30 Nov 2012

Understanding Colour: Colour Gamuts

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Colour Colour RedShark


Gamut is one of the stranger words in the English language. Phil Rhodes sheds some light on this often misunderstood term

Recently we've talked about colour quite a bit, in terms of the characteristics of the lighting that illuminates objects, and the abilities of cameras and displays to record and reproduce those colours. This is particularly relevant right now, because the newly-announced Sony F55, like the F65, has a sensor designed to include a somewhat wider range of colours than previous electronic systems.

Film has a wide range of available colours

This is important for cinema applications because, quite apart from whatever else is wrong with traditional video cameras, filmmakers accustomed to photochemical emulsions are used to having a very wide range of colours available. Given an appropriately-tinted piece of film, a film projector is theoretically capable of projecting any colour which is emitted by its light source. A typical xenon lamp comfortably covers the entire range of human vision and some way beyond, and the film is free to filter that light to any desired colour. Practical film manufacturing limitations limit the range that's actually available, but it remains true that the colour performance of a film camera and projection system is somewhat user-definable.

Conversely, the display you're staring at as you read this – which is almost certainly a TFT liquid-crystal or possibly OLED – has colour performance which is very much set at manufacture. The backlight of a TFT display may have reasonable coverage of the entire human visual range, but it is tinted by filters which are silk-screened onto the back of the display in the factory. Once you've turned the blue pixels on, for instance, and the red and green pixels completely off, that's as blue as that display can ever get. The only way to achieve deeper colour is to put a filter over the front of the display, and you do not have the flexibility that film can give you.

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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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