Kitplus - The TV-Bay Magazine

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COMMENT Missing Parts Of The Spectrum Fig. 1 by Graham Jaggers (Lighting Director) S ince the phasing out of conventional filament light bulbs and their replacement with fluorescent and LED, many manufacturers indicate the ‘colour accuracy’ of their lamps with a CRI (Colour Rendering Index) number. It is possible however, to illuminate a scene using lamps with quite good CRI ratings and experience inaccurate colours on a TV system, despite looking ‘normal’ to our eyes. Colour science expert Alan Roberts has developed a different standard for the assessment of lights, which is geared towards illumination for TV cameras. This is becoming the benchmark that manufacturers and TV professionals are turning to in assessing the suitability of a lamp for accurate TV colour rendition. It is known as the Television Lighting Consistency Index (TLCI). The incandescent light bulb has been with us since the 1840’s. Although notoriously ineffi cient and as good at generating heat as light, it also has some excellent qualities, which have helped it survive so long. One of these is colour. The fi lament of a light bulb radiates a broad continuous spectrum of light. It is biased towards the red end of the spectrum, but, what we tend to call “white tungsten light” contains the full spectrum of colours that the human eye can see. Similarly, daylight is biased towards the blue end of the spectrum, but, what we tend to call “white daylight” contains the full spectrum of colours. Over millions of years our eyes have evolved into a design that principally utilises red green and blue sensors (cones) that enable us to have good colour vision in daylight conditions. 44 | KITPLUS - THE TV-BAY MAGAZINE: ISSUE 107 NOVEMBER 2015 Colour cameras work to the same concept, utilising red green and blue sensitive sensors to sample the image formed by a lens. Both camera and the human systems rely on the scene being illuminated by full spectrum white light in order to correctly interpret the refl ected colours within a scene. Quite by chance, the fi lament light bulb has been providing us with camera-friendly illumination that can be colour corrected with a simple white balance, or graded because there is enough colour information in the picture. Now at last, over a hundred and seventy years after the light bulb was patented, we have a number of more effi cient “energy saving” alternative light sources. Both the motion picture and the domestic home markets are using new generations of fl uorescent and LED units to massively cut down on power consumption. Unfortunately, unlike the fi lament light bulb, these units do not naturally produce a continuous spectrum of visible light. Gas fi lled fl uorescents, and LEDs often have parts of the spectrum missing. (See Fig. 1)