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TECHNOLOGY / LIGHT SOURCES 135


Lighting fixture companies are in the process of developing variable CCT LED fixtures but many of them will not meet lighting designers’ specifications. In the first of a two- part article, Geoff Archenhold reports on what is needed to develop a quality lighting system that can vary CCT.


DEFINING COLOUR TUNABILITY


After my presentation for the IALD at Light + Building it was evident from a series of Q&As with over 20 designers that many still had reservations about colour quality and consistency with standard and variable CCT LED fixtures. Interestingly, I picked up that colour consistency is still an issue for large installations using many LED fixtures even though the reputable LED manufacturers use ANSI binning. When I broached the sub- ject of variable colour temperature white lighting it provoked a mixed bag of emo- tions and reservations from those that don’t see the point to those that do but doubt if fixture manufacturers have the skills and knowledge to deliver perfectly consistent colour tuneable LED fixtures with high qual- ity light at reasonable prices. Interestingly, the feedback consistently mentioned that all units seen so far would look great in demonstration with one or two fixtures however in an installation the units would not be consistent with strange light- ing/colouring effects occurring between them. These comments made me wonder as most companies that demonstrate such units only use one or two lighting fixtures and not 10’s or 100’s in an installation. I have been experimenting with LED lighting systems for well over fifteen years and in the process have even developed colour tu- neable LED fixture prototypes back beyond 2000 so I thought it would be interesting to create a series of articles to explain the do’s and don’ts of system design as well as how to specify colour tuneable lighting in your applications. In this first part I will cover the fundamen- tals of light and colour so that the reader is able to understand the limitations of colour tuneable lighting fixtures and what is needed in order to be able to provide consistency of colour.


THE HUMAN PERCEPTION OF COLOUR In order to create a variable CCT LED sys- tem we need to define the challenges that have to be overcome to create a “near” perfect system. The first challenge is the human eye and its perception of colour which is the ability of the eye to distinguish objects based on the wavelengths (or fre- quencies) of the light they reflect, emit, or


Figure 1: The modern model of colour perception in the human eye according to both the trichromatic and opponent process theories.


transmit. Indeed, a colour can be measured and quantified in various ways but within humans it is a subjective process whereby the brain responds to the stimuli that are produced when incoming light reacts with the several types of cone photoreceptors in the eye. In humans, there are three types of cones sensitive to three different spectra, result- ing in trichromatic colour vision as shown by the models in figure 1. Two complementary theories of colour vision are the trichromatic theory and the opponent process theory. The trichromatic theory states that the retina’s three types


of cones are preferentially sensitive to blue, green and red. However the opponent process theory states that the visual system interprets colour in an antagonistic way: red vs. green, blue vs. yellow, black vs. white. Both theories are actually correct and de- scribe different stages in visual physiology. The cones are conventionally labelled ac- cording to the ordering of the wavelengths of the peaks of their spectral sensitivities: short (S), medium (M), and long (L) cone types whose spectral sensitivities are shown in figure 2. Unfortunately, these three cone types do not correspond well to particular colours as


Figure 2: The spectral sensitivity of the S, M and L cones in the human eye.


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