observers prefer different CCTs at different illuminance values

I have a personal theorem (unchallenged theory), that the reason we prefer daylight and incandescent sources is not just about color – it’s about variability. Variability in light levels, as well as CCT. My theorem further suggests that the reason fluorescent lighting is considered inferior, creates eye strain, headaches, etc… is that they are too perfect. They produce a uniform color, and a uniform illuminance value, without relief. Supporting this: LEED recognizes the importance of daylighting to the quality of the visual environment; Studies show that individual control of lighting improves occupant comfort, while saving energy (I contend that this includes their changing the light level throughout the day as well); The Kruithoff Study and CIE have found that our light color preference changes as illuminance levels are changed. My concept is that the future of light will be enabled by SSL, in delivering lighting systems that not only deliver the proper amount of light, at an appropriate color, but not to a fixed single point in space. I suggest the ultimate lighting solution will be creating an operating range of illuminance, of color variation. This will be controlled over time to suit daylight conditions, circadian rythum, occupant activity, task engagement, and a random factor to remove any possible descernable pattern to it all. Some changes in color/intensity may be slow enough to not be noticed – like the passing of daylight hours, while others may be visible – like clouds passing.

Of course, this will not happen within todays market structure. To make this a reality will require manufacturers to standardize on both color mixing profile, but on an agreed upon controls protocol, so that all lighting within a space can be working in concert. If every luminaire within a space acts on its own, the potential of truly ugly conditions will emerge. Downlights moving to bright 6,000K color, while wall sconces move to a modified 2,700K mode based on a dimmer setting, will create a cyan-orange color contrast that would be seen as objectionable to anyone. To work, all lighting within a visual field need to be operating in concert – at least in color, and changing at exactly the same rate and to the same CCT and location against the Plankian curve (one shifting above and one below will create objectionable shifts as well.) Many will see this as the perfect excuse to ignore the entire concept, and brush it off as invalid – delaying the concepts deployment, even if some adopt it.


Energy conservation is about suffering the least amount of consumption to produce the maximum gain.The human eye is a variable receiver that adjusts to color and intensity continually, so does not require constant color or intensity to function properly. Not every task requires the same amount of light. For example, typing a letter into a word processor requires less than 5 Fc ambient illuminance, lowering the power used in the monitor as well. Meanwhile, reading fine print in a technical manual requires at least 100 Fc, more if one is past the age of 40. Better still, walking around a desk requires less than 2 Fc, even if the task at the desk demands 50 Fc. True energy conservative design would deliver only 5Fc when and where it is needed, 2 Fc where it is needed, and 100Fc only when and where it is required. We now live in compromised conditions where we get 35-50Fc uniformly. If we consider that the goal is to deliver only the light required and no more, we must consider that this will press the illuminance levels nearer the lower range of photopic vision. At these levels, it has been studied and proven that color has a large impact on our visual acuity. One cannot simply deliver 3500K at 50Fc and dim it to 10Fc and expact to have satisfied occupants – especially if the CRI remains at 80. The lower the light level, the greater the need for color depth in the light delivered. Highly critical tasks also demand high color rendering to support total eye response. This means that rather than pumping more light into the eye, deliver fuller light first, illuminance levels cannot efficiently compensate for poor color delivery. We know this from factory lighting environments, where even at 100Fc +, vision seems somewhat impaired, while we read small print under a halogen lamp just fine at 10% of that level.

The solution is to take lighting to the true next level. We need to pull together research already completed, and engage in new, to determine the specific ties between illuminance needs (intensity), CCT, CRI, and color/intensity variability on human vision. I contend that this will prove to be the end of conventional lighting as we know it. Not only can SSL deliver an underlying energy efficiency and optical advantages, these can be amplified greatly by delivering the variability required to enhance vision enough to substantially reduce illuminance values, and to vary illuminance and light qualities to specifically suit occupant needs, reducing energy demands even further.

While the gravity of the past is strong, escaping that gravity is necessary to any revolution. The lighting industry is rich in transformations that created large new markets for those who engaged the movements. Downlights were the result of rethinking lighting application to apply electric lighting in a way that other technologies could never produce. The fluorescent troffer is the result of rethinking light and applying the new tubular fluorescent lamp. The indirect linear concept is the result of rethinking illuminance levels as much as it was about eliminating shadows and veiling reflection. The future of LEDs and SSL will come from the creation of new lighting systems and approaches that deliver all new and unique desirable value to the market, not from simply stuffing LEDs into old fixture concepts. Stuffing LEDs into existing conventional products is no different than the creation of candle shaped incandescent lamps to electrify an even older obsolete design approach.

Just what might these new products look like? How will they be deployed?  I have several ideas, and would be a fool to share them here (I will share with you for a fee, that’s my business in fact.) Let’s just say that looking intensely at what is available in the market today, conventional or SSL, is the longest and most costly road to discovery.

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