LIGHTING SPECIAL
into a deeper blue. White-emitting LEDs have long struggled with a lack of deep blue-violet colours, which affects how white fabrics and even skin are rendered. Overcoming this challenge required low- level engineering. While blue LEDs were tricky to develop, violet was even tougher and has only recently become practical for high-power lighting. Even before that, the demand for features had already led designers to combine phosphor-converted white LEDs with other types. Take a tungsten- balanced phosphor white, add a blue emitter, and fading in that blue emitter moves the light towards daylight. Add red and green, and we can achieve any colour we like, including plus- or minus- green adjustments. Big-name manufacturers have built essentially that design, but the further from the original white we go, the more it relies on narrow-spectrum colour emitters – and the worse the colour quality gets. Some early lights with variable colour temperature had noticeably poorer colour rendering in daylight than tungsten. More advanced designs incorporate two different white emitters. The latest might not include a white emitter at all, instead relying on other colours to build a high-quality white. This invariably involves phosphor conversion to colours called cyan, mint, teal, lime, amber or orange. Similar to the phosphor white emitters, these colours offer a broader spectrum. The result is a smoother white spectrum and fewer caveats. MEASURING COLOUR QUALITY This has given us the most capable lighting we’ve ever known. However, it heavily applies technology to something we mostly prefer to keep simple. Daylight should look like daylight, tungsten should appear as tungsten, and when illuminating brightly coloured objects or people, the results should be free of unpleasant surprises. It’s natural, perhaps, to look for a straightforward and numeric score which might express how well a light performs, but it’s difficult (actually, it’s impossible) to boil a spectrum of light down to a single number. Even interacting with the user interface in a modern light might
EVEN mediocre lights CAN ILLUMINATE TEST PATCHES WHICH REFLECT A wide range of colours ”
mean knowing how one or more colour charts work. One of the simplest ways to evaluate light quality is by shining it on a series of colours and measuring how well each is illuminated. Averaging the results across many colours provides an overall indicator of a white light’s overall colour quality. For TLCI, the colour patches are taken from the famous Macbeth chart, which works reasonably well because the chart includes some saturated shades of the primary and secondary colours. If the light lacks cyan, for instance, the cyan chip will look dull and the score will reflect that deficiency. The counterexample is CRI, which uses only a fairly pale, desaturated set of colours. Even mediocre lights can
illuminate test patches which reflect a wide range of colours, masking some underlying problems. As a result, CRI is far less sensitive than TLCI. While TLCI is TV- orientated and no set of colour patches can cover every possibility, it’s rare for a light to measure a high TLCI and still create serious problems. More sensitive measurements include the Spectral Similarity Index (SSI), which compares any two light sources. It is often used to evaluate how closely an artificial light matches sunlight, which can be expressed as SSI[CIE D55] for a standard 5500K daylight source. Colorimeters typically make measurements to any of those standards – but still, none of these methods provide us with a definitive yes-or-no answer.
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