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Now that we have solid state lighting that can generate white light with changeable colour temperature, what can we do with it?
Solid state lighting has given us something new: the ability to generate white light with changeable colour temperature. The principle is simple: blend the light generated by warm white and cool white LEDs or vary the proportions of light emitted by red, green and blue LEDs. The result is… magical. Lighting designers who have seen it are entranced by the possibilities.
We spend much of our lives indoors under fluorescent or incandescent lighting. If we are fortunate, we enjoy some daylight through windows or skylights, but otherwise we are disconnected from the outside environment. This is regrettable, as our moods and psychological states are influenced by the daily cycle of sunlight and skylight. What if we could not only dim the lighting but also change the colour temperature to follow the daily cycle or to suit our moods?
This idea is not new – there are US patents dating back to 1996, and at least one European manufacturer (www.spectral-online.de) offers direct/ indirect pendant luminaries and luminous ceilings with colour temperatures that can be varied from 3,000 to 10,000 Kelvin. However, the idea becomes truly practical with solid-state lighting.
People – What can we do with it?
There is no question colour temperature influences our perception of the visual environment. In 1941 AA Kruithof conducted an informal pilot study for the Philips Research Laboratory in which he concluded the following:
- Observers prefer lower colour temperatures at low light levels, and higher colour temperatures and high light levels;
- Low colour temperatures at high light levels make the environment appear artificial and overly colourful; and
- High colour temperatures at low light levels make the environment appear cold and dim.
He further produced the “Kruithof effect” chart shown in Figure 1.
If you read Chapter 3 of the IESNA Lighting Handbook, you will find there is scientific evidence both for and against the existence of this effect. In particular, some researchers have noted that Kruithof likely used fluorescent lamps with low colour rendering indices (CRIs), and so his results may have been influenced by the observers’ colour preferences. Other researchers have noted that the effect disappears after the observer adapts to the colour temperature of the light source.
On the other hand, these more exacting studies have necessarily been performed using luminaires with the same colour temperature. Most environments have a complex mix of fluorescent and incandescent lamps, not to mention daylight entering through windows and skylights. In such situations, our adaptation is constantly changing as we shift our attention from one visual task to another. Scientific studies notwithstanding, the Kruithof effect is likely relevant to some extent for everyday visual environments.
The key phrase, however, is “colour preference.” Our colour preferences are notoriously difficult to measure in the laboratory, let alone quantify as a lighting design rule. As such, Figure 1 should be viewed as an informal design guide, to be interpreted with due consideration of other knowledge and design experience.
Speaking of colour preferences, fluorescent lamp sales have consistently shown that consumers in the southern United States prefer cool white lamps for residential use, while those in more temperature climates prefer warm white. Similarly, consumers in India and other tropical climates prefer cool white. This makes intuitive sense in that people in temperate climates seek a sense of warmth for their homes, while those in tropical climates seek relief from the heat. (See, however, the sidebar The Language of Colour for another possible explanation.) Even if the Kruithof effect is discounted, something is determining consumer preferences.
If you choose to design with Figure 1 as a guideline, you should keep chromatic adaptation in mind. If the colour temperature of the lamps illuminating an environment suddenly changes, we initially perceive both the light sources and white surfaces as being either yellowish or bluish in colour (depending on the change in colour temperature). After about sixty seconds, change and we see the light sources and white surfaces as being white again.
Depending on the skill of the lighting designer, this can be a good situation. Mixed illumination can result in visually interesting and pleasing environments. Having the ability to dynamically change the colour temperature provides the designer with a new palette of lighting design options. Think of high-end residences, long-term care facilities, restaurants, hotel lobbies and retail store displays as possibilities. These environments already make good use of dimmable incandescent lamps; solid state lighting with changeable colour temperature greatly expands the design opportunities.
Consider the reception desk in a hotel lobby. If there is a considerable amount of daylight entering the lobby, the colour temperature of the luminaires illuminating the desk can track or otherwise complement the spectral distribution of the daylight. High colour temperatures during sunny days may be appropriate, but cloudy days with rain may benefit from warm colour temperatures to offset the mood of the inclement weather.
What about long-term care facilities? Rather than spending endless hours under constant illumination, residents may enjoy recreational spaces in which the lighting intensity and colour temperature vary throughout the day. It may be easy to overdo the effect with faddish lighting, but subtle lighting changes may provide welcome relief from the monotony.
Lighting Research – What about it?
There are numerous scientific studies concerning colour temperature and colour preferences that demonstrate the importance of temperature. For example, warm colour temperatures apparently encourage people to resolve interpersonal conflicts through collaboration rather than avoidance, while cool colour temperatures make it easier to read printed books and papers. Interestingly, it is the surround light in our peripheral vision rather than the task lighting that most influences our visual acuity. This implies that we can have warm white task lighting for reading (warmth and comfort) with cool white ambient lighting (bright and cheerful).
