Kitplus - The TV-Bay Magazine

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Fig. 2 If pictures illuminated with such lamps are compared to continuous spectrum lighting, there is a colour shift. (See Fig. 2) In the picture each coloured panel and the portrait is a split between a daylight exposure and an LED illuminated exposure. The white balance is correct in both, hence the grey scale matches, but the colours vary widely. It takes complexity in the manufacture of LEDs and fl uorescents to get them good enough for use with TV cameras, so there needs to be a means of assessing and categorising their suitability. Back in the 1960s the system named the Colour Rendering Index was devised for comparing a light source with an ideal or natural light source. The highest possible value was 100 (full spectrum), which would include incandescent lamps and daylight. Fluorescent lights range from about CRI 50 for the basic types, up to about 90 for the best tri-phosphor type. The CRI was developed for domestic and architectural lighting, intended for direct viewing with the human eye. According to Alan Roberts, its colour samples and mathematics are crude by today’s standards and it is not really suitable for use with modern television systems. Some light sources with a high CRI score will still not reproduce accurate colours on TV. However, it is the only colour-difference metric approved by the CIE (the standards body for lighting and colour), so CRI values are commonly displayed for products from domestic compact fl uorescent bulbs to sophisticated TV lights. The Americans made an attempt to improve on the CRI and developed a system named the Colour Quality Scale (CQS), but it was deemed little improvement and it was not adopted. Alan thought there should be a more meaningful index, suited to TV. He remembered that there had been some work done at the BBC in the 1970s that proposed to mimic an entire television camera/display channel to analyse the performance of a luminaire relative to a reference. This forms the basis of Alan’s TLCI system. The highest possible value is still 100, but the scale is stretched at the high quality end to give more discerning results and the calculations more accurately represent a camera system. Trials were held where a number of experienced colourists were given sample material to correct. Lamps of different quality and TLCI value illuminated the samples and the colourists reported how diffi cult the samples were to colour correct. The result confi rmed that the TLCI was an improvement over previous systems and that the scale can be related to the potential diffi culty of post- production colour-correction. (See Fig. 3) Many manufacturers are now getting their lamps tested by Alan Roberts and are adopting TLCI values as an improved way to indicate their spectral quality. Those of us using lamps professionally should be grateful for the determined effort he has made to develop a system that will help us select lamps of appropriate quality for the job. Fig. 3 KITPLUS - THE TV-BAY MAGAZINE: ISSUE 107 NOVEMBER 2015 | 45