A more complex issue is whether simulating the daily cycle of sunlight and skylight with changes in both light levels and colour temperature is beneficial for night shift workers. A study done by the Lighting Research Center (www.lrc.rpi.edu) in 1997 found that mimicking changes in daylight intensity and colour temperature improved workers’ ability to handle mentally demanding tasks throughout eight-hour night shifts. However, the study did not attempt to separate the influence of changes intensity and colour temperature.
The Lighting Research Center has since been particularly active in studying the relationship between lighting intensity, spectral power distribution, and circadian rhythms (e.g., “Research Matters,” LD+A, May 2006). In brief, our retinas convert light into neural signals that regulate our bodies’ sleep patterns and wakefulness. Whether mimicking the changes in daylight colour temperature has an effect on circadian rhythms is apparently an open question. If it does, then this may be useful in designing lighting system for night-shift workers and control-room operators.
There is, however, a caveat to interpreting lighting and vision research for design guidelines. Many experiments necessarily involve simplified visual environments that attempt to isolate and study single factors. However, we live and work in visually complex environments, and it is precisely these confounding influences that affect our overall perception and mental states. Therefore, it is best to consider research results as points of reference when making design decisions and as a possible explanation when a particular design succeeds or fails.
Displays – What are the uses and benefits?
Displays are an obvious candidate for solid state lighting. Gold jewellery, for example, looks best when illuminated with warm white light because it emphasizes the yellow highlights. Silver jewellery, on the other hand, looks best under cool white light because silver highlights are spectrally neutral. Having controllable colour temperature enables the shop owner to easily tune the lighting digitally as needed to best showcase the items on display (Figure 2).
More generally, most art galleries and museums currently choose incandescent light sources with a colour temperature of 3,000K to illuminate their displays. However, several small-scale studies have shown that viewers prefer lighting with slightly higher colour temperatures (around 3,600K) because it maximizes the chromatic diversity of the artwork. More generally, higher colour temperatures are preferred for works of art with predominant blues and violets, while lower colour temperatures are preferred for predominant yellows and reds.
Other advantages of higher colour temperatures include increased colour saturation, a greater sense of depth (due to the effects of chromatic aberration in the human eye), and improved brightness and clarity. This must be balanced, however, against the intentions of the artist or sensibilities of the museum curator.
As an example, landscape paintings that are painted outdoors are illuminated with perhaps 20,000 lux and colour temperatures ranging from 5,000 to 15,000 kelvin. When they are viewed indoors with electric lighting, incandescent lamps ma provide 200 to 500 lux. It is perhaps not a coincidence that the preferred colour temperatures follow Kruithof’s recommendation of 3,000 to 3,500 kelvin.
Perhaps the best feature of solid state lighting for displays is that the colour temperature can be easily changed independently of the intensity, and without the need for colour correction filters or constant latter time by maintenance staff. Other benefits include complete freedom from damaging ultraviolet and infrared radiation.
Specialty Lighting – Who needs what?
There are numerous niche applications that can benefit from controllable colour temperature. Trumpf Medical Systems (www.trumpf-med.com) has recently introduced LED-based surgical lights that allow the surgeon to change the colour temperature according to the type of procedure is useful for examining skin and light tissue parts, while cool white light (6,500 K) is preferred for viewing internal organs and performing lengthy surgical procedures. Higher colour temperatures are also preferred for minimizing eyestrain and maintaining concentration.
The same principle will likely apply to any craftsperson involved in visually demanding tasks that require lengthy periods of concentration, from hand-sewing to diamond-cutting. A high colour temperature in the surround light causes our pupils to contract, which improves the depth of field and reduces spherical and chromatic aberrations in the eye. We also perceive the illumination as being brighter than low-colour-temperature lighting. However, being able to change the colour temperature on demand enables the user to view the object under normal indoor lighting conditions when desired.
Finally, dentists prefer high colour temperatures because it is easier to identify caries during examinations and to differentiate dentin from enamel when performing dental work. At the same time, they need to match porcelain crowns and composite resin fillings with tooth colouration. Having the ability to vary the colour temperature could be a valuable asset.
Conclusions
Solid State lighting providing white light with changeable colour temperature is something new. Like all new technologies, lighting designers need to learn from experience what works and what does not. We will undoubtedly see signature applications that will capture our attention today, but which in five to ten years’ time will prompt us to ask, “What were we thinking?”
More likely, however, we will discover applications for white light with controllable colour temperature that will in five to ten years’ time prompt us to ask, “How did we ever live without it?” It is a new capability for professional lighting design, and ours to discover.
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